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THE SPECTRO-PAEDIA

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absorbance
adsorption
Atomization
bandwidth
Beamsplitters
bioluminescence
chemiluminescence
chromatography
electroluminescence
electromagnetic
emission
Emissivity
Fluorescence
luminescence
Michelson
monochromators
photo-multiplier
Phosphorescence
photodiodes
photoelectric
photoluminescence
Rayleigh
Raman
spectrofluorometer
spectrometer
spectrophotometer
Spectrum
Transmittance
ultraviolet
Visible
wavelength
Wavenumber
UV Light
Beer's Law
Diode Array
Deuterium Discharge Lamp
Low Pressure Zinc Discharge Lamp
High Pressure Mercury Discharge Lamp
Low Pressure Cadmium Lamp
Tungsten Halogen Lamp
Lens and Window Material in Spectrometers
Cuvettes
Luminescence
Photoluminescence
Fluorescence
Phosphorescence
Bioluminescence
Radioluminescence
Electroluminescence
Fluorescence Reagents
Spectrum
Diffraction Grating
Interferogram
Fourier Transform IR Spectrometer
FT-IR
Halide Disks
Mull Samples
Film Samples for IR Spectroscopy
Light Pipes
Attenuated Total Reflectance Spectroscopy
Multiple Internal Reflectance
External Reflectance
Specular Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Beam Splitter
Raman Scattering
Rayleigh Scattering
Raman Spectroscopy
Atomic Spectroscopy
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
The Inductively Coupled Plasma Torch
The Helium Plasma Torch
Emission Spectrometer
Atomic Absorption Spectrometry
Flame Atomic Absorption Spectrometer
Flame AA
Hollow Cathode Lamp
Electrothermal Atomization
Graphite Furnace
L’vov Platform
Electron Paramagnetic Resonance
Zeeman Effect
Continuous Wave
Electron Paramagnetic Resonance
Pulsed EPR
Electron Spin Echo
Multple Resonance Spectroscopy
Magnetic Resonance Spectroscopy
NMR
Precessing
Nucleus Spin Decoupling in NMR
Superconducting Magnets
NMR Microcells
Electron Impact Ionisation
Chemical Ionization
Inductively Coupled Plasma Ionization
Secondary Ion Mass Spectrometry
Fast Atom Bombardment
Plasma Desorption Mass Spectrometry
Laser Desorption Mass Spectrometry
Matrix Assisted Desorption mass Spectrometry
Field Desorption Ionization
Thermospray Ionization
Electrospray Ionization
Atmospheric Pressure Ionization
Particle Beam Interface
Permeable Membrane Interface
Sector Mass Spectrometer
Quadrupole Mass Spectrometer
Ion Trap Mass Spectrometer
Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
Circularly Polarized Light
Verdet Constant
Faraday Effect
 

The Thermospray Interface

The thermospray interface evolved from the simple direct inlet system, by the rather simple modification of heating the tip of the entry tube. When the tip is heated, the solvent is vaporized right at the tip, and not somewhere inside the delivery tube. This results in much better control of both the nebulizing process and the ionizing process. One of the first reports of the successful use of the thermospray was by Covey and Henion [12]. Subsequently a number of different forms of the device were described, but the relatively simple form of the thermospray interface described by De Wit et al. [13] in 1987, will be used as an example to describe the operating principle. A diagram of the thermospray interface, devised by De Wit et al. is shown in figure 19. The device consists of a stainless steel tube, terminating in a metal cap made from a high-conductivity metal such as copper. Through the center of the stainless steel tube and copper cap passes a conduit carrying the reagent gas. In the center of the reagent conduit passes a section of a fused silica open tubular column (which carries the solution of the sample) and which projects slightly beyond the reagent conduit, into the ion source of the mass spectrometer. The sample tube first passes through a T union, to allow the reagent gas to be introduced into the annular space between the inner tube and the conduit, and then into the thermospray probe itself. A cartridge heater is placed in the copper cap together with a thermocouple, which measures the temperature of the probe tip, and provides a controlling signal to maintain the tip at a selected temperature.

Figure 19. The Thermospray Liquid Sampler

In a similar manner to the direct inlet system, the flow of sample solution had to be restricted, because the pumping rate of the mass spectrometer vacuum system was limited. The properties of the thermospray system were examined by Voyksner et al. [14]. They noted that the thermal spray system frequently produced molecular weight information (parent ions), and exhibited lower detection limits than the other sampling techniques. They also noted that using a thermal spray system with a 0.1 M ammonia acetate buffer, and a solvent that contained a high proportion of water, very high sensitivities could be achieved. The optimum interface temperature varied with solvent composition and could be determined by maximizing the solvent buffer ion intensities.

Figure 20. The Vestec Model 210 Thermospray Sampling Device

The spectra obtained from thermospray ionization resemble Chemical Ionization using ammonia as the chemical ionization reagent. The system produces protenation, ammonium addition and proton-bound solvent molecular clusters. The ionization procedure with this system was reported to be very soft and very few molecular fragment ions were formed. The system was successfully used for the analysis of triazine herbicides and organo-phosphorus pesticides.

Blakely and Vestal [15] employed the thermospray sample inlet system with the quadrupole mass spectrometer, and demonstrated that it could cope with sample flow rates up to 2 ml/min, with an aqueous mobile phase. Weakly ionized mobile phases require a conventional electron beam to be used to provide gas-phase reagent ions for the Chemical Ionization of the solute.

A more recent form of thermospray liquid sampling system, the Vestec Model 201 used by Via and Taylor [16] is shown in figure 20. This interface can handle flow rates of up to 1.5 ml/min, and incorporates two oil diffusion pumps backed by a single mechanical pump. The two pumps differentially exhaust the vacuum manifold, the source, and the analyzer regions of a Quadrupole Mass Spectrometer. A further two diffusion pumps, backed by a single mechanical pump, are coupled directly to the source, opposite the sample inlet. This pump removes about 99% of the vaporized solvent, whereas the heavier molecules pass through an ion aperture in the sampling cone, and into the mass spectrometer. The sample solution passes to the spray orifice, through a fused silica tube, which is joined by a 1/16 in. union to a length of stainless steel tubing, in the manner shown in figure 21.

Figure 21. Sample Solution Conduit to the Thermal Spray interface.

The ions are formed immediately after the nebulization, and pass alongside a repeller plate, held at a high potential, that impels the ions through a hole into the ion source. Once in the ion source, the ion optical system of the mass spectrometer directs them into the analyzer section of the spectrometer. The reagent gas is methane and its flow is controlled by separate needle valves.

A common problem in the explosives industry is the identification of stabilizer derivatives in the explosive itself, the presence of which will indicate its age or stability. Many nitrocellulose propellants are stabilized with diphenylamine . This stabilizer is thought to react with any NO2 that is released during aging or decomposition, to produce nitrated derivatives of the stabilizer. Consequently, an analytical method that will identify and measure the presence of nitro-diphenylamines in an explosive is highly desirable. Via and Taylor developed a chromatographic method to separate, identify and assay the amount of nitrated diphenylamines present in a nitrocellulose explosive sample. The results from a test sample, containing the stabilizer and its derivatives, that was separated by SFC and analyzed by the mass spectrometer are shown in figure 22.

 

  1. 2,6-dinitrotoluene, 2. impurity, 3. 2-nitrodiphenylamine, 4. 4-nitrodiphenylamine.

Figure 22. The Separation of a Propellant Test Mixture

The reconstructed total ion current chromatogram of the separation is shown at the top of figure 22. It is seen that a good separation of the solutes of interest was obtained. It is also seen that the spectra for 2,4 dinitrotoluene and 2-nitrodiphenylamine are clear and unambiguous and would allow the substance to be identified with certainty. The authors claimed that the sensitivity of the analytical procedure was about 1 ng. However, according to Via and Taylor, in a private communication, Wilkes of the Vestec Corp. claimed that satisfactory spectra could be obtained from samples present at the picogram level. For example, 60 pg of 2-nitrodiphenylamine injected on the column could provide an identifiable mass Spectrum.


 

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.

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