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Anal. Chem. 1983, 55, 578-580
ion since the intensitv variations of these mecies are small and the instrumental I;arameters can be o p t i k e d . However, since the position computing electronics of the RAE saturate above lo5 cps, any attempt to focus the instrument on a matrix level species is impossible. It is of course possible to adjust the ion optics on trace component signals, but this procedure is fraught with difficulties and potential artifacts. This issue of dynamic range limitation is not unique to the RAE but is common to any instrumental system when detection is required over 6 to 9 decades in intensity. We are presently investigating methods for extending the count rate capacity of the RAE to the lo6 cps range and are also pursuing methods to incorporate both an RAE and a microchannel plate/fluorescent screen image detector on the IMS-3f. This dual detector configuration would permit matrix ion tune-up of the instrument and provide pulse counting, position computing imaging of those signal intensities which are below the count rate capacity of the RAE.
LITERATURE CITED Liebi, H. "Proceedings of Workshop on SIMS and Ion Microprobe Analysis"; Heinrich, K. F. J.. Newbury, D. E., Eds.; National Bureau of Standards: Washington, DC, 1974; NBS Spec. Publ. No. 427. Morrison, G. H.; Siodzian, G. Anal. Chem. 1975, 4 7 , 932A. Wilson, R. G.;Vasudev, P. K.; Jamba, D. M.; Evans, C. A., Jr.; Deiine, V. R. Appl. Phys. Lett. 1980, 36, 215. Huggins, R. A,, Ed. "Annual Review of Materials Science"; Annual Reviews, Inc.: Paio Alto, CA, 1980; Vol. 10. Burns-Bellhorn, M. S. I n "Microbeam Analysis in Biology"; Lechence, C. P., Warner, R. R., Eds.; Academic Press: New York, 1979; pp 129-15 1. Schilling, J. H.; Buger, P. A. Int. J. Mass Spectrom. Ion Phys. 1978. 2 7 , 283. Rudenauer, F. G.;Steiger, W. Mlcrochim. Acta 1981, 11, 375. Furman, F. K.; Morrison, G. H. Anal. Chem. 1980, 5 2 , 2305. Patkin, A. J.; Morrison, G. H. Anal. Chem. 1882, 5 4 , 2.
(10) LamDton. M.: Paresce. F. Rev. Sei. Instrum. 1974. 45. 1098. and refeiences therein. (11) Poate, J. M.; Tu, K. N.; Mayer, J. W. "Thin Films-Interdiffusion and Reactions"; Wiiey-Interscience: New York, 1978. (12) Williams, P.; Evans, C. A., Jr. "Proceedings of Workshop on SIMS and Ion Microprobe Analysis"; Heinrich, K. F. J., Newbury, D. E., Eds.; National Bureau of Standards: Washington, DC, 1974; NBS Spec. Pubi. No. 427.
'
Present address: IBM Corporation, P.O. Box 390 South Road, Bidg. 052, Dept. A46, Poughkeepsie, NY 12602.
Robert W. Odom* Bruce K. Furman' Charles A. Evans, Jr. Charles Evans & Associates 1670 South Amphlett Boulevard, Suite 120 San Mateo, California 94402
Charles E. Bryson William A. Petersen Michael A. Kelly Surface Science Laboratories, Inc. 1206 Charleston Road Mountain View, California 94043
Donald H. Wayne Cameca Instruments, Inc. 2001 West Main Street, Room 105 Stamford, Connecticut 06902
RECEIVED for review July 8, 1982. Accepted November 12, 1982. We wish to gratefully acknowledge support for this research from a National Science Foundation/Small Business Innovative Research (NSF/SBIR) Grant No. DMR-8113779.
Evaluation of Lutetium for Volumetric Calibration by Isotope Dilution Mass Spectrometry Sir: One of the problems faced by safeguards is that of determining the quantities of fissile material in holding tanks. It is difficult to measure the volume of the contents of the irregularly shaped, partially filled vessels that hold radioactive waste generated in the nuclear fuel cycle. Through the choice of a suitable element, the double spike technique of isotope dilution analysis is a viable method for addressing this problem. A suitable element must have at least two naturally occurring isotopes and an isotopically enriched spike must be available. Addition of this element to the solution to be sampled must not interfere with subsequent chemical processing and the element must be amenable to mass spectrometric analysis. Two elements being considered for this purpose are lithium and magnesium. Neither of these elements is ideal for the purpose. Both are very common elements, making it necessary to take stringent precautions against contamination. Because both are light in mass, isotopic fractionation during mass spectrometric analysis can be a serious problem. This can largely be surmounted by use of standards and meticulous attention to procedural details, but the entire procedure is cumbersome and time-consuming. Considerations such as these led us to consider using lutetium for this purpose. It meets all the requirements listed above. Although produced in fission, the quantities are not large enough to be measurable. Rare earth elements in general are highly amenable to thermal emission mass spectrometry;
lutetium has the lowest ionization potential of the rare earths (5.3 eV) (1)and is thus particularly suited for this application. Its isotopic masses (175 and 176) are high enough that fractionation is controllable without the extensive analysis of standards required by lithium.
EXPERIMENTAL SECTION The mass spectrometer used in this experiment is a two-stage instrument with two 30 cm radius, 90° sector magnets ( 2 ) . It is equipped with a pulse-counting detection system, making possible analysis of subnanogram samples. The instrument is equipped with an ion source of new design that yields improved performance in comparison to the one previously used (3). The technique of using anion resin beads for introduction of uranium and plutonium samples into the mass spectrometer has been described in previous publications ( 4 , 5 ) . The mechanism of the bead-sample-filament interaction has been the subject of a study by secondary ion mass spectrometry (6). Lutetium samples as small as 0.2 ng were analyzed with no difficulty. A resin bead was added to 1pL of sample solution in the V-shaped rhenium filament (7); this allowed us to take advantage of the benefits of forming metal ions from resin beads without performing a chemical separation. The isotopic composition of the lutetium spikes were as follows: natural Lu, 97.393% 175Lu,2.607% '16Lu; enriched Lu, 28.577% lI6Lu, 71.423% l16Lu. The high abundance of 176Luin the naturally occurring element is advantageous for its use in this application. Substantial quantities of the original spike are required for direct spiking of the contents of the tank, making use of an
0003-2700/83/0355-0578$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
Table I. Expected Concentrations and Weights nominal volume, L 40 80 120 140 200
normal Lu concn, Pgk
1.4636 0.73196 0.48818 0.36605 0.29283
RESULTS AND DISCUSSION Table I[ contains the gravimetrically expected values for the concentration and weight for each volume sampled. All weights here and elsewhere have been corrected for buoyancy. Table I1 presents the measured values; each value listed in the average of‘two replicate analyses, and values for all four
579
Table 11. Measured Concentrations and Weights nominal volume, L 40
weight, kg 40.04 80.06 120.05 160.10 200.13
isotopically enriched element for the original spike highly undesirable because of limited availability and high cost. The spike concentration was calibrated through the isotope dilution technique by spiking with a gravimetrially prepared solution of natural lutetium. The spike concentration was 3.166 bg/g of solution. A spark source mass spectrometric analysis assigned a purity of 99.93% to the natural LulOa powder that we used as our primary reference. Ytterbium and hafnium are the only elements besides lutetium that have isotopes at mass 176;there is no other element with an isotope of mass 176. Both the natural lutetium and the en. riched spike were analyzed for these possible contaminants. Neither ytterbium nor hafnium was identifiable by its isotopic pattern (BaCl* interfered wiith the Yb spectrum). By assuming all counts at mass 174 were due to ytterbium, an upper limit of two parts in 10000 can be placed on its concentration in our natural lutetium, with hafnium being at significantly lower levels. The correspondinglevel of Yb in the enriched lutetium wae 7 parta in 100OOO. A reagent blank was analyzed and gave an upper limit of 2 x W4g of Lu/g of solution, Since there is some variation in the distribution of the isotopes of lutetium in nature, it will be necessary to characterize each batch of natural lutetium intended for this purpose. A large drum (208L capacity) was obtained and a plastic liner inserted to prevent attack by the acid of the sample solution. Enough distilled water was acidified with 400 mL of concentrated HN09 to give a final volume of about 40 L, and 58.542 mg of natural lutetium was added as the initial spike. The resulting solution was sparged by bubbling nitrogen through it for 30 min and allowing it to stand overnight. The solution was weighed (scale accuracy 115 g) and a 20-g aliquot removed. This process was repeated, except for addition of lutetium, in successive 40-L increments until a final volume of 200 L was reached. The total contents of the drum were adjusted for the amount removed for each 40-IL increment. Each 20-g aliquot served as a sample for isotope dilution analysis. Approximately0.5g of enriched spike solution containing about 1.6 fig of lutetium was added to enough sample solution (0.5-1.5g) to produce an isotopic ratio of about 1.0. This quantity of spike was chosen to keep weighing errors from becoming significant, and an isotope ratio of near unity results in mass spectrometric analyses of the highest precision. Each aliquot was weighed (&l mg) and spiked independently four times to improve the statistical reliability of the study; each such spiking from each aliquot was analyzed in duplicate, resulting in 40 analyses. In an early experiment, natural erbium was added to a solution of natural lutetium and evaluated as an internal standard. The known ratio of two erbium isotopes was used to calculate the bias correction required for that particular filament; this correction was then applied to the lutetium. Unfortunately, the results thus obtained were statistically no better than those obtained without an internal standard, probably due to different fractionation rates of the two elements. The results reported below were obtained by applying the appropriate average bias correction determined by analyzing natural lutetium (0.14% per mass) and not through use of an internal standard.
41
_ ~ ~ _
~
spiking 1 2 3
4 av std devu 80
1
2 3 4 av std deva 120
av std dev 160
av std dev 200
av std deva
1 2 3 4 1 2 3 4 1 2 3 4
concn, pgig 1.4635 1.4629 1.465 1 1.4649 1.4641 0.0011 0.732 09 0.731 44 0.73141 0.73271 0.731 91 0.00062 0.488 08 0.48821 0.488 18 0.48906 0.48838 0,00046 0.366 39 0.366 47 0.36602 0.36636 0.366 31 0.00020 0.29263 0.292 96 0.29269 0.292 99 0.29277 0.00024
calcd wt, kg 40.043 40.060 40.000 40.005 40.027 0.029 80.050 80.121 80.124 79.982 80.069 0.067 120.069 120.038 120.045 119.829 119.996 0.112 169.949 159.914 160.110 169.962 159.984 0.087 200.333 200.046 200.292 200.019 200.173 0.163
SD = [ ( X I - x ) ’ / ( n - l)]l’z where x is the average and the individual values of n determinations.
(I
xi
-
Table 111. Bias Evaluation of Volume Measurement nominal volume, L
concn, measured/ expected
wt, measured/ expected
40 80
1.00034 0.99993
0.99968
120
1.00041 1.00071
0.99954 0.99928 1.00021
160 200
0.999 80
1.000 11
spikings of each aliquot are included. Averages and standard deviations for each volume are also listed. Precisions of concentration measurements are about 0.1. % . Table I11 presents data comparing mass spectrometric with gravimetric results. For each volume sampled the average measured value was divided by the expected value. Agreement between the two techniques is about 0.1%. It would probably be worthwhile to investigate the use of erbium as the spiking material. Several isotopes (166, 167, 168, 170) are availstble a t purities greater than 90%. In addition, if a spike is available with insignificant amounts of two isotopes of the natural element, those two isotopes could be used as an internal standard. Previous work with strontium (8)and uranium ( 9 , I O ) has shown that much better precision is attainable through use of an internal standard than by using a bias correction which can only reflect average conditions.
CONCLUSIONS The results of Tables I1 and I11 show that precisions of about 0.1% can be obtained on isotope ratio measurements of lutetium; biases of similar magnitude are shown for volume measurements made under controlled laboratory conditions. We recommend thorough evaluation of lutetium on holding tanks for volumetric calibration.
580
Anal. Chem. 1983,
55,580-583
ACKNOWLEDGMENT
(8) Kesler, S. E.; Jones, L. M.; Walker, R . L. €con. Geol. 1975, 70, 5 15-526. (9) Kietz, L. A,; Puchuckl, C. F.; Land, G. A. Anal. Chem. 1982, 3 4 , 709-7 10. (10) Ridley, R. G.; Daly, N. R.; Dean, M. H. Nucl. Instrum. Methods 1985, 3 4 , 163-164.
We thank L. Landau for performing the spark source mass spectrometric analysis, H. Simmons for performing some of the early isotope dilution analyses, and H. C. Smith for preparing the samples. Registry No. l’ISLu,14391-25-4;l’16Lu,14452-47-2.
David H. Smith* R. L. Walker C. A. Pritchard J. A. Carter
LITERATURE CITED (1) Hertel, G. R. J . Chem. Phys. 1988, 4 8 , 2053-2058. (2) Smith, D. H.; Christie, W. H.: McKown, M. S.: Walker, R. L.; Hertel, G. R. Int. J . Mass Spectrom. Ion Phys. 1972, 10, 343-351. (3) Todd, P. J.; McKown, H. S.; Smith, D. H. I n t . J . Mass Spectrom. Ion Phys ., in press. (4) Walker, R. L.; Eby, R. E.; Pritchard, C. A,; Carter, J. A. Anal. Lett. 1974, 7, 563-574. ( 5 ) Smith, D. H.; Walker, R. L.; Carter, J. A. J . Inst. Nucl. Mterials Management 1980, 8(4), 66-71. (6) Smith, D. H.; Christie, W. H.; Eby, R. E. Int. J . Mass Spectrom. Ion Phys. 1980, 36, 301. (7) Christie, W. H.; Cameron, A. E. Rev. Scl. Instrum. 1988, 3 7 , 336-337.
Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37830
RECEIVED for review July 29, 1982. Accepted December 7, 1982. Research sponsored by the U S . Department of Energy, Office of Safeguards and Security, and Office of Basic Energy Sciences under Contract W-7405-eng-26 with the Union Carbide Corporation.
AIDS FOR ANALYTICAL CHEMISTS Nanoliter Injection System for Microcolumn Liquid Chromatography V. L. McGuffln and Milos Novotny” Department of Chemlstry, Indiana Unlverslty, Bloomlngton, Indiana 47405
In high-performance liquid chromatography (HPLC), the separation efficiency may be critically influenced by extracolumn contributions to band broadening, especially those which originate from the injection system. Precise, low-volume sample introduction is particularly important in high-speed liquid chromatography ( I ) and in microcolumn HPLC (24% where flow rates are typically on the order of microliters per minute and total peak volume may frequently be less than 1 pL. If the peak variance due t o the injection technique becomes significant in comparison with the variance due t o the column processes, themselves, then the column performance will be adversely affected. The maximum permissible injection volume (V,,,), which will produce a fractional (e2) increase in the volumetric variance (u:) of a nonretained peak, is given by the following equation (6): Vmm2 =
( T ~ K ~ ~ E ~ L ) ~ = Pa: N
for a chromatographic column of radius I“, length L , total porosity tT, and plate number N . The constant is characteristic of the injection profile and is equal to 12 for an ideal plug injection. Maximum injection volumes were calculated assuming an ideal injection profile for conventional, microbore, packed capillary, and open tubular columns. If the injection profile is not ideal, then permissible injection volumes h a y be considerably less than those indicated in Table I. The required injection volumes for conventional and microbore packed columns clearly appear to be within the capabilities of state-of-the-art technology. However, the stringent requirements of capillary columns preclude the use of conventional syringe or valve injection techniques. Several approaches to the injection problem in microcolumn HPLC have been reported, including various methods of split injection (4, 7, 8) and internal-loop valve injection (3, 9). Low-volume injection has also been achieved by filling a short length of stainless-steel or fused-silica capillary tubing with 0003-2700/83/0355-0580$0 1.50/0
sample and subsequently connecting this sampling tube to the microcolumn with shrinkable P T F E tubing (10). Direct sample introduction has recently been described in which the inlet of the microcolumn is briefly heated and sample is drawn into the column by capillary action (11). These injection techniques require a great deal of manual skill; they are generally cumbersome and imprecise and deliver only a limited range of injection volumes. A novel injection system has been constructed in our laboratory which is compatible with both packed and open tubular microcolumns. This system is capable of delivering injection volumes which range from 1 nL to 1 pL or more. Furthermore, the system is constructed of readily available materials and can be easily automated to improve precision and simplify operation for routine applications.
EXPERIMENTAL SECTION A high-pressure syringe pump (Model 8500, Varian Instrument Division, Palo Alto, CA) was utilized in this investigation, which allowed operation in both the constant-pressureand constant-flow modes. A schematic diagram of the injection system is shown in Figure 1. This device was based on a high-pressure six-port injection valve (Model AH-CV-6-UHPa-NG0,Valco Instruments Co., Inc., Houston, TX), equipped with a 10-pL sample loop. Two stainless-steel union tees (Swagelok SS-100-3,Crawford Fitting Co., Solon, OH) were connected in series at the valve outlet by using narrow-bore stainless-steel tubing. The branch of the first tee was connected to a restricting capillary and, subsequently, to a high-pressure shutoff valve (“SPLIT”). The second branch tee was connected with wide-bore stainless-steel tubing directly to a shutoff valve (“PURGE”). The HPLC microcolumn was inserted through both tees, and extended approximately 1 cm into the connecting tubing, so that turbulence and mixing at the column inlet were minimized. Two modes of operation were investigated with this sampling system: split injection and “heart-cut” injection techniques. Split injection was achieved with the syringe pump in the constant@ 1983 American Chemical Society
