Anal. Chem. 1999, 71, 3254-3257
Low-Temperature Thermal Decomposition of Sulfates to SO2 for On-Line 34S/32S Analysis Stanislaw Halas* and Janina Szaran
Mass Spectrometry Laboratory, Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland
We describe a fast, inexpensive, and safe method of direct SO2 extraction from BaSO4 for sulfur isotopic analysis by mass spectrometry. Only two reagents are used: (1) pure NaPO3, which is mixed with BaSO4 sample, and (2) Cu foil, from which reaction boats are manufactured. The extraction precedes in the Cu boat placed into a quartz tube connected to a vacuum line. The boat is heated to 650-700 °C while pure SO2 produced is collected in a “cold finger”. Reaction is complete in 7-10 min. We have proven by means of 18O-enriched BaSO4 specimens that the oxygen isotopic composition of the SO2 is totally controlled by 18O content in NaPO3, when the weight ratio of the reagent to sample exceeds 6:1. The method described can be used for “on-line” SO2 preparation for isotopic analysis. Agreement between laboratories measuring the sulfur isotope ratio is still not good. The spread of the results obtained in different laboratories can exceed a considerable margin that is elucidated on the basis of precision and reproducibility. Reasons for this poor agreement were investigated by Rees,1 Halas et al.,2 and Halas.3,4 It has been noticed that much larger discrepancies in isotope ratios are attributed to analysis in SO2 gas than in SF6 gas. On the other hand, preparation of SO2 is easier, more safe, and less expensive than SF6. Moreover, the analysis can be carried out on a smaller mass spectrometer (e.g., on an instrument usually used for CO2 analysis with mass range up to 70) because the heavier molecular species SF5+ with m/z ) 127 and 129 are analyzed when SF6 is introduced into mass spectrometers. It has been argued that the major error in the mass spectrometric analysis of 34S/32S in SO2+ or SO+ spectra comes from uncertainty of the oxygen isotopic composition of the SO2 gas. It should be emphasized here that even total conversion of sulfate sulfur to SO2 cannot guarantee the reproducibility of 66/64 or 48/ 50 ion currents of SO2 due to varying conditions during SO2 preparation. We will discuss this problem extensively below. It is the purpose of this paper to describe a convenient method of SO2 production from sulfates which has the following features: (1) it is a low-temperature decomposition of sulfate anions, (2) SO3 is totally converted in situ to SO2, (3) the memory effect in sulfur * Corresponding author: (e-mail)
[email protected]. (1) Rees, C. E. Geochim. Cosmochim. Acta 1978, 42, 383-389. (2) Halas, S.; Lis, J.; Szaran, J.; Trembaczowski, A. Ann. UMCS 1979, 34/35 (section AAA), 37-43. (3) Halas, S. In Studies on sulfur isotope variations in nature; IAEA: Vienna, 1987; pp 105-111. (4) Halas, S. RMZ-Mater. Geoenviron. 1998, 45, 48-50.
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and oxygen isotope ratios is entirely eliminated, (4) the oxygen isotope composition of SO2 is constant despite the δ18O of prepared BaSO4, and (5) it ensures on-line preparation of SO2 for isotopic analysis. HISTORY OF THE METHOD A critical review of the methods used recently among laboratories leads us to the conclusion that no one has all the features listed above. The most frequently used method is thermal decomposition in a quartz tube in the presence of V2O5 5,6 or in the presence of Cu2O,7 requiring temperatures of 1050 and 1120 °C, respectively. The first method was adopted in some laboratories in continuous-flow mass spectrometric (CFMS) analysis. However, the results obtained by these methods show a considerable dispersion, most likely due to memory effects in oxygen isotopic composition. The reason for this effect is the oxygen exchange between SO2 and the hot surface of the quartz tube. The exchange rate can be radically suppressed by lowering the reaction temperature. Halas and Wolacewicz8 and Halas et al.9 reported extraction methods in which conversion of sulfate sulfur to SO3 proceeds in a ceramic boat in the presence of sodium metaphosphate (NaPO3) somewhat above its melting point (610 °C). In these methods, SO3 is reduced to SO2 on a hot “plug” made of copper wire which is kept at 750 °C. The NaPO3 method has been applied by the authors for more than 15 years with few modifications. However, when an accuracy better than 0.10‰ was required for 1-mg samples of BaSO4, the method was not good enough. In the course of numerous experiments, we have investigated the reasons for the lack of accuracy of this method. The most serious reason is the presence of trace sulfates in both the ceramic boat and the reagent. Additionally, the memory effect for oxygen from the copper plug, which was used for a large number of extractions, is significant. Therefore, we applied additional cleaning of the reagent by melting and degassing in a vacuum. The next significant improvement was the replacement of the ceramic boat by a pure copper boat. In the first series of experiments with the copper boat, we noticed that the copper plug was always ideally clean while the surface of the Cu boat became dark due to formation of CuO. We concluded (5) Haur, A.; Hladikova, J.; Smejkal, V. Isotopenpraxis 1973, 9, 329-331. (6) Yanagisawa, F.; Sakai, H. Anal. Chem. 1983, 55, 985-987. (7) Coleman, M. L.; Moore, M. P. Anal. Chem. 1978, 50, 1594-1595. (8) Halas, S.; Wolacewicz, W. P. Anal. Chem. 1981, 53, 686-689. (9) Halas, S.; Shakur, A.; Krouse, H. R. Isotopenpraxis 1982, 18, 433-436. 10.1021/ac9900174 CCC: $18.00
© 1999 American Chemical Society Published on Web 06/25/1999
Table 1. Reproducibility of δ34S Measurements sample date
SO-5
SO-6
NBS-127
9.05.1998 14.05.1998 23.04.1998
0.89 ( 0.04 0.93 ( 0.05 0.94 ( 0.05
-32.09 ( 0.03 -32.05 ( 0.04 -32.32 ( 0.05
20.30 ( 0.05 20.34 ( 0.04 20.19 ( 0.06
average
0.92 ( 0.03
-32.12 ( 0.02
20.30 ( 0.03
Figure 1. Vacuum line for SO2 extraction for isotopic analysis.
in a Cu boat in a vacuum for each series of extractions. After cooling, it is crushed into small pieces which are pulverized in a small agate mortar together with a portion of the heated BaSO4 sample. We load typically 10 mg of BaSO4 and about 60 mg of NaPO3 into a clean Cu boat. The reactor is evacuated to about 0.13 Pa (1 mTorr) while the boat is heated to 200 °C by a small-size external furnace (see Figure 1). After outgassing, which takes about 10 min, the system is isolated from the pump, the furnace temperature is set to 650-700 °C, and the coldfinger is immersed in liquid nitrogen. One can observe the reaction rate by means of a Pirani gauge. The reaction is completed in 7-10 min. Then, the coldfinger is isolated from the reactor by closing the stopcock between them. Subsequently, the coldfinger is heated to room temperature, and SO2 gas is admitted to the inlet system. Neither water nor CO2 is present in the prepared SO2 gas.
Figure 2. Plot of δ50 vs δ18O of BaSO4 specimens with constant δ34S ) -0.30‰. Error bars represent double standard deviation values.
that the Cu plug and the furnace for heating this plug could be eliminated. In this way, the apparatus and the procedure were significantly simplified. Moreover, the novel method is faster and more economic (also in terms of materials and energy consumption) than any method described so far. APPARATUS AND PROCEDURE The apparatus, which can be used for on-line SO2 extraction from precipitates of BaSO4 produced in the laboratory or from natural sulfate minerals, is shown schematically in Figure 1. The apparatus can be simply arranged from commercially available stopcocks and joints. An ungreased joint between the quartz reactor and the remaining parts is recommended (e.g., Cajon Ultra-Torr fitting). The volume between the stopcocks should be as small as possible. This volume is connected to the inlet system of the isotope ratio mass spectrometer by means of a flexible and narrow tube made of stainless steel. The procedure requires only two chemicals: analytical grade copper foil (thickness of 0.1 mm or less) from which boats are produced and the NaPO3 reagent. We used Cu foil from BDH Chemicals, Poole, England. A set of boats produced by means of a simple press was heated at 750 °C in high vacuum prior to use. The NaPO3 should be purified from water and sulfur by melting
REPRODUCIBILITY The reproducibility of the present method was tested with various amounts of BaSO4 between 20 and 1 mg. When sulfur and water were eliminated from the reagent (as described above), no difficulties were encountered with analysis of small samples. Table 1 demonstrates excellent reproducibility obtained for two new barium sulfate standards, SO-5 and SO-6, distributed by the International Atomic Energy in Vienna, and NBS 127 barium sulfate. No memory effect was detected in alternative preparations of two samples, differing significantly in isotopic composition. An illustrative example is demonstrated in the next section of this paper. OXYGEN ISOTOPE FRACTIONATION To examine the oxygen isotope fractionation between SO2 and BaSO4, we performed a series of extractions from BaSO4 samples with identical sulfur isotopic composition but largely differing in 18O/16O ratios. These BaSO samples were produced in the 4 following way. Portions of 50 mg of isotopically homogeneous sphalerite (ZnS) with δ34S ) -0.30‰ vs V-CDT10 were mixed with Cu2O reagent in an appropriate proportion (1:8 w/w), and aliquots of SO2 were produced by quantitative combustion at 800 °C in a vacuum line according to the procedure described by Robinson and Kusakabe.11 The SO2 gas was collected in separate ampules for subsequent quantitative conversion to SO42- in a vacuum line comprising a manifold with Rittenberg’s flasks (reversed Y-shape tubes) filled with analytical grade hydrogen peroxide (commercially available 30% H2O2). The oxygen isotopic composition (10) Robinson, B. W. In Reference and intercomparison materials for stable isotopes of light elements. Proceedings of a consultants meeting held in Vienna, 1-3 December 1993; IAEA-TECDOC-825, pp 39-45. (11) Robinson, B. W.; Kusakabe, M. Anal. Chem. 1975, 47, 1179-1181.
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and
Table 2. Results of Mass Spectrometrical Determinations of δ50 Defined by Formula 1 and δ18O for BaSO4 Samples with Different δ18Oa δ18OVSMOW
δ50
2 3
145.65 ( 0.08 215.0 ( 0.1
4 L NH H
218.64 ( 0.07 3.79 ( 0.02 17.46 ( 0.04 22.47 ( 0.07
0.59 ( 0.03 0.80 ( 0.02 0.46 ( 0.03b 0.94 ( 0.08 0.18 ( 0.03 0.38 ( 0.02 0.27 ( 0.03
sample no.
18 [66] 34S O ) 32 + 2 16 [64] S O
The above equation can be rewritten in terms of commonly used δ values. From the recent determinations of the absolute isotope ratios in V-SMOW14,15 and V-CDT16 standard, one obtains the following relation between δ values:4
a The δ values are given with their standard deviations. b Double amount of NaPO3 reagent.
of the H2O2 + H2O mixture was adjusted by addition of isotopically light or heavy water. For example, to produce SO42- ion with δ18O ) +218.6‰ vs V-SMOW, we added about 6 µL of H218O (94.4% 18O and 1.4% 17O) to 4 mL of the reagent (30% H O ). 2 2 The quantitative oxidation of SO2 gas to sulfate ion was achieved by complete transfer of SO2 to outgassed Rittenberg’s flasks and a long-term reaction (1 week) of SO2 with the H2O2 + H2O mixture. Prior to admission of the SO2 aliquot, the mixture was outgassed from air by freezing with liquid nitrogen. To avoid breaking of the glass flasks, we always froze the mixture starting from the top of the liquid by pouring liquid nitrogen onto a wool ring wound around the tube. The sulfate ions produced in this way were quantatively recovered as BaSO4 by precipitation with BaCl2 solution. To test the influence of the variable 18O content in the analyzed sulfate on the ion current ratios determined mass spectrometrically for δ34S evaluation, we determined δ18O in BaSO4 by the method described by Mizutani12 and
δ50 ) ((R50sample/R50standard) - 1) × 103
(1)
where R50 is the ion current ratio of the isotopic beam m/z ) 50 to the major beam (m/z ) 48) in the SO+ spectrum (the alternative pair of the ion beams with m/z ) 66 and 64 in the SO2+ spectrum was not achievable for our collector system). The results of δ50 determination are shown in Table 2 along with δ18O values. A small correlation exists between obtained δ50 and δ18O values. However, the slope of the correlation line is only 0.003. This may be safely considered as meaningless in the case of natural variations of δ18O in sulfates (see discussion below). DISCUSSION Taking into account the isobaric contributions, Thode et al.13 found the following relationship between measured ion beam ratios and the isotopic ratios for respective SO+ and SO2+ spectra:
[50] 34S 18O + ) [48] 32S 16O
δ34S ) 1.04822δ50 - 0.04822δ18O
for SO+
(4)
δ34S ) 1.09644δ66 - 0.09644δ18O
for SO2+
(5)
and
It follows from eq 4 that the expected slope of the correlation line between δ50 and δ18O for δ34S ) constant is 0.04822/1.04822 ) 0.046. The above value, of 4.6‰ per 100‰ range of the natural variation of oxygen isotopic ratios would lead to unacceptably large systematic errors in the δ34S analysis. Therefore, various methods (see introduction section) are used to diminish the 18O/16O variations in the SO2. As was demonstrated above in the method described herein, the slope of the correlation line is only 0.3‰ per 100‰ range of the natural variation. It is most likely that oxygen isotopes are predominantly equilibrated with the excess of oxygen in the reagent, NaPO3. The reaction proceeds at temperatures above the melting point of the reagent (610 °C). This temperature is sufficiently high to minimize the value of isotope fractionation between NaPO3 and SO2 gas and promote fast isotope exchange. For the excess of the reagent, as described in the procedure, the SO2 product will have δ18O nearly totally adjusted to that of the NaPO3. For this reason, we have no conditions that allow a memory effect due to an oxygen isotope exchange between hot parts of the quartz glass envelope and the SO2 product. To further diminish the influence of δ18O variation in BaSO4, one may use a larger excess of NaPO3 reagent. We have found that at double the amount of reagent (120 mg of NaPO3/10 mg of BaSO4), the influence of the high content of isotope18O in BaSO4 is negligible (see Table 2). Other advantages of the novel method are as follows: (1) the method is suitable for small-size samples because it excludes an admixture of sulfur from the reagent, (2) SO3 is reduced quantatively to SO2 in the copper boat during reaction, (3) neither H2O nor CO2 is produced, hence no time-consuming purification is needed, (4) reaction time is short, (5) the method is highly economical, in terms of materials and energy consumption, and
(2)
(12) Mizutani, Y. Geochem. J. 1971, 5, 69-77. (13) Thode, H. G.; Macnamara, J.; Collins C. B. Can. J. Res. 1949, B27, 361373.
3256 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
(3)
(14) Wasserburg, G. J.; Jacobsen, S B.; DePaolo, D. J.; McCulloch, M. T.; Wen, T. Geochim. Cosmochim. Acta 1981, 45, 2311-2323. (15) Makishima, A.; Shimizu, H.; Masuda, A. Mass Spectrosc. 1987, 35, 64. (16) Ding, T.; Valkiers, S.; Kipphardt, H.; Taylor, P. D. P.; De Bievre, P.; Gonfiantini, R. Chin. Sci. Bull. Suppl. 1998, 43, 33 (Abstr. of ICOG-9).
(6) the method is safe because no dangereous chemicals are used or need disposal; moreover, copper may be collected for recycling. ACKNOWLEDGMENT We are grateful to Dr. Harro Meijer and Dr. Henk Visser, University of Groningen, for kindly supplying a sample of H218O. Thanks are due to Dr. Tyler Coplen from the U.S. Geological Survey, Reston, VA, for numerous corrections in the draft of this
paper. We thank Mr. Jan Szaran for editing the manuscript. This study was supported by State Committee for Scientific Research, Warsaw, Grant 6 PO4D 014 14.
Received for review January 12, 1999. Accepted May 5, 1999. AC9900174
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