356
Anal. Chern. 1980, 52, 356-358
Oxygen-18 and Deuterium Determination on a Single Water Sample of a Few Milligrams Noriaki Kishima" and Hitoshi Sakai Institute for Thermal Springs Research, Okayarna University, Misasa-cho, Tottori-ken, 682-02, Japan
A small quantity of water may be analyzed for the D / H ratio by the conventional hot-uranium technique ( I , 2) or for the ls0/l6O ratio by t h e BrF, technique ( 3 ) . In both of these techniques, water cannot be recovered after analysis and, therefore, a water sample must be split into two aliquots when both t h e hydrogen and oxygen isotopic ratios need t o be analyzed. However, with decreasing amounts of available water samples, it becomes increasingly more difficult to split samples into fractions of predicted amounts without deteriorating the original isotopic compositions. Thus, for a water sample of less t h a n about 5 mg, which one would often encounter in analyzing such samples as fluid inclusions in ore minerals, a procedure by which one of the two isotopic ratios can be analyzed without destroying the water sample would be very useful. Suzuoki and Itoh ( 4 ) analyzed t h e 1sO/160ratios of 9- to 18-mg water samples by equilibrating water vapor with COz under the catalytic action of a heated platinum wire ( 5 ) . The water vapor was recovered after the equilibration for the D/H measurement. However, as has been observed by several investigators (6-9), this procedure is not fully controllable under the described conditions and it is difficult to maintain routine, reliable analyses. T h e purpose of the present paper is t o describe a simple micro C02-water equilibration (MCE) technique for t h e precise 1sO/160analysis of a few milligrams of water. T h e D / H ratios can also be analyzed on the same aliquots of water after they are separated from COP. The MCE is a refinement of techniques used by many investigators for t h e determination of l80content of milligram quantities of ls0-enriched water. T h e present technique has been used successfully for the isotope analysis of atmospheric moisture in this laboratory a n d for t h e study of isotopic fractionation factors between water vapor a n d liquid water by Kakiuchi and Matsuo (10).
factor between COP and water at 25 "C. Approximately 5- and 2-mg alliquots of these waters were sealed in fine glass capillaries prior to analysis. Procedure. After an equilibration vessel (EV) was attached to the vacuum system at the position shown in Figure 1 and the lines on the right hand side of a stopcock S,were thoroughly evacuated by gentle heating, each water sample was released from a capillary at the capillary-tube breaker (CB) and condensed into the next U-shaped trap cooled with liquid air. A small volume of air liberated from the capillary was pumped away through the high-vacuum line (HV). Then the water sample was transferred once to the finger trap (IT and ) finally frozen into the small cavity of EV. On the other hand, approximately 1 cm3 at N.T.P. of COP was introduced from the storage flask ((20,)to the mercury manometer (MM) and was measured volumetrically within an accuracy of 0.170.After this COz was also frozen into the cavity of EV, the stopcock of EV was turned off and fastened by a rubber band so as not to be pushed out by the enclosed COz. E\' was placed in a thermoregulated box a t 25.0 f 0.1 "C for a period slightly longer than ten times the half-time of the equilibration reaction (see below), except in the case of the rate study. EV was taken out of the box, chilled at the cavity with liquid air, and attached with an adaptor to the vacuum line at the position of CB. When the content of EV was allowed to warm up and vaporize to the U-shaped trap immersed in liquid air, the COPand H 2 0 condensed into two separate rings. The C 0 2 component was liberated from the trap by changing the coolant to acetone slush, collected into MM in the watch of a Pirani vacuum gauge (PG),again measured precisely for its volume to check the recovery, and stored in a sample tube (ST). The water component was returned from the trap to EV, conveyed to the reduction line, and converted to hydrogen. The hydrogen gas was measured volumetrically to determine the amount of the water brought into equilibration, and stored in a sample tube. Before each group of H- and L-water samples was run, the uranium furnace was flushed by reducing two 5-mg samples of each water. The COz and the H2 samples thus prepared were submitted to the isotopic analysis by the mass spectrometers. In order to obtain true P O values for COz samples, Craig's correction (17) and instrumental corrections (18) were applied to the measured values. In the practical analyses, water samples are introduced to the U-trap from an appropriate extraction system (ES) and analyzed as described above. Calculations. The equation for 180-balancebefore and after the equilibration can be written as follows, ignoring the presence of "0,
EXPERIMENTAL The vacuum line used in the present study is shown in Figure 1. A separate line equipped with a uranium furnace for reduction of water and a Toepler pump (2)was used also. The equilibration vessel was made from a commercial vacuum glass stopcock as illustrated in the same figure. The cavity to admit the COz-water mixture was 4 cm long and the inner volume was adjusted to 0.5 cm3 by inserting a glass rod. This volume was determined by considering the requisite COP, 1 cm3 at N.T.P., for our mass spectrometer and the desired high pressure of COS of about 2 atmospheres for accelerating the equilibration. Apiezon N Grease (Apiezon Products Ltd., England) was applied to the stopcock afresh at the beginning of this experiment. Two Hitachi mass spectrometers, RM-6RS (11) and RMS-HD, of the McKinney (12)type were used for the oxygen and hydrogen isotopic ratio measurements, respectively. The results are expressed in this report in the b notation: dample =
(-
Rs8IOplt.
-
-.1 +Rf Rf Nc where, respectively, R,. R,, and Rf are the 180/160 ratios of the water sample and the COz before and after the equilibration, N , and N , are the numbers of oxygen atoms of the water sample and the COz,N,' and N , are those of the liquid and vapor waters in the cavity (thus, N , = N,' + NJ,and a and a' are the l80 fractionation factor between COSand water, and water vapor and liquid water at the equilibration temperature (25 "C); (Y is 1.04120 as given above and a' is 0.9910 (19). Recalling the definition of d notation and approximating R*/(l + R,) = R,, etc., the above equation is reduced to 1 + S,/lOOO = (61 - 6,).!3/1000 + (1 + af/1000).
1) x 1000(%0)
Rstandard
where R, the isotopic abundance ratio, stands for D / H (for dD) or for 1sO/160(for 6l80), and the standard arbitrarily chosen is the "standard mean ocean water" (23). A flask of moisture-free COz and two bottles of purified water, labeled as H-water and L-water, differing in isotopic compositions, were prepared as the test samples for the following procedures. Their d values are as follows: C02, d1'0 = +58.90; H-water, dl8O = +36.82 and 6D = +128.2; L-water, P O = -9.33 and 8D = -51.0. These values were obtained by conventional techniques (2, 14) and by using a value of 1.04120 (15, 16) for the " 0 fractionation 0003-2700/80/0352-0356$01 .OO/O
(1 - Y
+ CY'Y)/Q
where /3 stands for N J N , and y for N,IN,. The 6l80 value of the water sample (6,) can be calculated from this equation by
C
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2 , FEBRUARY 1980 HV
h
Table I. Results of Oxygen and Hydrogen Isotope Analyses 6
u
u ST
MMxl
E V +
357
u11
H-5-a H-5-b H- 5-c H-5-d
+35.56 36.78 36.83 36.85 36.80 36.82 36.86 36.87 -9.59 -9.22 -9.30 -9.34
'Iim
6D, ' l o o
give na
1 1 1
CY,
CY',
RESULTS AND DISCUSSION Half-Time of Equilibration. The 6l80 value of COz in a n equilibration mixture approached an equilibrium value exponentially with time. The half-time of equilibration ( t l / * ) was determined for water samples of 0.5, 1, 2, 5, and 10 mg to be 5.8, 4.6, 3.7, 2.3 and 1.9 h, respectively. These values are fitted by a n empirical function, tllz = 5.2 x (mg of water)-'i3 - 0.6. Oxygen Isotope Analysis. The results of sample analyses are shown in Table I. In the first column of the table, H-5-a, for example, indicates that the analyzed sample was H-water, the amount was 5 mg, and the equilibration vessel used was "a" (one of four similar vessels). The 16 samples were run for 6l80 and 6D in the order shown there within the full length of a week. With the present method, faulty results may arise from (1) isotopic contamination of the sample by residual water or other materials in the vacuum line, (2) incomplete transfer of the sample from one segment to another of the vacuum line, (3) lack of equilibration, (4) incomplete separation of COSfrom water after equilibration, and (5) some memory effect due possibly to the grease in contact with the C02-water mixture. As for the oxygen isotope analysis, however, fairly good results were obtained: the observed 6lSO value was within f0.1%0of the given value in 13 analyses out of 16; the maximum error was only O.26%,; a good precision was retained even for 2-mg samples; and no memory effect was observed. Judging from these results, the present MCE technique is expected to be applicable to 1-mg samples or less within a tolerable error if due care is taken to avoid the pitfalls cited above. The uncertainty of 6, due to inaccuracy in volumetric and mass spectrometric measurements (fO.l plus =k0.2% and 10.02%0for 6f,respectively, in our case) was calculated to be f0.04,0.06,0.07, and 0.05%0for H-5, H-2, L-2, and L-5 groups, respectively. Suzuoki and Itoh ( 4 ) also have obtained a good reproducibility of &0.1%0with their method. Compared with theirs, however, the present technique is simpler, more manageable, and more flexible to varying amounts of water samples and these merits will compensate for its greatest disadvantage of the long time of equilibration. Hydrogen Isotope Analysis. The 6D value was reproduced within hl%oof a given value in 9 out of 16 analyses of the samples recovered from MCE. The scatter of the observed 6D values was about two times larger than the best currently obtainable reproducibility, but it was almost comparable with t h a t for routine analyses (20).
L- 5-a L-5-b L-5-c L-5-d
-9.34 -9.42 -9.13 -9.38
obsd
+125.6 128.5 128.5 125.2
+ 36.82
-9.33
Figure 1. Vacuum line for the oxygen isotope analysis
introducing the measured values of tii and tif and parameters p, and y.
obsd
H-2-a H-2-b H-2-c H-2-d L-2-a L-2-b L- 2-c L-2-d
CB
' 8 0 ,
run
1
givena
1
+12s.2
'
126.6 123.4 128.5 128.6 -47.2 -50.8 -49.0 -50.0
J
1
-51.0 -51.0 -50.9 -50.8 -53.2
J
a Determined by repeated analyses using conventional techniques ( 2 , 1 4 ) .
The isotopic composition of ambient water was close to that of L-water. If this water or its vapor entered the vacuum lines to mix with sample waters, some of , P O values for H-water might have been lowered in proportion to the lowering of 6D values. This, however, is not necessarily the case. Another and more probable cause of the observed lower 6D values for H-water is a memory effect of the uranium furnace. In fact, 6D analyses of small samples are often disturbed by the persistent absorption by uranium chips of hydrogen from previous water samples. This may also be responsible for the high 6D value for Run L-2-a. Better results, therefore, will be obtained if the vacuum line of Figure 1 is equipped with a small uranium furnace used exclusively for the present purpose. Runs with L-water samples indicate that the supposed memory effect of the grease, if any, is negligible. In conclusion, the observed 6D values as a whole indicate that the water samples are recovered after MCE procedure without essential alteration of their original hydrogen isotopic ratios.
ACKNOWLEDGMENT We are much indebted to 0. Matsubaya who took care of the water-reduction line and the mass spectrometers used in this study. We also express hearty thanks to S. Matsuo, M. Kusakabe, and M. Kakiuchi of the Tokyo Institute of Technology, who critically tested the present MCE technique in comparison with the hot Pt-wire method as soon as we presented it a t the 1977 annual meeting of the Geochemical Society of Japan. LITERATURE CITED (1) J. Bigeleisen, M. L. Perlman, and H. C. Prosser. Anal. Chem., 24, 1356 (1952). (2) I. Friedman and R. L. Smith, Geochim. Cosmochim Acta, 15, 218 (1958). (3) J. R. O'Neil and S. Epstein, J . Geophys. Res., 71, 4955 (1966). (4) T. Suzuoki and T. Itoh, Mass Spectrosc., 22, 135 (1974). (5) I. Dostrovsky and F. S. Klein, Anal. Chem., 24, 414 (1952). (6) P. D. Boyer, D. J. Graves, C. H. Suelter, and M. E. Dempsey, Anal. Chem., 33, 1906 (1961). (7) J. Tamis and I. Opauszky, Magy. Kem Foly., 71, 352 (1965). (8) H. Sakai, unpublished work, 1974. (9) M. Kusakabe, Tokyo Institute of Technology, personal communication, 1977. (IO) M. Kakiuchi and S. Matsuo, Abstracts 01 the 1978 Annual Meeting of the Geochemical Society of Japan, p 221, (1 1) H. Sakai, 0. Matsubaya, and Y. Nakajima, Mass Spectrosc., 18, 1195 f 1970). - -, (12) C. R. McKinney, J. M. McCrea, S. Epstein, H. H. Allen, and H. C. Urey. Rev. Sci. Instrum., 21, 724 (1950). \
358
Anal. Chem. 1980, 52, 358-360
(13) H. Craig, Science, 133, 1833 (1961). (14) S. Epstein and T. Mayeda, Geochim. Cosmochim. Acta, 4, 213 (1953). (15) Y . Matsuhisa, 0. Matsubaya, and H. Sakai, Mass Spectrosc., 19, 124 (1971). (16) J. R. O'Neii, L. H. Adami, and S . Epstein, J . Res. U . S . Geol. surv., 3, 623 (1975). (17) H. Craig, Geochim. Cosmochim. Acta, 12, 133 (1957). (18) Y . Horibe, Mass Spectrosc., 14, 113 (1966). (19) S. Szapiro and F. Steckel, Trans. Faraday Soc., 63,883 (1967).
(20) 0. Matsubaya, this
Institute, personal communication, 1976.
for review July 9, 1979. Accepted September 20, 1979. One of us (N.K) is grateful to the Ministry of Education for its financial support (General Research D, Grant No. 064127, 1975). RECEIVED
Electrochemical Determination of Trace Mercury in Aqueous Solution Danton D. Nygaard Chemistry Department, Bates College, Le wiston, Maine 04240
Mercury in aqueous solution is routinely determined by one of the many variations on the procedure of Hatch and Ott ( I ) , which involves reduction of aqueous mercury to mercury metal with acidic stannous ion and spectroscopic detection of the purged vapor. Atomic absorption spectroscopy was proposed by Hatch and Ott, a n d is probably t h e most widely used method, although plasma emission spectroscopy has also been proposed (2). Both spectroscopic detectors provide a degree of specificity which is probably unnecessary, since phase separation of volatile mercury metal from the solution matrix eliminates a n y potential interference a t t h e detector. Therefore, a less specific detector may be adequate. This paper investigates the possibility of using a n electrochemical detector, at which mercury metal vapor is determined through oxidation t o mercuric ion a t a n anodically polarized electrode. T h e membrane covered electrode of Clark e t al. ( 3 ) ,in which a thin, gas permeable polymer membrane separates the gas phase from the electrode and electrolyte, is used t o accomplish t h e necessary three-phase interface.
EXPERIMENTAL Apparatus. The sample reduction and purge cell is shown in Figure la. It consists of a 30-mL medium porosity fritted glass Gooch crucible, which is cemented to a 3.5-cm diameter funnel and is covered with a neoprene rubber stopper. Purge gas enters through the funnel stem, passes through the glass frit and the sample, and exits through a glass tube inserted through the stopper. Reagents are added to the sample by syringe through the serum cap. Tygon tubing is used to connect the sample cell with the purge gas supply and detector cell. The flow rate of the purge gas is controlled with a two-stage regulator valve and a needle valve, and is monitored with a float type flowmeter. The membrane covered electrode detector is shown in Figure lb. The gas compartment is a 1.5-cm diameter by 2-cm polystyrene vial through which holes have been drilled to accommodate the gas flow ports. The open end of the vial is pressed against the Neoprene rubber O-ring, which holds the membrane in place, forming a gas tight seal. The working electrode is a 0.5-cm diameter inlaid platinum electrode (Beckman Instruments). It forms one arm of a two electrode H-cell, with a saturated KCl in agar salt bridge connecting it to the saturated calomel reference electrode. Purge gas from the sample cell flows through the gas compartment and over the polymer membrane. The working electrode is polarized, and the resulting current amplified, with a PAR Model 174A Polarographic Analyzer. The current is displayed as a function of time on a Bausch and Lomb VOM5 strip chart recorder. Reagents. Reagent grade chemicals are used throughout. The reductant is 10% stannous chloride in 5% concentrated sulfuric acid, as suggested by Velghe et al. ( 4 ) . Mercury solutions (10 and 1 ppm) in 3% concentrated nitric acid and 0.01% potassium dichromate are prepared daily by dilution from a 1000-ppm standard solution. The electrolyte for the detector cell is 0.1 M 0003-2700/80/0352-0358$01 .OO/O
Table I. Sensitivity and Detection Limit for Membrane Covered Electrode sensitivity detection membrane pa/ng Hg limit ng Hg 0.5-mil Teflon 0.9-mil Polyethylene 1.0-mil MEM-213
0.01 2 20
2000 10 1
KNOBin 0.1% concentrated nitric acid, as suggested by Barikov and Songina ( 5 ) . Both nitrogen and air are used interchangeably as the purge gas. Procedure. Ten milliliters of sample are placed in the sample cell and acidified to three molar with concentrated sulfuric acid. The sample cell is sealed with the neoprene stopper, and the purge gas flow rate is adjusted to the desired value. When a base-line current is established for the detector, 1 mL of stannous chloride reductant is injected into the sample cell. After mercury metal is purged from the cell and the detector current has returned to the base-line value, mercury standard solution may be added to the cell for the purpose of standard addition analysis, or the sample may be rinsed from the cell and the next sample added.
RESULTS AND DISCUSSION Reducing Medium. Hydrochloric, sulfuric, nitric, and various mixtures of these acids have been used in different laboratories to acidify the sample before reduction with stannous ion. It has been reported (6) that a sulfuric acid medium yields a more rapid reduction reaction. Three molar hydrochloric and three molar sulfuric acid were compared as reducing media; it was found t h a t t h e sulfuric acid medium yields a detector response peak which is 40% higher than that which results from the hydrochloric acid medium. Nitric acid is not satisfactory as a reducing medium, because the gaseous nitrogen oxides present in the acid are electroactive, and cause excessively large background currents. Polarizing Potential. T h e detector response to mercury is independent of polarizing potential a t potentials of +0.60 V vs. SCE and above. However, t h e background current increases with increasingly anodic polarizing potentials. Therefore, +0.70 V vs. SCE was chosen as the operating potential. Purge Gas Flow Rate. A plot of detector response to mercury as a function of purge gas flow rate yields a broad maximum plateau a t flow rates between 120 and 240 mL/min. T h e electrode noise is independent of flow rate. Therefore, 180 mL/min was chosen as the optimum flow rate. Membrane Covered Electrode Detector. T h e peak current response and detection limit of the membrane covered electrode detector depend on the membrane material; they are shown for Teflon, polyethylene, and MEM-213 (General C 1980 American Chemical Society
