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Report for Analytical Chemists

hollow cathode source and in the flame, respectively, usually is con­siderably less than one. For sources in thermal equilibrium, J, < Jpi. For nonthermal sources, Js may be much larger than Jri, as in various electrical discharges such as hollow cathode lamps, permit­ting the great superiority of AA at short wavelengths. Much hotter flames are required to improve F E in the deep ultraviolet. For lines at longer wavelengths, evidently the radiance ratio does not outweigh the line profile ratio—i.e., their product does not exceed unity, for many elements, and FE thus can be more sensitive than AA. Note that the relation says nothing at all about populations of energy levels. The reader should consult Alke-made's paper (17) for a more de­tailed discussion of this and other commonly accepted fallacies con­cerning AA.

A systematic series of measure­ments of noise levels from the vari­ous sources, and of relative signal strengths, by the two methods and by AF, for a number of elements and flames, would help to put the field of flame photometry in better

order. Several publications by Winefordner et al. have discussed the subject but without presenting much data {18-21). The treat­ment of Alkemade (17) outlined above brings to mind this need for actual measurements of noise levels and of line strengths, independent of each other. No amount of mea­surements of detection limits—i.e., signal-to-noise ratios—can serve the purpose. The treatment of the measurement of background, espe­cially important in determining sig­nal-to-noise ratios in FE, is taken up in the section on interferences below.

ATOM FORMATION QUESTION

The efficiency with which the flame produces neutral atoms of the test element is of equal importance in F E and AA. This efficiency was not known, even approximately, for most elements until recently. Se­lected data from two studies (22, 28) are presented in Table I I .

The table shows that many ele­ments give free atom fractions not far below unity. This means that a rather large percentage of the metal

TABLE II. Free Atom Formation Fractions in Flames (The ratio of the number of free atoms to the total number of

atoms in all states of an element present in the flame at any instant)

In the Air-CoHu f lame Ref. (23)

Ref. (22) Lean Rich

In the N20-C2H2 flame

Ref. (23) Lean Rich

Ag 0.66 AI <0.00001 <0.0001 <0.0001 0.29 0.29 Ba 0.0011 0.0019 0.0034 0.11 0.074 Ca 0.14 0.069 0.052 0.50 0.34 Cd 0.50 0.77 0.80 0.67 0.60

Co 0.052 0.13 0.023 0.13 0.11 Cr 0.064 0.20 0.54 1.00 1.02 Cu 0.98 0.38 0.40 0.54 0.49 Fe 0.66 Ga 0.16

In 0.67 0.13 0.10 0.45 0.37 Li 0.20 Mg 0.59 1.09 1.05 1.00 1.07 Mn 0.45 0.33 0.33 0.38 0.39 Na 0.50 0.68 0.63 0.31 0.32

Pb 0.44 Sn <0.0001 0.003 0.061 0.24 0.35 Sr 0.13 0.087 0.068 0.98 0.57 V <0.0004 <0.0004 Zn 0.45 0.62 0.66 0.54 0.49

32 A · ANALYTICAL CHEMISTRY, VOL. 4 1 , NO. 14, DECEMBER 1969