Lithium Isotopic Analysis by Nuclear Magnetic Resonance Spectrometry Albert Attalla and Ralph R. Eckstein Monsanto Research Corporation, Mound L a b o r a t o r y , Miamisburg, Ohio 45342
THE ISOTOPIC ABUNDANCE of lithium in a wide variety of samples has been determined by means of optical and emission spectrometry (I-3), neutron activation (4), and mass spectrometry (5-9). All of these methods either involve intricate sample preparations and complicated instrument calibrations or are restricted in the concentration range of analysis and the limits of precision. The method described in this paper can be used not only for isotopic abundance ratio determinations but also for quantitative analysis of 6Liand ’Li. To overcome some of the problems associated with the techniques discussed above, nuclear magnetic resonance spectrometry (NMR) was investigated as a possible alternative method for the analysis. Quantitative analysis by NMR (IO) can be quite precise and accurate, especially when the sample is in solution and gives a single characteristic absorption peak for each element such as the isotopes of lithium.
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EXPERIMENTAL
Apparatus. The absorption spectra were obtained using a Varian Associates HR-60 High Resolution NMR spectrometer equipped with a Varian-designed dual-frequency probe (Varian SK 22268) tuned to the resonance frequencies (8.8 and 23.3 MHz, respectively) of 6Liand 7Li in a constant magnetic field of 14,078 oersteds. The dual-frequency probe was designed to detect both isotopes of lithium while maintaining all experimental variables constant. The sample rests in the probe within a vertically situated glass insert around which is wound a single receiver coil. A single transmitter coil is located on the surface of the probe sample cavity and the magnetic field sweep coils are embedded into the sides of the probe facing the magnet poles. Both the receiver and the transmitter circuits are pretuned to the 6Li and 7Li resonance frequencies by manually adjusting the rf circuits for the probe. For a given frequency, the receiver and transmitter coils are connected in parallel and the two frequencies are received and transmitted one at a time by a two-way switch. The NMR signal was displayed as a function of magnetic field sweep on a Sargent Model SR recorder. Spectral peak areas were measured with an Ott-Planimeter (Burrell Corp., Pittsburgh, Pa.). Sample Preparation. Standards were prepared from mass spectrographically analyzed mixtures of the 6Li and 7Li isotopes. All lithium samples were converted to their chlorides by reacting with concentrated HCl, evaporating to dry(1) J. K. Brody, M. Fred, and F. S. Tomkins, Spectrochim. Acta, 6, 383 (1954). ( 2 ) V. A. Fassel and H. J. Hettel, ibid., 7, 175 (1955). (3) G. K. Werner, D. D. Smith, S. J. Ovenshine, 0. B. Rudolph, and J. R. McNally, Jr., J . O p f . SOC.Amer., 45, 202 (1955). (4) L. Kaplan and K. E. Wilzbach, ANAL.CHEM., 26, 1797 (1954). (5) M. J. Higatsberger, A c f a P h y s . Austr., 9, 179 (1955). (6) H. Wanke and E. V. Monse, 2.Naturforsch., loa, 667 (1955). (7) A. H. Gillieson and R. P. Thorne, Nature, 176, 1228 (1955). (8) I. Omura and N. Morito, J . Phys. SOC.Japan, 13, 659 (1958). (9) H. J. Svec and A. R. Anderson, J. Sci.Znstrum., 43, 134 (1966). (10) J. A. Pople, W. G. Schneider, and H. J. Berstein, “HighResolution Nuclear Magnetic Resonance,” McGraw-Hill Book Company, New York, N.Y., 1959.
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Figure 1. Ratio of area of 6Li NMR absorption spectrum to sum of spectral areas of both isotopes as a function of 6Li content
ness in an oven at 115 “C overnight, and then dissolving the dry LiCl in a 0.1M aqueous solution of MnCh (11). (LiCl is very hygroscopic and must be handled under anhydrous conditions.) Several drops of concentrated HCl were added to each sample to facilitate dissolution. The LiCl solutions were then pipetted into 5-mm precision bore NMR sample tubes for analysis. Procedure. The instrument controls were adjusted so that all samples throughout the complete concentration range could be handled under identical operating conditions. Using the ratios of the areas of the NMR absorption peak of the 6Liisotope to the sum of the areas of both isotopes, the standard curve shown in Figure 1 was plotted as a function of the 6Licontent in atom per cent. The isotopic abundances of analytical samples are interpolated directly from the standard curve. In practice, the lithium isotope abundance is determined approximately from the complete standard curve and is then determined precisely from a plot of several standards which lie closely in concentration to the unknown, which is run in duplicate. Typical data for plotting the 6Li and ’Li standard curves are given in Tables I and 11, respectively. DISCUSSION
General. Two types of analysis are inherent in the NMR method. The ratio analysis requires no special handling practices. Enough sample is simply dissolved in several milliliters of MnCh solution to give a useful NMR spectrum. (11) B. E. Holder and M. P. Klein, Phys. Rev.,98, 265 (1955). ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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Table I. Area of 6LiNMR Absorption Spectrum as a Function of Concentration Molarity, Area, arbitrary units moles/liter 11 0.75 28 1.88 2.47 38 2.97 46 52 3.30 78 4.96 97 6.17 Table 11. Area of 7Li NMR Absorption Spectrum as a Function of Concentration Molarity, Area, moles/liter arbitrary units 0.46 26 0.99 60 1.07 65 1.66 100 2.17 134 3.63 224 5.65 350
On the other hand, quantitative analysis of 6Li and ’Li requires a knowledge of the mass of the dry lithium sample and the volume of the final solution. Obviously, the last procedure can also be used to calculate the isotopic abundance ratio.
The smallest practical amount of BLidetectable was about 0.7 mole/liter of solution. In the case of 7Li approximately 0.2 mole/liter was necessary to produce a satisfactory spectrum. Precision and Accuracy. If it is assumed that the mass spectrographic analyses of the standard lithium samples are unbiased (no systematic errors), then it can be reasonably assumed that the accuracy of the NMR analysis is identical to the precision. An average deviation of 0.84 arbitrary unit was obtained in the NMR spectral area measurements. This value was calculated from at least five determinations on each of several hundred NMR spectra obtained over a period of 2 years and is in agreement with the fact that the planimeter can be read only to the nearest whole number. Excellent reproducibility of the average NMR spectral area measurement is indicated by a standard deviation of 0.1 obtained from three or more successive runs of the same sample. The standard deviation of a series of 6Li isotopic analyses of duplicate samples was found to be 0.02 at. %. Quantitative determinations of 6Li and 7Li were performed on standard samples prepared from mixtures of the pure isotopes. The amount of isotope found was within 0.01 mole/liter of solution of the amount taken. RECEIVED for review December 30, 1970. Accepted March 1, 1971. Mound Laboratory is operated by Monsanto Research Corporation for the US. Atomic Energy Commission under Contract No. AT-33-1-GEN-53.
Gas Chromatographic Determination of Organic Mercury Compounds by Emission Spectrometry in a Helium Plasma Application to the Analysis of Methylmercuric Salts in Fish Carl A. Bache and Donald J. Lisk Pesticide Residue Laboratory, Department of Entomology, Cornel1 University, Ithaca, N . Y. 14850
THERECENT WIDESPREAD concern about relatively high levels of mercury in fish and other species has been publicized (1-5). Methods of analysis for total mercury in biological materials have included colorimetric (6-lo), neutron activation (11, 12), (1) (2) (3) (4)
Chem. Eng. News, 48 (26), 36, (1970). Daniel Zwerdling, The New Republic, p 17, Aug. 1, 1970. Carl E. Parker, The Conservationist, p 6, Aug.-Sept. 1970. Chem. Eng. News, 48 (42), 8 (1970). ( 5 ) Ind. Res., p 25, Oct. 1970. (6) Assoc. Offic. Agr. Chemists, Washington, D. C., “Official Methods of Analysis,” 10th ed., pp 375-77, 1965. (7) D. M. Goldberg and A. D. Clarke, J . Clin. Pathol., 23, 178
(1970). (8) V. L. Miller and F. Swanberg, Jr., ANAL.CHEM., 29,391 (1957). (9) W. H. Gutenmann and D. J. Lisk, J . Agr. Food Chem., 8, 306 (1960). (10) C. A. Bache, C. E. Mckone, and D. J. Lisk, J . Ass. Ofic.Anal. Chem., in press. (11) Bernt Sjostrand, ANAL.CHEM.,36, 814 (1964). (12) M. Szkolnik, K. D. Hickey, E. J. Broderick, and D. J. Lisk, Plant Dis. Rep., 49, 568 (1965). 950
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flameless atomic absorption (13-15), mercury vapor meter absorption (16-23), isotope dilution (24) and microelectrolysis (25). The analysis of certain intact organic mercury fungi(13) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40, 2085 (1968). (14) J. F. Uthe, F. A. J. Armstrong, and M. P. Stainton, J. Fish. Res. Bd. Canada, 27, 805 (1970). (15) Marvin J. Fishman, ANAL.CHEM., 42, 1462 (1970). (16) M. B. Jacobs, S. Yamaguchi, L. J. Goldwater, and H. Gilbert, Amer. Ind. Hyg. Ass. J . , 21, 475 (1960). (17) Myron M. Schachter,J . Ass. Ofic.Anal. Chem., 49,778 (1966). (18) W. W. Vaughn, U. S. Geol. Surcey Circ., 540,1967. (19) T. Y. Toribara and C. P. Shields, Amer. Ind. H y g . Assoc. J . , 29, 87 (1968). (20) A. 0. Rathje, ibid., 30, 126 (1969). (21) M. E. Hinkle and R. E. Learned, U. S. Geol. Sur., Prof. Pap. 650-D, pp D251-D254 (1969). (22) J. C. Gage and J. M. Warren, Ann. Occup. H y g . , 13, 115 (1970). (23) G. Lindstedt, Analyst, 95, 264 (1970). (24) J. RJiirka and C. G. Lamm, Talarrta, 16, 157 (1969). (25) D. Pavlovid and S. ASperger, ANAL.CHEM., 31,939 (1959).
