Isotope Analysis Using Dimethylmercury V E R N O N H. DIBELER National Bureau o f Standards, Washington,
D.
C.
A simple technique is described for the complete conversion of milligram quantities of elemental mercury to dimethylmercury and for mass spectrometric analysis of isotope abundance ratios. Mercuric chloride, prepared by direct combination of the elements, is reacted with a slight excess of dimethylzinc to form the mercury alkyl. Experiments using mercury-I98 establish the absence of measurable fractionation in the synthesis, or of exchange, with mercury vapor during analysis. Measurements of isotope abundance ratios using dimethylmercury are compared with results obtained by other laboratories measuring mercury vapor.
V
OLATILE organometallic compounds for isotope abundance measurements were first used by Aston ( 1 ) in his early studies of the isotopic constitution of the elements. Although measurements were usually made on the atom ions, the formation of hydride ions by rearrangement complicated the mass spectrum and the calculation of abundance ratios. Consequently, the use of the elements or appropriate inorganic compounds have been preferred especially for establishing upper limits to the existence of rare isotopes. In 1951, however, on the basis of accumulated experience in mass spectra and the development of semimicrotechniques for the preparation of tetramethyllead, this compound was suggested (2, 3 ) for use in routine isotopic analysis of milligram quantities of lead. Since that time, a number of laboratories have attained considerable experience with this method and a t least one study ( 4 )has shown that analytical results obtained by using the volatile lead alkyl are comparable with those obtained by evaporating lead chloride or lead iodide. A41thoughno measurable variation in the natural abundance of mercury isotopes has been reported, the element is now available in a wide range of isotopic compositions for various research purposes, including nearly pure mercury-198 and -202 for use in spectroscopic lamps as standards of length. Kecessary isotope abundance measurements associated with these researches are commonly made by using the vapor of the element. However, accurate measurements on samples differing appreciably from normal composition require rigorous exclusion or removal of mercury background, usually accomplished by prolonged bakeout and frequent cleaning of the ion source. Furthermore, laboratories also engaged in the precise measurement of lead-204 abundance using lead atom ions prefer to exclude mercury vapor from their mass spectrometers a t all times, Thus, the use of a volatile organometallic compound appears to be particularly attractive for routine abundance measurements of mercury isotopes. It is the purpose of this paper, therefore, to describe a simple method for the preparation of milligram quantities of dimethylmercury and a mass spectrometric technique using this compound for isotope assay. Further, the method is evaluated by experiments using a slightly contaminated sample of mercury-198 and by means of comparative measurements by several laboratories on identical samples of a stock of ordinary mercury prepared as a density standard at the Sational Physical Laboratory, Teddington. EXPERIMENTAL DET 41 LS
Preparation of Dimethylmercury. -4 schematic diagram of the apparatus used in the preparation of the mercury alkyl is shown in Figure 1. The manifold and connections are con-
structed of borosilicate glass tubing, 5 mm. in inside diameter. All stopcocks and joints are lubricated with a low vapor pressure fluorocarbon grease. Several 15-cm. lengths of 3-mm.-outside diameter borosilicate glass tubing, A , are flame-sealed a t one end (the addition of a 2-ml. bulb a t this end facilitates vacuum handling of the volatile reactants) and sealed to inner 14/20 standard tapers, B, a t the opposite end. One or 2 mg. of the mercury to be analyzed is placed in each dry, clean tube by means of a calibrated micropipet. The sample tubes are then connected to the manifold as shown in Figure 1. The manifold and tubes are evacuated to a few microns pressure by means of a mechanical pump through a trap refrigerated with solid carbon dioxide. Stopcock C is then closed, and chlorine gas is admitted t o the system a t a pressure slightly less than 1 atm. All D stopcocks are then closed and the mercury droplets warmed gently in the tubes with a microburner until a rapid, vigorous reaction occurs. The resultant mercuric chloride sublimes readily t o the cooler walls of each tube, ermitting complete reaction of the mercury. Then the excess cilorine is pumped away and dimethylzinc (commercially available in high purity) is metered in slight excess of the calculated stoichiometric quantity by means of the gas pipet, E, and condensed into the appropriate sample tube held a t liquid nitrogen temperature. The D stopcocks are manipulated pro erly throughout the procedure to prevent cross contamination o?mercury samples. Finally, the tubes are flamesealed, cut from the standard tapers, and stored a t room temperature until analyzed by the mass spectrometer.
T O DRY-ICE T R E AND VACUUM P U M P
Figure 1. Schematic diagram of apparatus used in preparation of dimethylmercury
Pressure-volume-temperature measurements and mass spectrometric analyses made immediately after the tubes warm to room temperature indicate a rapid, essentially quantitative conversion to the mercury alkyl. I n several reparations, small amounts of water vapor were condensed into t i e tubes after formation of the dimethylmercury in order to destroy any excess dimethylzinc. No significant differences in results were observed. There was also no evidence for fractionation effects in preparations using slightly less than stoichiometric quantities of dimethylzinc. Several samples of dimethylmercury synthesized from approximately 95 atom % mercury-198 were sealed in vacuo together with droplets of ordinary mercury. No measurable decrease in mercury-198 abundance Ras observed in the dimethylmercury after standing several hours a t room temperature. Known mixtures of mercury-198 and ordinary mercury were also converted to dimethylmercury. The mercury alkyl exhibited the expected isotope abundance ratios within the estimated experimental errors. 1958
V O L U M E 27, N O . 12, D E C E M B E R 1 9 5 5 Table I.
1959
Comparison of Isotope Analyses Using Dimethylmercury and Mercury Vapor
Observer X P L sample number Form Isotope
This Research 9 Hg(CHdz
Fleming ( 5 )
\’on Ubisch and Salg ( 8 ) 8 13, 14, 15 H g vapor H g vapor Abundance, Atom 72
Palmer ( 7 )
G
Hg vapor
196 0.156 i 0.01 0 . 1 5 1 f 0.002‘ 0 . 1 5 4 zt 0 O O 6 b 0.18 10.02 10.00 i0 . 0 2 1 0 . 0 0 i 0.11 198 1 0 . 1 2 i 0.10 9.98C 199 16.99 i 0 . 0 9 16.93 1 0 . 0 2 16.95 16.91 1 0 . 0 8 23.19 23.11 i0.06 200 23.07 1 0 . 1 2 23.15 1 0 . 0 2 13.21 13.18 i0.05 201 13.27 i 0.07 13.16 i0.04 202 2 9 . 6 4 i 0.1; 29.83 0.02 29.74 29.83 f 0.16 204 6.79 1 0 . 0 5 6.77 1 0 . 0 3 6.78 6.78 10.03 Mean of three runs a n d standard deviation. Single determination. F Weighted mean of three samples. Individual analyses differed by less than 0.2%;,relatively.
+
Mass Spectrometric Measurements. The published ( 3 ) mass spectrum of dimethylmercury shows the Hg+, Hg( CH3) +,and Hg(CH3)2+ ions to be of comparable abundance. Of these, however, only the molecule ion is free of interference from mercury background and from the rearrangement hydride ions that complicate the atom ion and monomethyl ion spectra. Furthermore, it is convenient and advantageous to measure the abundances of the isotopic molecule ions by using ionizing electron energies sufficient to produce the molecule ions without forming ions by the dissociation of hydrogen atoms from the molecule. The nearly monoisotopic dimethylmercury-198, synthesized in the previous section, provides a sensitive means of determining the optimum electron energy for the measurement of Hg( CH3)2 ions in the absence of Hg(CH3) ( CH2)+ and other ions resulting from further loss of hydrogen atoms. +
Abundance measurements are made using a 180” mass spectrometer designed for routine analysis of gases and volatile liquids. The instrument has a 5-inch radius of curvature and an estimated resolving power of 1/350. Sample vapor enters the ion source from a 4-liter reservoir through a molecular leak, and data are obtained by electrostatic scanning. The molecule ions of the dimethylmercury samples were measured several times using, alternately, 70- and 10-volt (uncorrected) electrons. The relative abundances of the mercury isotopes are calculated directly from the peak heighte observed a t 10 volts after correction for ions containing carbon-13 and deuterium atoms in normal abundance. However, a more precise measure of the mercury196 abundance is obtained in the following manner: The Hg196(CH3)2 +/Hg198(CH3)2+ ratio is measured using 70-volt electrons and a correction made for dissociated ions as determined from the pure Hg198(CH3)2spectrum a t 70 volts. This ratio is then used to calculate the 10-volt Hg196(CH3)2+peak height from the observed IO-volt HgIg8(CH3)2+ peak. The calculated Hg196(CH&r peak height a t 10 volts usually agreed with the observed value well within the record reading error.
Blthough the abundance ratios are measured under different experimental conditions, the results obtained using dimethylmercury and routine techniques generally agree well within the stated limits with careful measurements obtained for mercury from the same source using the vapor. The mean of the collected values shown in Table I is not in complete agreement with Nier’s 1950 measurements (6) on laboratory-supply mercury. However, the differences may result from alteration in the abundance ratio of the carefully purified sample used in this research. The agreement shown in Table I is particularly gratifying in view of the simple synthesis for milligram quantities of dimethylmercury and the apparent suitability of the physical and chemical properties of this compound to mass spectrometric analysis. For example, the use of the mercury alkyl in a properly designed inlet system, such as that used in commercially available gas analysis mass spectrometers, avoids the uncertain degree of isotopic fractionation occurring in the evaporation of a liquid or a solid. Also, when used in sample introduction systems free of lubricated stopcocks, only elementary precautions are required in evacuating a previous sample of dimethylmercury in order to prevent memory effects in subsequent samples. Furthermore, dimethylmercury analyses are unaffected by a background of mercury vapor or absorbed mercury in the inlet system or analyzer tube. This is particularly useful in analyzing isotopic compositions differing appreciably from ordinary mercury. Finally, there is no interference between ions commonly used in the analysis of tetramethyllead-Le., Pb(CH3)3+ions (m/e = 248 t o 254) and the molecule ions of dimethylmercury (m/e = 226 to 234) thus permitting routine isotope analysis of both these elements using the same mass spectrometer. ACKNOWLEDGMENT
R E S U L T S AND DISCUSSION
Mean values obtained for the isotopic analysis of five samples of dimethylmercury synthesized from Xational Physical Laboratory mercury sample No. 9 are given in column 2 of Table I. The indicated uncertainties, which are t v o to five times the mean deviations for each analysis, are estimated only from possible errors in the recording system. Columns 3, 4, and 5 give results for samples from the same source analyzed, respectively, by W. H. Fleming, Hamilton College, Hamilton, Ontario; by Hans von Ubisch and S. E. Salg, Aktiebolaget Atomenergi, Stockholm; and by G. H. Palmer, United Kingdom Atomic Energy Authority, Harwell. Fleming ( 5 ) used a 180” mass spectrometer of conventional design with magnetic scanning to measure mercury atom ions in mercury vapor. \‘on Ubisch and Salg (8) and Palmer (7‘) used conventional 90” instruments with magnetic scanning to measure mercury atom ions in mercury vapor. Fleming and von Ubisch and Salg introduced mercury vapor directly into the ion source, whereas Palmer introduced vapor through a molecular leak.
The author is indebted to W.H. Fleming, Hans von Ubisch, S. E. Salg, and G. H. Palmer for communicating the results of their analyses and to Fred L. Mohler for much helpful discussion. LITERATURE CITED
(1) Aston, F. W., “Mass Spectra and Isotopes,” 2nd ed., Longmans, Green, New York, 1942. (2) Collins, C. B., Freeman, J. R., and Wilson, J. T., Phys. Rev., 82,
966 (1951). (3) Dibeler, V. H., and Mohler, F. L., J . Research iVat2. Bur. Standards, 47, 337 (1951). (4) Farquhar, R. Palmer, G. H., and Aitken, K. L., N a t w e , 172,
860 (1953). ( 5 ) Fleming, W. H., private communication. (6) Nier, A. O., Phys. Rev., 79, 450 (1950). (7) Palmer, W. H., private communication. (8) von Ubisch, Hans, and Salg, S.E., private communication.
RECEIVED for review
J u l y 26. 1955.
Accepted September 8, 1955.
