Identification by Wavelength Measurement

A search of wavelength tables, arranged by wavelength, reveals a sensitive cadmium line at 3261.1 A thus, cadmium may be the element producing the observed spectral line. To verify this tentative identification, a table of spectral lines of cadmium should be examined and the spectrum should be inspected to determine if other cadmium lines are present. Three or four lines in the spectrum coinciding with the wavelengths and relative intensities of other cadmium lines is considered positive proof that cadmium is present in the sample. 

Linear interpolation can be used with grating spectrometers operated in the normal position. It also can be used to obtain approximate wavelengths with prism instruments if the wavelength difference between the two known spectral lines is small. For prism instruments interpolation is possible using the Hartmann formula  

The three constants Aq, C, and do may be evaluated from three known wavelengths if the separations of the three lines are obtained as rfj, d2, and d. The three simultaneous equations can then be solved for Aq, C, and do and the resulting equation used to determine unknown spectral lines. 

Some commercially available mixtures of elements are available to produce comparison spectra containing more than one element and are especially useful for qualitative analysis. The Jarrell-Ash Division, Fisher Scientific Co., produces a powder standard that contains 1.30% of 45 different elements and another containing 1.20% of 48 elements. The one powder standard includes Al, Sb, As, B, Ba, Be, Bi, Ca, Cd, Ce, Cs, Cr, Co, Cb, Cu, Ga, Ge, Hf, In, Fe, Pb, Li, Mg, Mn, Hg, Mo, Ni, P, K, Rb, Si, Ag, Na, Sr, Ta, Tl, Th, Sn, Ti, W, U, V, Zn, and Zr. The second powder standard includes all the above elements plus Au, Pd, and Pt. The powders can be used to produce reference spectra for qualitative analysis and also can serve for semiquantitative procedures by proper dilutions with other matrices. 

Johnson-Mathey Chemicals, Ltd., produces several standard mixtures. One contains 53 elements at 1.18 wt % of each individual reference element. A second mixture of eight elements, (Au, Ir, Os, Pd, Pt, Rh, Ru, and Ag) is available for identifying precious metals and a third powder is available for the rare earth elements. Johnson-Mathey recommend use of lithium fluoride as a spectroscopic buffer if one is needed to suppress matrix effects. If the recommended line is observed in the unknown spectrum, the presence of the element should be confirmed by identifying two or three other spectral lines of the same element. 

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