Quantitative Production of H2 by Pyrolysis of Gas Chromatographic

Anal. Chem. 1998, 70, 5136-5141

Quantitative Production of H2 by Pyrolysis of Gas Chromatographic Effluents Thomas W. Burgoyne†

Biogeochemical Laboratories, Departments of Chemistry and Geological Sciences, Indiana University, Bloomington, Indiana 47405 John M. Hayes*

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Hydrogen gas can be produced quantitatively from nanomole amounts of organic H in continuously flowing gas streams. The system described here is suitable for use in isotope-ratio-monitoring mass spectrometric systems and is based on a pyrolysis reactor consisting of a graphitized alumina tube heated to 1450 °C. Methane forms as an intermediate product at temperatures above 750 °C, but, for all tested analytes, yields of H2 were quantitative at temperatures between 1430 and 1460 °C, provided residence times in the reactor were greater than 300 ms. Quantitative yields of H2 were obtained for all components of a homologous series of n-alkanes (C15 to C30). Analyses of low-molecular-weight alcohols demonstrated that O-bound H was also quantitatively converted to H2 and, thus, that H2O, if formed, was quantitatively reduced to H2. On-line analyses of the hydrogen isotopic compositions of organic compounds in gas chromatographic effluents will require a method for the quantitative production of H2 from organic H in the continuously flowing gas stream. Two approaches can be envisioned: (i) combustion, followed by reduction of the H2O in order to produce H2, and (ii) use of pyrolytic or catalytic techniques in order to produce H2 in a single step. The first approach can draw on the extensive body of techniques that have evolved for the analysis of H2O,1-6 but any coupling of combustion and reduction reactors (e.g., ref 7) is inherently problematic, since O2 usually bleeds continuously from the combustion reactor and consumes resources in the reduction reactor. Accordingly, analysts attacking this problem are increasingly turning to the second approach (e.g., refs 8 and 9). * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Department of Chemistry, Indiana University, Bloomington, IN 47405. E-mail: [email protected]. (1) Schimmelmann, A.; DeNiro, M. J. Anal. Chem. 1989, 65, 789-792. (2) Vennemann, T. W.; O’Neil, J. R. Chem. Geol. 1993, 103, 227-234. (3) Bigeleisen, J.; Perlman, M. L.; Prosser, H. C. Anal. Chem. 1952, 24, 13561357. (4) Harting, P. Isotopenpraxis 1989, 25, 347-348. (5) Gehre, M.; Hoefling, R.; Kowski, P.; Strauch, G. Anal. Chem. 1996, 68, 4414-4417. (6) Prosser, S. J.; Scrimgeour, C. M. Anal. Chem. 1995, 67, 1992-1997. (7) Tobias, H. J.; Brenna, J. T. Anal. Chem. 1996, 68, 3002-3007.

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In fact, the first description of the production of H2 using U as a reductant3 referred to the “conversion of hydrogenic materials [water being only one example] to H2” and described the pyrolytic conversion, with a yield of 96%, of the H in ethanol or propane to H2. More recently, Harting4 has described the production of H2 from methane, ethane, or heptane by pyrolysis on Cr at 1027 °C and shown that accurate and precise ((2‰) measurements of δD can be based on this procedure. These pyrolytic reactions also yield metal carbides. The effectiveness of metal-based pyrolytic reactors over extended periods during which all surface sites might be converted to carbides is unknown. At still higher temperatures, high yields of H2 can be obtained by pyrolysis of hydrocarbons in the absence of any metallic catalyst or carbon-consuming reactant. The potential of this procedure for use in hydrogen isotopic analyses was examined by Sofer and Schiefelbein,10 who pointed out that incomplete yields of H2 result mainly from the formation of CH4, which is itself extremely stable, and who thus focused on the equilibrium

CH4 a C + 2H2

(1)

Values of KP exceed 1000 at temperatures greater than 1430 °C,11 and conversion of CH4 to H2 would exceed 99.9% for PCH4 e 10-3 atm. The same conclusion follows more generally from the calculations of Duff and Bauer,12 who considered all molecular and solid species in the C-H system and who, for T ) 1430 °C, found XH2/XCH4 g 103. This was true even for C/H ) 103, the large amount of C being considered in order to account for the presence of solid C in the system. Amounts of H in all other species were 1000-fold lower than those in CH4. Using sealed quartz tubes and thus being limited to relatively low temperatures (1140 °C) and high pressures (0.5 atm), Sofer and Schiefelbein10 were nevertheless able to obtain a standard deviation of 3‰ in measurements of δD for the C15+ aliphatic fractions of crude oils. (8) Tobias, H. J.; Brenna, J. T. Anal. Chem. 1997, 69, 3148-3152. (9) Begley, I. S.; Scrimgeour, C. M. Anal. Chem. 1997, 69, 1530-1535. (10) Sofer, Z.; Schiefelbein, C. F. Anal. Chem. 1986, 58, 2033-2036. (11) Gulbransen, E. A.; Andrews, K. F. Ind. Eng. Chem. 1952, 44, 1034-1038. (12) Duff, R. E.; Bauer, S. H. J. Chem. Phys. 1962, 36, 1754-1767. 10.1021/ac980248v CCC: $15.00

© 1998 American Chemical Society Published on Web 11/06/1998

Figure 1. Instrument setup showing the three modes of sample input: (A′ and A′′) continuous flow, (B) peaks generated via eight-port valve, and (C) gas chromatography. Restrictor 1 is a 25-µm-i.d. fused-silica capillary, 130 cm long, and provides flows ranging between 1 and 10 µL/min. Restrictor 2 is a 320-µm-i.d. capillary column, 50 m long, and provides flows of from 0.5 to 3 mL/min. Restrictor 3 is a 110-µm-i.d. fused-silica capillary, 2 m long, and provides flows of 0.5 to 3 mL/min. The sample loops on the eight-port valve have volumes of 50 µL.

Studying the kinetics and mechanism of the pyrolysis of CH4, Arutyunov et al.13 showed that ethane, ethylene, and acetylene form as intermediates but that H2 is the dominant gaseous product above 927 °C. Summarizing their own and a wide variety of results obtained by others, these authors concluded that the first-order rate constant for pyrolysis of methane at temperatures ranging from 830 to 1430 °C is given by k ) (3 × 1012) exp(-343 000/ RT) s-1, where R ) 8.314 J/(mol‚K). At 1430 °C, therefore, k ) 87 s-1, and the half-life of CH4 is 8 ms. In fact, this was observed directly by Kozlov and Knorre,14 who examined the thermal decomposition of CH4 in an Ar/CH4 gas stream at 1430 °C and 1 atm pressure (i.e., under conditions closely comparable to those which would prevail in a reactor installed downstream from a chromatographic column). We have concluded that the generation of H2 by pyrolysis of organic matter and the subsequent mass spectrometric measurement of δD from differential comparison of ion currents from “sample” and “standard” gases provides the best means of converting organic H to H2 for isotopic analysis. Here, we examine the practicality of this concept. EXPERIMENTAL SECTION The layout of the experimental system is shown in Figure 1. Gases were introduced to the pyrolytic reactor from any of three sample sources. In sources A′ and A′′, a continuous flow of He mixed with either propane (to examine pyrolytic production of (13) Arutyunov, V. S.; Vedeneev, V. I.; Moshkina, R. I.; Ushakov, V. A. Kinet. Catal. 1991, 32, 234(14) Kozlov, G. I.; Knorre, V. G. Combust. Flame 1962, 6, 253-263.

H2) or hydrogen (to calibrate instrument response) was sent to the reactor. In source B, peaks of propane or H2 were added to a continuous flow of He. The size of each peak could be regulated by adjusting the pressure of the H2 or propane source, thus varying the flow of H2 or propane mixed into the He upstream from the eight-port sampling valve. In source C, a gas chromatograph was used to produce peaks of H-bearing organic molecules in He carrier gas. For compactness in presentation, Figure 1 depicts all three modes of sample introduction, though only one was used at any given time. In the continuous-flow mode (A′, Figure 1), propane flowing at 1-10 µL/min was mixed with He flows ranging from 0.5 to 2 mL/min. Alternatively (A′′, Figure 1), a continuous flow of 50 ppm of propane in He or 200 ppm of H2 in He (Indiana Oxygen, Indianapolis, IN) was used. In source A′′, a 0-10 cm3/min Brooks thermal mass flow controller (accuracy ) (1% full scale, Brooks Instrument Division, Hatfield, PA) was used to regulate flow into the reactor. To generate peaks (B, Figure 1), streams of pure He and He/propane were attached to an eight-port valve with two 50-µL sample loops. Both of these sample introduction methods used a fused-silica capillary (0.22 µm i.d.) to carry the sample gas to the reactor. A gas chromatograph (Shimadzu GC-9A I, Shimadzu Corp., Kyoto, Japan) was used as a final method of sample introduction (C, Figure 1). It was equipped with an open split injection port (split ratio 20:1) and was fitted with a Hewlett-Packard Ultra-1 capillary column (Hewlett-Packard, Palo Alto, CA). The gas chromatographic effluent was carried to the reactor through a Analytical Chemistry, Vol. 70, No. 24, December 15, 1998

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heated, fused-silica capillary (0.22 µm i.d., approximately 10 cm long). The gas stream from any one of the three sample introduction systems was led to a graphitized alumina tube (see below), 0.5 mm i.d., 3 mm o.d., and 305 mm long (Bolt Technical Ceramics, Conroe, TX), using a Valco 1/8-in. to 1/32-in. reducing union (Valco, Houston, TX). This alumina tube was placed in an insulated furnace in order to provide a convenient pyrolysis reactor. The heating element for the furnace is a Globar, a silicon carbide electrical heating element (Cesiwid, Niagara Falls, NY). The particular heating element used was 1.27 cm o.d., 25.4 cm long, with a heated-zone length of 17 cm. The Globar was powered with a high-current (22 A) autotransformer (Powerstat, Bristol, CT). The Globar and alumina tube were placed in a 1-in.-i.d. alumina tube and encased in firebrick for insulation (not shown in Figure 1). The temperature was monitored with a type B thermocouple. The maximum measured temperature obtained with the Globar was 1550 °C, with the autotransformer set to deliver 25% of the 110-V line voltage. Analytes are pyrolyzed within the heated alumina tube. Better peak shapes and reproducibilities are obtained after a basal layer of graphite has been deposited inside the tube, covering the aluminum oxide surface. Accordingly, tubes to be used as reactors were graphitized by passing a flow of pure propane (approximately 10 mL/min) through the tube while heating it with an oxygen-methane flame. Starting at the inlet, the tube is heated until glowing and then, over a period of 2 min, the torch is steadily moved toward the outlet end. This heating process is then repeated with He flowing through the alumina tube in order to remove volatile products resulting from incomplete pyrolysis. At the downstream end of the reactor, a fused-silica capillary was coupled to the alumina tube in order to transfer the He and pyrolysis products to the open split. The open split, in which the capillary leak leading to the mass spectrometer was inserted into the bore of the capillary leading from the pyrolysis reactor, was housed in a Swagelok tee (Swagelok Co., Solon, OH), with the side port providing a vent for the excess gas that could not be accepted by the mass spectrometer. The flow of the gas into the mass spectrometer was limited by either a 280-cm-long, 25-µmi.d. fused-silica capillary that transmitted 80 µL/min (Delta E, see below for mass spectrometer details) or a 200-cm-long, 110-µmi.d. fused-silica capillary that provided an approximate flow rate of 200 µL/min (MAT 252). Yields of H2 were determined using Delta E or MAT 252 mass spectrometers (Finnigan MAT, Bremen, Germany). For the Delta E, a special inlet line and valve were added to the standard instrument in order to allow admission of the gas stream from the open split. The mass spectrometer, which was operated at 3000 V accelerating potential, has two sets of collectors. One, at short ion-orbit radii, allows collection of masses 2 and 3 (for isotopic analysis of H2); the other allows collection of masses 44, 45, and 46, and is normally used for the isotopic analysis of CO2. Faraday cups and feedback electrometers are used for ion collection and signal conversion. For detection and quantitation of H2, the mass 2 collector was used with a feedback resistor of 3 × 1010 Ω (different from the normally installed value of 1 × 109 Ω, the higher resistance providing adequate signals from the traces of H2 present in the relative large flow of He). For detection 5138 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998

Figure 2. Ion currents representative of propane (m/z 44), methane (m/z 16), and hydrogen (m/z 2) as a function of furnace temperature.

and quantitation of propane (MW ) 44), its molecular-ion beam was directed to the collector normally used for either mass 45 or mass 46 during analysis of CO2, thus providing feedback resistances of 3 × 1010 or 1 × 1011 Ω. The 252 mass spectrometer, which operates at 10 kV accelerating potential, was used unmodified. Unless indicated, experiments were conducted using the Delta E mass spectrometer. Ion current and thermocouple signals were recorded with an ADB/IO (an 8-bit A/D board, BeeHive Technologies, Pasadena, CA) or a DAQCard-1200 (National Instruments, Austin, TX) on a Powerbook 160 or Powerbook 5300c (Apple Computer, Cupertino, CA) using Igor Pro Software (WaveMetrics, Lake Oswego, OR). Peak areas were additionally measured with an HP3380S integrator (Hewlett-Packard). RESULTS AND DISCUSSION Temperature and Residence Time. To determine the optimal temperature for production of H2 by pyrolysis of organic compounds, a steady stream of helium containing 64 ppmv propane was introduced into the heated alumina tube (see A′, Figure 1). From a maximum temperature of approximately 1500 °C, the furnace was slowly cooled while both the temperature and selected ion currents were recorded. Results of three successive experiments in which masses 44 (representing propane), 16 (methane), and 2 (H2) were monitored are summarized in Figure 2. As shown, removal of propane or, more specifically, attenuation of species generating m/z 44, is not complete until 900 °C. Interestingly, production of methane (as an intermediate product) becomes significant at this same temperature and maximizes at about 1000 °C. The hydrogen signal maximizes at 1430 °C, just as the methane signal disappears. For all later experiments, the temperature of the furnace was held between 1430 and 1460 °C. The mass-2 signal decreases at temperatures above 1470 °C. When the furnace was held at temperatures near 1500 °C, small fractures (visible after cooling) formed in the alumina tube, with attendant increases in ion currents associated with water, nitrogen, and oxygen. Alumina tubes are unusable after such fractures have formed. It seems likely that porosity begins to develop at 1470 °C and that this accounts for the loss of signal above that

Table 1. Pyrolysis Mass 2 Signal of Propane (Normalized to Hydrogen Signal) for Variable Gas Flows

Figure 3. Schematic diagram of gas flows in an open split, where (Q1 + Q2) is the total input, Q2 is the flow accepted by the mass spectrometer, Pa is atmospheric pressure, and ∆P is a finite pressure difference ()Ps - Pa) between the inlet of the capillary leading to the mass spectrometer and the outlet of the larger-bore capillary. See text for details.

temperature. It also seemed possible that this phenomenon might be obscuring an optimum pyrolysis temperature above 1500 °C. Further evidence, discussed below, suggests that this is not the case. In principle, the minimal reaction time (or residence time in the reactor) allowing quantitative production of H2 could be determined simply by supplying a mixture of C3H8 and He to the reactor, varying the flow rate (and thus the residence time), and noting the point at which the yield of H2 decreased. The observation is, however, complicated by two additional factors. First, the observed ion currents must be corrected for variations in the split ratio which occur as the flow rate is changed. Second, the split ratio itself must be measured at each flow rate because the characteristics of the split are flow-rate-dependent. Specifically, as shown in Figure 3, neither the split ratio [defined here as Q2/(Q1 + Q2)] nor Q2 will remain constant as Q1 + Q2, the total throughput, increased. The split operates nonideally because there is a finite, and flowrate-dependent, pressure difference between the vent of the open split and the inlet of the capillary leading to the mass spectrometer. Flow Q1 is limited by the conductance of the annular space between the inner and outer capillaries and by any restrictions between the end of the larger capillary and the vent, which is at atmospheric pressure. Flow Q2 is limited by the conductance of the capillary leading to the mass spectrometer. For any limiting conductance, Q ) G(Pi - Po), where Q is the throughput (TorrL/s), G is the conductance (L/s), and Pi - Po is the pressure difference (Torr) between the inlet and the outlet. Under conditions of laminar flow, G is proportional to the average pressure within the limiting conductance. If this is approximated by (Pi + Po)/2, an estimate of the characteristics of the system is provided by

Q ) K(Pi2 - Po2)/2

(2)

where K is a constant dependent on the size and shape of the flow pathway. Here, for Q1 (as defined in Figure 3), Pi ) Pa + ∆p, where Pa is the atmospheric pressure, and Po ) Pa. For Q2, Pi ) Pa + ∆p, and Po, the pressure within the mass spectrometer, is effectively zero. Insertion of these values in eq 2 yields the relative flow rates shown in Figure 3. The dimensions of the capillaries must be chosen such that the desired flow rate of gas reaches the mass spectrometer and such that the conductance of the annulus formed when one capillary is inserted into the other is small enough to prevent back diffusion of atmospheric gases at the lowest flow rates. That

gas flow (mL/min)

residence time (s)

C3H8/H2 signal ratio

0.25 0.50 0.75 1.00 1.05 1.075 1.09 1.10 1.11 1.25 1.75 2.00 2.50 3.00 4.00 5.00

1.222 0.611 0.407 0.306 0.291 0.284 0.280 0.278 0.275 0.244 0.175 0.153 0.122 0.102 0.076 0.061

1.064 0.988 1.074 1.018 0.999 0.991 1.008 0.595 0.548 0.540 0.467 0.527 0.476 0.479 0.491 0.480

conductance determines the magnitude of ∆p. That pressure difference cannot be measured conveniently, but calculations based on eq 2 show that, if capillary dimensions are chosen such that ∆p ) 10 Torr at the lowest flow rates (i.e., 0.25 mL/min, see Table 1), ∆p will rise to 250 Torr at the maximal flow rate of 5 mL/min. This wide range of flow rates is required in order to cover a significant range of residence times for samples passing through the pyrolysis reactor. If the low-flow ∆p is 30 Torr, that at 5 mL/min will approximate 600 Torr. As a result of such changes, both the conductance and the throughput of the capillary leading to the mass spectrometer will increase significantly. To overcome this problem, measurements of the relationship between yields of H2 and reactor residence times have been made differentially. For each entry in Table 1, we report in column 3 the H2 signal resulting from the pyrolysis of 50 ppm C3H8 relative to the signal resulting from the introduction of He + 200 ppm H2 at the same flow rate. This differential comparison neutralizes the effects of variations in ∆p. A signal ratio of 1.00 thus corresponds to a 100% yield. The residence time of gas in the reactor was changed by changing flow rates from 0.25 to 5 mL/min using a mass flow controller. This resulted in residence times ranging from 1.2 to 0.06 s (calculated from measured flow rates and from the temperature-corrected volume of the hot reactor). To avoid any possible systematic errors associated with changes in characteristics of the reactor, gas flows were randomly set, and then the H2 (m/z ) 2) signals were alternately recorded for both hydrogen and propane gases. As shown in Figure 4, the signal from inputs of C3H8, expressed relative to that from H2, drops off dramatically at higher flow rates. For propane, quantitative yields of H2 are attained at residence times of 300 ms or more, corresponding to He flow rates of 1.1 ( 0.1 mL/min or less. Dynamic characteristics of the pyrolysis reactor were investigated by introducing peaks of propane and hydrogen using an eight-port valve (see B, Figure 1). Figure 5 shows signals resulting from the injection of 9.4 nmol of propane (A, m/z ) 44, reactor at room temperature; B, m/z ) 2, reactor at 1450 °C) or 23 nmol of H2 (C, m/z ) 2, reactor at 1450 °C). The He gas flow into the reactor was 1.3 ( 0.2 mL/min. Peak widths for the Analytical Chemistry, Vol. 70, No. 24, December 15, 1998

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Figure 4. Ion-current ratio representative of (H2 produced by pyrolysis of 50 ppm propane in He)/(200 ppm H2 in He) versus residence time in the pyrolysis reactor. Quantitative pyrolytic yields of H2 are obtained at residence times of approximately 300 ms or more (total gas flows of 1.1 mL/min or less). See Table 1 for details.

Figure 6. Comparisons of mass spectrometric and flame ionization detector signals for homologous n-alkanes. A y ) x line is drawn for reference. Yields are relative to those obtained from the C22 n-alkane, which served as a point of normalization. As explained in the text, the differing mechanisms of response (FID signal proportional to carbon number, mass spectrometric signal proportional to H2) have been taken into account in the normalization.

Figure 5. (A and B) Ion-current recordings obtained by injection of 9.4-nmol peaks of propane (see B, Figure 1): (A) m/z ) 44 (propane M+), furnace off; B, m/z ) 2 (H2+), furnace on, reactor at 1450 °C. (C) Mass 2 ion-current recording obtained by injection of a 23-nmol peak of H2 with the reactor at 1450 °C.

Figure 7. Hydrogen mass chromatogram for a mixture of lowmolecular-weight alcohols in hexane solvent. Amounts of the individual compounds correspond to 370-920 nmol of H2. Other peaks, not labeled in the chromatogram, are attributed to the mixture of isomeric hexanes and other hydrocarbons in the solvent.

propane and propane-derived H2 peaks were calculated by fitting a Gaussian curve to each peak. At 0.607 × peak height, the average propane peak width was 2.05 ( 0.3 s, and the average propane-derived H2 peak width was 2.04 ( 0.02 s. Tailing of the propane-derived H2 peaks was not significantly greater than that of the peaks resulting from the direct injection of H2. The noticeably greater tailing of the peaks resulting from the transmission of unreacted propane at 25 °C presumably reflects significant sorption of the propane on the graphitized walls of the reactor at that low temperature, far from normal operating conditions. Areas of the H2 peaks resulting from injection of propane were compared to those of peaks resulting from direct injection of H2 in order to examine the completeness of production of H2 under these dynamic conditions. Ten peak areas were measured for both propane and hydrogen sample gases and compared to the

respective molar quantities of analyte (determined from the ideal gas law, sample loop volume, and relative flow rates of helium and sample gas). Hydrogen and propane flow rates were 11.2 and 5.3 parts per thousand by volume, respectively. This experiment was conducted twice. Errors were determined from standard deviations of measured flow rates and peak areas. The peak area/mole ratio resulting from injection of H2 was (7.2 ( 0.2) × 108 V-s/mol of H2, and that resulting from injection of propane was (7.0 ( 0.1) × 108 V-s/mol of H2. The standard deviations (noted here as ( values) for these values result from propagation of errors of flow-rate and peak-area measurements. Accordingly, we conclude that the pyrolysis is complete and that yields of H2 are quantitative. Gas Chromatography. Experiments were conducted with a gas chromatograph coupled to the reactor (see C, Figure 1). First

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Table 2. Yields of H2 from Chromatographically Separated Alcohols relative yield of H2 compound

quantitative

if H2O lost

observed

yield (%)

methanol ethanol 2-propanol tert-butyl alcohol 1-propanol

0.754 1 1.497 1.117 0.603

0.995 1 1.684 1.340 0.678

0.772 1 1.463 1.192 0.609

102.4 100.0 97.7 106.7 101.0

used in the investigation was a C15 to C30 n-alkane mixture. Amounts injected range from 1 × 10-7 to 3 × 10-8 mol of n-alkane. Peak areas obtained during analysis of the n-alkane standard were tabulated from five FID and five mass spectrometer runs and then averaged. These areas were then normalized to the corresponding C22 peak area. Additionally, the signal ratios were corrected for the difference in mechanisms of response (that is, for n-alkanes, FID signals are proportional to carbon number, whereas the mass spectrometer signal is proportional to H2). As shown in Figure 6, all points lie close to the y ) x line, indicating that the yield of H2 from pyrolysis is quantitative. Chromatograms based on mass 2 ion current were also obtained for a mixture of low-molecular-weight alcohols (methanol, ethanol, 2-propanol, tert-butyl alcohol, and 1-propanol, see Figure 7) dissolved in mixed hexanes. Within this series of compounds, the ratio of C-bound H to O-bound H varies from 3 to 9. Accordingly, differences in the efficiency of production of H2 from

H bound to C and O should become apparent when the yields of H2 from these compounds are compared. Areas and measured amounts injected were normalized to ethanol and are shown in Table 2. The observed yields are consistent with the expected yields from quantitative conversion of both C-bound and O-bound H, indicating either that water is not formed in the reactor or that, if formed, it is reduced to yield H2 (and, presumably, CO). CONCLUSION A method has been developed for on-line preparation of H2 from nanomole quantities of organic H and for continuous monitoring of the abundance of H2 in the gas stream. Further work, showing that the method can be integrated with isotoperatio-monitoring mass spectrometry in order to provide accurate and precise isotopic analyses, will be reported in a separate manuscript. ACKNOWLEDGMENT We thank Arndt Schimmelmann and Alex Sessions for advice and Stephen Studley for assistance in maintaining the Delta E mass spectrometer. We also thank Prof. G. M. Hieftje for the loan of a gas chromatograph. Research support provided by NSF OCE9711284. Support for T.W.B. was provided by an NSF EarthSciences Postdoctoral Fellowship. This is Woods Hole Oceanographic Institution Contribution No. 9798. Received for review March 5, 1998. Accepted September 23, 1998. AC980248V

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