The L’vov Platform

The sample must be in good contact with the hot graphite surface for as long as possible and be in a uniform temperature environment to ensure good element selectivity. The sample has intimate contact with a flame although for a limited time. If the sample is applied directly to the wall of a cold Graphite Furnace and subsequently stepwise heated to a high temperature, however, the tube wall will heat faster than the gas producing temperature differentials.

Figure 19. A Graphite Furnace Containing a L’vov Platform

The poor uniformity of temperature can cause condensation or recombination of atoms. L’vov suggested that a small graphite platform coated with pyrolytic carbon was inserted in the furnace. A diagram of a furnace containing a L’vov Platform is shown in figure 19. The platform was heated by radiation from the hot walls and by the hot gas molecules; thus, the vaporization an Atomization was delayed until the tube had obtained constant temperature. The net result was that the sample was always vaporized into an environment that was hotter than the surface from which it left. As a consequence, the chance of condensation and recombination was significantly reduced and any molecular species that were rendered volatile were more likely to suffer dissociation. The L’vov Platform has shown a great improvement in performance, particularly sensitivity, and without surface modification. Matrix modification has shown even greater improvement in performance. The relative performance of a simple pyrolytic carbon tube, a standard graphite tube and a graphite tube containing a L’vov platform is shown in figure 20.

The platforms are often prepared from a pyrolytically coated graphite tray that has been further heated after treatment with a tantalum solution.

It was also found that the sensitivity of the analysis employing the graphite tube was improved by inserting a tungsten or tantalum tube inside the graphite tube and depositing the sample on the metal surface (as opposed to that of the graphite tube). It appeared that the metal surface acted in a similar way to the L’vov Platform. It was found that, by this procedure that detection limit for some 23 elements could be lowered by 3 to 100 fold and, at the same time, reduce the Atomization temperature by 200 to 900 C.

The possible use of wire filaments and springs have also been examined and various degrees of improved performance obtained.

Figure 20. The Relative Performance of a Simple Pyrolytic Carbon Tube, a Standard Graphite Tube and a Graphite Tube Containing a L’vov Platform

(Sample 1 ppm of Arsenic)

An example of the performance of the commercially available Perkin Elmer AAnalyst 700 instrument is shown in figure 21. The advantages of the technique for trace analysis is clearly demonstrated.

When a quantum of light is emitted, the atom responsible suffers a change in energy level (i.e from E₂ to E₁ where E₂ >E₁). Thus, the change in energy, from equation (1), (ΔE) is given by,

ΔE = E₂ – E₁ = hν.

Courtesy of the Perkin Elmer Corporation

Figure 21, The Results from the Analysis of a 1μg/ml Solution of Chromium using a Graphite Furnace.

Thus if (λ) is the wavelength of the light and (c) is the velocity of light,

Then, c = νλ thus, ΔE = hc/λ

Consequently, the energy change is proportional to the reciprocal of the light emitted and thus, the greater the energy change the shorter its wavelength. The emission Spectrum of the element hydrogen is shown in figure 22.

Figure 22. The Spectrum of Hydrogen Taken in the Visible Light Region

The Spectrum lines show a regular increase in frequency (decrease in wavelength) as the lines progress towards the lower frequencies.

The series of lines is called the Balmer Series after Balmer who deduced the following relationship for the wavelength of the emitted light;

where (λ) is the wavelength of the light,

(n’) is a member of a series of consecutive integers starting with three.

and (G) is a constant.

In figure 23 the value for (n’) the first line of the series shown at a wavelength of 656.3 nm (65639Å) is 3 (the values of the other lines in the series are 486.1 Å, 434.0 Å, 410 .2 Å 399.8 Å that correspond to values of n= 4,5 and 6 respectively).

Figure 23. Graph of Wavelength (nm) against ‘Balmer’ Function

It is clear from the graph that the higher the energy change the shorter the wavelength of the emitted photon and that the individual spectral lines become closer and closer together. Examination of the equation of Balmer, it is seen that by simple algebraic manipulation and noting that, ν = c/λ, Ballmer’s equation can be put in the form of ,

where, (R_H) is known as Rydberg’s constant.

Rydberg’s constant, (R_H), is given by the following equation,

where (e) is the charge on the electron,

(h) is Plank’s constant,

(ε_o) is the Vacuum Permativity,

and (μ) is reduced mass of the electron and proton and is given by,

It is interesting to observe that the number of electrons involved becomes greater as the atomic weight of the element becomes greater. The atomic Spectrum of helium is shown in figure 24

Figure 23. The Atomic emission Spectrum of helium

In the Spectrum shown in figure 22 the spectral lines are closely matched to the precise wavelengths given on the wavelength scale. However, in the spectra given in figures 24 and 25 the relationship between the lines and the scale are not so precise but are sufficiently accurate to give a correct impression of the forms of the spectral line distribution. It is clearly seen that there are considerably more lines in the Visible region spectra and this is because the extra electron in orbit round the nucleus adds many energy levels to the atom and, thus, many more emission lines are apparent.

The increased complex nature of the line pattern is emphasized by the Spectrum for nitrogen shown in figure 24. As the atomic number increases and, thus, the number of electrons orbiting the nucleus also increase then the number of lines can increase immensely.

Figure 23. The Atomic emission Spectrum of nitrogen

It should be noted that nitrogen has only an atomic number of 14 and one of the popular areas of application for Atomic Spectroscopy is for the detection and analysis of trace levels of heavy metals. However, it should also be pointed out that all the emission lines are not of the same intensity and in practice the number of lines examined for identification purposes or quantitative assessment can be relatively few. The line wavelengths chosen are usually from those having relatively high intensity and of wavelengths that are appropriate for the measuring spectrometer involved and unique the element of interest.