114
Anal. Chem. 1980, 6 1 , 114-118
viation. For the 9.6-mm segment length the two measures of the band-broadening rate agree within 3 standard deviations of each other. It has been observed that the steady-state extraction o c c m uniformly across all parts of the interface (i.e. kl/k,m = aend/aaeg), and so the non-steady-state extraction might also be expected to occur uniformly over all of the interface. The absorbances in segment -1 a t time zero, A-l,t=o,determined from the segment -1 absorbances using the successive reactions model are shown in Table 111. Also shown in the last column of this same table is the iodine absorbance which would be predicted for uniform non-steady-state extraction across all of the interface. It can be seen that these two sets of values are in excellent agreement.
ACKNOWLEDGMENT Wayne Bexon of the Chemistry Department Electronics Shop made the on-tube photometric detector and Tony Walford of the Chemistry Department Machine Shop made the single segment injector.
(8) Shelly, D. C.; Rossl, T. M.; Warner, I . M. Anal. Chem. 1982. 5 4 , 87-91. (9) Cantwell, F. F.; Swelleh, J. A. Anel. Chem. 1985, 57, 329-331. (10) Lucy, C. A. PhD Thesis, University of Alberta, 1988. (11) McClintock, S. A.; Weber, J. R.; Purdy, W. C. J. Chem. Educ. 1985, 62, 65-67. (12) BetterMge, D.; Dagless, E. L.; Fields, B.; Graves, N. F. Analyst (London) 1978, 103, 897-908. (13) Dye, J. L; Nicely, V. A. J. Chem. Educ. 1971, 48, 443-448. (14) Luque de Castro, M. D. J. Autom. Chem. 1988, 8, 56-62. (15) Horvath, C.; Solomon, B. A.; Engasser, J.-M. Ind. Eng. Chem. Fundam. 1973. 12, 431-439. (16) Chen, Jlng-Den J. Couoid Interface Sci. 1988, 709, 341-349. (17) Betherton, F. P. J. FluidMech. 1981, 10, 166-188. (18) International Crltlcal Tables of NumericalDate : Physics, Chemistry and Technology. 1st ed.; National Research Council; McC3aw-Hlll: New York, 1929; Vol. I V . (19) International Critical Tables of Numerical Data : Physics, Chem/stry and Technology, 1st ed.; National Research Council; McGraw-HIII: New York, 1929; Vol. V. (20) Bugliarello, G.; Hsiao, G. C. 8brh.30lOgv 1970. 7, 5-36. (21) Lambert, J. L.: Flna. G. T. J. Chem. Educ. 1984, 6 1 , 1037-1038. (22) Cantwell, F. F.; Frelser, H. Anal. Chem. 1988, 60, 226-230. (23) Guy, R. H.; Fleming, R. J. Colloid Interface Scl. 1981, 8 3 , 130-137. (24) Crank, J. The MaathemeHcs of Diffuslon, 2nd ed.; Oxford University Press (Clarendon): Oxford, 1975; p 49-53. (25) Szabd, 2. G. I n Comprehensive Chemicai Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1969; Vol. 2, Chapter 1.
LITERATURE CITED (1) Nord, L.; Backstrom, K.; Danielsson, L.-G.; Ingman, F.; Karlberg, B. Anal. Chim. Acte 1987, 194, 221-233. (2) Lucy, C. A,; Cantwell, F. F., Anal. Chem., preceding paper In this Issue. (3) Imasaka, T.; Harada, T.; Ishibashi, N. Anal. Chim. Acta 1981, 129, 195-203. (4) Snyder, L. R.; Adler, H. J. Afl8l. Chem. 1978, 48, 1017-1022. (5) Snyder, L. R.; Adler, H. J. Anal. Chem. 1978, 48. 1022-1027. (6) Pedersen, H.; Horvlth, C. Ind. Eng. Chem. Fundam. 1981, 20, 181-1 86. (7) Nord, L.; Karlberg, B. Anal. Chim. Acta 1984, 164, 233-249.
RECEIVED for review June 30, 1988. Accepted October 12, 1988. This work was supported by the Natural Sciences and Engineering Research Council of Canada and by the University of Alberta. The postgraduate scholarship provided for C.A.L. by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Presented in part at the 3rd North American Chemical Congress, Toronto, Ontario, June 1988.
Gas Chromatographic Determination of Volatile Sulfides at Trace Levels in Natural Freshwaters Franpois Caron and James R. Kramer* Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4Ml
Volatlle sulfides in the aqueous phase were analyzed by degassing the sduUon and cryogenically trapplng the vdatlles. A gas chromatograph equlpped wlth a Hal electrdytlc conductivlty detector Is used In the sulfur mode. Water samples were analyzed with the Internal standard method because of varlable recoverles. Recovery efflclencies of the Internal standard show a good llnear relatlonshlp with the analytes. Detectlon limits (as S) of 0.03 ng/L for H2S, 0.02 ng/L for COS, 0.06 ng/L for CHISH, 0.09 ng/L for CS,, 0.05 ng/L for CHSSCH3,0.07 ng/L for CH,CH,CH,SH, and 0.10 ng/L for CH,SSCH, can be achleved for a IOO-mL sample. The preclslon Is better than 10% for the range of 1-4000 ng of S/L.
INTRODUCTION The importance of dimethyl sulfide (DMS, or CH3SCH3) in the natural environment and its significance toward the global sulfur budget have been well documented (1-7). Other volatile sulfides can be more significant than DMS, particularly with emissions from marshes and soils (5, 8-10). For example, HzS, CH3SH (or MeSH, methanethiol), COS, CSz, CH3SSCH3 (or DMDS, dimethyl disulfide), and sometimes 0003-2700/89/0361-0114$01.50/0
SOz and CH3CH2CH2SH(PrSH or 1-propanethiol) can contribute significantly to the sulfur flux in the atmosphere. Organic volatile sulfides (OVS) are ubiquitous in the environment because they result from exudates of algal primary production, bacterial decomposition, or a combination of both (6, 9, 11, 12). OVS analysis has emphasized the oceanic environment where DMS is the dominant sulfur species, and its flux to the atmosphere is important. Freshwater environments, however, have not been widely studied. Concentrations of OVS are apparently low (13). Determinations of DMS in seawater have been published but some of these methods still suffer from important limitations of selectivity (14,15)and complicated procedures (15, 16),which may be a problem when many sulfur species other than DMS are present. A recent method applied to freshwaters (17) lacked the required detection limit. Most methods tend to be either limited to or optimized for DMS. This study describes a gas chromatographic technique that uses a modified Hall electrolytic conductivity detector (HECD) (18)for the simultaneous determination of seven or more different volatile sulfur species in water. The sample is purged with helium, and the sulfides are trapped cryogenically in a Teflon tube, which serves as a sampling loop 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
115
Table I. Gas Chromatograph and Detector Conditions Hewlett-Packard 5890 gas chromatograph Hall electrolytic conductivity detector, 0.1. Corp. Model 4420, operated on sulfur mode recorder Hewlett-Packard 3390A peak integrator column Chromosil 330, 8 f t (2.4 m), packed on 6 f t (1.8 in. (0.32 cm) 0.d. in FEP Teflon m) X tube (Supelco, Inc., Bellefonte, PA) Helium ultrahigh purity (Canadian Liquid Air), carrier gas with on-line filters: Molecular sieve 5A (Supelco) and Oxytrap (Chromatographic Specialties, Brockville, ON); flow rate, 12.5 mL/min detector gas air ultra zero grade (Canadian Liquid Air), with glass moisture trap (Chromatographic Specialties);flow rate, 90 mL/min detector solvent methanol HPLC grade, added to Millipore water (200 mL of water completed to 1 L with methanol); solvent flow rate, 50 bL/min detector output 0-10 mV injector temp 200 "C detector temp 125 "C (base), 950 "C reactor oven temp 6 min at 40 "C, followed by temperature program 30 OC/min for 1 min followed by a 9-min hold at 70 "C
apparatus detector
for injection into the gas chromatograph. The system features are simple and easy to operate, requiring only 100 mL of water sample. A detection limit of 0.02-0.10 ng of S / L is attained for different sulfur species.
EXPERIMENTAL SECTION Apparatus and Analytical Conditions. A gas chromatograph (Hewlett-Packard Model 5890A) equipped with an electrolytic conductivitydetector (0-1Corp. Model 4420), a Chromosil 330 column (Supelco),and a reporting integrator (HewletbPackard 3390A) were the major components of the system. The instrument parameters are summarized in Table I. The flow rates of the gases were measured with a soap bubble flowmeter, whereas the solvent flow rate through the conductivity cell was measured with a 0.1-mL pipet graduated into 0.01-mL divisions. Calibration. Permeation tubes were used for the calibration of all the sulfur species. The tubes for the gaseous species (COS, H2S, MeSH, and SOz) were obtained as certified Dynacal calibrated permeation tubes (Vici Metronics, Santa Clara, CA). The tubes of the other sulfides, which are liquid at room temperature (CSz,DMS, PrSH, DES, DMDS), were custom made and gravimetrically calibrated according to standard procedures (19,20). The permeation tube consists of a reservoir of pure liquid sulfide hermetically sealed with FEP Teflon tubing (which is semipermeable to sulfides) and stoppered with a glass rod. The device emits the sulfide at a constant rate with time when held at constant temperature. These tubes are inserted in a condenser and attached to a mixing line (20). Helium is passed through the line at various known flow rates, and volumes of the gas are then withdrawn with a syringe and injected onto the GC column. Small concentrations of analytes (0.01-10 ng of S/mL) are obtained by using this procedure. All the permeation tubes are stored in a chamber maintained at 25 "C (rt 0.1 "C). They are individually sealed in polyethylene test tubes to avoid cross-contamination problems. Silica gel pouches are enclosed in the chamber to keep the air dry, and a flow of dry air continuously purges the chamber in order to minimize cross-contamination among permeation tubes and problems caused by humidity (19). Calibration curves can be obtained simultaneously for up to seven different gases. Distillation Apparatus. Description. Figure 1shows a sketch of the custom-made distillation line. The two-necked 100-mL flask (B) is attached to a 10-cm condenser (F) and then to a drying trap (G).A coil trap (J)and a removable injection/trap loop (N) complete the line. The whole line is made out of Pyrex glass, except for the injection/trap loop. The inlet tube (9 mm 0.d.) (D) is connected to the helium supply with a 3 / 8 in. Swagelok connection and a
D
W
/
A
I
\
I I
I
IS
Q I
I
1I
I
I
I
~
4
1
Figure 1. Distillation line: A, heating mantle; B, sample flask; C, tap water inlet; D, helium carrier gas; E, ground glass joint; F, 10-cm condenser; G, drying trap; H,ball and socket joint; I, tweway stopcock; J, coil trap: K, three-way valve; L, vacuum outlet; M, Luer tip Junction; N, Teflon sample ioop/trap; 0, outlet; P, sample loop male tip; Q, R, sample loop valves; S, sample loop female tip; T, packed Teflon tape; U, Bostik glue; V, Male Luer tip (cut off); W, Teflon tubing (1/16 in. 0.d.); X, three-way medical stopcock; Y, liquid nitrogen Dewars.
Teflon ferrule. The ground glass joints (E) are either 3 19/22 or 19/26, whereas (H) is a ball and socket joint (ball and socket 12/5). The drying trap (G) (100 mm X 19 mm 0.d.) is filled to approximately one-fourth of its volume with anhydrous CaC12. The first cryogenic trap (J) is made of a 1.8 m long, 9 mm 0.d. Pyrex glass tubing coiled on 4 cm x 15 cm, and the second (N) is a custom-made removable Teflon loop. The latter (Figure 1, inset) is made of 40 cm of FEP Teflon tubing (1.6 mm 0.d.) (W) glued between two three-way polyethylene medical stopcocks (Seamless Corp.) (X) with Bostik glue. The internal volume of the injection/trap loop is only 0.4 mL. The connections between the tubing and the Luer fittings are sealed with Teflon tape (T). This loop is attached to the line with the body of a Multifit glass syringe (M), which is affixed to the Pyrex line with Teflon tape and Bostik glue. The three-way stopcock (K) connects the line with a vacuum pump (L), and the two-way stopcock (I)can isolate the right-hand part of the line under low pressure when the stopcocks (R) or (Q)are also closed. The ground glass joints and the stopcocks are lubricated with a thin film of silicone-based vacuum grease (Dow Corning). Procedure. For purging the sample, stopcocks I and Q are closed, R is opened, and K is switched to vacuum to evacuate residual gases in the right-hand part of the line. After approximately 2 min, the traps J and N are immersed in liquid-nitrogen Dewars (Y). The water sample is poured in the sample flask (B), an internal standard (DES) is added to it and the helium line (D) is attached to the flask. Vaccum is turned off when stopcock (K) is switched to the line, helium is turned on with a flow of 100-120 mL/min, heat is provided to the heating mantle (A), and valves I and Q are opened in the same operation. The sample is heated just below the boiling point (95-100 "C) in 8-10 min and held at this temperature for an additional 10 min. After the set time, helium and the heating mantle are turned off, stopcocks I and Q are closed and K is opened to vacuum for 30 s and closed again. The liquid nitrogen Dewar under trap J is replaced by hot water (approximately90 "C) and held in place for 20 min for the sample to transfer from J to N. Vacuum is switched on the sample loop only (for 30 seconds) by rotating stopcock (K) the appropriate way, then valve (R)is closed. For the injection, the loop (N)is removed from the line and a syringe (previously filled with 2.5 mL of air) is attached on the
118
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15,
1989
8.0
Table 11. Detection Limits for Selected Volatile Sulfides
n
v1 3
.PI
species
7.0
3 H2S
cos MeSH cs2
e
26.0
so2
cd
DMS PrSH DES DMDS
0,
2 5.0
%i
o HZS: Y A YcSB: Y
e,
a
Y DYS: Y PrSE: Y DES: Y DYDS: Y = 0.972X
0 SO2: CS2
Q
F4.0
+
c7 x t
= 1.029X + = 1.052X + = 0.BSlX + = 0.99OX + = 1.044X + = 0.976X + = 1.040X +
Y
1
3.0
0
I
-1.0
I
0.0
I
1.0
+
I
2.0
5.776 5.724
detectability
LD (LD=
( D = 2N/S),
DWo,6),pg of
pg of S/s
Wo.s,s
compound
0.45 0.27 0.50 0.52 0.47 0.30 0.37 0.26 0.26
6.0 6.0 10.8 15.0 14.7 14.3 15.2 21.3 24.4
2.9 3.0 8.0
9.2 13.7 8.2 13.4 15.8 9.3
method limit: ng of S/L 0.03
0.02 0.06
0.09 0.08 0.05
0.07 N/Ab 0.10
"Method limit: calculated by using the detection limit of the individual sulfides, considering a sample volume of 0.1 L and a 90% efficiency extraction as estimated from the DES spike. b N / A, not applicable
5.786 5.726
6.021
6.003
6.125 6.160
3
Log amount of sulfide (as ng S Flgure 2. Calibration curves and working ranges for selected sulfides (attenuation, 5; reactor temperature, 950 "C).
female luer tip (S). A needle is fixed at the other end (P). The loop is immersed in hot water (90 "C) for 30 s to volatilize the sample. The needle is inserted into the injection port of the GC, stopcocks Q and R are opened, and the volume inside the loop is injected into the GC using the syringe. Glassware Preparation. All the glassware in contact with the sulfides must be thoroughly cleaned and deactivated with a suitable chemical agent because gaseous sulfides strongly adsorb to glass (21). The cleaning procedure consists of soaking the glassware in Chromerge for at least 15 min and rinsing with Millipore water. It is followed by a 1 h (minimum) soak in 10% HCl, then the glassware is rinsed again with water and dried with acetone. A premixed silicone-based reagent, dimethyl dichlorosilane 5% (v/v) in toluene (Sylon CT, Supelco, Bellafonte, CA), is applied according to the manufacturer's instructions: the reagent is poured into the glassware, followed by rinsing with toluene and methanol. Each sample flask is cleaned and silanized after approximately 10 uses, whereas the distillation line undergoes this treatment every 2 months. Spikes and Internal Standards. Small amounts of the liquid sulfides (CS2,DMS, DES, DMDS) are weighed and diluted to about 0.05 pg/mL in ethylene glycol to make standard solutions (the ethylene glycol was previously degassed at 170 "C and purged with helium for 3 h). The resulting solutions are stored in a freezer (approximately -5 "C). The concentrations of all the sulfides are monitored periodically for up to 6 months, after which a new standard solution is required. For the standardization procedure of the distillation line, 200 pL of internal standard solution (DES) and volumes ranging from 25 to 500 pL of the other standard solutions are added to artificial lake water and analyzed as with normal samples. For the real samples, only the internal standard DES is added.
RESULTS AND DISCUSSION Detector Performance. Figure 2 shows the log-log calibration curves for nine sulfur species commonly found in the environment. Each individual curve is made up of 7-14 data points. The two symbols show the upper and lower limits of the working range of the calibration curves. The figure shows that all nine sulfur species have similar slopes and intercepts. This is in contrast to the variable slope values often observed for flame photometric detectors (17,22). The slopes of the curves obtained with the HECD are close to unity (between 0.95 and 1.05), which means that the signal varies linearly with the amount of sulfur. All of the intercepts are in the range of 6, with minor differences in values between two sets of sulfur species: the first one (COS, DMS, PrSH, DES, and DMDS) at about 6.0, and the second (H2S,MeSH,
SOz, and CS2) at around 5.75. The cause and the significance of these differences in intercepts are unknown. The slopes for all the nine sulfur compounds remained consistent over months of use of the detector a t 950 "C. However, a lower oxidation temperature of 850 "C gave slopes between 0.88 and 1.06 for DMS and between 0.76 and 0.82 for DES. Thus the oxidation temperature is critical for obtaining similar calibration curves among sulfur species. The consistency of the slopes and intercepts among sulfur species suggest that the oxidative reaction of the reduced sulfides to yield S02-S03 is nearly quantitative and independent of the nature of these molecules at 950 "C. Therefore other unknown sulfur-containing molecules can be detected and quantitated on the basis of their sulfur content. Figure 2 shows a working range of greater than 3 orders of magnitude. This range is limited partly by the dynamic range of the integrator. However, the linear range of the detector has never been exceeded a t any attenuation. Optimal performance was obtained when the detector attenuation was set at 5 or 10. Detection Limit. The detection limit of the detector was obtained following the approach given in Massart et al. (23). The results of the calculations are given in Table I1 for all the sulfides under study. The detectability (D) is defined as the smallest detectable signal from an analyte. D is acquired from an analyte giving a response 2 times the value of the background noise (N) and the sensitivity of the analyte (S),so that D = 2 N / S . The sensitivity of the analyte is given by the intercept of the log-log plot. The background noise was measured at full scale on the integrator a t the two temperatures of the chromatographic runs (40 and 70 OC). The peak width at half height ( Wo.5)was measured at full scale on the integrator for peaks having between 3 and 8 times the value of the background noise. The detection limit of the compound is defined as the product of the detectability (D) and (Wo.& The method limit is calculated for a 0.1-L sample and a 90% efficiency of extraction as estimated by the internal standard (see below). The ability to detect picogram levels of sulfur is requisite for many environmental samples. The HECD used here achieves this level of detection and surpasses other commonly used detectors (24, W ) ,as well as a new fluorescence detector (26), but is less than a recently developed SF,-ECD (27). Base-line stability is essential to maintain low detection limits. The solvent and the resins in the recirculating unit play a crucial part: the solvent is replaced after either 150 runs or 3 to 4 weeks, whereas the ion exchange resins are changed every 2 months. The nickel tube and the scrubber require less frequent changes, but they are also important for the base-line maintenance.
AI\IALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
Distillation Line and Internal Standardization Method. The internal standardization method was used for quantitation. An internal standard (DES) is added to the sample in order to normalize for chromatographic and/or preparative conditions that may vary among samples. A known amount of DES (which is not present in freshwater samples and algal cultures) is added to the water sample at each run to ensure a quantitative recovery of the analytes. The necessity of this procedure comes from the fact that spike recoveries varied widely (10-94%, average 30-50%) for different runs under the same conditions. In addition, different sulfides may not necessarily have the same partition coefficient (helium-water) as DES at 95-100 OC. It is therefore important to verify whether the extraction efficiency of DES varies linearly with that of the other sulfides. Similar recoveries between DES and other sulfides would allow the use of the internal standard. Artificial lake water (Bold's basic medium) (28) was spiked with standard ethylene glycol solutions containing the internal standard and other sulfides (see earlier section). A good linear relationship between the recovery efficiency of DES vs DMS was obtained (%DMS = l.OO%DES + 2.7; n = 25, r = 0.94; where %DES, etc. is expressed as percentage of recovery). The recovery efficiency varied between 2.7% and 92% for this set of data, and 30-50% was typical. A similar relationship was found for CS2 and DMDS (r = 0.88, n = 25) with slopes of 0.98 and 0.68, respectively. The extraction efficiency of the other volatile gaseous species (SO2, H2S, MeSH and COS) could not be determined; a slope of 1 is assumed. The reason for the variable recovery efficiencies of the internal standard is conjunctural at present. Sample heating and purging with helium seems to be critical. For instance, the recovery of DES is much lower if the sample is boiled. In addition, reruns of samples show some residual sulfides in solution but not in the rest of the distillation line. A longer purge time gives no significant increase in efficiency. Optimal recoveries are obtained for a flow of 100 mL/min of helium gas, but a lower or higher flow rate does not increase the recovery of analytes. Different sample matrices, i.e. deionized, distilled, or artificial lake water, do not affect recovery of DES. There is no pH effect observed between 4 and 10, and solution ionic strength does not correlate with recovery efficiency. Different amounts of spikes do not correlate with stripping efficiencies. Sulfide concentrations were corrected by using the internal standard according to where S is the amount of sulfide, expressed in nanograms of sulfur (which is obtained from the calibration curves of the individual sulfides), A is the slope of the extraction efficiency, V is the sample volume, and E is the extraction efficiency (expressed as fraction, i.e. 0 < E 5 1). The subscripts are as follows: expt, amount found experimentally; s, amount of sulfide corrected, in ng of S/L. Freshwater Samples and Precision of the Method. Figure 3 shows a typical chromatogram obtained from an unfiltered Anabaena Sp. unialgal culture (30 days old) grown in Bold's basic medium. Methanethiol and dimethyl sulfide are the most important species, followed by DMDS. These species are usually more predominant in algal cultures as well as in natural water analyses. The unknown species (1)can be 2-propanethiol or methyl ethyl sulfide. The other unknown peak (2) is a signal from ethylene glycol. The DES recovery (28%) was lower than the average. The retention times for C02,COS, and H2S are respectively 0.98, 1.25, and 1.35 min, producing a poor to moderate resolution for these gases. The C02peak is usually sharp and relatively weak and can always be resolved from the other two, whereas sometimes the COS
117
.co2
15l Species
Conc. n g S / L
cos
22 55
H2S
MeSH
497
DM S
586
?1 DMDS DES
est.
31
54 iSTD
2.6ng
28% recov.
.
Figure 3. Typical chromatogram of unfiltered Anabaena sp algae grown in Bold's basic medium. Peak 72 is ethylene glycol.
Table 111. Analysis of OVS in Samples from the Desjardins Canal concn, ng/L Oct 15 subsample 1 2 3 4 av std dev Oct 13 av std dev (n = 3)
COS MeSH CS2 DMS DMDS % DES 47.7 48.0 47.8 N.R" 47.8 0.2
28.1 27.7 54.3 35.4 36.4 12.5
13.7 14.4 12.3 12.1 13.1 1.1
3.5 3.7 3.4 3.3 3.5 0.2
2.9 9.1 9.3 5.4 6.7 3.1
47.3 6.1
52.9 0.9
15.1 2.5
6.3 2.6
10.5 2.7
1.7
8.4
5.7
25.7
% re1 std dev 0.4
50.4 47.8 51.2 41.5
N.R, not reported.
and H2S peaks cannot be separated on the integrator. A cryogenic unit in the GC would undoubtedly help to separate the COS and H2S peaks. Table I11 shows the volatile sulfides concentrations obtained for water samples taken from an eutrophic environment (Desjardins Canal,Cootes Paradise, Dundas, ON). The sample was collected in a 1-L polypropylene Nalgene bottle and divided into subsamples within 30 min. Each subsample was poured into a 125-mL bottle, leaving no headspace. They were stored on ice in a cooler (0-4"C)and in the dark until analysis. The DMS concentration values obtained are comparable in magnitude but somewhat lower than these reported elsewhere for nearby Hamilton Harbour and Cootes Paradise (17). Replication of analysis suggest that the reproducibility can be within a margin of 5% for COS, MeSH, CS2, and DMS, and 10- to 25% for DMDS. However, results vary with time of storage among subsamples. Aberrant values in Table 111, giving relatively high standard deviation, are not believed to be artifacts of the method, but rather the result of microbial activity in the stored sample. Other sets of samples (not shown) were taken from Desjardins Canal to observe changes in analytical results at various times after sampling. Important
118
Anal. Chem. 1989, 6 1 , 118-122
changes occur within 3 h of storage. A microbial intoxicant, m-cresol(29), was added to some stored subsamples 4 h after sampling to test whether microbial activity causes changes in sulfide concentrations during storage. The concentrations of some sulfide species stabilized, but some others decreased with time (particularly MeSH) after addition of the phenol. The data are not necessarily conclusive with regard to m-cresol suppressing microbial effects, but they show that sulfide analysis do change in the sample after collection. Thus concentrations of sulfides may be biased in samples stored for only a few hours.
CONCLUSIONS A simple and reliable method of analysis for volatile sulfides was developed for freshwater samples. The HECD is a key factor for achieving low detectability, multiple component calibration, and reproducibility of the calibration curves. The signal of the detector is linear with the amount of analyte, and the working range is greater than 3 orders of magnitude. Detection limits in the range of picograms of sulfur are generally better than those found in other studies. The column, however, does not decisively separate H a from COS at 40 "C. Replicate analyses of water samples show changes in reduced sulfide concentrations during storage of only a few hours, probably due to microbial activity. An effective preservative agent for these compounds in water is needed. Chemical agents and containers for sample preservation are under investigation at the moment. ACKNOWLEDGMENT Critical comments on the manuscript by Y. K. Chau and P. Takats, technical advice from P. Brassard, and the use of some laboratory installations by H. P. Schwarcz are appreciated. We thank J. 0. Nriagu for sharing with us an unpublished manuscript on DMS in freshwaters (13).
&&try NO.DMS, 75-18-3; DES, 352-93-2; DMDS, 624-92-0; HzS, 7783-06-4; COS, 463-58-1; MeSH, 75-18-3; CSz,75-15-0; SOz, 7446-09-5; PrSH, 111-47-7; HzO, 7732-18-5.
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RECEIVED for review April 18, 1988. Accepted October 13, 1988. This research was supported in part by a Natural Science and Engineering Research Council (NSERC) operating grant for J.R.K.
Measurement of Diffusion Coefficients of C,* Unsaturated Fatty Acid Methyl Esters, Naphthalene, and Benzene in Supercritical Carbon Dioxide by a Tracer Response Technique Toshitaka Funazukuri, Sumito Hachisu, and Noriaki Wakao* Department of Chemical Engineering, Yokohama National University, Yokohama 240, Japan
Blnary dmuslon coeffklents of C,, unsaturated fatty acld methyl esters, naphthalene, and benzene In supercritlcal carbon dloxlde were measured by the Taylor-Ark tracer response technique. The dWfushm coeffldentp of deic, linoleic, and y-tlnolenlc acld methyl esters In SC-C02 at 313 K and 16.0 MPa were found to be inappreciably dmerent from each other. For #nolelc acld methyl ester the dWuslon coeffklents were correlated with temperature In the range from 308 to 328 K, respectively, at constant pressure of 19.0 MPa and at constant denslty of 800 kg/ms.
* Author to whom correspondence should be addressed. 0003-2700/89/0361-0118$01.50/0
INTRODUCTION A considerable attention is being paid to extraction by using supercritical fluid. Especially in the field of food industries, COZ has widely been used as a supercritical fluid. This is due to the fact that COZ is nontoxic and nonflammable, in addition to the low critical temperature and some other advantageous points. One of the physical properties needed for the design of supercritical extractors/separators is the molecular diffusion coefficients under the supercritical conditions. However, measurements of the diffusion coefficients have been limited (1-7). In this work the binary diffusion coefficients for C18 unsaturated fatty acid methyl esters, naphthalene, and 0 1989 American Chemical Society
