Anal. Chem. 1996, 68, 4414-4417
Sample Preparation Device for Quantitative Hydrogen Isotope Analysis Using Chromium Metal M. Gehre,* R. Hoefling, P. Kowski, and G. Strauch
UFZ-Centre for Environmental Research LeipzigsHalle, Umweltforschungszentrum LeipzigsHalle GmbH, Permoserstrasse 15, D-04318 Leipzig, Germany
A new technique for the sample preparation, directly coupled to an isotope ratio mass spectrometer (IRMS) for D/H determination, is described. The method is suitable for the preparation of fresh and saline waters as well as different organic compounds (gaseous or liquid). One microliter of water or hydrogen equivalent is injected and reduced by means of chromium metal in a specially designed reaction furnace, and then the hydrogen gas flows directly into the IRMS to be analyzed by standard/ sample comparison. The reproducibility is about 1‰. The accuracy of this method is confirmed by analysis of IAEA standard waters VSMOW, GISP, and SLAP. All aqueous samples including liquid food samples as well as methane gas were injected as original compounds and directly measured without any preparation. The stable isotopes deuterium and oxygen-18 are excellent for studies of the natural water cycle.1 The isotopic characterization of different water bodies enables the determination of their origin and history as well as the study of transport and physicalchemical processes which take place within water bodies. It is necessary to measure both nuclides in parallel because of their close correlation. However, the D/H determination is much more complicated than the measurement of the 18O/16O ratio. Several off-line and directly coupled preparation methods for the D/H analysis are known. In general, two different principles/ techniques for sample preparation of hydrogen gas are carried out: (1) reduction of water to form hydrogen using metals as reagent and (2) catalytic isotope exchange between water and hydrogen gas. Zinc2-7 and uranium8-10 are the most usable reduction means. Other reagents are manganese,11 nickel,12 and chromium.13-17 An alternative method to the decomposition of
water is the catalytic isotope exchange between water and hydrogen gas introduced and described by Horita.10,18,19 When using uranium, zinc, or manganese for the conversion of water to hydrogen gas, several problems have to be taken into consideration, i.e., sorption of hydrogen in the metals, memory effect, metal charge, and the nature of the sample. In the case of the catalytic isotope exchange, a relatively large quantity of the water sample is necessary. Furthermore, the strong temperature dependence of the isotope separation factor requires a very precise temperature control during the exchange process. Generally, impurities in the water samples influence the accuracy as well as the reproducibility of the δD values when using the reduction reagents uranium, zinc, and manganese or the catalytic exchange. Since the 1960s, chromium as a reduction means has been known.13,14 Particularly, water samples containing organics could be processed to hydrogen, yielding highly pure measuring gas. Based on this chromium technique, a very promising method was developed to convert a large variety of samples (water and organic compounds) to pure hydrogen gas and to measure the δD values on IRMS by means of a directly coupled sample preparation device. This technique makes the D/H analysis accessible to many users in hydrology, geology, climatology, ecology, medicine, biology, etc. EXPERIMENTAL SECTION Principles. Chromium reduces water to hydrogen gas at a temperature >700 °C, simultaneously binding other elements of the sample to thermally stable compounds, i.e., nitride (N), carbide (C), oxide (O), and halogenide:
2Cr + 3H2O f Cr2O3 + 3H2
A one-step conversion of water to hydrogen on hot chromium contact was developed by slow evaporation of the sample, reducing the water quantitatively to hydrogen, and collecting the entire gas in ampules (Figure 1). The principle of the new technique involves a very fast evaporation (flash evaporation) and reduction of the sample at the hot chromium contact. The specially designed reaction tube is directly coupled to the dual-inlet system of the mass spectrom-
(1) Craig, H. Science 1961, 113, 1833. (2) Coleman, M. L.; Sherpherd, T. J.; Durham, J. J.; Rouse, J. E.; Moore, G. R. Anal. Chem. 1982, 54, 993-995. (3) Kendall, C.; Coplen, T. B. Anal. Chem. 1985, 57, 1437-1440. (4) Tanweer, A. Anal Chem. 1990, 62, 2158-2160. (5) Vennemann, T. W.; O’Neil, J. R. Chem. Geol. (Isot. Geosci. Sect.) 1993, 103, 227-234. (6) Tanweer, A. Analyst 1993, 118, 835-838. (7) Schimmelmann, A.; DeNiro, M. J. Anal. Chem. 1993, 65, 789-792. (8) Bigeleisen, J.; Perlman, M L.; Prosser, H. C. Anal. Chem. 1952, 24, 13561357. (9) Nief, G.; Botter, R. Patent Specification 904,165, Aug 27, 1959; No. 29315/ 59, Patent Office London. (10) Horita, J. Chem. Geol. (Isot. Geosci. Sect.) 1988, 72, 89-94. (11) Tanweer, A.; Han, L.-F. Isot. Environ. Health Stud. 1996, 32, 97-103. (12) Tobias H. J.; Goodman, K. J.; Blacken, C. E.; Brenna, J. T. Anal. Chem. 1995, 67, 2486. (13) Rolle, W.; Huebner, H. Fresenius Z. Anal. Chem. 1967, 232, 328 (14) Runge, A. Isotopenpraxis 1980, 16, 2.
(15) Gehre, M.; Hoefling, R.; Kowski, P. Patentschrift 1994; Aktenzeichen P 44 37 120.9, Patentbuero Berlin, Deutschland. (16) Gehre, M.; Hoefling, R.; Kowski, P. Patentschrift, 1995; Internationales Aktenzeichen PCT/EP95/03889, Patentbuero Berlin, Deutschland. (17) Gehre, M.; Hoefling, R.; Kowski, P. Isot. Environ. Health Stud., in press. (18) Horita, J. Chem. Geol. (Isot. Geosci. Sect.) 1989, 79, 107-112. (19) Horita, J.; Ueda, A.; Mizukami, K.; Takatori, I. Appl. Radiat. Isot. 1989, 40, 801-805.
4414 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
S0003-2700(96)00676-2 CCC: $12.00
© 1996 American Chemical Society
Table 1. Comparison of the δD Values from Different Samples Converted into Off-Line Method and Directly Coupled Methoda δD values (‰) test samples
Figure 1. Schematics of the off-line sample preparation device.
offline
ocean water (MAOW) +0.7 Leipzig winter precipitation -100.8 Antarctic precipitation -290.4 natural gas field brineb -67.4 champagne 1 -8.1 champagne 2 -1.2 methane -162.6
directly coupled
n
SD (‰) directly coupled
+0.5 -100.1 -289.8 -66.9 -7.9 -0.8 -162.6
10 10 10 5 3 3 5
1.2 1.4 1.1 0.7 0.4 0.7 1.1
a All samples were measured originally without any preparation. Salt content: Na+, 24 700 mg/L; Ca2+, 1039 mg/L; Mg2+, 385 mg/L; Cl-, 37 750 mg/L; SO42-, 5300 mg/L.
b
Figure 2. Schematic of the equipment directly coupled to the IRMS.
eter.15,16 The coupling of the reduction furnace to the IRMS is arranged in such a way that the hydrogen gas flows directly into the inlet system due to the pressure difference between the reaction zone and the variable volume of the mass spectrometer (Figure 2). Apparatus and Reagents. Off-Line Technique. The previous successfully used off-line technique consists of the evaporation volume, the electrically heated quartz tube (inner diameter 15 mm, length 220 mm) filled with chromium, and an automatic Toepler pump unit. The hydrogen formed is quantitatively collected in glass ampules using the Toepler pump (Figure 1). At the IRMS, the sealed ampules are broken. For the chromium method, the reagent is chromium powder Patinal 1 bar) by means of a capillary (0.1 mm diameter) via the injection unit into the furnace. The sample amount is controlled by time of flow-through. For one run, the amount of injected methane corresponds to a hydrogen equivalent from a water sample. When using the off-line technique, the methane gas (around 4 µl water equivalent) is injected via an injection unit fixed at the sample entry of the furnace in the same procedure described above. RESULTS AND DISCUSSION The results of the isotope analyses of the liquid samples and the standards are shown in Table 1. To compare the accuracy and the reproducibility of the analyses, all standards and samples were studied by both the off-line and the directly coupled methods. The measurements agree well for both techniques. Furthermore, the reproducibility of the δD values from the directly coupled technique varies within 0.4 and 1.4‰ for all liquid and gas samples. For duplicate analysis using the off-line technique, the reproducibility of the mean values is about 0.6‰. To prove the accuracy of the δD values using the directly coupled technique, the three international standards VSMOW, GISP, and SLAP21 were determined. The measured δD values of VSMOW, GISP, and SLAP agree with the defined standard values. Particularly, the δD value of GISP corresponds excellently to the defined IAEA standard GISP corrected to the VSMOW-SLAP scale (Table 2). Usually, natural aqueous samples differ in their isotopic compositions, which requires measuring conditions without any influence on the δD values of the next sample. Therefore, the new directly coupled technique has to be tested for any memory effect when isotopically different samples are converted at the hot chromium contact. Following the procedure for liquid samples, we compared two test waters, differing about 400‰, which is typical for water within the natural abundance. Each water was measured three times in an alternate sequence (Table 3). The standard deviations of the mean values from all measurements of test water 1 with δD of -395.6 ( 1.2‰ and of test water 2 with δD of +9.6 ( 0.9‰ lay within the accuracy of the chromium method generally reported in Table 2. The mean value of the standard deviations of the test waters also does not differ from the standard deviation of the chromium technique (Table 3, column 5). Because of the ability of chromium to reduce organic compounds directly to pure hydrogen, we chose methane as gaseous (20) Harting, P. Isotopenpraxis 1989, 25, 347-348. (21) Gonfiantini, R. IAEA-Rep. Vienna 1984, 77p.
4416 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
Table 3. Memory Test for the Directly Coupled Chromium Method Using Two Test Waters of Natural Origin with Different Isotopic Composition within the Natural Abundance order of measurements 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 jx (‰) σn-1 (‰)
test water 1 (‰)
test water 2 (‰)
-394.7 -396.0 -395.9 +8.3 +9.0 +9.1 -395.6 -395.0 -395.3 +9.7 +9.0 +10.5 -393.5 -396.9 -397.2 +8.9 +10.5 +10.9 -395.6 (1.2
+9.6 (0.9
jx (‰)
σn-1 (‰)
-395.5
(0.8
+8.8
(0.5
-395.3
(0.3
+9.8
(0.8
-395.9
(2.1
+10.1
(1.1 0.94 (0.7
a
b
Figure 3. (a) Gas analysis spectra of the methane sample. (b) Gas analysis spectra of the methane sample after the reduction to hydrogen by means of the directly coupled technique.
hydrogen-containing compound to determine the deuterium content. Methane as a chemically very stable organic compound requires a longer reaction time than is needed for the reduction of aqueous samples. In contrast to the preparation of liquid samples, the complete conversion of methane to pure hydrogen requires 45 min at 1000 °C. During the reaction time, the valve between the furnace and the IRMS remains closed. The complete conversion of methane gas at the chromium contact has to be proved by mass spectrometric determination of the gas composition before and after reduction. As shown in Figure 3, the spectra of the gas analysis point to a complete conversion of methane. It is necessary to prevent methane from entering the ion source because of its influence on the H3+ formation and thus on the δD
value. The nitrogen content of the technical methane gas could be reduced from 7.07% to 0.92% during the reaction time. Due to the chemical sorption ability of chromium, small impurities of nitrogen can be eliminated to get pure hydrogen measuring gas.22 Despite the extended residence time of methane in the closed furnace over 45 min, the δD values agree with the results of the off-line technique (Table 1). It is an important result of the procedure because of the well-known diffusion effect of hydrogen through quartz glass at high temperature. Compared with the other reduction means, i.e., zinc, uranium, or manganese, which are mostly suitable for water only, chromium yields pure hydrogen from hydrogen-bearing compounds, including organics. This advantage is due to the ability of chromium to form thermally stable compounds of other gaseous products from C, N, O, S, and halogens. No isotope fractionation is observed due to any sorption processes of hydrogen at the hot chromium surface (Tables 1-3).
efficient and correct D/H analysis. Advantages of the new technique include (1) using only microliter quantities of original liquid samples of different chemical composition and origin as well as measuring hydrogen-containing gases like methane, (2) measuring of δD values within an 1‰ reproducibility and without memory effect, and (3) the fact that use of an autosampler can make this method more effective. The described method opens new possibilities for the D/H analysis in many fields of research, especially in the field of medicine and biochemistry. The technical components of this method are registered for a patent.
CONCLUSIONS The comparison of results of the off-line and the directly coupled techniques shows that the new method is suitable for an
Received for review July 9, 1996. Accepted September 18, 1996.X
(22) Nitzsche, H.-M.; Stiehl, G. Isotopenpraxis 1985, 21, 439-441.
ACKNOWLEDGMENT All research work was carried out at the Centre for Environmental Research LeipzigsHalle, Germany, and we thank the scientific director, Professor Peter Fritz, for his helpful discussions and support.
AC9606766 X
Abstract published in Advance ACS Abstracts, November 1, 1996.
Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
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