~~
Table 11. Detection Limits-Plasma Emission us. Atomic Absorption (14) Plasma, pg/ml A.A., grams 1-ml sample 10-111sample Ag (no matrix)
Cu (no matrix) Cu (with matrix) Fe (with matrix) Cr (with matrix) Co (with matrix)
x
0.01 0.005
...
0.01 0.005
0.007
+
4 of the total signal obtained (background sample signal). The per cent standard deviation at the detection limit for each of the metals is approximately 40 Z. At 10-'0 gram, the per cent standard deviation is 7 z . For the 10-l' gram determination of cobalt, manganese acetylacetonate was substituted for chromium because of the lower background signal. To verify that the use of the matrix technique eliminates interferences from other metals, a copper sample for example containing gram of copper, lo-* gram of cobalt, and 10-8 gram of iron was analyzed. A blank determination was made on a similar solution to which the copper was not added. The corrected emission intensity for the mixture was identical to that obtained from a similar copper sample containing only the chromium matrix. Similar experiments were carried out with various other combinations including those metals mentioned above in addition to Mn, Cr, and A1 and several nonmetals including C, C1, Br, and I. In no case were interference effects noted. A study was made to determine the usefulness of the method for less volatile inorganic salts. Figure 15 shows results obtained for the evaporation of silver iodide and copper chloride samples, Analytical curves were obtained which were similar to those for the chelates. The log log plot for silver iodide (excess HI added to silver nitrate solution) has a slope of unity and shows a detection limit similar to that for the chelates with matrix addition. The copper chloride curve indicates a slope of nearly unity, but a detection limit similar to that determined with a pure copper chelate without matrix. The attempt to obtain a satisfactory metallic matrix to add to either silver iodide or copper chloride has not been successful. The difficulty appears to lie in the relative evaporation rates of the matrix and of the sample. It is essential that these rates be nearly equal so that both the matrix metal and the sample enter the discharge simultaneously. Another problem which has arisen in the study of the feasibility of analyzing inorganic metal salts has been the difficulty in the evaporation of these materials from the surface of a hot filament. For example, silver as the nitrate does not give a reproducible analytical signal. Recent work indicates that these problems can be overcome by revising the sample introduction technique and work is being conducted to determine a general procedure for the analysis of inorganic compounds and to devise a satisfactory matrix technique. A comparison of detection limits for atomic absorption and the plasma method on the basis of concentration and total grams is shown in Table 11. For sufficiently large samples (one milliliter or more of solution) the detection limits by this technique are at least equal to those of atomic absorption, assuming no preconcentration of the sample. The major
0.001 0.03 o.oO01 0.001 0.001 0.001
x
1
x
5
x 5x 7x 1
Plasma, grams
10-8 10-9
...
10-8
10-9 10-9
1 x 10-11 x 10-10 1 x 10-1s 1 x 10-11 1 x 10-11 1 x 10-11
5
advantage of the plasma method is that these parts per million sensitivities can be obtained on 10 p1 of sample. On the basis of minimum grams detectable, however, the plasma is several orders of magnitude more sensitive (assuming approximately 1 ml of sample is necessary per determination by atomic absorption). RECEIVED for review March 16, 1967. Accepted July 31,1967. Presented in part, Midwest Regional American Chemical Society Meeting, Lawrence, Kansas, October 27, 1966. Work supported by National Science Foundation under Grant NSF GP5095. (14) W. Slavin, Appl. Spectry., 20,281 (1966).
Corrections Estimation of Preexponential Factor from Thermal Decomposition Curve of an Unweighed Sample In this article by R. N. Rogers and L. C.Smith [ANAL. CHEM.39,1024 (1967)], the equation given for determining the preexponential factor from DSC or DTA curves is not original, but has, in fact, appeared in the literature several times previously in the same context (1,2), The authors extend their apologies to the readers and especially to Murray, White, and Kissinger. (1) P. Murray and J. White, Trans. Brit. Cerurn. Soc., 54, 204 (1955). (2) H. E. Kissinger, J . Res. Natl. Bur. Standards, 57, 217 (1956); ANAL.CHEM., 29, 1702 (1957).
A Mechanistic Investigation of Molybdenum Blue Method for Determination of Phosphate In this article by S. R. Crouch and H.V. Malmstadt [ANAL. CHEM., 39,1084 (1967)l on pages 1087-8, Figures 3 and 5 are incorrect. The figures themselves have been interchanged, but the captions are correct as they are.
VOL. 39, NO. 12, OCTOBER 1967
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