789
Anal. Chem. 1998, 65, 789-792
Preparation of Organic and Water Hydrogen for Stable Isotope Analysis: Effects Due to Reaction Vessels and Zinc Reagent Arndt Schimmelmann' University of California a t San Diego, Scripps Institution of Oceanography, La Jolla, California 92093-0215
Michael J. DeNiro Department of Geological Sciences, University of California, Santa Barbara, California 93106
Coralbwtlon of organic matter In u a k d Pyrex, Vycor, and quartz ampules at temperature8 between 520 and 900 "C ylelda krr than stolchknntrlc amount8 of water. The lorr of hydrogen to hydration reaction8 b8tween water vapor and gladquartz Interfereswith the dotennlnath of C/H and N/H elemental ratlor In organic matter. The effect Increasesfrom quartz to Vycor to Pyrex, but the Incomplete yield d m not sign~antiyaffect the precklon and accuracy of the determlnatbnof 8tabk hydrogenkotopt~ rat&$. ReacUonsbetween water and Pyrex do not affect the convenlon of water to hydrogen with zlnc In Pyrex ampules at 500 O C , whkh k quantitative,but even prooutgamed zlnc contains a deuterhm depleted hydrogen blank. D/H ratlor In hydrogen from the Zn method requirea nonHnearcorrectionto achievecompati,tlblllty wtth 60 values from the uranium method.
the loss of hydrogen to reactions of water with glasses at high temperatures. Our results show that an apparent hydrogen loss does not significantly affect the determination of the stable hydrogen isotope ratios in organic matter. The systematic artifact of decreased elemental H/C ratios, however, needs to be taken into account for mass-balance calculations,*for comparison with the "hydrogen indexwused in fossil fuel geochemistry,and for the calculation of the CHN elemental composition of combusted organic compounds. Demonstration of water reactions with glass led us to examine the effects of such reactions on the determination of hydrogen isotope ratios in water by the method in which water is reduced to elemental hydrogen by high-temperature reaction with zinc in Pyrex reaction vessels. Although hydrogen yields from this reaction are about quantitative, we found that the resulting D/H ratios need to be corrected for the presence of an isotopically light hydrogen blank in zinc.
INTRODUCTION
EXPERIMENTAL SECTION
The determination of stable hydrogen isotope ratios in organic matter requires the conversion of organic hydrogen to hydrogen gas (Hz) prior to isotopic analysis in a mass spectrometer. As a first step, organic material is usually combusted in evacuated and sealed quartz, Vycor (quartz with about 4% Pyrex mixed in), or Pyrex ampules, which are loaded additionally with copper(I1) oxide, copper, and silver foil.lP2 After the resulting water is separated from the combustion products COZand Nz on a vacuum line, the water is reduced to elemental hydrogen in contact with hot uranium or zinc.3-6 Manometric quantification of the gases Hz, COZ, and Nz is frequently used for the calculation of elemental ratios in the original organic compound based on the assumption of 100% yields. Low hydrogen yields from sealed-ampule combustion of organic compounds with known stoichiometry has been observed earlier,6J but without evidence for isotope fractionation. A need for further investigation arose when a new method for controlled isotopic equilibration of exchangeable hydrogens required the knowledge of accurate and reproducible hydrogen yields for mass-balance calculations. Here we report the results of a series of experiments that ascribes
Reagents. Water standards SMOW (from the International Atomic Energy Agency, IAEA), GISP, and SLAP of known hydrogen isotopic compositionwere obtained from the National Institute of Standardsand Technology (NIST;formerlyNational Bureau of Standards, NBS). Laboratory water standards were calibrated against SMOW. Cellulose nitrate was prepared according to standard techniques9 from a-cellulose purchased from Sigma Chemical Co. Other materials include granular copper (Baker),copper(II)oxidewire (Fisher),silver foil (Aldrich), and tubing of Pyrex (Corning), Vycor (Corning), and quartz (Quartzand Borosilicate Instnunenta Corp., Santa Ana, CA) with outer diameters (0.d.) of 9 and 6 mm and inner diameters of 7 and 4 mm, respectively. Zinc was purchased from the Biogeochemical Laboratories, Departments of Geology and Chemistry, Indiana University, Bloomington, IN 47405-5101. Procedure. Three types of studies were done. In the first, cellulose nitrate was combusted in Pyrex, Vycor, or quartz ampules. In these studies, weighed aliquota of cellulose nitrate were loaded into 9-cm-long ampules 9-mm 0.d. together with 1 g of copper(I1) oxide wire, 1g of granular copper, and a piece of silver foil 10 mm X 4 mm X 0.025 mm. In the second type of study, water was heated inside Pyrex, Vycor, or quartz ampules. For these studies, capillaries prepared by heating and stretching 9-mm-0.d.tubing of the same material as the ampule were sized to hold 2-6 mg of water. One end of each capillary was "Z"shaped to facilitate ita fracturing inside evacuated and sealed ampules before heating. The weight of the londsd water was determined with an accuracy of fl X 10-6 g by subtracting the weight of a preweighed, empty capillary from the final totalweight of the loaded capillary together with the sealed-off dried ends, using a Perkin-Elmer AD-6-autobalance. The "Z"-shaped end of each loaded capillary was first inserted into a 1.5-cm-long, 6-mm-0.d. Pyrex, quartz, or Vycor tube ("hammer", when unit is inverted) which was closed at ita bottom. Such a unit was then
~~~
* To whom correspondence should be addressed.
(1) Stump, R. K.; Frazer, J. W. Nucl. Sci. Ab&. 1973,28, 746. (2) Northfelt, Donald, W.; DeNiro, Michael J., Epstein, Samuel Geochim. Coemochim. Acta 1981,45, 18961898. (3) Bigeleieen,Jacob;Perlman, M. L.; Proeeer,H. C. Anal. Chem. 1952,
24. ---1R.M-1367. --7
(4) Sudzuki, Norio Geochem. J. 1987,21,29-33. (5) Tanweer,A.; Hut, G.; Burgman, J. 0. Chem. Geol. 1988,73,199203. (6) DeNiro, Michael J. Earth Planet. Sci. Lett. 1981,54, 177-185. ( 7 ) Schimmelmann,Amdt; DeNiro, Michael J. Geochim. Cosmochim. Acta 1986,50,148&1496. (8) Schimmelmann,Arndt Anal. Chem. 1991,63,2456-2459. 0003-2700/83/0365-0789$04.00/0
(9) Sternberg,LeonelPlantFibers; ModernMethodsofPlnuthalysis, New Series; Springer: New York, 1989; Vol. 10, pp 89-99. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993
Table I. Combustion of Cellulose Nitrate for 3 h at 520 "C in AmDules with 1 I[ of CuO (nd = Not Determined)
Pyrex Pyrex Pyrex Pyrex mean: Vycor Vycor Vycor Vycor mean: quartz quartz quartz quartz mean:
101.6 100.5 101.5 101.6 101.3 100.4 101.8 101.6 101.0 101.2 100.5 100.9 101.4 100.7 100.9
101.9 101.0 101.1 102.1 101.5 100.1 100.8 100.3 101.2 100.6 99.4 101.5 99.9 99.0 100.0
90.9 90.6 91.6 91.9 91.3 93.4 95.2 93.5 93.0 93.8 96.3 97.0 97.0 97.2 96.9
1.99 1.99 2.01 1.99 2.00 2.01 2.02 2.02 2.00 2.01 2.02 1.99 2.03 2.03 2.02
1.04 1.05 1.05 1.06 1.05 1.09 1.09 1.07 1.07 1.08 1.12 1.12 1.12 1.13 1.12
-63.1 -61.8 -60.0 -58.9 -61.0f 1.6 -63.9 -60.0 -60.5 -61.5 -61.5f 1.5 -61.0
nd -61.3 -62.4 -61.6i 0.6
loaded into an 9-mm-0.d.ampule. All loaded ampules containing cellulose nitrate or capillaries were evacuated on a vacuum line for 2 h while being immersed in boiling water and were finally sealed under vacuum, forming break-seals. The ampules containing the capillaries were then tapped in inverted position so as to break the "Z"-shaped ends of the capillaries by the fall of the hammers inside of the evacuated amples. We avoided explosion of loaded, previouslyunbroken capillariesin the course of heating,because hydrothermal, supracritical conditions inside of the heated capillary before explosion might have caused rections between water and capillary that are irrelevant to, and may interfere with, the focus of this study. The subsequent heating of the samples, and the conversion of water to Hz,followed routine methods.lV2 Engel and Maynard'slo observation regarding the possibilityof carbon isotope fractionation within combusted ampules after cooling is likely to be valid for hydrogen as well, due to the formation of hydrated sulfateell and hydrated copper carbonates.1° In the third series of studies, we weighed aliquots of zinc into 20-cm-long6-mm-0.d.Pyrex ampules. Smallamounts of annealed quartz wool were placed 3-4 cm above the zinc, to support watercontaining capillaries with "Z"-shaped ends and annealed 2-cmlong solid glass rods (0.d. 2 mm) 4-5 cm above the zinc. The ampules received "Z"-shaped constrictions 14 cm above their bottoms and were attached to a vacuum line. Depending on the protocol, either an entire ampule was immersed in boiling water bath and evacuated for 1h or the zinc was selectivelyoutgassed at 350 O C for 5 or 30 min using an electric resistance furnace around the portion of the ampule containing the zinc. Selective outgassing of the Zn in the lower portion of an ampule was accompanied by heating the upper portion of the ampule (containing the water sample) to approximately 100 O C with a heat gun. Shaking of the sealed ampules let the glass rods crush the capillariesand evenlydistribute the zinc over the entire length of the ampules. The samples were then placed horizontally into a 500 O C oven for 1 h. All samples were processed on a vacuum line within hours (for experiments with capillaries) to 2 days (for experiments with cellulose nitrate) following combustion/reduction. Gas yields were measured manometrically. Isotopic results are expressed in bDs~ownotation in per mil referring to standard mean ocean water (SMOW) isotope standard prepared by the uranium method: bDsMow(per mil) = lo3 SMOW
The mass-spectrometric precision of the measurements is f2 per mil for bDs~owvalues. (10) Engel, M. H.; Maynard,R. J. Anal. Chem. 1989,61, 1996-1998. (11) Krishnamurthy,R.V.; DeNiro, Michael J. Anal. Chem. 1982,54,
153-154.
Table 11. Heating of Water Standard (6D= -183 per mil) in Ampules for 5 h at 520 OC, Compared with Processing at Room Temperature hydrogen yield typeof temp samplewt % of 6D ampule ("C) (mg) rmol theory (per mil) Pyrex 520 2.67 130.0 87.7 -181.8 Pyrex 520 2.68 130.8 87.9 -182.9 Pyrex 520 2.09 102.2 88.1 -178.4 520 1.75 83.9 86.4 -179.4 Pyrex mean: 87.5 -180.6 1.8 Vycor 520 2.98 151.6 91.6 -183.8 Vycor 520 2.26 115.8 92.3 -184.0 Vycor 520 1.84 91.3 89.4 -181.5 520 2.46 124.4 91.1 -183.2 Vycor mean 91.1 -183.1 i 1.0 quartz 520 2.56 133.4 93.8 -185.1 520 2.60 138.8 96.2 quartz -184.8 520 2.32 124.0 quartz 96.3 -180.7 520 2.37 quartz 96.4 126.9 -184.4 mean 95.7 -183.7 f 1.8 25 1.29 Pyrex 70.4 98.3 -184.6 Pyrex 25 1.53 84.0 98.9 -180.7 Pyrex 25 1.80 100.0 100.1 -184.8 quartz 25 3.05 170.8 100.9 -180.0 mean 99.5 -182.5 f 2.2
*
Table 111. Heating of Water Standards in Ampules for 5 h at 700 and 900 OC type of additives temp meanyield ampule in ampule ("0 (%)" n Vycor 700 88.7 2 Vycor Cu,CuO,Ag 700 89.8 2 quartz 700 91.5 3 quartz 900 89.5 4 Mean hydrogen yield in % of theory.
RESULTS AND DISCUSSION Combustion of 12 aliquota of cellulose nitrate in quartz, Vycor, and Pyrex ampules for 3 h at 520 'C resulted invariably in lower than theoretical hydrogen yields, decreasing from quartz(96,9%),overVycor (93.8%)toPyrex(91.3%) (Table I). The respective H/C ratios of 1.05, 1.08,and 1.12 are all smaller than the theoretical one of 1.17. Only about 96% of the hydroxyl groups in cellulose can be nitrated, thus increasing the theoretical H/C ratio to about 1.19.618 The stable isotope ratios of the hydrogen samples that were obtained from combusted aliquota of cellulose nitrate agree closely, within the precision of our isotopic measurements (Table I). The choice of Pyrex, Vycor, or quartz ampules for combustion of cellulose nitrate at 520 O C therefore does not introduce any significant isotopic variance. We caution, however, that other organic substrates that are less labile than cellulose nitrate require temperatures higher than 520 "C for complete combustion. Pyrex ampules cannot be used above 560 OC. We tested for isotope fractionation between hydrogen in an uncombusted sampleand the recoverable water hydrogen after combustion by heating known amounts of water isotope standards in different types of ampules at various temperatures for 5 h before determining the yield and the isotopic composition of the water hydrogen. Stoichiometric yields and accurate isotope ratios observed in four initial experiments of "combustion at room temperature (25 "C)" (Table 11) assured us that our technique of sealing water in "Z"-shaped capillaries, without subsequent heating, does not introduce analytical artifacta.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, lQQ3 791
Table IV. Conversion of Water Standards to Hydrogen Gas in Pyrex Ampules Heated at 500 O C for 1 h Using Zinc (Preoutgassedin Vacuo at 350 "C, 5 min) type and 6Da (per mil) of water used lab EM;+286 lab std;+286
SMOW; 10.0 SMOW; tO.0 SMOW; tO.0 lab sM; -87.0 lab std; -87.0 lab std;-87.0 GISP; -189.7 GISP; -189.7 GISP; -189.7 SLAP; -428 SLAP; -428 SLAP; -428
amt of zinc ( m d
amt of water (mg)
48.6 52.1 51.2 50.3 49.0 47.8 46.4 49.8 102.9 203.0 405.0 51.4 102.3 203.3
1.02 2.11 2.24 2.53 2.24 0.69 1.08 1.11 1.02 1.14 4.16 3.60 2.25 2.47
hydrogen yield pmol % of theory
57.2 117.0 125.3 138.9 123.1 38.5 59.7 61.0 55.6 62.6 225.9 201.4 123.8 135.9
mean a
100.7 100.0 100.8 98.9 99.0 101.1 100.1 98.8 98.1 98.9 97.8 100.8 99.1 99.1 99.5t 1.1
6Da of Hz (per mil) +272.0 +273.9 -7.4 -9.1 -8.0 -95.7 -95.5 -94.2 -193.0 -194.3 -197.6 -425.3 -426.3 -426.1
isotopic difference Zn va U (per mil)
-14.0 -12.1 -7.4 -9.1 -8.0 -8.7 -8.5 -7.2 -3.2 -4.5 -7.8 +2.7 +1.7 +1.9
6D values are given relative to SMOW (prepared by the uranium method).
Heating of water at 520 "C in ampules invariably caused decreased hydrogen yields, with Pyrex showing the lowest yields, followed by Vycor and by quartz (Table11). Additional experiments showed that, at 700 OC, the yield from quartz (91.3 f 1.17% ,n = 2) was slightly better than from Vycor (89.1 f 0.6%, n = 2; Table 111). Increasingthe temperature to 900 OC slightly decreased the yield from quartz to 89.5 % (Table
111). Experiments with water heated in Vycor at 700 OC, with and without the presence of copper, copper(I1) oxide, and silver foil in ampules, resulted in no differences of yields, indicating that the decreased yield of hydrogen is not caused by reactions of water vapor with non-siliceous additives (Table 111). Loss of water via diffusion through the walls of ampules can be excluded on the basis of our observation that heating of 3 mg of water for 30 h a t 800 "C in asealed 6-mm-0.d. Vycor ampule inside of an evacuated and sealed 9-mm-0.d. Vycor ampule did not render any measureable amount of water hydrogen in the interspace inside of the 9-mm ampule. The diffusion of elemental hydrogen in glass and quartz at high temperatures12J3 cannot account for the loss, because of the low partial pressure of Hz in the equilibrium 2H2 + 02 = 2Hz0 in the presence of excess elemental oxygen released from copper(I1) oxide at high temperature. Water can be lost by the formation of a hydrated siliceous layer at the inner surface of the ampule, as described for silicate glasses at lower than criticaltemperature.14 Our yield data suggest that hydration decreases in the order Pyrex Vycor quartz and that quartz is therefore best suited as the ampule material. The use of quartz ampules is also recommended for combustion of organic materials, which, with few exceptions (most notably cellulose nitrate), require heating at temperature of at least 700 OC to effect complete combustion of the sample to COZ,Nz, and HzO. The complete hydrogen yields from reactions of zinc with water indicate that the reduction of water by Zn (proceeding fast at temperatures significantly below 500 "C'5) is kinetically favored over hydration reactions of siliceous phases in Pyrex (Table IV). Using Zn in amounts larger than 50 mg does not improve the yield. Previous workers reported that the pretreatment of zinc is crucial for the size of the hydrogen blank in zinc but did not indicate the quantity or isotopic composition of the blank
-
(12) Diffus. Data 1969,3, 476; 1970,4, 239-240; 1971,5,89-90. (13) Hartley, P. E. Anal. Chem. 1980,52, 2232-2234. (14) Petit, J.-C.;Della Mea, G.;Dran,J . 4 . ; Magonthier,M.-C.;Mando, P. A.; Paccagnella, A. Geochim, Cosmochim. Acta 1990,54,1941-1955. (15) Graff, J.; Rittenberg, D. Anal. Chem. 1952, 24, 878-881.
(see refs 4 and 16-19 and references therein). A separate batch of zinc was leached briefly in 30 % nitric acid (toremove the oxide crust),rinsed with deionized water, and dried under vacuum. In spite of additionally drying this zinc at 100 "C in vacuo prior to sealing 0.5-1-g aliquots in Pyrex ampules, subsequent heating to 450 "C for 1h liberated up to 44 pmol of Hdg of Zn, with 6D values ranging between -163 and -229 per mil. Without the leach in nitric acid, the hydrogen blank was reduced to 3 pmol of hydrogen/g of Zn, with a 6D value of -281 per mil. The use of zinc with significant hydrogen blanks characterized by very negative 6D values resulted in poor reproducibility of isotopic results when used for conversion of water to hydrogen. The recommended preparation procedure for zinc avoids the leaching step but mandates outgassing of Zn in vacuo at 350 "C for 5 min (Fong, personal communication, 1992). Heating of such properly outgassed Zn to 500 "C for 1 h liberates less than 0.5 pmol of Hdg of Zn. Extending the preheating phase to 30 min further reduces the blank to below 0.2 pmol/g of Zn. In spite of the small size of Hz blanks, even in 50 mg of Zn, the extreme deuterium depletion has a noticeableinfluence on measured 6D values of reduced waters (Table IV). The isotopic shift in the direction of a hydrogen blank 6D value of -200 to -300 per mil increases in GISP and SLAP experiments with increasingloads of zinc. We suggest that this effect is mainly responsible for the observed systematic isotopic differences between zinc- and uraniumbased methods of sample preparation."-19 We caution that the assumptionin ref 17 of a linear isotopic bias is onlyjustified if the ratio of zinc versus water is kept constant. Further optimization of parameters for the reaction of Zn with water, similar to that in refs 5 and 17 for other types of zinc, should result in better precision and accuracy than offered in Table IV.
CONCLUSIONS Combustion of organic matter in sealed tubes of Pyrex, Vycor, and quartz at temperatures between 520 and 900 OC yields less than theoreticalamounta of water, due to hydration of glass and quartz. The accuracy of the determination of C/H and N/H elemental ratios in organic matter is thus reduced. The partial loss of hydrogen does not affect the determination of stable hydrogen isotope ratios. D/H ratios (16) HQbner,G . Kernenergie 1960,3,888-890. (17) Kendall, C.; Coplen, T. B. Anal. Chem. 1985,57, 1437-1440. (18) Tanweer, A. Anal. Chem. 1990,62,215&2160. (19) Florkowski, T. Znt. J . Appl. Radiat. Zsot. 1985, 36, 991-992.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993
from hydrogen prepared by reduction of water with zinc are not affected by reactions between water and Pyrex but are influenced nonlinearly by a deuterium-depleted hydrogen blank in the zinc. Proper outgassing of zinc and calibration with isotopically diverse water standards should achieve mutualcompatibility of the uranium- and zinc-basedmethds.
DE-FG03-92ER14245,and by NSF Grant No. BNS-84-18280. We acknowledge generous support from the Stable Isotope Laboratory a t Southern Methodist University (Dallas) where some of our hydrogen gases for this study were measured mass spectrometrically. We thank Steven Chambers, Jon Fong, and three anonymous reviewers for helpful advice.
ACKNOWLEDGMENT This study was supported by a grant from the Scripps Industrial Associates, by Department of Energy Grant No.
RECEIVEDfor review September 4, 1992. Accepted December 21, 1992.
