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Anal. Chem. 1884, 56,386-389
Absolute Determination of Deuterium Content of Heavy Water Standards by Distillation and Mass Spectrometry Warren M. Thurston* and Murray W. D. James Atomic Energy of Canada Limited Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO
An absolute method for heavy water standardlzatlon Is described which Is based on llnear extrapolation to 100% D20. The method uses a reflux column attached to a boller contalnlng the sample to be standardized. Wlth the column operatlng at equlllbrlum under total reflux, samples from both the top and bottom of the column are converted to hydrogen and mass analyzed by use of an arbllrary D20scale. To minlmise mass spectrometer errors, the dlfference between top and bottom HD/total Ion current Is measured at the sample concentration and at several other slightly hlgher HD concentrations. The dlfference values vary llnearly wlth the HDO concentratlon In the boller. The hear relatlanship Is extrapolated to zero HD difference as would be observed If 100% D,O were used. The extrapolated value on the arMtrary D20 scale Is compared wlth 100% and ahy dlscrepancy represents an adjustment requlred to establish an absolute scale. The method has been tested at both the D20 and H20 ends of the range and has shown that the accuracy of the method Is within the preclslon of 0.0006 mass % D,O.
The calibration of the absolute deuterium content of heavy water has always been a problem since no international standard for DzO exists and no one has produced or has proven the existence of DzO isotopically pure in deuterium. The Canadian standards have been based on correlation measurements between mass spectrometry and infrared spectrometry done in 1962 (unpublished). At that time a primary standard was established with secondary standards internally consistent to better than fO.OO1 mass % covering the range 99.54 to 99.97 mass '70. In France, Ceccaldi (1)made absolute determinations by using a technique which compared the infrared spectra of liquid and solid DzO to determine the values of very high D20 standards "with a precision of at least 40 ppm - 20 ppm and very likely f 5 ppm" and later (2) quoted an accuracy of ( f 5 ppm) for this work. Babeliowsky and De Bolle (3) have used a method based on the equilibration of deuterium gas samples with DzOfollowed by mass spectrometry which gives an accuracy stated to be f0.001'70 at 99.98 mol %, The ASTM standard method D2184-81 (4) quotes a single laboratory precision of 0.02 atom % D. With the growing commerce in heavy water, it was desirable to develop a simple independent analytical method which could be performed with large samples (20 cm3) to minimize sample handling problems characteristic of small samples ( 4 ) and which would have an accuracy of better than fO.OO1 mass % DzO, i.e., an accuracy which is significantly better than the routine precision of f0.003 mass % for infrared techniques above 99.5 mass % DzO (5). The method investigated here is based on two arguments. (a) If a small vacuum still containing 100% DzO (no protium) and operating in total reflux were connected to a mass spectrometer (via a uranium reduction furnace for converting DzO and HDO to D, and HD), there would be no difference in the mass 3 (HD+) abundance for vapor sampled above the
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0003-2700/84/0356-0386$01.50/0
boiler liquid and vapor sampled just above the condenser; Le., only the ever-present HD background in the mass spectrometer would be seen. (b) Additions of protium to the distillation system would cause proportional increases in the difference between top vapor and bottom vapor HDO concentrations provided the separation efficiency of the distillation column remains constant. If these arguments are valid, then it should be possible to measure differences in deuterium concentration between top and bottom vapors under total reflux for a series of liquids of known relative isotopic content to obtain an experimentally based relationship which is valid for extrapolation to pure DzO. A method based on these premises has been developed to determine the absolute value of DzO standards; it has resulted in a direct linear approach to the problem with improvements in convenience and as good or better accuracy than previous methods.
EXPERIMENTAL SECTION Mass Spectrometer. The system shown schematically in Figure 1 uses a Vacuum Generators Micro Mass MMlC mass spectrometer. The "1C" denotes a spectrometer that includes a totalion collector and a 0.5 mm defining slit on the peak collector to assure a more stable, nonscanning mode of operation. The spectrometer vacuum is provided by a Leybold-Heraeus TMP-150 turbomolecular pump and the inlet vacuum by a 100 L/min roughing pump and a -45 OC cold trap. The "total" ion current is monitored by the Micro Mass amplifier provided with the MMlC, and a Cary 401 vibrating reed electrometer is used for amplification of the collected "peak" ion current. Spectrometer pressure is monitored by a Vacuum Generators VIG-21 ion gauge and the inlet pressure is monitored by a PVGlSC pirani gauge. A modified Edwards Vacuum, Type CG3 0-40 torr (0-5.2 kPa) capsule gauge monitors the intermediate pressure region of the inlet section, where pressure is decreased to 0.1 kPa. The inlet section being monitored is connected to the aneroid element and the roughing pump vacuum is used as the reference in the gauge case. The above components were assembled into an analytical system which uses a simple automatic cycle (6) and is capable of measuring the ratios mass Pltotal ion, mass 3/total ion, mass 4ltotal ion, or any other specific mass up to 50/total ion, to a precision of better than 10.5%. For this work we used the ratio of mass 3/total ion for both ends of the D20-H20 concentration range and occasionally mass 14/total ion for impurity monitoring. The vapor being sampled passes through one leak (Figure 1)which drops the pressure to 1W a and then through a second leak where the pressure is reduced further t o near that of the mass spectrometer (1.3 X Pa). A uranium furnace (7) at 600 "C used between the second leak and the mass spectrometer to reduce water vapor to H2,HD, and D2. The cycle solenoid valve allows either the pumping away of incoming vapor or, on closing, the building up of pressure between the leaks, which in turn produces a rising pressure in the spectrometer. As the total ion current reaches the preset level, a read and print command is sent to read the output from the mass 3 amplifier followed by a pump-out command to the cycle solenoid valve. The system, which contains only one moving part (the solenoid valve), monitors a sample stream with a readout every 10 to 180 s. For this work a 45-s cycle 0 1984 American Chemical Society
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Flgure 1. Schematic of MMlC mass spectrometer system: A, inlet from still; B, crimp leaks; C, uranium furnace; D, mass spectrometer total ion collector; E, mass spectrometer; F, total ion amplifier; G, sigma relay meter; H, peak amplifier; I, recorder and printout; J, ion gauge; K, turbomolecular pump; L, mechanical pumps; M, differential gauge: N, cycle solenoid valve. r--
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0
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TO MASS SPECTROMETER
t
c
FROSTED COLUMN
SPIRAL
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Figure 2. Distillation apparatus: 0.5 cm X 25 cm frosted column for vacuum distillation wlth spiral dripper installed for reflux flow measurements.
was used which includes about 20 s for the fresh sample entering the inlet from the experiment to be pumped away before the pressure is allowed to rise for the next readout. Both the print and pump-out signals are provided through the Sigma relay Model 9270-21-D-VB and have proven reproducible to better than 0.03% of the signal level. Distillation Apparatus. The distillation apparatus shown in Figure 2 consisted of a vacuum jacketed column 25 cm high and 0.5 cm diameter with a 5-cm condenser on a 50-cm3roundbottomed flask. The sampling lines were integral with the Pyrex system and were each 25 cm long and 1.0 mm in bore. Three J. Young (Scientific Glassware),Ltd., Catalog No. POR-lRA, l-mm valves were used, one for evacuating and degassing and the other two for selecting top or bottom vapor for analysis. The still assembly was connected to the mass spectrometer by a 1/4 in. to in. (6.5 mm to 1.5 mm) Cajon adaptor, Cajon Co., Cleveland, OH. A 0.5-mm wire spiral in a 1.5-mm Refrasil woven silica (H.I. Thompson Fiber Glass Co.) sleeve was inserted at the bottom of the column as shown in Figure 2 to collect the reflux flow from the column wall and return it to the boiler a drop at a time. Timing the drops gave a measure of the relative flow rates at different boiler temperatures. The distillation column was frosted on the inside surface with a sand blast of #SO grit SiOz during fabrication. This gave a uniformly wettable surface and stable column performance. The still boiler sat in a constant temperature water bath and the water flow to the condenser was provided by a second bath, both stable to hO.01 'C.
Flgure 3. Deuterium concentration (mass % D,O on an arbitrary, self-consistent scale) in the still boiler vs. mass spectrometer readout (mass Wtotal ion current ratio in arbitrary units) for distillation Column top and bottom vapor sample points. Also plotted are the differences between top and bottom values.
Still Operation. Twenty cubic centimeters of DzO was introduced to the boiler flask through the access port, degassed several times over a 10-min period by pumping in short bursts through the top Young valve with no detectable isotopic separation, and refluxed for 2-3 h with less than 0.1% draw off of the reflux flow from the top sample point for final degassing. This stream of vapor was sampled by the mass spectrometer and used to equilibrate the uranium furnace and the mass spectrometer to the very high deuterium levels before taking the final top and bottom values used in the determination. During the conditioning period a constant difference in the deuterium concentration across the still was established. Measurements were made with the mas8 spectrometer after sampling the top of the column continually for 2 to 3 h and only after the system had shown stable operation (no drift) for at least 30 min. The small amount of vapor drawn off for analyses (0.1% of the reflux stream) had no measurable effect on either the top or bottom values or the reflux rate. Each reading listed in Tables I and I1 represents results averaged over a 15-min period (20 analyses). After each set of top and bottom values the still was f l d e d with dry nitrogen and opened, a liquid sample was taken for precise infrared analysis against existing standards, protium was added in the form of lower grade D20, the still was resealed and degassed, and a second set of values was determined. Two to four sets of points constituted a single determination as presented in Figure 3.
RESULTS AND DISCUSSION High End (D20). Figure 3 shows a four-point determination of the D20 scale and represents a run of about 14 h. In this example linearity of both top and bottom vapor values for the mass 3/total ion current ratio with changing boiler D,O concentrations is demonstrated as well as linearity of the differences. In some determinations top and bottom values were nonlinear due to changes in background in the mass spectrometer; however, in these cases the differences plotted against still boiler liquid DzO concentration gave a linear plot. This procedure cancels the contribution of background to the mass 3/total ion current signal and minimizes the effect of background changes by keeping the interval between top and bottom measurements short (15 min). The results of eight separate experiments are shown in Table I and are plotted in Figure 4 based on the difference between topbottom vapor values vs. the D20concentration. Any arbitrary DzO scale in the range 99.8 to 99.99 mass % D20would be satisfactory provided it was internally consistent and sufficiently precise.
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Table I. Summary of Results at the D,O End of the Range
expt no. and conditions no. 5 @ 26/21 'C: 3.8 cm3/h no. 6 @ 26/21 O C , 3.8 cm3/h no. 7 @ 24/21 'C, 2.0 cm3/h no. 8 @ 25/21 'C, 3.0 cm3/h no. 9 @ 25/21 'C, 3.0 cm3/h no. 1 0 @ 25/21 'C, 3.0 cm3/h
no. 11 @ 25/21 'C, 3.0 cm3/h no. 12 @ 25/21 'C, 3.0 cm3jh
mass spectrometer readings' top top vap. bottom vap. bottom 32.3 56.8 79.0 98.0 100.5 30.9 64.5 21 8.2 86.8 161.4 155.5 103.9 63.9 160.0 147.1 52.0 100.1 49.9 166.9 178.5 86.5 128.8 173.5
28.8 35.7 42.1 48.0 51.4 27.4 36.4 95.3 70.6 87.0 98.7 96.2 53.3 73.6 65.6 37.3 48.7 35.4 65.8 78.3 72.9 79.6 90.3
3.5 21.1 36.9 50.0 49.1 3.5 28.1 122.9 16.2 74.4 56.8 7.7 10.6 86.4 81.5 14.7 51.4 14.5 101.1 100.2 13.6 49.2 83.2
still boiler,d mass % D,O
correctionse graphic
99.967 99.916 99.866 99.827 99.830 99.967 99.893 99.893 99.967 99.926 99.893 99.967 99.967 99.897 99.897 99.963 99.927 99.963 99.873 99.873 99.963 99.929 99.891
mean std dev
computed
0.0213
0.0217
0.0225
0.0225
0.0222
0.0219
0.0219
0.0214
0.0228
0.0233
0.0230
0.0224
0.020
0.0218
0.0233
0.0220 0.0221 0.0006
0.022 +0.001
a Still boiler temperature/condenser temperature. Reflux stream flow rate in cm3/h. Top and bottom vapor analyses and their differences. D,O concentration based on existing standards. e Graphic (Figure 4) and computed corrections.
Table 11. Summary of Results at the H,O End of the Range mass spectrometer readings' expt no. and conditions no. 1 @ 25/21 "C, 3 cm3/h no. 2 @ 32/21 'C, 8.5 cm3/h no. 3 @ 29/21 O C , 6 cm3/h no. 4 @ 29/21 "C, 6 cm3/h
top vap. 83.4 84.0 81.0 33.6 48.8 72.5 90.9 34.4 28.1 38.5 47.1 96.Bb 102.9 99.8
bottom bottom vap. top 83.7 111.3 159.8 34.2 72.3 135.1 182.1 34.4 27.7 58.4 85.2 97.2b 109.6 115.8
t 0.3 27.3 78.8 t 0.6 23.5 62.6 91.2 0 -0.4 19.9 38.1 t 0.4 6.7 16.0
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still boiler, mass % H,O 99.99998 99.9556 99.8860 99.99998 99.9556 99.8860 99.8362 99.99998 99.99998 99.9657 99.9358 99.99998 99.9960 99.9920
a Top vapor, bottom vapor, and bottom - top differences at varying H,O concentrations. Starting water D/(H t D). has an atom fraction of less than 0.2 X Experiment no. 4 at three times the amplifier sensitivity of experiments no. 1 to no. 3.
In our work we used a scale based on exisiting standards. Slope changes observed in Figure 4 are due to (a) different reflux flow rates, (b) changing mass Spectrometer background conditions, and (c), in the case of run 8, a rising laboratory temperature throughout the course of a run which would probably affect (a) and (b). In no case, however, did these changing conditions affect the final results noted below. As required by our initial arguments, the top-bottom difference values decrease linearly with decreasing HDO concentration in the still boiler, i.e., with increasing mass percent DzO. Linear regression analysis of the data gives an average correlation coefficient of -0.9998 for the six runs in which there
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DIFFERENCE IN M S READOUTS ( T - 8 )
Figure 4. Deuterlum concentration (mass % D,O) vs. difference in mass spectrometer readouts (arbitrary units) of top - bottom vapor values for eight experiments at different reflux rates. The difference term (100% - intercept on the Y axis) graphically represents the correction to be added to exlsting standards for absolute calibration.
were more than two determinations, compared with a correlation factor of unity for a perfectly linear relationship. Further confidence in the method is gained by noting that the top-bottom difference lines all converge on extrapolation to very nearly the same mass percent D20value, as discussed below, even though their slopes differ by more than a factor of 4. Table I lists the correction (100% - Y intercept) for the difference line of each run found graphically (Figure 4), and computed with a least-squares fit of all points in each run in which there were more than two determinations. The cor-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
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1 ! -L 40 50 60 70 8 0 90 D I F F E R E N C E I N M S READOUTS ( 8 - T )
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Figure 5. The low end (H,O) equivalent of Figure 3, i.e., H20 concentration vs. mass spectrometer readout of top and bottom vapor at different H20concentrations and differences of bottom -top vs. mass
% H20.
rection is the value for each determination that must be added to the original DzO scale to give a zero difference between the top and bottom vapor streams at 100% DzO concentration. The mean correction for the eight experiments is +0.0221 f 0.0006 for conversion to an absolute concentration scale. The highest standard used in this work is therefore 99.9670 + 0.0221 = 99.9891 mass % DzO with a standard deviation (g,) of f0.0006 mass % based on eight independent determinations. Low End (H20). Further tests of the basic arguments underlying the method are possible at the low end of the range, i.e., where the HDO concentration also decreases to zero as 100% H 2 0 is approached. Since we have pure H2O known to have an atomic fraction less than 0.2 X lo4 D/(H+D) content determined by a similar low end technique where more precise instrumentation can be used (yet to be published), and by comparison with IAEA standards, several analogous experiments were performed starting with this HzO. The same distillation conditions, uranium reduction, and mass spectrometer system (i.e., HD/total ion ratio measurement) were used after uranium replacement and equilibration with HzO. Results are tabulated in Table I1 and shown graphically in Figures 5 and 6. It is apparent that the top-bottom difference decreases monotonically as we approach zero deuterium concentration as measured by the HD/total ion current ratio and that the method does maintain linearity within the experimental error. The extrapolated top-bottom difference lines converge at very nearly the same mass percent HzO value, as shown in Table 11. These observations at the HzO end of the D20-Hz0 scale further illustrate the validity of the method. Precision and Accuracy. The differences between top and bottom with 99.999 98 mass % HzO in the still boiler best demonstrate the precision of the method using the MMIC mass spectrometer. The standard deviation of these five determinations in terms of concentration is fO.OOO 22 mass % HzO. The standard deviation of f0.0006 mass % D 2 0 of the eight DzO determinations reflects an added uncertainty introduced by the infrared analyses of the DzOcontent of the still boiler (Table I) which at best are determined to a precision of fO.OO1 mass %. The segment of the experiment most likely to introduce a bias in the results is the performance of the mass spectrometer including the H3+ and background contributions. Since differences have been used throughout the experiment, these
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Figure 6. The low end equivalent (H,O) of Figure 4 showing system linearity near 100% H,O. Plotted are bottom - top differences VS. mass % HO , from Table 11.
variables are effectively cancelled, allowing us to conclude, using the definitions of Barry (8),that the accuracy of the method is within the experimentally determined precision of f0.0006 mass % D20.
CONCLUSION The arguments on which the method is based have been resolved. The difference values at both the D20 and HzO ends of the range extrapolate linearly to the same value, within the experimental error, representing what is unattainable in practice, DzO and HzO containing on HDO. Thus an absolute method has been derived which forms the basis for calibrating practical standards of lower concentration. Advantages over other techniques include a simple, direct, and relatively quick method with no intermediate steps. Samples are easily managed and are isolated from atmospheric contamination. Quantitative determination of the instrumental background is not required. Accuracy of the method is equal to or better than other techniques and appears to be limited only by the precision of the analytical instrumentation and therefore is subject to further improvement.
ACKNOWLEDGMENT The authors are indebted to J. H. Rolston and E. A. Symons, Physical Chemistry Branch, and T. A. Eastwood, Chemistry and Materials Division, for their valuable discussions and J. G. Wesanko, Neutron and Solid State Physics Branch, for glassblowing and still fabrication. Registry No. D2, 7782-39-0; DzO,7789-20-0.
LITERATURE CITED (1) Ceccaldi, M. Centre D’Etudes Nuclealres de Saclay CEA-R.2441, 1984. (2) Ceccaldl, M.; Girard, G.; Menache, M.; Rledlnger, M. Metrologia 1975, 7 1 , 53-65. (3) Babellowsky, T.; De Bolle, W. E. Metrologia 1974, 10, 129-138. (4) ASTM Standard Method, D2184-81. p 1061. Annu. Book ASTM Stand. 1982. Part 45, Nuclear Standards, p 733. (5) Stevens, W. H.; Thurston, W. M. Atomic Energy of Canada Report AECL-295, March 1954. (6) Thurston, W. M. Rev. Scl. Instrum. 1970, 41 (No. 7), 983-966. (7) Nief, G.; Botter. R. “Advances in Mass Spectroscopy”; Pergamon Press: London, 1959; Voi. 1, pp 515-525. (6) Barry, B. Austin “Errors In Practical Measurement in Science, Engineering, and Technology”; Why: New York, 1978; Chapter 3.
RECEIVED for review August 17,1983. Accepted November 28, 1983.
