Ultratrace Analysis for Beryllium in Terrestrial, Meteoritic, and Apollo 11 and 12 Lunar Samples Using Electron Capture Gas Chromatography Kent J. Eisentraut, Deborah J. Griest, and Robert E. Sievers Aerospace Research Laboratories, ARLILJ, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio 45433
A rapid and sensitive method has been developed and applied to the ultratrace analysis for beryllium in terrestrial, meteoritic, and lunar samples using electron capture gas chromatography. Beryllium was determined in lunar rock and dust samples returned from the Apollo 11 and 12 sites. The technique was also used to determine the amount of beryllium present in some U.S. Geological Survey standard rock samples, a sample of the titanium-rich Buddington’s Brown Point gabbro, and samples of the Allende, Murchison, and St. Severin meteorites. In addition, a method is reported for the analysis for beryllium rocket exhaust contamination in soil. This research has revealed an interesting phenomenon regarding the concentration of beryllium in the lunar samples. The beryllium content of the lunar dust is much higher than that of any of the lunar crystalline rocks analyzed. The concentration of beryllium in the crystalline rocks is less than 1 ppm while lunar dust contains as much as five times more beryllium. The detection limit is approximately 4 x gram beryllium; the sensitivity of the electron capture detector to beryllium trifluoroacetylacetonate is much greater at higher carrier gas flow rates.
The method consists of converting the beryllium in the sample t o the volatile trifluoroacetylacetonate chelate
and extracting it into benzene with subsequent separation and analysis by electron capture ga$ chromatography. Only very small samples (10-100 mg) are required for analysis at the ppm and sub-ppm concentration level. Extremely small quantities of beryllium can be detected by this technique; in this study, analysis was conducted at the picogram level with a lower detection limit of 4 X 10-14 gram Be. This technique is more sensitive for beryllium than any other analytical method known. EXPERIMENTAL
GASCHROMATOGRAPHY has become a n important technique in metal analysis (1, 2). The electron capture detector is especially sensitive t o volatile fluorinated metal chelates. Besides its application to beryllium, it has been used, for example, in the analysis of chromium (3-6), rhodium (7), and aluminum (5, 7). I n the case of beryllium, electron capture gas chromatography has been used t o analyze for ultratrace quantities in aqueous solutions (8, 9), in biological fluids (10, I]), and in polluted air (11, 12). This paper extends the technique of electron capture gas chromatography t o the ultratrace analysis for beryllium in terrestrial, meteoritic, and lunar dust and rock samples. (1) R. E. Severs and R . W. Moshier, “Gas Chromatography of
Metal Chelates,” Pergamon Press, Oxford, 1965, and references therein. (2) W. D. Ross and R. E. Sievers, “Developments in Applied Spectroscopy,” Vol. 8, E. L . Grove, Ed., Plenum Press, New York, N.Y., 1970, pp 181-192. (3) J. Savory, P. Mushak, F. W. Sunderman, Jr., R. Estes, and N. Roszel, ANAL.CHEM.,42, 294 (1970). (4) W. D. Ross and R. E. Severs, ihid., 41, 1109 (1969). (5) W. D. Ross, ibid., 35, 1596 (1963). (6) C . Genty, C. Houin, P. Malherbe, and R. Schott, ibid., 43, 235 (1971). (7) W. D. Ross, R . E. Severs, and G. Wheeler, Jr., ibid., 37, 598 (1 965). (8) W. D. Ross and R. E. Severs, “Gas Chromatography 1966,” A. Littlewood, Ed., The Institute of Petroleum, London, 1967, p 272. (9) W. D. Ross and R. E. Sievers, Tdaiitn. 15,87 (1968). (10) M . L. Taylor, E. L. Arnold, and R . E. Severs. At7crl. L e / / . , 1, 735 (1968). (11) M. H. Nowcir and J. Cholak, Emirou. Sci. T e c h o l . , 3, 927 ( 1969). ( 12) W. D. Ross and R. E. Severs, ibid., in press.
Reagents. Chemicals used were reagent grade unless specified otherwise. The hydrochloric acid used was Ultrex (J. T. Baker Co.). The benzene used was Nanograde quality (Mallinckrodt Chemical Works). Trifluoroacetylacetone, H(tfa), was supplied either by Pierce Chemical Co. o r PCR, Inc., and was freshly distilled prior t o use (bp 106-7 “C a t atmospheric pressure). All borosilicate glassware that contained beryllium trifluoroacetylacetonate, Be(tfa)p, solutions of less than gram Be/ml was silanized prior t o use with a 2 5 % by volume solution of hexamethyldisilazane in benzene. Aqueous standard beryllium solutions were prepared by weighing and dissolving 99.5 beryllium metal powder (Alfa Inorganics, Inc.) in dilute nitric acid and diluting with distilled demineralized water t o the desired concentration. Beryllium trifluoroacetylacetonate standard solutions were prepared by dissolving freshly sublimed Be(tfa)?, weighed on a Cahn electrobalance, in Nanograde benzene. The organic standards were freshly prepared each week by diluting a stock solution (10P gram Be/ml) of Be(tfa).? in benzene. Distilled demineralized water was used exclusively throughout this study. Analytical Procedure. TERRESTRIAL, METEORITIC,A N D LUNARSAMPLES. Samples in the form of rock chips are powdered in a “diamond” mortar. The sample of rock powder or lunar fines weighing between 10 and 100 mg is fused in a platinum crucible using approximately five times its weight of sodium carbonate. After dissolving the fusion mixture in 1 to 5 ml of 6 N hydrochloric acid, the solution is quantitatively transferred to a 30-1111 polyethylene bottle fitted with a screw cap. Ten-milligram samples were analyzed in all cases except for the meteorites where 40- t o 100-mg samples were used. The following procedure is suitable for a 10-mg sample. The pH of the solution is adjusted t o approximately 4 with 6 N NaOH. A sodium acetate-acetic acid buffer is added t o bring the solution t o a p H of 5 , followed by adding I ml of
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0.1N Na2EDTA. (Addition of Na2EDTA is necessary t o mask potentially interfering metal ions.) The polyethylene bottle is shaken for 5 minutes on a shaker and then heated for 5 minutes a t 95 O C . After the solution cools, 10 ml of a 0.082N solution of trifluoroacetylacetone in benzene is added and the polyethylene bottle is shaken on the shaker for 15 minutes. This was an acceptable shaking time since shaking for longer periods resulted in the extraction of unacceptable amounts of Al(tfa), which would interfere in the detection of the Be(tfa)2. Also, essentially all the beryllium is converted to the chelate and extracted in that time period. The mixture is placed in a separatory funnel, the aqueous layer drained, and the benzene layer washed for 5 seconds with 15 ml of 0.1N NaOH t o remove any excess ligand (which if not removed would interfere with the analysis). The washing time is very critical; longer contact time results in loss of beryllium while insufficient time results in residual H(tfa) in the benzene layer. The aqueous sodium hydroxide layer is immediately discarded and the benzene layer is ready for analysis by gas chromatography. Generally 1- t o 4-pl samples of the benzene layer are injected using a microliter syringe. ROCKETEXHAUST PRODUCT.High fired BeO, the rocket exhaust product studied, was not completely dissolved using the Na,C03 fusion procedure above. High fired B e 0 is very difficult to dissolve using most standard chemical techniques. However. we have discovered that the high fired B e 0 can be quantitatively dissolved by simply boiling with a 75 NaOH solution containing the suspended material for 2 minutes. Concentrated nitric acid is added t o neutralize the solution, and then the p H is adjusted t o about 4 with dilute nitric acid. The remainder of the analytical procedure for the B e 0 rocket exhaust material is the same as that given above for the terrestrial, meteoritic, and lunar samples. The efficacy of both experimental procedures was established through separate analysis of aqueous beryllium nitrate solutions of various known concentrations a t the ppb level, each microliter injection containing from 0.5 t o 4.0 X lo-'* gram Be. These standard aqueous solutions were analyzed at the same concentration levels as the unknowns. The average of 33 separate determinations showed 99.8% recovery with a relative standard deviation of 1 2 . 7 z . The average of the relative standard deviations for all of the geological samples reported in this paper is 8.4z. Standard solutions of Be(tfa)l were injected continually during the analysis of unknowns. Fresh standards were prepared each week and a new calibration curve was generated each day. The number of fusions of each sample ranged from 1 t o 4. Where enough sample was available, a larger amount was fused than 10 mg, and individual aliquots equivalent to 10 mg were taken from the fusion solution and analyzed independently. Each fusion was then divided into 5 aliquots and each aliquot treated as described above. The organic layer from each aliquot was injected at least five times. This represents an aggregate of from 25 t o 100 data points for each sample. Measurement of peak heights was used for the analyses; the Be(tfa)2 peak was extremely sharp with little tailing. Blank determinations were made daily to check all reagents and vessels, and there was no significant contribution t o the beryllium peak during the period in which the analyses were performed. However, periodic checks o n the reagents, especially the ligand, are necessary since a small peak, which forms in relation t o the amount of time elapsed since the ligand was distilled, elutes at the same retention time as Be(tfa)l. In this study it was necessary t o redistill the trifluoroacetylacetone approximately every month during the course of the analyses t o eliminate this interfering peak. Calibration curves were obtained and results calculated by the use of a Hewlett-Packard Corp. 9100A Calculator/9125A Plotter system. 2004
0
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100
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260
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340
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FLOW RATE (ml/minl
Figure 1. Sensitivity of electron capture detector to Be(tfa), as a function of carrier gas flow rate Electron Capture Gas Chromatographic Conditions. A Hewlett-Packard Corp. Model 810 Research G a s Chromatograph equipped with a tritium electron capture detector was used for the analyses. The chromatograph was modified by incorporation of a Hewlett-Packard Model 5750 injection port assembly. The column was fabricated and fitted in such a way as to provide for on-column injection. The column used was constructed from thick-walled Teflon (Du Pont) tubing (56 cm long, 3 mm inside diameter) packed with 10% SE-30 on 70/80 mesh Gas-Chrom Z (Applied Science Laboratories). The tubing had a wall thickness of approximately 2 mm. The carrier gas was 10% methane/ 90% argon at a measured flow rate of 60 ml/min which was dried by passing through a bed of molecular sieves and Drierite prior t o entering the chromatograph. N o purge gas was used and the purge entry opening was sealed off. The column was operated isothermally a t 120 "C with the electron capture detector a t 176 "C, and the injection port a t 161 O C . The pulse interval was 150 psec and the electrometer was operated generally a t a n attenuation of 1 X 32. (This is equivalent t o a current for full scale display of 1.28 x 10-lo A.) The peak for Be(tfa)2 appeared 1.7 min after injection; the chart speed was 4 min/in. It was discovered later in this work that the electron capture detector sensitivity t o Be(tfa)t is a critical function of argon/ methane carrier gas flow rate through the column. Although flow rates less than 100 ml/min can be used, much greater sensitivities can be achieved by operation a t a higher column flow. Optimum flow rates for this analysis generally lie between 200 and 300 ml 10% methane/90%argon per minute. Sensitivities t o Be(tfa)t can be enhanced by nearly 50 times over that a t 20 ml/min by operation at the higher flow rates. This information was taken into account in the later stages of this work a t which time the flow rates were 200 to 300 ml/min. The tremendous sensitivity t o carrier gas flow rate is shown in Figure 1. Figure 1 shows this dependence o n flow rate as a function of peak height. We have also found a similar effect for peak area. In addition, in the early stages of this work after letting air into the cell from either solvent cleaning or column change, it was necessary to allow the electron capture detector t o condition with 60 ml/min carrier flow rate for as long as from 3 t o 5 weeks before the sensitivities t o Be(tfa)s became optimum. Now with operation a t the higher column flow rates, optimum sensitivities are achieved in from 2 t o 12 hours. This discovery has allowed many more analyses t o be performed per unit time. The limit of detectability was experimentally determined. The injection of a sample containing 4 X 10-14 gram Be produced a peak with a signal t o noise ratio of 3 :1. RESULTS AND DISCUSSION
The gas chromatographic method was applied to the analysis for beryllium in some U S . Geological Survey stan-
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APOLLO II BRECCIA SAMPLE ’ 10069
APOLLO I I CRYSTALLINE ROCK
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Table I. Beryllium Concentrations in Terrestrial and Meteoritic Rocks, ppm Spectrochemical analysis Sample This work (13) Fluorimetry 1.53 It 0.18
