Anal. Chem. MID ( 4 8 5 - 4 9 5 and 6 3 0 - 6 4 0
1082, 5 4 , 957-960
amu)
1 Sample Blank
1
957
analysis is encouraging and should provide stimulus to explore other instances where thermally labile compounds cannot be analyzed by existing methods. The use of ammonia as reagent gas is particularly useful where oxygen atoms are available (mainly hydroxyls) as possible sites of protonation. However, the utility of ammonia gas in cases involving pure chlorohydrocarbons such as Mirex resulted in the expected nonappearance of molecular derived species as well as the possibility of ipso substitution product ions which might confuse the characterization process. The design of the LCMS interface unit has contributed in a 2-fold manner to overcome thermal lability. Flash vaporization does provide a convenient mechanism to minimize thermal degradation while the physical arrangement of the belt entrance to the ion source is optimized to encourage production of protonated molecular ions under CI conditions.
LITERATURE CITED 24
5 2
8 0
McFadden, W. H. J. Chromatogr. Sci. 1980, 78,97-115. Games, D. E.; Hlrter, P.; Kuhnz, W.; Lewis, E.; Weerasinghe, N. C. A,; Westwood, S. A. J. Chromatogr. 1981, 203, 131-138. Arplno, P. J.; Krlen, P.; Vajta, S.; Derant, G. J. Chromatogr. 198.1, 203, 117-130. Games, D. E.;Lewis, E. Biomed. Mass Spectrom. 1980, 7 , 433-436. Thruston, A. D.; McGuire, J. M. Biomed. Mass Spectrom. 1981, 8 , 47-50. Cotter, R. J.; Fenselau, C. Biorned. Mass Spectrom. 1979, 6, 287-293. Grlffen, G. W.; Price, A. K. J. Org. Chem. 1964, 29, 3192-3196. Cairns, T. Q. Bull.---Assoc. Food Drug Off. 1978, 42,3-24. Harless, R. L.; Harris, D. E.; Sovocol, G. W.; Zehr, R. D.; Wilson, N. K.; Oswald, E. 0. Biomed. Mass Spectrom. 1978. 5 , 232-237. Laseter, L.; DeLeon, I. R.; Remale, P. C. Anal. Chem. 1978, 5 0 , 1169-1 172. Borsettei, A. P.; Roach, J. A. G. Bull. Environ. Contamin. Toxicor’. 1978, 20,241-247. Uk, S.; Hlmel, C. M.; Dlrks, T. F. Bull. Environ. Confamin. Toxicol. 1972, 7 , 207-215. Carlson, D. A,; Konyka, K. D.; Wheeler, W. B.; Marshall, G. P.; Zayiskle, R. G. Scrence 1978, 939-941. Luke, M. A.; Froberg, J. E.; Doose, G. M.; Masumoto, H. T. J. Assoc. Off. Anal. Chem. 1981, 6 4 , 1187-1190. Luijten, W. C. M. M.; Onkenhout, W.; van Thuyl, J. Org. Mass Spectrom. 1980, 75, 329-330. Cairns, T.; Siegmund, E. G.; Froberg, J. E. Biomed. Mass Spectrom, 1981, 8 , 569-574. Cairns, T.; Siegmund, E. G. Org. Mass Spectrom. 1981, 76, 555. Tabet, J. C . ; Fralsse, D. Org. Mass Spectrom. 1981, 76,45-47. Demark, B. R.; Klein, P. D. J. L/pid Res. 1981, 22, 166-171.
10.4
Tlme (mlns)
Figure 6. Multiple ion detection chromatograms (LCMS-CI-CH,) of (A)
Kepone hydrate and Kelevan standard mixture (100 ng injected on column), (B) banana sample blank, (C) banana sample spiked with Kepone hydrate and Kelevan at the 1 ppm level; recording conditions: 485-495 and 630-640 amu scanned, normalized to maximum intensity recorded for (A). covery of Kepono hydrate and Kelevan was about 80%.
CONCLUSIONS In summary, the application of LCMS to the characterization of perchloro cage compounds has revealed certain advantages and disadvantages. I t is obvious that the LCMS interface system has certain inherent design features which impart enhanced production of protonated molecular ions over the conventional introduction via GCMS. The application of LCMS-CI-CH, in residue analysis to distinguish Kelevan from Kepone hydrate has demonstrated the powerful selectivity of mass spectrometry in solving a difficult analytical problem. Analytical findings can now be advanced to include the assignment of Kelevan to incurred residues previously reported as Kepone. The capability of LCMS to detect these high molecular weight compounds at the 1ppm level in residue
RECEIVED for review June 18, 1981. Accepted January 27, 1982.
Thermal Ionization Mass Spectrometry of Uranium with Electrodeposition as a Loading Technique Donald J. Rokop,” Rlchard E. Perrin, Gordon W. Knobeloch, Voncille M. Armljo, and Wllliam R. Shields Los Alamos National Laboratory, MS-514, Los Alamos, New Mexico 87545
Uranlum samples are electroplated on degassed zone-refined rhenium filaments and overplated wlth rhenlum to permlt accurate Isotopic analyses at nanogram levels. The method provides an lonizatlon efflclency of 1.5 X for 10 ng at a temperature of 1620 f 10 ‘C. Isobarlc interferences are less than or equal to the dark current of the measurlng system, 0.05 countsh. Accuracles achieved on 10 ng of NBS U-500, a 111 2351238 uranium Isotopic standard, are 0.05% t a at the the 95 % confidence level. The effects of counting system dead tlme and chemlcal bank are dlscussed.
Programatic demands at Los Alamos National Laboratory
have necessitated the development of methods to do accurate isotopic analysis on a number of elements from samples with varying matrices and high specific radioactivity. The methods are designed to utilize the minimum sample size required for analysis in order to reduce the amount of radioactivity handled. This limitation frequently requires working on samplef, where the element of interest is at the nanogram level. For this reason “cleannchemical laboratories and procedures were developed to permit preparation of samples for isotopic analyses free of environmental contamination. The laboratories housing the mass spectrometers are also “clean”, -class 10 000 air, and include laminar flow clean hoods and benches, -class loo+ air.
0003-2700/82/0354-0957$01.25/0 0 1982 American Chemical Society
958
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6 , MAY 1982
The specific method herein described permits highly accurate isotopic analyses on 10 ng of uranium. Both the instrumentalist and the chemist are required to reduce and/or eliminate impurities t o a level not previously attempted in this type of work. Intense ion beams, even from elements whose mass is far removed from the range of interest, can cause scattered ions which significantly contribute t o the background. Small amounts of hydrocarbons can cause additional significant background contributions. Electrodeposition was chosen as a means of loading uranium onto the filament because of its inherent cleanliness and specificity. Some cations, such as Na, K, and Ca, which are difficult for the chemist t o remove at such levels and are especially detrimental t o mass spectrometric analyses are eliminated. Electrodeposition also permits easy elimination of hydrocarbon impurities. The instrumentalist faces the same problem in that he must produce a high enough ionization efficiency at a low enough temperature so that source parts are not heated to the point where the same type of background impurities occur. Rec and co-workers have reported a technique ( I ) for improving single filament ionization of U and P u by sputtering a coating of Re over the sample mounted on a rhenium filament. For our purposes, sputtering was neither clean enough from impurities nor reproducible enough in thickness t o permit achievement of the accuracies desired. The simple process of adding Re t o the electrolyte subsequent to the deposition of the uranium yielded a reproducibly uniform and clean coating of Re. We thus utilized the benefits of both methods.
EXPERIMENTAL SECTION Instrumentation. The mass spectrometer used was a single stage magnetic sector type with a thermal ionization source and both Faraday cage and pulse counting ion detection capabilities. The instrument was originally developed at the National Bureau of Standards under the direction of Shields (2). The particular instrument used for this work was built at Los Alamos National Laboratory by Shields with some design changes. A brief description follows: The ion source is a modified Nier thin lens with ''2" axis focusing (2). The analyzer has a 30.5 cm radius of curvature and 90' deflection. The analyzer optical system uses an extended flight path, 1 . 1 O off axis and ion beam with a mismatched flight tube and magnet, 8 2 O , to compensate for fringe field effects. The vacuum envelope is all stainless steel with semisheer gold gaskets. In addition, there is a beam valve between the source and analyzer sections which is an integral part of the source mount and has an oversize orifice to avoid interference with the ion beam. The pumping system includes a Hg diffusion pump with a liquid nitrogen cold trap on the source and a 140 L/s double ended ion pump mounted on the analyzer. The ion pump is located so as to maximize pumping speed while minimizing the effects of stray particles. Also included is a glass cold finger mounted above and behind the source to facilitate removing condensables, primarily water, and acetone. The latter is not accomplished unless torr before liquid the system is pumped down below 7 X nitrogen is added. Ion detection can be done by two separate systems. The first is a deep Faraday cage mounted behind a "Z"axis baffle, electron suppression slit, and a defining slit. The signal is amplified by a Cary 401 MR vibrating reed electrometer and then converted to digital form by a 0.01% linear voltage to frequency converter (Analog Devices 460L). The output from the V-F converter is then sent to a 5-MHz scaler. This collector has an ion beam collection efficiency of 99% or better. This is not a cubic suppression system. The second system utilizes: a 17-stageelectron multiplier with a gain of lo*, a Shideler discriminator (3) capable of supplyinga 1-V, 7 ns wide output pulse when triggered by pulses from 6 mV to 2 V; a voltage divider to eliminate line reflections; and a 250-MHz scaler. The ion counting system achieves efficiencies of 100% with a deadtime of 31 ns and a dark current of 2 counts in 25 s. The system utilizes an automated data collecting system
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PLATING MODE
RINSING MODE
Figure 1. Single filament electro position apparatus. comprised of a Hewlett-Packard 9830B desk calculator with extended memory and line printer. The calculator controls various switching functions of the mass spectrometer in addition to processing data. Chemistry. Rock samples from the Nevada Test Site are dissolved with hydroflouric and perchloric acids. The solution is converted to a clear chloride with particular care on removing all fluoride. A n3U tracer is added and isotopic exchange achieved with fuming perchloric acid. The uranium is extracted into ethyl acetate and back-extracted into water. The sample is converted to a sulfate and loaded onto an anion column. After several washing steps, the uranium is eluted from the column with nitric acid, converted to a chloride, and loaded on a macroporous anion resin column. The uranium is eluted with water and treated by one of two methods. The first method is simply to fume with HN03-HC104 at
