Anal. Chem. 1988, 60, 1244-1246
1244
Ical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 9-13. 1987. (2) Dicksteln, Held1 L.; Curran, D. J.. University of Massachusettes, Oct. 1987, private communication. (3) Anderson, Jeffrey E.; Tallman, Dennis E.; Chesney, David J.; Anderson, James L., Anal. Chem. 1978, 50, 1051-1056. (4) Gueshi. Tatsuro; Tokuda, Koichi; Matsuda, Hlroaki. J . Nectroanal. Chem. Interfacial Electrochem. 1979, 101, 29-38. (5) Welsshaar, Duane E.; Tallman, Dennis E.; Anderson, James L. Anal. Chem. 1981, 53, 1809-1813. (6) Weisshaar. Duane; Tallman, Dennis E. Anal. Chern. 1983, 55, 1146-1 151. (7) Swofford, Harold S..Jr.; Carman, Roy L., I11 Anal. Chem 1986, 38, 966-969. (8) Faiat, Ladlslav; Cheng, ti.-Y. Anal. Chem. 1982, 54, 2111-2113. (9) Engstrom, Royce C.; Weber, Michael: Werth. Jane Anal. Chem. 1985, 57,933-936.
(IO) Peerce. Pamela J.; Bard, Allen J. J. Nectroanal. Chem. Interfacial
Electrochem. 1980, 114, 89-1 15. (11) Peerce, Pamela J.; Bard, Allen J. J. Nectroanal. Chem. Interfacial Nectrochem. 1980, 108, 121-125. (12) Kannuck, Rosanne M.; Bellama, Jon M.; Biubaugh, Elmo A,; Durst, Richard A. Anal. Chem. 1987, 59, 1473-1475. (13) Santos, Leonel. M.; Baldwin, Richard P. Anal. Chem. 1986, 58, 848-852.
RECEIVED for review October 23, 1987. Accepted January 4, 1988. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Research Corporation for support of this research.
Single-Step Method for Hydrogen Isotope Ratio Measurement of Water in Porous Media Jeffrey V. Turner* and Vic Gailitis CSIRO Division of Water Resources, Private Bag, P. 0. Wembley, 6014 Western Australia, Australia The standard zinc reduction technique for stable hydrogen isotope analysis (1, 2) has been modified to a single-step method which enables rapid measurement of the deuterium to hydrogen (2H/1H)ratio in water from porous media. The novel feature of the method is microdistillation of water from the porous medium within the zinc reduction tube. The need for such a technique mose from a requirement for 2H/1Hratio measurement on microliter quantities of water from unsaturated porous media. Conventional methods using reduction of water over zinc (1, 2 ) or uranium (3) require free water which must be extracted from the porous medium by either immiscible fluid displacement, vacuum distillation, azeotropic distillation, or squeezing. Each of these methods is timeconsuming, can present extraction difficulties a t low water content, and may lead to isotope fractionation where phase changes occur during the extraction procedure. Because the technique presented here requires only 100-300 mg,of sample (5-30 mg of water, depending on water content) it has a wide application to experimental studies of water movement in porous media using water either artificially enriched in deuterium or of natural abundance. We have used the method successfully in field and laboratory experiments at bot,h environmental and enriched concentrations of deuterium. The method is equally applicable to saturated porous media and also gives a measure of the water content of the sample. The sample is taken from the bulk porous medium using a small coring device and extruded into a short glass tube. This is quickly transferred to a sidearm in a preprepared reaction vessel. The reaction vessel is then sealed and the microdistillation and reduction of water vapor with zinc takes place. EXPERIMENTAL SECTION A 36 cm long, 28 mm 0.d. medium wall Pyrex glass reaction vessel is fitted with a 10 mm high vacuum Teflon stopcock with elastomer O-rings. A 4.5 cm long, 12 mm o.d. sidearm is positioned below the stopcock at an angle of 45" such that when the reaction vessel is placed in a 450 "C heating block, the sidearm tip is positioned 5-6 cm above the block surface (Figure 1). In this position, convection from the heating block heats the sidearm to between 100 and 110 " C . The sample coring tube is made from a 10 cm length of 3.3 mm o.d. stainless steel tube. A 2.4 mm 0.d. stainless steel rod with a Teflon tip acts as a plunger. The glass sample tube is 2 ern long and 4 mm 0.d. Zinc shot (BDH AnalaR, 0.5-2.0 mm diameter) is used following preparation and storage methods described in ref 1 but with the addition of ultrasonic cleaning hefore the acid wash to remove fine particles. 0003-2700/88/0360-1244$01 SO/O
The stopcock of the reaction vessel is removed and approximately 0.6 g of zinc shot is added to the bottom of the tube. After the stopcock is replaced, the tube is evacuated and adsorbed water vapor is outgassed by heating with a cool flame. Once cool, the reaction vessel is refied with dry nitrogen to atmospheric pressure while still attached to the vacuum manifold. With free water samples, the stopcock is quickly removed and a gentle flow of dry nitrogen is maintained over the mouth of the reaction vessel to prevent ingress of atmospheric water vapor during sample loading. The water sample is injected by micropipet directly onto the zinc and immediately frozen with liquid nitrogen. With the modified technique, the sample of porous medium is taken from the bulk sample with the coring tube and extruded into the sample tube. The stopcock is again quickly removed under the flow of dry nitrogen and the weighed sample tube containing the porous medium is placed in the sidearm with the aid of tweezers. After the stopcock is replaced, the sidearm containing the sample tube is frozen with liquid nitrogen and the reaction vessel is reevacuated, sealed, and placed in a heating block at 450 O C for 2.5 h. Following reaction, the hydrogen pressure in the reaction vessel is approximately 35 kPa (at room temperature for 30 mg of water) and is transferred by expansion to a gas sample bottle. Hydrogen samples are prepared in batches of 10 and the sample bottles attached directly to the mass spectrometer for isotope ratio determination. Sample tubes containing the dried porous medium are recovered and weighed and the weight loss is compared to the expected yield of hydrogen measured by using the calibrated volume of the mass spectrometer inlet system. Sample tube constant weights are checked at 105 "C. Provided a sufficient number of reaction vessels are available, up to 30 samples a day can be processed in this way. Isotope ratios are measured on a VG SIRA 9 mass spectrometer with an automatic inlet suited to batch preparation of this type. Results are reported in standard 6 notation relative to Vienna Standard Mean Ocean Water (V-SMOW) where 6sample-V.SMOW = ((R,,j,/Rv.sMow) - 1)X lOOO%o and R is the ratio 2H/'H. The standard deviation (1uI5)for conversion of free water is 0.9%. Values are corrected for the H3+ contribution to the HD ion beam. RESULTS AND DISCUSSION Reaction times for the reduction of free water with zinc are normally 30-40 min (1). With porous media samples of 150 mg and water content of less than 2070, the modified technique requires an increase in reaction time to 2.5 h. The effect of this longer reaction time on analytical precision was investigated in view of the exchange of gaseous hydrogen with hydrogen in the Pyrex glass walls reported in ref 2. Six standard water samples with ranging from -13.6 to -270%0,including one saline groundwater (total dissolved 0 1988 American Chemical Society
1245
ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
Teflon stopcock -40 O
tI
Reaction Time (mtn ) 0
30
0
60
6 0 minutes
Sample 120 minutes
Sample holding tube
Standard 3 - f r e e w a t e r
J
1
3 heating block (450'C)
Figure 1. Schematic diagram of the apparatus used for simultaneous microdistillation of water from porous media and reduction to hydrogen for D / H ratio measurement.
Table I. 6%
of Hydrogen Heated in Reaction Vessels
std 1, no prior heating 120 min heating initial a2H -13.6,
63
= 0.1 -15.4,
84
= 0.2 -270.0,
std 1, prior heating with st 6 120 min initial a2H heating -13.6.
U?
= 0.2 -14.0,
UR
std 6, no prior heating 120 min initial a2H heating 64
= 0.2 -269.1,
64
= 0.4
std 6, prior heating with std 1 120 min initial PH heating
= 0.3 -270.2,
UR
= 0.2 -268.2,
~ f i
0.4
Table 11. Effect of Reaction Time on a2H of Free Water reaction time, min 40 120 200
standard 1 -13.6 -15.3 -14.9
63 64
= 0.1 = 0.1 = 0.3
standard 5 -97.8 -97.5 -97.9
63 63 US
= 0.5 = 1.5 = 0.8
standard 6 -269.1 -269.3 -266.3
63
= 0.4
63
= 0.2
63
= 1.2
solids 10 400 mg/L) were used to test various aspects of the technique. Table I summarizes the results of experiments using standards 1 and 6 to determine the extent of gaseous hydrogen exchange with the Pyrex glass reaction vessel walls. Reaction vessels were heated to approximately 100 O C under vacuum to remove adsorbed water vapor before each experiment commenced. Hydrogen of known isotopic composition was then introduced at a pressure of 19 kPa (25 "C). The vessels were heated at 450 OC for 2 h and the isotopic composition of the hydrogen was remeasured. The 2-h heating period resulted in changes in Lj2H of between 0.4 and 2%0. Heating experiments run with hydrogen initially of 62H = -13.6%0 in reaction vessels that had previously been heated with hydrogen of 62H = -270%0,and vice versa, showed no Table 111. a2H of Water Extracted from Porous Media
standard 1 (distilled) free water water from: sand sandy clay kaolinitic clay kaolinitic clay (corrected)
standard 2 (deionized)
-13.6
US
= 0.7
-19.0
~6
= 0.6
-14.5
64
= 0.3
-20.0
66
= 0.6
-20.7 -13.6
UT
= 0.6
standard 3 (deioiiized,
= 0.1 -25.0
-20.4
~2
-23.1 -25.7 -19.6
u4 = 0.3 U(
standard 4 (groundwater) (TDS = 10400 mg/L)
= 0.2
-26.8 -29.5 -31.3 -25.6
613 65
= 1.0
= 0.4 = 1.4 = 1.0
standard 5 (distilled)
standard 6 (distilled)
-96.5
US
= 1.3
-267.6
1711
= 1.7
-96.8
63
= 0.6
-267.0
62
= 0.9
UT
= 1.7
-256.5 -267.6
Ug
= 1.6
U I ~ 64
-100.1 -99.9
1246
Anal. Chem. 1900, 60, 1246-1248
for enriched water (standards 1 to 4) and toward isotopic enrichment of ,Hfor depleted water (standard 6) is observed. Its magnitude ranges from less than 2% for water of all isotopic compositions extracted from sand, to 10%0for isotopically depleted water extracted from kaolinite. For standard 5 (6'H = -96.57~)there is no difference in the measured 6% of water extracted from sand and a depletion of 3%0in water extracted from the kaolinite. Despite the bias in measured isotopic compositions, particularly for water extracted from kaolinite, the reproducibility is good with l a values for replicate analyses between 0.2760 and 1.7%. The bias in J2H may be corrected if high precision is required. In the case of kaolinite, the correction based on data from standards 1 and 6 is given by the simple linear relation a2H, = 1.086'Hm + 8.2 where c and m correspond to the corrected and measured d2H values, respectively. Corrected values are shown in Table I11 for kaolinite. Comparison of results shown in Tables I and I1 with those of Table III shows that the small bias in measured 6% of water extracted from sand is of the same magnitude and direction as that from the hydrogen heating experiments and the reaction time experiments. As before, the bias can be accounted for by hydrogen exchange with the Pyrex glass walls. In the case of water extracted from kaolinite, the magnitude of the bias is larger and there appears to be an additional source of exchangeable hydrogen in the clay, The hydrogen fractionation factor for hydroxyl hydrogen in kaolinite and interlayer is approximately 0.97 ( 4 ) . The 6'H of water, (lLkaolini+,,&r hydroxyl hydrogen in the kaolinite used in the experiments is expected to be in the range -60% to -80760, based on the 6'H of meteoric water at the source of the kaolinite used. Isotopic exchange between the interlayer water, i.e. the introduced standards in the experiments, and hydroxyl hydrogen
with 6'H in the range -60% to -8OYw can account for the additional magnitude of the bias observed with the extraction of the standards from kaolinite. This technique was successfully applied to laboratory experiments investigated a2H relations during evaporation from sand columns using both environmental and artificially enriched deuterium concentrations. Examples of 62Hanalyses obtained by using this technique in porous media from these experiments and from the field are given in ref 5 and 6. We conclude that this is a rapid and accurate method for 'H/'H ratio determination on water from porous media. Of the porous media studied, kaolinite showed the largest but consistent bias in measured 6,H. Where high precision is required in measuring environmental h2H values, the bias may be corrected for by use of standard waters of known isotopic composition. The method is suited to experimental studies of water movement in porous media and may be used with either environmental or artificially enriched deuterium concentrations. Registry No. D,, 7782-39-0;H,, 1333-74-0;H20,7732-18-5; Zn, 7440-66-6. LITERATURE C I T E D (1) Coleman, M. L.; Shepherd, T. J.; Durham, J. J.; Douse, J. E.; Moore, G. J. Anal. Chem. 1982, 5 4 , 993-995. (2) Kendall, C.; Coplen, T. B. A n d . Chem. 1985, 5 7 , 1437-1440. (3) Godfrey, J. D. Geochim. Cosmochlm. Acta 1962, 2 6 , 1215-1245. (4) Savin, S. M.; Epstein, S. Geochim. Cosmochim. Acta 1970, 3 4 ,
25-42.
(5) Shimojima, E.; Curtis, A. A.; Turner, J. V., submitted for publication in
.
J . Hydro1 (6) Turner, J. V.; Arad, A,; Johnston, C. D. J. Hydro/. 1987, 0 4 , 89-107.
RECEIVED for review December 24,1686. Resubmitted August 25, 1987. Accepted January 4, 1988.
Investigation of a Fusion Technique for the Determination of Total Sulfur in Geological Samples by Ion Chromatography Ellen A. Stallings,* L i n d a M.Candelaria, and Ernest S. Gladney
Health and Environmental Chemistry, M S K-484, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 We have encountered the need for a rapid, accurate, precise, and sensitive method for measuring total sulfur in large numbers of geological samples. Numerous environmental studies of sulfur deposition due to sulfur dioxide emission from combustion and industry are currently under way. Several techniques for measuring total sulfur in soils and other silicate materials have been published including neutron activation analysis (I), thermal neutron capture prompt y-ray spectrometry ( 2 , 3 ) ,inductively coupled plasma atomic emission spectrometry (4,isotope dilution mass spectrometry (5),X-ray fluorescence (6),turbidimetry (3, ion chromatography (IC) (8-12), iodimetric titration (13),and fluorometry (14).However, none of these methods is completely satisfactory for routine analysis of large numbers of samples. Many have levels of detection that are inadequate for measuring low levels of total sulfur (17 MQ cm resistivity. Sulfate standards were prepared from the sodium salt in a background matrix of fused Na202in the same concentration as the samples. The eluant consisted of 1.8 mM NaHC0,/1.2 mM Na2C03and the anion membrane suppressor regenerant was 0.025 N H2S04. A Dionex series 4000i ion chromatograph equipped with an autosampler and a Spectra-Physics4270 integrator was employed in conjunction with a Dionex AS4 column and guard column for the sulfate determination. Sampleswere injected through a 100-pL injection loop into the eluant stream flowing at 2 mL/min. Sulfate detection was accomplished by use of a conductivity cell and the output range was set at 10 pS. Sulfate concentrations were determined by using integrated peak area calculations. Sulfate retention time was 8 min. Sample Preparation. Air-dried samples were fused in covered zirconium crucibles using 0.15 g of sample and 1.0 g of NazOzin an 800 "C furnace for 3-5 min. The fusion melt was reacted with 15 mL of deionized water, and the resulting slurry was transferred into a 50-mL volumetric flask and diluted to volume with deionized water. A 5-mL aliquot of the slurry was filtered through a 0.45-pm filter syringe and injected into a 5-mL autosampler vial.
0003-2700/88/0360-1246$01.50/0 0 1988 American Chemical Society
