The Selection of Optimum Conditions M. 1. Parsons1 W. J. McCarthy and J. D. Winefordner University o f Florida Gainesville, 32601
for Spectrochemital Methods 111.
Sensitivity o f atomic fluorescence,
absorption,
and emission flame
spectrometry
Expressions for the limiting detectable atomic conrcntration (and sample solution concentration aspirated into the flame gases) in atomic emission (46), atomic absorption (47,48), and atomic fluorescence (49)flame spectrometry have been previously presented. Using these expressions, it is possible to compare directly the calculated limiting detectable atomic concentrations for these three flame spectrometric methods under any experimental conditions. Since the measured signal for the three flame spectrometric methods vary in the same manner with the concentration of ground state atoms in the flame gases, the three methods will therefore vary approximately in a similar fashion with solution concentration aspirated into the flame gases under similar flame conditions (50). It is the purpose of this paper to demonstrate that this comparison will provide the spectroscopist and analyst with the facility to: choose the most sensitive flame spectrometric method for a given analysis; choose the most sensitive instrument for the given analysis once the method has been chosen; and choose the optimum exponential conditions for the analysis once the method and instrument have been chosen.
trometric methods, a practical experimental system (see Fig. 4) is assumed and the parameters of the components in the experimental system are listed in Table 4. All of the data needed for calculating limiting debectable atomic concentrations for the three elements with resonance linesooccurring at ~ i d e l ydifferent waveJengths A) was (Cd-2288A, Cu-3248 A, and Ka-5890 either compiled from literature data or experimentally measured in the authors' laboratory and is also given in Table 4. For all of the calculations performed, the amplifierreadout noise was found to be nedilinible comuared to the other noises in any of the systems and therefore did not enter into the calculations. Atomic Emission Flame Spectrometry ( A E ) . See Table 3 and ref. (46). For the experimental system described in Table 4, the following simplification has been made: The noise due to the fluctuation in thermal
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M)NOOlRDMPTO6 SUT
Equations used for Comparison
For the sake of simplicity, only the resonance transitions for three elements are compared. All of the expressions are given in Tahle 3. The equations are given in the final form which is useful for calculation, and the reader is referred to the appropriate references (cited in Tahle 3) for complete derivations and discussion of the expressions. To facilitate a comparison of the three flame specInvestigation sponsored by AFOSR (SRC)-OAR, U.S.A.F. Grant No. AF-AFOSR-1033-66 with supplementary funds from Edgewood Arsenal, Md. 'Present address: Phillips Petroleum Co., Research and Development Dept., Bartlesville, Oklahoma, 74004.
EDITOR'S NOTE Part I of this paper, discussing the use of signal-to-noise ratio theory, appeared in the February issue of THIS J O U R N ~ L (44, 80, 1967). Part 11, on the quantum efficiency and decay time of Inminescent molecules, appeared in the hfarch issue of THIS JOURNAL(44, 80, 1967). The numbers of the figures, tables, and literatwe cited in this paper, iallow consecutively those in Part I and Part 11.
214 / Journal of Chemical Education
I
t LENS
l'.
;
Figure 4. Schemdic diagram of entrance optin for atomic emission, otomic absorption, and atomic flvorercence flame spectrometry.
Table 3. Equations for Limiting Detectoble Atomic Concentrations N, by the Three Flame Spectrometric Meth0dsa.b Atomic Emission Flame Spectrometry (48) N,
=
2dg[ko(is
+ r n k ~ R d W ~ 1+~ )( E ~ ~ ~ M R ~ W *(1)I B ) ~ ] ~ I ~ rnUknrWK~L
Atomic Fluorescence Flame Spectrometry Using A Continuous Source of Excitation (49)hC Nmc = 2&f[k~(id
+ mknrRsWa[Is+ yJc0I) + (frnknrRdW'I~)~ + (xyrnknrRdW*I~~)~]~" (2) mknrWKcJce@L
Atomic Fluorescence Flame Spectrometry Using A Line Source of Excitation (49)" N d
=
+
+
+
+
2 d r f [ k ~ [ i d mkxW(1~RdW UYIL')] ( E r n k ~ R d w T ~ )(~x ~ ~ ~ U ~ . W W I L(3) ~)~]II? rnUmYKLIL%~
.4tomic Absorption Flame Spectrometry Using a Continuous Source' (48) N,,
=
(mk",^&?L)
[ko(G
+ mknrRsW21cD)+ (*mnknrRdWPJc")'+ ( ~ ~ M R ~ W ~ (I4B) ) ~ ] ~ ' ~
Atomic Absorption Flame Spectrometry Using a Line Source (47) N,G
=
(-K~daf-) Ikd& mUkaaW1~"L
+ rnUknrWI~")+ ( W L ~ U ~ M W I+L "(
