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absorbance click here for a wiki article on absorbance


The energy carried by an electromagnetic wave is not continuous, but is propagated in finite parcels called packets. Radiation is absorbed by a substance when the energy of the radiation corresponds to that needed to increase the potential energy of the substance by one or more increments. The transfer of energy is achieved by the interaction of its electric vector with the substance. Absorption of UV/Visible radiation changes the electronic state of a molecule and can raise an electron from ground state to an excited state. The ground state is one in which all of the electrons are in their most stable orbits. An excited state is one in which at least one of the electrons occupies an orbit of higher energy than the ground state. If the wavelength of the radiation passing through a substance is programmed, and the transmitted light monitored, then the curve relating sensor output to wavelength will show a series of sharp adsorption peaks or bands that occur at frequencies where the radiation energy (h?) is equal to that of specific electronic transitions in the molecules of the substance. This curve is called a spectrum. In solution, a molecule may exhibit numerous adsorption levels having energies very close to one another. The bands may be so close that they cannot be observed individually and, as a result, they occur under one envelope giving a broad band often seen in a UV adsorption spectrum.

Beamsplitters click here for a wiki article on Beamsplitters


A beam splitter consists of a very thin film of germanium supported on a transparent window. A beam splitter reflects 50% of the light falling on it and allows 50% of the light to pass through it. If the splitter is set at an angle then 50% of the light will be reflected away from the incident path and 50% will continue along the line of the incident light. A beam splitter is often used to produce two beams of light that are allowed to follow different optical paths and finally made to join and produce an interferogram that will be a function of the difference in path lengths.

Visible click here for a wiki article on Visible


"The wavelength range between 400 nm to 700 nm is considered the whole of the visible spectrum. The eye does not have the same sensitivity to the different wavelengths or colours in the visible region. The eye is most sensitive to green light and fairly sensitive to yellow and orange light. Thus, most road warning signs are now in orange and yellow as opposed to the older signs that were in red. Police jackets and road workers jackets are in yellow and orange so that they can be more clearly seen. The response of the retina of the eye to red and blue is poor and so signs with black or blue letters on a red background are difficult for older people to see, The eye, however, is very sensitive to slight changes in light intensity and so accurate measurements can be made visually for analytical purposes if very similar light intensities are compared (i.e. by comparators, comparing the colour intensity of a sample with that of a standard).

Beer's Law click here for a wiki article on Beer's Law


The relationship between the intensity of light transmitted through a cell and the concentration of the absorbing solute in the cell is given by Beer's Law. Beer’s Law states that the intensity of the transmitted light is proportional to the intensity of the incident light times the exponent of the product of a constant, the path length and the concentration of the solute in the cell. The constant is called the extinction coefficient. It follows that the logarithm of the ratio of the intensity of the transmitted light to the intensity of the incident light is equal to product of the extinction coefficient, the path length and the concentration of the solute in the cell. This product is called the Absorbance. It is also seen that the logarithm of the ratio of the intensity of the transmitted light to the intensity of the incident light is linearly related to the solute concentration and/or the path length. It follows that measurement of this ratio will allow the solute concentration to be calculated.

Diode Array click here for a wiki article on Diode Array


A diode array consists of a series of tiny photo diodes that may be many hundreds in number lined up, side-by-side and electrically insulated from one another. Each photo diode gives an output that is proportional to the intensity of light falling on it. Diode arrays are used extensively in many types of spectrometer. A beam of light can be dispersed by means of a prism or diffraction grating across a diode array and the each diode will then measure the intensity of the light of that particular wavelength that is striking its surface. By reading, sequentially, the output from each diode, a curve can be obtained relating the intensity of light falling on each diode against the wavelength of the light i.e. a spectrum. The resolution that is obtained from the system will depend on the extent of the dispersion and the number of photo diodes in the array.

Deuterium Discharge Lamp click here for a wiki article on Deuterium Discharge Lamp


The deuterium discharge lamp emits UV light over a broad wavelength range from about 120 nm to 400 nm but not at strictly the same intensity. It normally operates with a constant discharge current. The deuterium lamp is the preferred source of UV light for UV/Vis spectrometers. If the envelope is made from impure silica then light emission is restricted to a minimum of about 200 nm and, thus, eliminates the production of ozone. For wavelengths down to 150 nm, then a pure synthetic silica envelope must be used and steps must be taken to remove the ozone that will be produced.

Low Pressure Zinc Discharge Lamp click here for a wiki article on Low Pressure Zinc Discharge Lamp


"Low Pressure Zinc Discharge Lamp"

High Pressure Mercury Discharge Lamp click here for a wiki article on High Pressure Mercury Discharge Lamp


The low-pressure mercury discharge lamp emits light almost exclusively (>96%) at 253.7 nm and so is not strictly monochromatic. The low-pressure mercury discharge lamp, however, is relatively inexpensive and, consequently, it is a popular UV light source for UV absorption chromatography detectors. It does emit some light below 200nm but most of this light is absorbed by the material from which the envelope is made.

Low Pressure Cadmium Lamp click here for a wiki article on Low Pressure Cadmium Lamp


The low-pressure cadmium discharge lamp, although emitting light at specific wavelengths (including those in the UV region), has so many emission lines that it is basically a polychromatic light source. It's major emissions lines are at 228.8 nm, 326.1 nm, 335.4 nm, 340.3 nm, and 346.6 nm. There are, however, at least seven other emission lines of significant intensity going down to 214.4 nm. Employing suitable interference filters, certain specific wavelengths may be selected for particular applications but, in general, the cadmium low pressure discharge lamp is not in common use. "

Tungsten Halogen Lamp click here for a wiki article on Tungsten Halogen Lamp


The tungsten halogen lamp is the most popular choice to provide visible and near infra red light. One of the reasons is that it has a smooth spectral emission curve and thus, does not produce peak outputs at specific wavelengths as many other lamps do. The filaments of tungsten halogen lamps are doped to help provide a strong emission and are contained in a silica or quartz envelope which transmits light well below 350nm. The filaments operate at about 3000 K. The envelope is usually filled with one of the rare gases containing a trace of a halogen gas.

Lens and Window Material in Spectrometers click here for a wiki article on Lens and Window Material in Spectrometers


Lens and windows in the optical system of spectrometers must efficiently transmit light of the wavelength pertinent to the specific spectrometer. The UV spectrometer must use lens and window materials transparent to UV. The three most common materials used in the UV and visible wavelength range for lenses and windows are borosilicate glass (Pyrex), impure quartz, and very pure synthetic silica (pure quartz). Pure silica glass can transmit light down to a wavelength of about 150 micron but impure silica can only transmit down to about 200 micron. All three transmit visible radiation although borosilicate glass is more often used to make coloured glass filters (e.g. cobalt blue filters). The substances transparent to infrared light are mostly halide salts of the alkali and alkali earth metals, e.g., sodium and potassium chlorides, potassium and caesium bromides. caesium iodide and calcium and barium fluorides. Two exceptions that are also in common use as windows are silver chloride (horn silver) and antimony sulphide. Care must be taken to ensure water does not come in contact with certain materials (e.g. the alkali chlorides) also some are soluble in alcohol (e.g. sodium chloride).

Cuvettes click here for a wiki article on Cuvettes


Cuvette is the name given to a sample cell used for measuring UV or visible light absorption by liquids or solutions. Cuvettes are usually manufactured from quartz or pure fused silica and have carefully fabricated parallel faces with accurately known separation distances (which is the path length of the cell). Cells with path lengths ranging from one or two millimetres to 10 cm are available. If only visible light is to be used by the spectrometer then the cells can be made of Pyrex glass.

Luminescence click here for a wiki article on Luminescence


Light emitted from material at high temperatures is called incandescence. Light emitted from a body by processes other than high temperature emission is called luminescence. When molecules are excited by electromagnetic radiation to produce luminescence, the luminescence is called photoluminescence. If the release of electromagnetic energy is immediate, or stops on the removal of the exciting radiation, the substance is said to be fluorescent. If the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent. There are other forms of luminescence. Light emitted from a gas discharge lamp (e.g. a neon lamp) is called electroluminescence and light emitted during radioactive decay is called radio luminescence. If light is emitted by a chemical reaction then this process is called chemiluminescence and if the reaction has a biological origin (e.g. the firefly) then the light emission is called bioluminescence.

Photoluminescence click here for a wiki article on Photoluminescence


Photoluminescence is a form of luminescence that is produced when molecules are excited by electromagnetic radiation. If the release of electromagnetic energy is immediate, or stops on the removal of the exciting radiation, the photoluminescence is called fluorescence. If the release of energy is delayed, or persists after the removal of the exciting radiation, then the photoluminescence is called phosphorescence. Both the light emitted by fluorescence and phosphorescence has a longer wavelength than the excitation light.

Fluorescence click here for a wiki article on Fluorescence


Fluorescence is a form of photoluminescence which, in turn, is a form of luminescence. When molecules are excited by electromagnetic radiation to produce photoluminescence and the release of electromagnetic energy is immediate, or stops on the removal of the exciting radiation, the substance is said to be fluorescent. During fluorescence light is absorbed at a particular wavelength and emitted at a different wavelength. In the process of absorption and electron is raised to and excited state and when fluorescence takes place it returns to its original energy level and emits light. As some energy is lost during the process, the emitted light has a longer wavelength than the excitation light.

Phosphorescence click here for a wiki article on Phosphorescence


When molecules are excited by electromagnetic radiation to produce luminescence, the luminescence is called photoluminescence. If the release of electromagnetic energy is immediate, or stops on the removal of the exciting radiation, the substance is said to be fluorescent. If the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent. During phosphorescence light is absorbed at a particular wavelength and emitted at a different wavelength. In the process of absorption and electron is raised to and excited state and when phosphorescence takes place it returns to its original energy level and emits light As some energy is lost during the process the emitted light has a longer wavelength than the excitation light.

Bioluminescence click here for a wiki article on Bioluminescence


Luminescence can occur due to a variety of physical and chemical reactions. Light can be emitted by living organisms which is a form of luminescence called bioluminescence. For example the light emitted by the firefly is the result of a chemical reaction involving an enzyme called luminase.

Radioluminescence click here for a wiki article on Radioluminescence


Some radioactive materials are luminescent as a result of light having a wavelength within the visible region being emitted during radioactive decay. This type of luminescence is called is called radioluminescence. Radioluminous materials were once used to provide luminous numbers etc. on hand watches.

Electroluminescence click here for a wiki article on Electroluminescence


The luminescence due to light emitted from a gas discharge lamp (e.g. a neon lamp) is called electroluminescence. The wavelength of the emitted light will depend on the nature of the gas surrounding the discharge electrodes and is usually one of the rare gasses or a mixture of the rare gasses.

Fluorescence Reagents click here for a wiki article on Fluorescence Reagents


In chromatography many substances are not detectable by UV absorption and sometimes other detection methods are insufficiently sensitive. Under such circumstances, fluorescent derivatives are prepared so that they can be sensed by a fluorescence detector. The fluorescent substance can be seen even when mixed with other substances that do not fluoresce and can be detected at levels down to 10 ng/ml or lower. There are a number of reagents available for the preparation of fluorescence derivatives; the following are two examples. One of the most popular fluorescent reagents is 5-dimethyl aminonaphthalene-1-sulphonyl chloride (dansyl chloride, DNS-chloride or DNS-Cl). Dansyl chloride reacts with phenols and primary and secondary amines under slightly basic conditions to form a fluorescent suphonate ester or suphonamide. The detection limits of the dansyl derivatives are often in the low nanogram range (ca 1 x 10-9 g/ml) and the excitation and emission maxima can vary between 350-370 nm for excitation and 490-540 nm for emission. Fluorescamine (4-phenylspiro(furan-2-(3H),1'-phthalan)3,3'-dione) is also a commonly used fluorescence reagent. It reacts almost instantly and selectively with primary amines, while the excess of the reagent is hydrolyzed to a non-fluorescent product. The reagent itself is non-fluorescent. The excitation and emission wavelengths are 390 nm and 475 nm respectively. Fluorescamine has been employed in the analysis of polyamines, catecholamines and amino acids.

Spectrum click here for a wiki article on Spectrum


There are two types of spectrum, the transmission spectrum and the absorption spectrum. The transmission spectrum is a curve relating light transmitted though the medium or sample to either the frequency or the wavelength of the incident light. If the incident light is in the infrared range then the intensity of the transmitted light is sometimes related to the reciprocal of the wavelength called the wave number. The absorption spectrum is a curve relating the intensity of the light absorbed by the medium or sample solution to the wavelength, frequency or wave number of the incident light.

Diffraction Grating click here for a wiki article on Diffraction Grating


A diffraction grating is an optical device, which, in effect, consists of an array of narrow slits, or grooves that generate a large number parallel beams of light. These parallel beams of light can optically interfere and produce a spectrum. Such gratings are used to disperse electromagnetic waves and are frequently employed in the construction of the monochromators that are used in both UV/Visible and IR instruments. Gratings are far more efficient than prisms that originally were used for the same purpose. Gratings can be constructed by ruling lines on a suitable substrate.

Interferogram click here for a wiki article on Interferogram


An interferogram is a signal that is produced by the change of path length between two interfering light beams. In Fourier Transform IR, it is the change in light intensity produced as a function of the actual path length difference between two light beams that is recorded. The two domains of distance and frequency are interconvertable by the mathematical procedure called `Fourier Transformation. Thus, by calculation, a relationship between intensity and path length difference can be transformed to a relationship between intensity and frequency which takes the form of the normal spectrum.

Fourier Transform IR Spectrometer click here for a wiki article on Fourier Transform IR Spectrometer


The FT-IR spectrometer operates in the following manner. Light from a broadband infrared source impinges on a beam splitter that comprises a very thin film of germanium. Approximately 50% of the light passes through the film and is reflected back along its path by a fixed mirror, where half of the light intensity (25% of the original light intensity) is reflected by the same beam splitter, through the sample cell, to the infrared sensor. The other 50% fraction of the incident light is reflected at right angles to its incident path onto a moving mirror. Light from the moving mirror returns along its original path and half of the light intensity is transmitted through the beam splitter, through the sample cell, to the infrared sensor. As a result, 25% of the incident collimated light from the source reaches the sensor from the fixed mirror and 25% from the movable mirror. Now the path length of the two light beams striking the sensor will be different so there will be destructive and constructive interference. The system constitutes a form of the Michelson interferometer. The light falling on the sensor is monitored relative to the movement of the mirror producing and interferogram. Carrying out a Fourier Transform on the interferogram data produces a spectrum that relates intensity to frequency (or wavelength or wave number).

FT-IR click here for a wiki article on FT-IR


The Fourier Transform IR Spectrometer spectrometer operates in the following manner. Light from a broadband infrared source impinges on a beam splitter that comprises a very thin film of germanium. Approximately 50% of the light passes through the film and is reflected back along its path by a fixed mirror, where half of the light intensity (25% of the original light intensity) is reflected by the same beam splitter, through the sample cell, to the infrared sensor. The other 50% fraction of the incident light is reflected at right angles to its incident path onto a moving mirror. Light from the moving mirror returns along its original path and half of the light intensity is transmitted through the beam splitter, through the sample cell, to the infrared sensor. As a result, 25% of the incident collimated light from the source reaches the sensor from the fixed mirror and 25% from the movable mirror. Now the path length of the two light beams striking the sensor will be different so there will be destructive and constructive interference. The system constitutes a form of the Michelson interferometer. The light falling on the sensor is monitored relative to the movement of the mirror producing and interferogram. Carrying out a Fourier Transform on the interferogram data produces a spectrum that relates intensity to frequency (or wavelength or wave number).

Halide Disks click here for a wiki article on Halide Disks


The halide disk is a device for placing solid samples into an IR spectrometer. A number of different alkyl halides can be used but potassium bromide is one of the more common. A few milligrams of the sample is mixed with about 150 mg of dry halide powder and the two substances intimately ground together in a mortar. The mixture is then transferred to a circular dye a few millimeters in diameter and a millimeter or so thick. The mixture is pressed under vacuum into a disk in an IR disk press. The pressure employed is usually about 1.6 x 105 kgcm-2. At this pressure the material is sintered and a clear transparent disk should be produced. The disk of potassium bromide incorporating the sample is then placed in a sample disk holder and inserted into the spectrometer. Potassium bromide is transparent to light in the mid-infra red region.

Mull Samples click here for a wiki article on Mull Samples


The preparation of a mull sample is a means of placing solid material into the IR spectrometer for absorption measurement. Ideally, the mull sample will contain the substance of interest dispersed in a liquid that does not absorb infrared radiation over that range of wavelengths that would interfere with the sample measurement, One of the more common mulling agents is liquid paraffin (Nujol). To prepare a mull, the sample is first ground to a fine powder in a pestle and mortar and then about 50 mg is suspended in about 20 ?l of the mulling agent. The suspension is further ground into a smooth paste. To obtain suitable spectra the proportions of sample to Nujol may need to be adjusted and the particle size of the sample must be sufficiently small. The sample may then be sandwiched between two polished sodium chloride disks to form a thin film of the mull, the disks mounted in a suitable disk holder and the absorption spectra taken. For some samples it may be helpful to subtract the spectrum of Nujol from the sample spectrum to obtain a spectrum that is more exclusive to the sample.

Film Samples for IR Spectroscopy click here for a wiki article on Film Samples for IR Spectroscopy


The production of a thin film of the material of interest is an excellent method of sample preparation for subsequent infrared examination. Film samples are particularly useful for the examination of polymers and low melting point solids. Suitable thin films can be formed by deposition from a solvent onto a suitable surface or by melting the sample and pressing the melt between two plates. The film is then pealed from the plates and supported appropriately in the infrared light path or placed between two halide disks which are placed in the infra light path. The film can also be formed on an infrared window using a similar procedure. If the sample melts at an appropriate temperature and remains stable, it can be hot-pressed into a film by means of a hydraulic press.

Light Pipes click here for a wiki article on Light Pipes


A light pipe is a small volume gas cell for measuring infrared spectra of gasses, vapors or gasses containing vapors. The light pipe was originally designed to provide on line spectra from peaks eluted from a gas chromatography capillary column where the amount of solute eluted may be less than one microgram. Light pipes consist of tubes of circular or rectangular cross-section with highly reflecting internal surfaces that are usually produced by gold plating. The light source is moved back from the face of the light pipe, so that the focus, which normally coincides with the entrance of a gas cell, is transferred to the exit of the light pipe, however the process is not completely efficient. Internal reflections at the walls of the light pipe results in the 'apparent path length' being increased by about 33% with a consequent increase in sensitivity. When used with a capillary column as a gas chromatography/infrared interface, the capillary column is passed some way into the entry tube and a scavenger gas is introduced between the column and inlet tube. The scavenger gas removes the solutes rapidly from the light pipe so that more than one solute can not exist in the light pipe at the same time. Many modern GC/IR systems employ light pipes to augment the IR signal.

Attenuated Total Reflectance Spectroscopy click here for a wiki article on Attenuated Total Reflectance Spectroscopy


ATR is a technique used in infrared spectroscopy. A light beam entering a crystal will undergo complete internal reflection if the angle of incidence is greater than the critical angle which will be a function of the refractive indices of the two surfaces. On striking the surface, the beam will penetrate the surface slightly and, if the substance absorbs light at the wavelength of the incident light, some of the light will be absorbed. The intensity of the attenuated radiation can then be plotted against the wavelength of the incident light and an absorption spectrum will be obtained. Typical materials that can be used as crystals for total reflectance spectroscopy are zinc selenide (RI 2.4, 20000-500 cm-1), germanium (RI 4.0, 5000-550 cm-1) and thallium/iodide (RI 2.4, 17000-250 cm-1). The common factors between all the materials are that they are relatively insoluble in water and have high refractive indicies.

Multiple Internal Reflectance click here for a wiki article on Multiple Internal Reflectance


Multiple internal reflectance techniques are used in infrared spectroscopy and are similar to internal reflection (ATR) methods but produce more intense spectra as a result of an optical system that allows multiple reflections. In contrast to attenuated total reflectance that usually employs a prism, multiple internal reflectance techniques employ specially shaped crystals that allow multiple reflections inside the crystal. Such crystals may produce as many as 25 multiple reflections.

External Reflectance click here for a wiki article on External Reflectance


If light is focused on the external surface of a sample two forms of reflectance can occur. The first is specular reflectance and the second diffuse reflectance. Both forms of reflectance can be used in infrared spectroscopy to produce spectra from a sample, however, in order to have effective and useful reflectance, the surface must either, itself be reflective or be attached to a reflective backing.

Specular Reflectance click here for a wiki article on Specular Reflectance


Specular reflection occurs when the angle of reflection equals the angle of incidence. The amount of reflected light in specular reflectance is determined by the angle of incidence, the refractive index of the sample, the surface roughness and the absorption properties of the sample. Thus, if a sample is coated on the surface of the reflector, the reflected light will be devoid of some of those frequencies absorbed by the sample. If grazing angles of incidence are employed, this, in effect, increases the path length through any coating on the surface and, consequently, increases the amount of absorption. For coatings on the reflective surface of one micron or more in thickness, then the angle of incidence and reflection are normally about 30o, however, grazing angles of up to 85o can be used if necessary.

Diffuse Reflectance click here for a wiki article on Diffuse Reflectance


Diffuse reflectance is a technique used in infrared spectroscopy. When incident light strikes a surface, the light that penetrates is reflected in all directions and this is called diffuse reflectance. As the light that leaves the surface has passed through a thin layer of the reflecting material, its wavelength content will have been modified by the optical properties of the matrix. Consequently, the wavelength and intensity distribution of the reflected light will contain structural information on the substrate. By analysing the diffuse reflected light the absorption characteristics of the sample can be determined. It is clear that this process would be applicable to the study of surfaces and coatings and could obviously be employed to scan TLC plates.

Photoacoustic Spectroscopy click here for a wiki article on Photoacoustic Spectroscopy


Photoacoustic Spectroscopy is a technique used in infrared spectroscopy to examine surfaces. The incident radiation is allowed to fall on the sample contained in a suitable enclosure. When infrared radiation is absorbed by the sample, the substance heats and cools in response to the absorption of the radiation received. Situated in the enclosure is an acoustic sensing device (a device that records small changes in pressure), which may be a simple microphone or a piezoelectric sensor. The sensor detects the acoustic (pressure) pulses (caused by the heating of the surrounding gas) due to the adsorption of the different IR frequencies to which the surface is exposed. Briefly, in photoacoustic spectroscopy, the heat energy evolved by the absorption of the infrared radiation causes coincident pressure pulses in the surrounding gas which are sensed by an appropriate acoustic measuring device.

Beam Splitter click here for a wiki article on Beam Splitter


A beam splitter consists of a very thin film of germanium supported on a transparent window. A beam splitter reflects 50% of the light falling on it and allows 50% of the light to pass through it. If the splitter is set at an angle then 50% of the light will be reflected away from the incident path and 50% will continue along the line of the incident light. A beam splitter is often used to produce two beams of light that are allowed to follow different optical paths and finally made to join and produce an interferogram that will be a function of the difference in path lengths.

Raman Scattering click here for a wiki article on Raman Scattering


The process of Raman scattering, it is similar, but not exactly the same, as fluorescence. If a molecule absorbs incident radiation and, as a consequence, is raised to a higher level of energy, the molecule then emits light at the Raman frequency and falls to a new energy level, usually somewhere between the initial and final states. This process is called Raman Scattering. The scattered light will contain frequencies that differ from that of the incident light and will be characteristic of the substance that causes the scattering. The Raman frequency is defined as the difference between the frequency of the incident light and that of the scattered light. There are two forms of Raman scattering; if the light excites the molecule from ground level and then falls back to an energy level emitting light of a frequency lower than the incident light then this is called Stokes-Raman scattering. If a molecule already exits at a raised energy level and is raised further to a higher level of excited state and then falls back to the ground state, the frequency of the scattered light will be greater than that of the incident light and this is called Anti-Stokes-Raman scattering.

Rayleigh Scattering click here for a wiki article on Rayleigh Scattering


If a substance is exposed to radiation of a defined frequency, then some light will be scattered at right angles. If the excitation energy raises the molecule from ground state to an excited level and then falls back to the ground state, the frequency of the light emitted will be the same as that of the incident light and this phenomenon is called Rayleigh scattering. The ratio of the intensity of the scattered light to the intensity of the incident light is equal to the product of the attenuation constant, a function of the refractive index and the Rayleigh constant. The Rayleigh constant, in turn, is a rather involved function of the molecular weight of the scattering material, the refractive index of the solvent, the wavelength of the incident light and Avogadro's number. It is seen that from the intensity of the scattered light the molecular weight of a substance could be determined or assessed.

Raman Spectroscopy click here for a wiki article on Raman Spectroscopy


Raman scattering can be observed as a change in frequency of a small percentage of the intensity in a monochromatic beam of light interacting with the material of interest. In Raman spectroscopy the frequency changes occur as the result of coupling between the incident radiation and the vibrational energy levels of the molecules of the scattering material. A monochromatic beam of light is used to measure Raman spectra and sometimes the effect can inadvertently be seen in the measurement of fluorescence. The frequency of the scattered light is determined by the structure of the molecules of the sample and, thus, the Raman spectrum can be used to confirm the identity of a substance or help elucidate its structure.

Atomic Spectroscopy click here for a wiki article on Atomic Spectroscopy


When gases or vapours are heated, spectroscopic examination of the light emitted discloses a series of lines, often very complicated in structure, at those specific wavelengths that are characteristic of the elements present. These bands, or lines of emitted light, represent energy changes that occur when electrons orbiting the nucleus of the respective atom change energy levels. Conversely, if light having a frequency(s) characteristic of a particular element is passed through a vapour sample containing this element, then some of the light will be absorbed. From the amount of light emitted or absorbed, the amount of the element present can be determined. A study of the emitted light or absorbed light from a heated vapour sample is called atomic spectroscopy

Atomic Emission Spectroscopy click here for a wiki article on Atomic Emission Spectroscopy


When gases or vapours are heated, light of different frequencies is emitted and the specific frequencies of emission are characteristic of the elements present. The bands, or lines in the spectrum of the emitted light, represent energy changes that occur when electrons orbiting the nucleus of the respective atom change energy levels. The study of this emission process is called atomic emission spectroscopy. Atomic emission spectroscopy can be used to identify the presence of an element from it line pattern or to identify the amount of material present from the intensity of one or more bands.

Atomic Absorption Spectroscopy click here for a wiki article on Atomic Absorption Spectroscopy


Atomic absorption spectroscopy is the compliment of atomic emission spectroscopy in that it is based on the measurement of the radiation energy absorbed by free atoms when in the gaseous state. To identify the presence or amount of a particular element that is present in a particular vapour sample, light frequencies characteristic of the element are passed through the vapour. The amount of light absorbed will be proportional to the amount of the element present in the vapour. The bands, or lines in the spectrum of the absorbed light, represent energy changes that occur when electrons orbiting the nucleus of the respective atom are excited by the radiation to higher energy levels.

The Inductively Coupled Plasma Torch click here for a wiki article on The Inductively Coupled Plasma Torch


The ICP torch consists of three concentric tubes made of quartz, A copper coil surrounds the top portion of the torch, though which water circulates for cooling purposes, and is connected to a radio frequency source. Argon or helium is commonly used as the nebulizing gas and a second flow of argon enters the base of the torch and acts as both a coolant and as part of the plasma-forming agent. The base of the ICP torch can also be cooled by a water jacket. A spark produced by a Tesla coil initiates the plasma formation by generating some electrons in the plasma area. The radio frequency produces electric and magnetic fields that accelerate the electrons by inductive coupling (hence the term inductively coupled plasma ICP). The high-energy electrons cause further argon ionization by collision and this process continues as a chain reaction producing the plasma. The temperature of the plasma ranges from 6000 C to 10,000 C and appears as an intense brilliant white 'tear-drop' shaped fireball in the end of the quartz tube. At such temperatures virtually all the elements are volatilized and will emit their characteristic radiation.

The Helium Plasma Torch click here for a wiki article on The Helium Plasma Torch


The helium plasma torch is similar in principle to the ICP torch but employs helium instead of argon as the plasma gas. In the helium plasma torch the volatilizing plasma is induced into the helium stream using a water-cooled microwave transducer. The sample, which is pre-mixed with the pure helium make-up gas, enters the plasma and the elements present in the solute are heated to the required high temperature and emit light at the wavelength characteristic of the elements present. The light emitted is transmitted through a quartz window, and focused by a quartz lens and spherical mirror onto the diffraction grating of a spectrometer.

Emission Spectrometer click here for a wiki article on Emission Spectrometer


The ICP Emission spectrometer identifies and quantitatively estimates the elements present in a sample. The sample is pre-mixed with the pure plasma make-up gas (which may be argon or helium) and enters the plasma. The elements present in the sample are heated to the required high temperature and emit light, of their characteristic wavelength. The light emitted is transmitted through a quartz window, and focused by an appropriate optical system onto a diffraction grating. The dispersed light from the grating (or a segment of it) is focused onto a diode array. Different wavelength ranges can be selected for monitoring, from the full spectrum provided by the grating. The elements present are identified from the frequency of the bands of light emitted and the quantity present estimated from the intensity of the light bands.

Atomic Absorption Spectrometry click here for a wiki article on Atomic Absorption Spectrometry


Atomic absorption spectroscopy is an element specific spectroscopic monitoring system that can determine the presence of specific elements when they exist at high temperature in a vapour or gaseous state. Light characteristic of that emitted by the element of interest is passed through the vaporized sample The amount of light absorbed is proportional to the amount of the element present, which, in turn, is proportional to the amount of the element that is continuously fed as a vapour or gas into the path of the radiation. By monitoring the fall in light intensity passing through the gas or vapour when the respective element is present, the amount of the element present can be determined.

Flame Atomic Absorption Spectrometer click here for a wiki article on Flame Atomic Absorption Spectrometer


In flame ionization a chosen volume or fixed aliquot of the sample solution is nebulized and transported to the flame in a current of gas. The gas must have enough energy to atomize the sample and the flame enough energy to vaporize the sample but not ionize it. The chemical composition of the flame and the chemical environment must be optimal for sample vaporization. The flame must also be transparent to the light frequencies of interest and the rate of combustion must be sufficiently slow to allow the atoms to remain in the absorption volume for adequate time. The volume of the flame is a factor that will determine the sensitivity of the measurement. The two common gas mixtures used in the flame AA are air-acetylene and nitrous oxide-acetylene. The fuel to oxidant ratio should be optimized for each particular sample.

Flame AA click here for a wiki article on Flame AA


"Flame AA"

Hollow Cathode Lamp click here for a wiki article on Hollow Cathode Lamp


Hollow cathode lamps are now the established light sources for atomic absorption spectroscopy. The hollow cathode lamp has a hollow, usually cylindrical cathode containing one or a few of the elements of interest. The anode is usually constructed from tungsten or nickel. The electrode system is enclosed in glass envelope often fitted with a quartz window. The envelope is filled with an inert gas (usually neon or argon) at a reduced pressure (ca. 1kPa). The voltage applied across the electrodes ranges from 100 and 200 volts depending on the electrode geometry. This potential sustains a glow discharge emitting light that contains the characteristic radiation frequencies of the elements present in the hollow cathode.

Electrothermal Atomization click here for a wiki article on Electrothermal Atomization


Electrothermal atomization is an alternative to flame atomization. The atomization efficiency of the flame is limited due to sample dilution by the high flow rate and only one atom in one hundred million actively absorbs the radiation to which the sample is exposed. The first use of electrothermal atomization was the now well-established method using an electrical furnace. The furnace increases the residence time of the sample in the atomizing volume by one hundred thousand times increasing the sensitivity and permits much smaller samples to be analyzed. Additionally, the physical characteristics of the sample solution (viscosity density etc.) have little, or no, effect on the atomization process. Another advantage is that, the use of inert gas in the furnace enhances the reducing properties of the carbon and, thus, improves the decomposition of metal oxides. Inert gases also permit the use of ultraviolet radiation for measurement. The main advantage of the electrothermal method of ionization is its exceedingly high sensitivity (i.e. at the low picrogram level). Most elements can be determined at a sensitivity that is one thousand times lower than that achieved by flame atomization.

Graphite Furnace click here for a wiki article on Graphite Furnace


The graphite furnace was the first electrothemal atomization device developed for atomic spectroscopy. The modern furnace consists of a straight graphite tube supported between two water cooled cones A potential of about 10 volts is applied across the tube which results in a current of about 500 amp producing an energy of 5kW and a temperature of about 2600 C. A stream of inert gas continually passes through the graphite tube. The sample is deposited in the tube while cold and the furnace then rapidly heated to atomize the sample. As little as 10-13 g of many elements has been determined in this way. The performance of the graphite furnace can be improved by coating it with pyrolytic carbon.

L’vov Platform click here for a wiki article on L’vov Platform


The graphite furnace used in atomic absorption spectroscopy has certain disadvantages. If the sample is applied directly to the wall of a cold graphite furnace and subsequently stepwise heated to a high temperature the tube wall will heat faster than the gas producing temperature differentials. The poor uniformity of temperature will cause condensation or recombination of atoms. L'vov invented a small graphite platform coated with pyrolytic carbon that was inserted into the furnace (the L’vov Platform). The platform was heated by radiation from the hot walls and by the hot the gas molecules. Thus, the vaporization and atomization was delayed until the tube had obtained a constant temperature. Consequently, the sample was always vaporized into an environment that was hotter than the surface from which it left. Accordingly, the chance of condensation and recombination was significantly reduced and any molecular species that were rendered volatile were likely to suffer dissociation. The L'vov platform has shown a great improvement in graphite furnace performance.

Electron Paramagnetic Resonance click here for a wiki article on Electron Paramagnetic Resonance


Electron paramagnetic resonance (EPR), is a spectroscopic technique that is also known as electron spin resonance (ESR) and, occasionally, as electron magnetic resonance (EMR). EPR is used to examine the environment of unpaired electrons that are often present in organic free radicals, transition metal complexes and, even more importantly, often present in biochemicals and generally in substances of biological origin. As a consequence of these unpaired electrons, any sample that may contain them will absorb electromagnetic radiation and produce a transition between electron energy levels. The EPR instrument measures the extent of the absorption of the electromagnetic radiation and the frequency at which the transition occurs. Such information allows the energy of transition to be calculated.

Zeeman Effect click here for a wiki article on Zeeman Effect


The Zeeman effect is a term given to the splitting of spectrum lines into several symmetrically disposed components that occurs when the source of electromagnetic radiation is placed in a strong magnetic field. The individual components are polarized and the direction of polarization will depend on the direction from which the source is viewed with respect to the lines of force. The majority of stable molecules do not have primary magnetic dipole moments but they do have small magnetic moments (ca one nuclear magneton) as a result of molecular rotation, interaction with higher electronic states, nuclear moments etc. and these produce Zeeman splitting. Zeeman splitting is usually resolved with a field of about 10 kgauss. Paramagnetic resonance takes place when a magnetic field is forced on paramagnetic substances that are subjected to electromagnetic radiation and entails the absorption of energy from the radiation. The observed transitions occur between the Zeeman components of particular interval levels. As the strength of absorption become more intense fairly rapidly with frequency, it is preferable to employ microwaves rather than radiation of lower frequencies when examining electron paramagnetic resonance.

Continuous Wave Electron Paramagnetic Resonance click here for a wiki article on Continuous Wave


Continuous wave techniques in electron paramagnetic measurements involve the continuous application of microwaves of a given frequency to the sample while the magnetic field is scanned and the signal (the energy absorbed) is measured as a function of the strength of the magnetic field; the apparatus consists of a microwave generator and detector, a strong magnetic field and a super imposable, weak, programmable magnetic field together with appropriate power supplies. In addition there will be a modulation and phase sensitive detection unit and a computer data acquisition and processing system. The spectrometer measures the energy absorbed during electronic transitions that occur at specific field strengths. Pulsed EPR

Pulsed EPR click here for a wiki article on Pulsed EPR


In pulsed electron paramagnetic measurements the sample is subjected to one or more pulses of electromagnetic radiation and the response of the electron spin observed after the completion of the pulse. In practice, for example, the induction decay could be observed after a single pulse has been applied. Subsequently, a Fourier transform of the data provides a frequency-domain spectrum. An extended microwave pulse can be applied to the sample to saturate the spin system to its higher energy level. The energy absorbed is then measured with low power continuous microwaves during the period during which equilibrium is recovering. The time for equilibrium to become re-established will be related to the electron spin relaxation processes.

Electron Spin Echo click here for a wiki article on Electron Spin Echo


In electron spin echo experiments, two microwave pulses are applied to the sample to form an 'echo'. At equilibrium, the magnetic field will appear as a magnetic vector precessing about the direction of the external magnetic field (when a force acts upon a spinning body so as to displace its axis of rotation the spinning body will precess). As in classical NMR, the precession frequency will depend on the environment of the electron. A pulse of radiation is applied to the sample at right angles to the magnetic field (called a 90o pulse). After a given time interval (?), a 180o pulse is applied to the system which flips the average magnetization vector and interchanges the behavior of the faster and slower precessing electrons. After a second interval (?) the system recovers from the 'echo'. In two-pulse electron spin echo envelope modulation (ESEEM the echo intensity is monitored during the time (?),

Multple Resonance Spectroscopy click here for a wiki article on Multple Resonance Spectroscopy


Excitation with a second frequency during an electron paramagnetic measurement experiment is more informative than excitation with a single wavelength. The three common multiple resonance methods are Electron-Nuclear Double Resonance Spectroscopy (ENDOR), Electron-Electron Double Resonance Spectroscopy (ELDOR) and TRIPLE. Monitoring the EPR intensity when one of the NMR transitions is irradiated is ENDOR spectroscopy and monitoring the intensity of one EPR transition while irradiating another EPR transition is ELDOR spectroscopy. The ENDOR experiment can be extended by employing two, simultaneous, radiofrequency fields, that is the ENDOR (TRIPLE) technique. Using a single radiofrequency only discloses the magnitude of the nuclear coupling constant. However, by employing a second radiation sources the signs of the coupling constant can also be determined. One EPR transition is saturated with one source of microwave radiation and a second EPR transition is observed with a non-saturating microwave field generated by a separate, source.

Magnetic Resonance Spectroscopy click here for a wiki article on Magnetic Resonance Spectroscopy


The nucleus of an atom spins and, thus, if the charge is not symmetrically placed on the nucleus, the spinning charge will constitute a circular current and possess an associated magnetic field similar to a small bar magnet. If the spinning nucleus and its associated magnetic field, is situated in a strong external magnetic field, the external field will act upon the spinning nucleus to try to change its spinning axis to be in line with the magnetic field. Now, according to Newton's law, "when a force acts upon a spinning body to change its axis of rotation then, to conserve the angular momentum, the spinning body will precess". Quantum rules predict that the precessing nucleus has only two possible orientations. Consequently, if energy is given to the spinning nucleus, employing electromagnetic radiation of the appropriate frequency, energy will be absorbed and the precessing nucleus displaced from one orientation to the other. The relationship between the absorbed energy and the applied magnetic field at a content irradiating frequency will provide an NMR spectrum. The spectrum, if sufficient resolution is obtained, will provide information as to the nature of a proton and the nature of its proton environment.

NMR click here for a wiki article on NMR


The nucleus of an atom spins and, thus, if the charge is not symmetrically placed on the nucleus, the spinning charge will constitute a circular current and possess an associated magnetic field similar to a small bar magnet. If the spinning nucleus and its associated magnetic field, is situated in a strong external magnetic field, the external field will act upon the spinning nucleus to try to change its spinning axis to be in line with the magnetic field. Now, according to Newton's law, "when a force acts upon a spinning body to change its axis of rotation then, to conserve the angular momentum, the spinning body will precess". Quantum rules predict that the precessing nucleus has only two possible orientations. Consequently, if energy is given to the spinning nucleus, employing electromagnetic radiation of the appropriate frequency, energy will be absorbed and the precessing nucleus displaced from one orientation to the other. The relationship between the absorbed energy and the applied magnetic field at a content irradiating frequency will provide an NMR spectrum. The spectrum, if sufficient resolution is obtained, will provide information as to the nature of a proton and the nature of its proton environment.

Precessing click here for a wiki article on Precessing


The nucleus of an atom spins, Consequently, if its charge is not symmetrically placed on the nucleus, the spinning charge will constitute a circular current. A circular current will produce an associated magnetic field similar to a magnet. If the spinning nucleus and its associated magnetic field is placed in a strong external magnetic field, the external field will apply a force to change its spinning axis to be in line with the magnetic field. Now, according to Newton's law, "when a force acts upon a spinning body to change its axis of rotation then, to conserve the angular momentum, the spinning body will precess". As a consequence of the applied magnetic field the proton will precess.

Nucleus Spin Decoupling in NMR click here for a wiki article on Nucleus Spin Decoupling in NMR


The true meaning of a peak or peaks in an NMR spectrum is often concealed by the presence of overlapping patterns along with difficulties arising from "non-first order" multiplets. Such spectra can often be simplified by chemically replacing a specific interfering proton with a deutron or by examining the sample using a higher magnetic field and consequently higher rf frequencies. There is, however, an easy electronic alternative called double resonance or spin decoupling. Spin decoupling involves the application of a strong and second electromagnetic wave having a frequency close to the resonance frequency of the interacting nucleus while scanning the spectrum in the usual manner to examine the remaining protons present in the structure. The second frequency saturates the chosen proton so that it remains at its high energy level and, thus, cannot contribute to the spectrum of the remaining protons.

Superconducting Magnets click here for a wiki article on Superconducting Magnets


The superconducting magnets consists of a large main-field superconducting coil together with a number of other smaller coils that control field gradients in different directions relative to the main field. These small coils are shim coils and are adjusted to improve the homogeneity of the field and, consequently, the resolution. The superconducting coils are submerged in liquid helium and maintained at 4.2 oK. The superconducting coils are made of NbTi, Nb3Sn, or (NbTa)3Sn alloys constructed in the form of multi-core wires enclosed in an outer copper sheath. The coil is located in a Dewer container that, in turn, is placed in a evacuated chamber that is held at a pressure of 10-6 torr. The inner Dewer container is situated in a second Dewer container filled with liquid nitrogen. And held at a temperature of 77oK. As some of the helium and nitrogen inevitably boils away, the cold gas is lead over joints and connecting rods to keep them cool and reduce heat entering the system by thermal conduction.

NMR Microcells click here for a wiki article on NMR Microcells


NMR micro cells can take different forms. They can be constructed as an all glass device with a sample volume of 75 ?l. Alternatively, they can be fabricated by the use of a Nylon insert that is introduced into a standard sample tube. These devices have a sample volume of about 25 ?l. Flow cells have now become necessary in many laboratories to allow the NMR spectrometer to be operated in-line with a liquid chromatograph (the LC/NMR tandem system). Unfortunately, the in-line cells have volumes of 50 to 300 ?l and such volumes can cause the resolution obtained from the chromatographic column to be seriously degraded.

Electron Impact Ionisation click here for a wiki article on Electron Impact Ionisation


Electron impact ionization is generally a fairly harsh method of ionization but the procedure can produce a range of molecular fragments that helps to elucidate the structure of the molecule. Electrons are formed by thermal emission from a heated tungsten or rhenium filament and accelerated by an appropriate potential to an anode trap. The accelerating potential ranges from 5 to 100 V depending on the electrode geometry and the ionization potential of the substances to be ionized. The filament current can be automatically controlled to provide a constant trap anode current and, thus, maintain steady ionizing conditions. The sample is introduced into the gas stream at the centre of the electron beam. The ions formed are repelled by a suitably placed electrode, through a hole in the wall of the ion source enclosure into the accelerating field of the mass spectrometer. A magnetic field of a few hundred gauss is often maintained along the axis of the electron beam, to confine the electrons to a narrow helical path. In general only about 0.1% of the molecules entering the ion source are ionized.

Chemical Ionization click here for a wiki article on Chemical Ionization


The chemical ionization is considered to be a gentle form of ionization, as the energy of the reagent ions never exceeds 5 electron volts and this includes those reagent ions that have relatively high ionization energies. In chemical ionization very little fragmentation takes place and parent ions + a proton or a +molecule of the reagent gas are produced. Consequently, the molecular weight of the parent ion is easily obtained. In practice, little modification to the normal electron impact source is necessary and an extra conduit for supplying the reagent gas is all that is necessary. The ionization process involves firstly the ionization of a reagent gas (e.g. methane) in a simple electron impact ion source; as there will be an excess of the reagent gas, the reagent molecules are preferentially ionized. The reagent ions then collide with the sample molecules and produce sample + reagent ions or in some cases protonated ions. The spectrum produced by chemical ionization is determined by the nature of the reagent ion and, consequently, different structural information can be obtained by employing different reagent gases. This adds another degree of freedom to the operation of the mass spectrometer.

Inductively Coupled Plasma Ionization click here for a wiki article on Inductively Coupled Plasma Ionization


The inductively coupled plasma (ICP) mass spectrometer ionization source was developed from the ICP torch of the atomic emission spectrometer. The ICP ion source is very similar in design to the volatilizing unit of the atomic emission spectrometer. The argon plasma is an electrodeless discharge, initiated by a Tesla coil spark, and maintained by radiofrequency radiation that is inductively coupled to the inside of the torch by an external coil wrapped round the torch stem. The plasma is maintained at atmospheric pressure at a mean temperature of about 800 K. The inductive coupling is accomplished using a coil consisting of 2-4 turns of water-cooled copper tubing, placed a few millimetres behind the mouth of the torch. The radiofrequency generator produces radiation at 27 or 40 MHz and about 1300 watts that induces a fluctuating magnetic field along the axis of the torch. The plasma temperature in the induction region can reach 10,000°K but in the ionizing region the temperature is 7000-9000 K. The sample ions account for less than 10-6 of the total number of atoms present in the plasma region (one part per million) and, consequently, there is little or no quenching due to the presence of the sample. At the plasma temperature, more than 50% of most elements are ionized.

Secondary Ion Mass Spectrometry click here for a wiki article on Secondary Ion Mass Spectrometry


In secondary ion mass spectrometry (SIMS), the excitation beam that ionizes the sample consists of a stream of ions having a kinetic energy of about one kilo-electron volt that are generated in a specially designed ion-gun. The commonly used ions are the Ar+, O2+ and Cs+ ions. If the sample, or sample support, is conducting (i.e. a metal) then the charge received by the target from the ion beam can leak away. If the target is not conducting, an electric charge may build up on the sample surface and interfere with the subsequent focusing of the ions produced. Any charge that accumulates can be neutralized by exposing the sample to low-energy electrons derived from a separate electron source. Copious molecular ions are formed by the secondary ion process, (e.g. (M+H)+ and (M+Ag)+) from thin layers of sample deposited on a metal (silver). Other materials can be used as a substrate, such as nitrocellulose, that can improve the ionization efficiency for very high molecular weight samples.

Fast Atom Bombardment click here for a wiki article on Fast Atom Bombardment


Fast atom bombardment spectrometry is an extension of secondary ion mass spectrometry. A beam of high-energy atoms is generated and focussed onto the sample, which is held in the form of a thin film deposited on a clean metal surface. The secondary ions so formed are extracted by an appropriate ion optical system into the mass spectrometer analyzer. The impact of high-energy atoms striking the surface creates an intense thermal spike. The energy resulting from this thermal spike results in the ejection of an ion and then is dispersed through the outer layers of the sample. Initially neutral atoms of argon and xenon were employed, but ultimately these were replaced by charged ions such as Ce+ and Xe+.

Plasma Desorption Mass Spectrometry click here for a wiki article on Plasma Desorption Mass Spectrometry


Plasma desorption ionization employs a radioactive source and the fission particles are used to ionize the sample. Employing 252Cf as the source, plasma desorption ionization can be used in conjunction with the time of flight (TOF) mass spectrometer in a unique way. 252Cf decays giving an alpha particle and two charged fission fragments simultaneously emitted in opposite directions. Typically a pair of fission fragments might be 106Te and 142Ba with energies of 104 and 79 MeV respectively. The sample is deposited as film on a thin aluminum sheet (or a sheet of aluminized polyester) that is connected to a high positive potential (assuming the sample ions will be positively charged). When fission takes place, one particle strikes the sample and produces ions, while the other particle, emitted in the opposite direction, is sensed by a trigger sensor that initiates the time of flight measurement. The ionized sample molecule or fragment is accelerated to its characteristic velocity, passes through the drift region of the spectrometer and is finally sensed by the stop sensor which then halts the time of flight measurement.

Laser Desorption Mass Spectrometry click here for a wiki article on Laser Desorption Mass Spectrometry


The laser can be employed in an ionizing system in two ways, one to desorb the sample as vapour into an ionizing system, and two, to actually ionize the sample. However, pulsed lasers produce bursts of ions which are obviously not suitable for use with a scanning spectrometer. Nevertheless, intermittent ion production is compatible with the time of flight (TOF) mass spectrometer, which can record all the ions produced by each laser pulse. An added advantage of the LASER/TOF combination is the nearly unlimited mass range of this particular type of mass spectrometer. The sample is loaded onto a probe that receives high-energy laser pulses from a carbon dioxide laser. The 'bursts' of vaporized sample are driven by an expanding argon stream expressed from a 100 ?m orifice, through a skimmer, into the accelerating section of the mass spectrometer. In the next section, the sample molecules are again exposed to a high-energy laser light beam, this time, however, from a dye laser, producing ions by photo-ionization. The ions so formed are immediately accelerated through the region by a suitable potential gradient into the drift section of the mass spectrometer, where they are deflected by an ion reflector, to an electron multiplier.

Matrix Assisted Desorption mass Spectrometry click here for a wiki article on Matrix Assisted Desorption mass Spectrometry


Matrix assisted laser desorption/ionization (MALDI) can ionize substances having extremely high molecular weights. The LASER employed is usually the Nd-Yg-Laser that has a wavelength of 286 nm, and a pulse width of about 10 ns. The sample is dispersed in an involatile liquid to prevent decomposition and to also permit the surface to be continually renewed. A range of liquids has been explored, including glycerol and nicotinic acid. Nicotinic acid is exceptionally appropriate as it absorbs very strongly at 286 nm, the wavelength of the laser light. The laser beam is arranged to strike the surface at a 45° angle, and the ions that are emitted are collimated, by a three element Einzel lens into an ion deflector to the time of flight mass spectrometer. The ions are then accelerated by a suitable voltage, and allowed to drift to an ion reflector and then back to the electron multiplier.

Field Desorption Ionization click here for a wiki article on Field Desorption Ionization


Field desorption (FDI) involves the extraction of ions from a sample (deposited on a specially prepared surface) by the use of extremely high electrical fields. The preparation of the emitting surface can be quite complicated. The emitter is attached to an insulated probe, which enters the ion source through a standard vacuum lock. After reaching the entry position, there is a counter electrode about 2 mm from the probe tip that is held at a potential of about 10,000 V relative to the probe. The ions produced by the emitter are then accelerated toward the counter electrode and pass through an aperture into the focusing section, and finally into the mass spectrometer analyzer. The procedure necessary to prepare the emitter surface is tedious and time consuming. The most commonly formed surface takes the form of carbon micro needles that are deposited on tungsten wire under vacuum. One substance that is employed for this procedure is benzonitrile. The benzonitrile is deposited on the wire and its temperature raised to gently pyrolyse the material, and in the process producing carbon spots along the wire. A high voltage is then applied to the wire and the pyrolysis continued; carbon needles are slowly formed that develop from the original carbon deposits on the wire surface. The sample for analysis is coated on the carbon needles and then inserted into in a strong electric field; the high potential gradient that exists close to the needle-points causes ions to be emitted from the sample. Field desorption ionization can provide spectra of high molecular weight substances.

Thermospray Ionization click here for a wiki article on Thermospray Ionization


The thermospray ionization source evolved from the direct inlet system, by simply modifying and heating the tip of the entry tube. Because only the tip is heated, the sample and solvent is vaporized only at the tip, and not somewhere inside the connecting tube. The modification results in far better control of the nebulizing process and also the ionizing process. A number of different forms of the device have been described in the literature but a relatively simple form is in general use. The thermospray ionization source consists of a length of stainless steel tube, terminating in a high conductivity metal cap (e.g. copper). Through the centre of the stainless steel tube and copper cap passes another tube carrying the reagent gas. In the centre of the reagent conduit passes a length of a fused silica open tubular column (which carries the sample solution) and which projects slightly beyond the reagent conduit, into the ion source of the mass spectrometer. The sample tube initially passes through a T union, to permit the reagent gas to enter the annular space between the inner tube and the conduit, and then into the thermospray probe itself. The copper cap contains a cartridge heater together with a thermocouple, which measures the temperature of the probe tip and provides the signal that maintains the tip at a selected temperature.

Electrospray Ionization click here for a wiki article on Electrospray Ionization


The electrospray ion-producing inlet system is probably the most commonly used interface used for liquid sample solutions. Electrospray ionization is produced by a strong electric field acting on the surface of a sample solution as it is sprayed into a dry gas such as nitrogen. A cloud of charged droplets is produced that evaporate rapidly and, as a consequence shrink in size. The associated increase in charge density resulting from the decrease in droplet volume, surface area, and radius of curvature causes very strong electric fields to be formed. As each drop continues to shrink, the electric fields become sufficiently strong to cause the droplets to explode, producing ions. Due to the strength of the electric field, and the large number of ions that are produced, many of the ions that are formed will contain multiple charges. Multiple ion production can extend the mass range of the spectrometer.

Atmospheric Pressure Ionization click here for a wiki article on Atmospheric Pressure Ionization


The atmospheric pressure ionization (API) source is similar in some ways to the electrospray ionization source; it can handle a range of sample solution (or column) flow rates to a maximum of about 2 ml/min. As a result, the total flow of sample solution can be utilized without splitting the flow. Unfortunately, although the total sample may enter the interface, only some of the solute molecules are ionized, and, in addition, not all the ions enter the mass spectrometer. There are three types of the API source: the first uses a heated nebulizer with a corona discharge, the second employs an atmospheric electrospray and the third uses an ion spray. Taking the first type as an example the solvent is nebulized by a gas flow that is then swept by a sheath gas or make-up gas through a quartz tube heater that vaporizes the solvent. The sample drifts through a chamber containing a corona discharge (developed by a potential of about 2000 volts applied across a simple electrode arrangement). The charged solvent vapour molecules act as chemical ionization agents. The reactant ions, formed in the corona discharge, collide with the sample molecules and give sample molecule plus a proton (hydrogen positive ions), i.e. [M+H]+. The ions so formed are then impelled by a charged electrode into the mass spectrometer.

Particle Beam Interface click here for a wiki article on Particle Beam Interface


The particle beam interface (PBI) nebulizes the sample solution, and the solvent free particles of solute pass into the ionization chamber of the mass spectrometer. Electron impact or chemical ionization spectra can be produced, and the system has been given the name 'monodispersed aerosol generation sampling interface' that has been endowed with the pretentious acronym (MAGIC). The interface consists of two parts, the aerosol generator and the momentum separator. The sample solution is nebulized at the end of a fused silica tube similar to that employed in capillary gas chromatography. The liquid jet that forms at the end of the small-diameter tubing seldom clogs. The body of the momentum separator is normally made of stainless steel and fitted with nozzles and skimmers that are machined from 6010 grade aluminum. The skimmers are designed to provide an undisturbed path for the particles from their point of generation, until they arrive in the ion source of the mass spectrometer (often an electron impact or a chemical ionization source).

Permeable Membrane Interface click here for a wiki article on Permeable Membrane Interface


The permeable membrane interface (PMI) is suitable only for relatively volatile samples. The principle of the interface is the diffusion of the sample through a supported membrane into an electron impact or chemical ionization source. The materials that have been explored as suitable membranes are silicone, types of latex, polyethylene, polyurethane (polyether), polyurethane (polyester) a copolymer of acrylonitrile and butadiene and polyvinyl chloride. The membrane is positioned between two stainless steel blocks, and the entire interface is heated a to a controlled temperature. The solution, column eluent or flow injection sample passes over the membrane through which the solutes diffuse. On the other side of the membrane, the solute evaporates into the high vacuum of the mass spectrometer and the vapour then passes down a heated tube into the ion source.

Sector Mass Spectrometer click here for a wiki article on Sector Mass Spectrometer


The sector mass spectrometer functions on the combined selectivity of an electrostatic field and a magnetic field. An instrument that utilizes both an electric and a magnetic field to resolve the ions is often called a double focusing sector mass analyzer. Each sector is a channel forming the quadrant of a circle. The first contains parallel plates having a positive potential on the outer plate and a negative potential on the inner plate The ions, having been accelerated to a given velocity, pass between the plates and those that have an outward centrifugal force equal to the inward electrostatic force will pass round the circular path an exit the quadrant. All the ions exiting at a specific radius will have the same kinetic energy. The ions of the same energy will then enter the magnetic quadrant where a magnetic field will act upon the moving ions and those ions where the inward magnetic force is equal the outward centrifugal force will, again, exit at a specific radius. The magnetic sector will discriminate, not on the basis of energy, but on the basis of momentum. The combination of the two sectors will result in the ions that exit from the magnetic sector having the same mass to charge ratio. This is a simple treatment of the double focusing mass spectrometer. If the slit entering the magnetic sector was sufficiently narrow to produce well-resolved mass peaks, the few ions would be collected and the system would be very insensitive. However, by the choice of a specific combination of electrostatic and magnetic sectors so that the velocity dispersion in the two sectors is equal and opposite, the poor ion throughput can be avoided and a larger slit provide the desired greater sensitivity.

Quadrupole Mass Spectrometer click here for a wiki article on Quadrupole Mass Spectrometer


The quadrupole spectrometer contains four rods that are precisely straight and parallel and arranged so that the ion beam from the ion source is directed axially between them. Theoretically the rods should have a hyperbolic cross section but less expensive cylindrical rods are nearly as satisfactory. A potential comprising a DC component (U) and a radio frequency component (Vocos?t) is applied between adjacent rods, opposite rods being electrically connected. Ions are accelerated into the centre, between the rods, by a relatively small potential ranging from 10 to 20 volts. Once inside the quadrupole, the ions oscillate in the (x) and (y) dimensions as a result of the high-frequency electric field. Only ions of a given mass are in a stable state and can pass through the quadrupoles to the detector. The m/z ratio is scanned by programming both U and Vo but keeping the ratio U/Vo constant.

Ion Trap Mass Spectrometer click here for a wiki article on Ion Trap Mass Spectrometer


The ion trap mass spectrometer is a modified form of the quadrupole mass spectrometer, and was originally designed as a chromatography detector. However, the combination of the ion trap mass spectrometer with its capacity for mass analysis and the chromatograph is a useful tandem technique. The ion trap mass spectrometer has a quite different electrode configuration to the quadrupole mass spectrometer and consists of three cylindrically symmetrical electrodes comprised of two end caps and a ring. The device can be easily miniaturized, the opposite internal electrode faces being only 2 cm apart. Each electrode has hyperbolic internal faces and in a similar manner to the quadrupole spectrometer, an rf voltage together with an DC voltage is applied to the ring and the end caps are grounded. As with the quadrupole mass spectrometer, the rf voltage causes rapid reversals of field direction, so any ions are alternately accelerated and decelerated in the axial direction and vice versa in the radial direction. The ion trap is small and the ring radius is about 1 cm. At a specific rf voltage, ions of a definite mass range are held oscillating in the trap. Initially, the ionizing electron beam produces ions and after a given time the beam is turned off. All the ions, except those selected by the magnitude of the applied rf voltage, are lost to the walls of the trap, and the remainder continue oscillating in the trap. The potential of the applied rf voltage is then increased, and the ions sequentially assume unstable trajectories and leave the trap via the aperture to the sensor. The ions exit the trap in order of their increasing m/z values. To improve stability and the quality of the spectra traces of helium are introduced into the ion. The spectra produced are satisfactory for solute identification by comparison with reference spectra.

Time of Flight Mass Spectrometer click here for a wiki article on Time of Flight Mass Spectrometer


The basic time of flight spectrometer (TOF) consists of a cylinder containing four electrodes. At one end is a cathode and at the other an ion collection electrode. Fairly close to the anode is another electrode that contains apertures and so is permeable to ions and is maintained at a negative potential relative to the cathode to extract any +ions produced near the cathode into the accelerating zone. The sample is volatilized into the space between the first and second electrodes and then a burst of electrons (over about a microsecond period) is allowed to produce ions. The extraction voltage is then applied for short period of time. As those ions further from the second electrode will experience a greater force than those closer to the second electrode, this will focus the ions (the ions farther from the electrode will experience a greater accelerating potential than those closer to the electrode and, thus, will catch them up). Between the extraction electrode and the anode and close to the extraction electrode is the accelerating electrode (which is also porous to ions). The accelerating potential (V) is applied for a much shorter period than that used for ion production (ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously and their final velocity will be inversely proportional to a function of their mass. The ions then pass through the third electrode into the drift zone and are collected at the sensor electrode(or detecting device). The ions will arrive in order of their masses, the lowest mass first and greatest mass last. The time taken for any particular ion to arrive will be a function of its mass.

Optical RotationCircular Dichroism click here for a wiki article on Optical RotationCircular Dichroism


"If light from an appropriate source passes through a polarizer (typically consisting of a set of crossed Nichol prisms) a beam of plain polarized light (usually polarized vertically) will be produced. If the vertically polarized light is then passed through a sample tube containing an optically active substance, the plane of the light is rotated in the sample cell and transmitted with the plane of polarization turned through an angle. The magnitude of this angle is determined by the nature of the substance and its concentration in solution. If the light is then passes through a second polarizer that is adjusted by rotation, until the angle of the transmitted light is again returned to the vertical, the angle through which the second polarizer is rotated is called the optical rotation of the sample. "

Circular Dichroism click here for a wiki article on Optical RotationCircular Dichroism


A linearly polarized beam of light can be considered to be the resultant of two, equal-intensity, in-phase components, one left, and the other right, circularly polarized, or of two orthogonal linear components at ± 45°. The differential absorption of these two ± 45° linear components in a medium is known as linear dichroism. If there is a differential velocity between the two ± 45° linear components, when passing through a medium (i.e. the refractive index of the medium to the two light components differ), then this is known as linear birefringence. In an analogous manner, the difference in the adsorption characteristics of a medium to the left and right circularly polarized light, is termed circular dichroism (CD).

Circularly Polarized Light click here for a wiki article on Circularly Polarized Light


Light consists of a sinusoidally changing electric field normal to, and in phase with, a sinusoidally changing magnetic field. The plane of the electric vector in normal light, takes no particular orientation, but in plane polarized light, the electric vector is either vertically or horizontally polarized. If the electric vector transcribes a helical path, either to the right or left, the light is said to be circularly polarized.

Verdet Constant click here for a wiki article on Verdet Constant


If a plane polarized beam of light passes through a medium that is subjected to a strong magnetic field the plane of the polarized beam will be rotated through a small angle (?), The angle (a) is equal to the product of the path length, the strength of the magnetic field and the Verdet constant.

Faraday Effect click here for a wiki article on Faraday Effect


If a plane polarized beam of light passes through a medium that is subjected to a strong magnetic field, the plane of the polarized beam will be rotated through a small angle (?), and (a) will be equal to the product of the path length, the strength of the magnetic field and the Verdet constant. This is called the Faraday Effect.

adsorption click here for a wiki article on adsorption


"adsorption "

Atomization click here for a wiki article on Atomization


"Atomization "

bandwidth click here for a wiki article on bandwidth


"bandwidth "

chromatography click here for a wiki article on chromatography


Click here for Access to the Author's Books on all aspects of Chromatography published on his Library4Science Website

electroluminescence click here for a wiki article on electroluminescence


"electroluminescence "

electromagnetic click here for a wiki article on electromagnetic


"electromagnetic "

emission click here for a wiki article on emission


"emission "

Emissivity click here for a wiki article on Emissivity


"Emissivity "

Fluorescence click here for a wiki article on Fluorescence


"Fluorescence "

luminescence click here for a wiki article on luminescence


"luminescence "

Michelson click here for a wiki article on Michelson


"Michelson "

monochromators click here for a wiki article on monochromators


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photo-multiplier click here for a wiki article on photo-multiplier


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Phosphorescence click here for a wiki article on Phosphorescence


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photodiodes click here for a wiki article on photodiodes


"photodiodes "

photoelectric click here for a wiki article on photoelectric


"photoelectric "

photoluminescence click here for a wiki article on photoluminescence


"photoluminescence "

Rayleigh click here for a wiki article on Rayleigh


"Rayleigh "

Raman click here for a wiki article on Raman


"Raman "

spectrofluorometer click here for a wiki article on spectrofluorometer


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spectrometer click here for a wiki article on spectrometer


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spectrophotometer click here for a wiki article on spectrophotometer


"spectrophotometer "

Spectrum click here for a wiki article on Spectrum


"Spectrum "

Transmittance click here for a wiki article on Transmittance


"Transmittance "

ultraviolet click here for a wiki article on ultraviolet


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wavelength click here for a wiki article on wavelength


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Electromagnetic Wave click here for a wiki article on Electromagnetic wave


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Wavenumber click here for a wiki article on Wavenumber


"Wavenumber "

UV Light click here for a wiki article on UV Light


"UV Light"

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

gamma rays