of 2.14 is only slightly lower than that of ethyl acetate and it is an easier solvent to work with. All other solvents studied produced less enhancement than did methyl isobutyl ketone and ethyl acetate. Matrix effects are small with copper in organic solvents since only the extractable species and the chelating agent are extracted, thereby, producing a relatively constant organic matrix. However, the aqueous matrix does have a significant On the Observed copper absorbance in aqueous solution, and can also influence the degree of extraction
(3, 7). Therefore, greater reproducibility of calibration curves in organic solvents is obtained. RECEIVED for review January 11, 1971. Accepted May 19, 1971. This work was supported in part by a Chemical Company Fellowship in Chemistry and an Eastman Kodak Research Fellowship. (7) M. Suzuki, M. Yanagisawa, and T. Takeuchi, Talanta, 12,
989 (1965).
Rapid Determination of Heavy Elements in Organometallic Compounds Using X=Ray Fluorescence J. M. McCall, Jr., and D. E. Leyden Department of Chemistry, University of Georgia, Athens, Ga., 30601
C. W. Blount Department of Geology, University of Georgia, Athens, Ga. 30601
INTHE RAPIDLY EXPANDING AREA of organometallic chemistry, new compounds appear by the hundreds each year. Frequently, these compounds are characterized by chemical analysis for carbon, hydrogen, and nitrogen content with an occasional analysis for halogens. Often elements such as the metals, phosphorus, and halogens are the major constituents. The cost of analysis for these elements by independent laboratories deters many researchers from requesting complete elemental analysis. There is also concern that sample treatment procedures such as ashing may alter the results when organometallic compounds are analyzed. X-ray spectrography has long been recognized as an accurate analytical tool. In principle, elements of atomic number thirteen or greater may be determined conveniently by X-ray methods. Although instrumental design, sample matrix, and other factors make firm statements of sensitivity difficult, an estimate of one part in 104 is conservative for modern instrumentation. Recent reports of ion exchange loaded paper ( I , 2), ion exchange beads pressed into pellets (3),and filter paper as the sample matrix for the determination of chloride (4), calcium (3, sulfur ( 6 ) , iron (7), and iodine ( 8 ) in blood has led to new applications of this technique. The use of filter paper is a major advantage because it acts as a support upon which the sample may be diluted. The result is an essentially constant matrix composition provided that the sample is kept at a sufficiently low level that self adsorption of the secondary radiation is not encountered. (1) P. D. Zemany, W. W. Welbon, and G. L. Gaines, ANAL. CHEM.,30, 299 (1958). (2) W. J. Campbell, E. F. Spano, and T. E. Green, ibid., 38, 987 (1966). (3) C. W. Blount, W. R. Morgan, and D. E. Leyden, Anal. Chim. Acta, 53, 463 (1971). (4) J. S. Rudolph and R. J. Nadalin, ANAL.CHEM.,36, 1815 (1964). (5) S. Natelson, M. R. Richelson, B. Sheid, and S. L. Bender, Clin. Chem., 5, 519 (1959). (6) S. Natelson and B. Sheid, ibid., 6, 299 (1960). (7) Ibid., 7, 115 (1961). (8) Ibid., 8, 17 (1962). 1324
EXPERIMENTAL
Apparatus. The instrument used in this work was a Phillips-Norelco Universal Vacuum Path X-Ray Spectrometer. A LiF 200 analyzing crystal was used in all cases, and a tungsten target tube was used for all analyses except those involving tungsten, for which a chromium target tube was substituted. The spectrograph was equipped with scintillation and gas flow proportional counters, the latter using P-10 gas. For each element the counter giving the largest peak to background ratio was determined, and used for subsequent analysis. Before each set of samples was analyzed, a check for any possible spectral interferences was made. A scan of 20 was conducted in the vicinity of the angle to be used for the analysis to check for overlapping peaks. Also a pulse height scan was made at the selected 20 angle to ensure correct setting of the counter voltage, and to check for higher order components in the radiation being counted. The X-ray source and counting equipment had adequate stability after an initial warm up time of approximately one hour. Reagents. The inorganic salts used for standards were analytical reagent grade chemicals and were used without further purification. The organometallic compounds were obtained from other groups within this Department. Whatman No. 40 filter paper was used for paper disks which were punched out using a die approximately one-half inch in diameter. The paper disks were mounted on aluminum disks of five-eights inch diameter using double adhesive tape. Procedure. The samples were prepared as follows. First, a series of aqueous solutions of the standard inorganic salt were carefully prepared such that exactly twenty-five microliters of each solution could be used to prepare a series of disks containing amounts of the element ranging from one to five micromoles. The twenty-five microliters were added slowly so as to saturate the paper disk but not allow the solution to overrun the paper. The standard disks were then dried at room temperature and sprayed with a very light coating of ten per cent collodion in acetone to protect them. With care these disks can be used as permanent standards; no loss of X-ray fluorescent intensity was observed over several months. The compounds to be analyzed were prepared in like manner, with the amount used selected to fall within the range of the standard disks. Solvents were
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
selected by the solubility of the particular compound. No effect of solvent was observed. Duplicate standards at intervals of one micromole, from one to five micromoles, and six disks for each sample were prepared. The sample was supported in the X-ray beam by a mylar window in the bottom of the sample holder provided with the instrument. An aluminum washer was used to keep the sample from shifting out of the X-ray beam. The samples were rotated in the beam to ensure even irradiation. The spectrometer was usually evacuated unless the compound was suspected of being volatile. The samples were irradiated until 106 or greater counts were accumulated. In all cases, the relative standard deviation in the net counts was maintained below 1.o %. Calibration curves were prepared by plotting the count rate us. the micromoles of the element of interest on the standard disks. The curves were linear up to about five micromoles of sample for most elements. Above this level, the counting rate shows a negative deviation because of “dead time” in the counter. This can either be compensated or smaller samples used. Sample sizes much lower than those used in this work may be employed if a microbalance is available or if the sample is extensively diluted. As little as 1.0 micromole of most elements gives a counting rate of 10a-104 per second. RESULTS AND DISCUSSION
The compounds analyzed were obtained from several inorganic research groups in this Department, and represent as wide a range in organometallic compounds as could be found in regard to the transition metal and molecular formula. The samples obtained by us were transition metal organometallic compounds. Interest was in the analysis for the central metal atom. The amount requested for analysis was on the order of 50-100 milligrams. This could conceivably be reduced to the amount needed for just one disk, or about one milligram, with some sacrifice in precision of the results. The compounds analyzed are shown in Table I with the elements determined represented in italics. Uncertainties are standard deviations from the mean of six replicate runs on each sample. The agreement between theoretical composition and analysis varied. In some cases, such as the PPN complexes, the compounds are thought to form solvated species during crystallization (9). Although an effort is made to remove the solvent molecules, the phenomenon could easily justify occasional low results. The CSH5Fe(C0)J sample obtained was an unpurified precursor to the product CbHSFe(CO) [PFzNEtJI. Both compounds gave low results for iron. There was no evidence to indicate loss of sample as a result of irradiation. In cases in which the agreement was particularly poor, an analysis was performed using an appropriate standard colorimetric method (10). The colorimetric 9) J. K. Ruff, Department of Chemistry, University of Georgia, Athens, Ga., personal communication 1970. (10) F. D. Snell and C. T. Snell, “Colorimetric Methods of Analysis,” 3rd ed., D. Van Nostrand, New York, N. Y . , 1954.
Table I. Results of Some Representative Analyses Compound HZCUEDTA HzNiEDTA
Theoretical
X-Raya
18.0 16.8
17.7 f 0 . 2 16.6 i 0.4
As { KCH&l*Jl& )zCr(CO)r 6 - 2 . 0 [(CH~)ZN]~A~-C~(CO)~ As = 18.8 Cr = 13.0 C5H5Fe(C0)21 18.4 C5H6Fe(CO)[PFzNEtz]I 13.4 COC~Z(CH~)ZNCHZCHZP(CeHs)zHCl 13.9 CUC~Z(CH~)ZNCHZCH~PO(CeH5)zHCl 14.3 Mn( CO)5Br 20.0 PPNCuClz, (PPN :(Ce,H5)3-
Colorimetric0
As
- = 1.95 Cr 19.2 f 0 . 4 12.5 i~0.3 12.8 f 0 . 2 1 3 . 5 i 0.8 9 . 2 1 0 . 2 10.6 13.9 f 0 . 1 14.1 f 0.3 17.6 f 0 . 2 16.6
+
P=N=P( CBH5)3) 9.4 9.5 f 0 . 1 PPNCr(C0)aSCN 6.6 6 . 3 f 0.3 PPNCuCla 9.0 9.1 f 0.2 PPN W(C0)500CCzFj 18.0 18.3 i 0.3 PPNFe(C0)3N0 7.9 7.6 f 0 . 1 [C~H~F~(CO)ZIZ 31.6 32.1 f 0.3 20.4 19.5 f 0 . 3 PPN W(C0)5C=CCHZ PPNW(C0)jSCFs 19.1 15.7 i. 0 . 2 15.6 + 0 . 6 PPN W(CO),Cl 20.5 17.5 i 0 . 2 17.3 (PPN)HF~~(CO)U 16.5 17.3 f 0 . 2 a Average and standard deviation of six replicates. Standard deviations shown for runs of three or more replicates, Standard deviations not shown for duplicates.
methods substantiated the X-ray results within reasonable expectations. The technique presented has several advantages. The filter paper matrix effectively eliminates matrix effects, or makes them essentially constant. The impregnation of the paper by evaporation of a solution eliminates variation in particle size which may affect the counting rate. The technique is rapid; approximately one hour is required for an isolated sample and fifteen minutes per sample for a batch to be analyzed for a given element. Decomposition of the sample is not important once weighed unless the element of interest forms a volatile decomposition product. Perhaps the most significant advantage is that two or more elements can be determined using the same sample. An example of this is [(CH3)2N]3A~-Cr(CO)5 in Table I. No pretreatment of the sample is required. ACKNOWLEDGMENT The authors thanks R. B. King, J. K. Ruff, and R. C . Taylor for some of the compounds used. RECEIVED for review April 5, 1971. Accepted May 19, 1971. This research was supported in part by a Research Grant from the National Science Foundation, GA-001667.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1325
