Anal. Chem. 1987, 59, 1016-1020
1016
Determination of Sulfide at the Parts-per-Billion Level by Ion Chromatography with Electrochemical Detection Kai H a n ' and William F. Koch* Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
An improved procedure for the determination of sulfkle uslng ion chromatography with electrochemical detection has been developed. Detection iimb have been extended down to 0.1 ng/mL, with linear response up to 1000 ng/mL. Several factors affecting the response of the system to sulfide have been investigated, including condltion of the columns, the arrangement of the columns, purity of reagents, composition of eluent, conatkn of the working electrode, Stabilty of sutflde solutlons, mechanism of retention, and temperature of the system. The two maln sources of error in the determination of sulfide are impurities In the etuent and adsorption of sulfide on the columns. Metal Impurities in the eluent and on the column must be removed to achieve sensitivities below 20 ng/mL. To accompILsh this, a new cokmn cleanlng procedure has been developed and a rearrangement in the positlonlng of the columns Is recommended.
Ion chromatography (IC) with electrochemical detection (EC) has proven to be a very useful technique for the determination of trace anions, such as cyanide, sulfide, iodide, and bromide. Bond e t al. used a gold working electrode to determine HS- and CN- (I). Their detection limits were 100 ng for HS- and 250 ng for CN- (100 pL injection volume). When a pulse method with a mercury-coated platinum electrode was used, the limits were reduced to 25 ng and 150 ng, respectively. Koch used a dual-piston pump t o reduce flow pulsations and, after optimizing chromatographic conditions, determined trace cyanide in dust samples (2). This method provided good linearity over the range 1-lo00 ng/mL. Rocklin e t al. separated and detected HS-, CN-, I-, and Br-. Their detection limits were 30, 2, 10, and 10 ng/mL, respectively (3). However, if HS- was below 20 ng/mL, the peak became irreproducible or a t times would disappear; they suggested several chemical reactions to explain this observation. Mu et al. determined HS- and CN- simultaneously in wastewater ( 4 ) . The detection limits were 8.9 and 0.25 ng/mL. Below 8.9 ng/mL of HS-, the calibration curve became nonlinear. Hence, IC-EC is a very useful technique for determining trace cyanide, but for sulfide there still remain many factors that limit its application and range. The goals of the research reported in this paper were to investigate these factors, which include contamination from the eluent, oxidation of sulfide, absorption of sulfide on the analytical columns, temperature effects, and electrode response factors, and to optimize the experimental conditions for the determination of sulfide at trace levels.
EXPERIMENTAL SECTION Reagents and Standard Solutions. High-purity water required for this research was prepared by passing distilled water through a commercially manufactured ion-exchange water-pu'Guest worker at the National Bureau of Standards. Current address: The Institute of Chemical Metallurgy, Academia Sinica, Beijing, The People's Republic of China.
rification system. The water was then boiled for 30 min to drive off dissolved oxygen. Reagent-gradeethylenediamine (EDA)with an assay of 99.7-99.8% was stored in a refrigerator and thawed at room temperature before use. Only colorless or faintly yellow EDA should be used. EDA that is dark yellow will contaminate the eluent. The sulfuric acid was purified by subboilingdistillation (5). All other chemicals used in this work met ACS reagent-grade specifications. The stock solution of sulfide (approximately 1.0 mg/mL) was prepared by dissolving NazS-9Hz0in 0.05 mol/L NaOH. The solution was stored in a polyethylene bottle in the refrigerator at 4 "C. It was standardized before use by the iodometric titration method (6). Working solutions of sulfide were prepared by serially diluting the stock solution with the eluent. Stock solutions of 1.0 mol/L NaHZBO3and 0.1 mol/L Na2C03were prepared and stored in polyethylene bottles. The eluents, admixtures of EDA, NaH2BO3,and NaZCO3as specified below, were prepared daily. Apparatus. The ion chromatograph was a modified commercial unit (Dionex,Model 10)with a dual-piston pump replacing the original single-piston pump in order to reduce pulsations. An electrochemical detector (Dionex, Model 35221) was used for amperometric detection. This system uses a three-electrode potentiostat. Data output was recorded with a computing integrator (Spectra Physics, SP4100) or with a strip chart recorder. The separations were achieved on commercial anion exchange columns (DionexAG2 and AS2). No suppressor column was used. The sample loop was 100 p L . Two compositions of eluents were used in this research. The first, designated E(I), was an admixture of 10 mmol/L NaH2B03,1.0 mmol/L NazC03,and 14.8 mmol/L EDA. The second, E(II), was an admixture containing 20 mmol/L NaH2BO3,2.0 mmol/L NazC03,and 14.8 mmol/L EDA. Eluent flow rate was 4.0 mL/min. The system pressure with the AG2 and AS2 columns installed was 15.9 kPa (430 psi) with a pulsation of less than 0.7 kPa (20 psi). The potentiostat used a silver working electrode, a silver/silver chloride reference electrode, and a stainless steel counter electrode. The applied potential at the working electrode, relative to the reference, was +0.06 V. Quantitation of the current was accomplished by measurement of the peak height from the analog output of the current-to-voltage converter of the detection unit. No special precautions were taken to thermostat the system; however, the temperature of the laboratory was maintained at 23.5 f 0.5 "C. Column Conditioning. After extended use of the AS2 column for sulfide determinations, it was found that some dark rings or bands appeared on the resin column. Spark source mass spectrometric analysis of these dark bands after elution from the column with dilute HCl indicated that these bands are due to sulfides of tin and transition metals. These impurities come from different sources, e.g., tin from the tin-lined pipes used in the distilled water system at NBS; iron, chromium, and nickel from the metal pump system; and copper, lead, and zinc from the reagents, especially from boric acid and sodium hydroxide. The accumulation rate for these metals on the column is about 2 pg per liter of flowing eluent. These metal sulfides could not be removed by using the customary basic solution of 0.1 M NaZCO3 nor by using sodium tartrate (7, 8). The accumulation of the metals causes deterioration in the ability to detect and quantitate sulfide. Hence, it was essential to develop a cleaning/conditioning procedure that could be used prior to sulfide determinations. The conditions for this procedure are much more critical for this application than for the determination of most other anions, due to the oxidizability of sulfide. It should be noted that if there
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
are metals in the sulfide samples, they will also accumulate on the column and eventually degrade the performance at low levels of salde. (These metals in the samples will have already affected the "free" concentration of sulfide in the individual samples.) More frequent cleaning of the columns will be necessary if metals are present in the samples. For convenience, a separate pumping system was used to perform the column cleanup. Various procedures using different acids and solvents were tried. The procedure that worked best used high-purity (metal-free) (5) 0.5 mol/L HzS04in filtered deionized water as the cleaning solution. An additional guard column (Dionex AG2) was placed between the pump and the columns to be cleaned in order to trap the impurities from the reagents and pump. The columns were first rinsed with deionized water for 10 min at 2.0 mL/min. Then the cleaning solution was pumped through the columns for 30 min at 2.0 mL/min. After the column was rinsed again with deionized water until the effluent pH was 7,O.l mol/L Na2C03was pumped through the columns for 30 min at 2.0 mL/min to regenerate the resin in the carbonate form. The columns were rinsed with deionized water for 5 min, and then reinstalled in the ion chromatograph. The columns could then be equilibrated with the desired eluent. This cleanup procedure differs from the manufacturer's recommended procedure in the use of the guard column prior to the columns being cleaned, in the use of high-purity sulfuric acid, and in the thorough rinsing with deionized water to elute all sulfate to maximize column efficiency. All columns used in this experiment were cleaned by this procedure, which provided columns with adequate efficiency to determine sulfide at the nanogram per milliliter level with good linearity and reproducibility. To check the efficiency of a cleaned column, a flow injection amperometric (FIA) method was used, i.e., removing the resin columns from the system, but leaving everything else intact. This method allows one to determine the maximum signal (peak height) for sulfide at the detector with no interference, broadening, or complication from the column but with the concomitant loss in selectivity for sulfide afforded by the ion-exchange separation. By use of the FIA, 0.5 ng/mL of sulfide could easily be detected at 300 nA/V output. The response was very linear from 0.5 to 20 ng/mL, with a correlation coefficient of 0.9998 and a response factor of approximately 7 nA/ (ng/mL). This method revealed that problems due to poor sensitivity and linearity are primarily caused by the columns. In general, the peak height obtained by using freshly cleaned columns was about 60% of that obtained by FIA. Cleaning and Equilibration of the Working Electrode. In order to obtain reproducible results, the silver working electrode had to be cleaned often and equilibrated with the eluent and analyte. Ordinary toothpaste and emery 302 powder were used to polish the silver working electrode. It was found that for a newly cleaned working electrode, the peak heights of the first few injections of 1 pg/mL were always low. Only after ten or more injections did the peak height gradually become constant. Even the FIA method produced these results. It was concluded that a certain, small amount of sulfide was needed to condition and equilibrate the surface of the working electrode before a stable steady state was established. This implies that having a newly cleaned working electrode does not guarantee having an electrode in the best working condition. As the cleaning agent, toothpaste performed better than emery powder. The peak height obtained with an electrode cleaned with toothpaste was about 17% higher than that obtained when emery powder was used. This difference is probably caused by the very fine particles of AlZ0, or other impurities in the emery powder adhering to the surface of the electrode and subsequently adversely affecting the electrochemical reaction of sulfide at the electrode. R E S U L T S A N D DISCUSSION Applied Potential at t h e Working Electrode. It is important to chose the applied potential at the working electrode so as to maximize the response to the analyte of interest and to minimize the effects of interferences. Because sulfide and cyanide can be determined simultaneously by IC-EC, it is advisable to use a mixture of the two to determine the optimal applied potential at the working electrode. Replicate injections
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Flgure 1. Effect of applied potential of silver working electrode vs. Ag/AgCI reference electrode: (a) e,300 ng/mL sulfide and 300 ng/mL cyanide, eluent E(I), guard column before injection loop, potential ramped from to -; (b) D, same as a, except potential ramped from - to +; (c), 0 , 400 ng/mL sulfide, no cyanide, eluent E(I), no guard column before injection loop, potential ramped from -k to -; (d) A, same as c, except potential ramped from - to +; (e) V, 300 ng/mL sulfide, no cyanide, eluent E(II), guard column before injection loop, potential ramped from ito
+
-.
of a mixture were made while varying the applied potential. The data obtained indicated that this seemingly simple approach became rather complicated. The optimum applied potential for sulfide in a mixture of 300 ng/mL sulfide and 300 ng/mL cyanide was dependent on the direction that the potential was ramped and varied from +O.OO to +0.06 V vs. Ag/AgC1 (Figure 1). Solutions containing only sulfide did not show this dependency on direction of ramping. At very low concentrations of sulfide and cyanide, this phenomenon was insignificant. A probable explanation is that the cyanide reacts with the silver working electrode to form AgCN or Ag(CN)2-, thereby changing the nature of the surface of the working electrode and causing the change in the optimal applied potential for sulfide. At high concentrations of CN-, this effect significantly affects the response of the electrode toward sulfide. Additional experiments were performed a t +0.02 and at +0.06 V vs. Ag/AgCl using solutions containing only sulfide. Various eluents were used. The results are shown in Table I. The peak height at 0.06 V was consistently higher than a t 0.02 V and is chosen as the optimal applied potential. Chromatograms of sulfide and cyanide are shown in Figure 2, indicating good resolution and sensitivity. P u r i t y and Concentration of Eluent. The purity and concentration of the eluent have a direct effect on the sensitivity and the linearity of sulfide determinations. It was observed that when ethylenediamine (EDA) which was old and dark yellow in color was used, the slope of a 1-20 ng/mL calibration curve declined by 4.4% relative to the case when a fresh bottle of EDA (water-white in appearance) was used. In addition, Figure 3a and Table I show that both a lack of EDA and an excess of EDA cause a decrease in peak height. Although the eluent containing 7.4 mmol/L EDA yielded a slightly higher peak height than that with 14.8 mmol/L EDA, the less concentrated eluent exhibited poorer linearity below
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
Table I. Effects of Eluent Composition and Applied Potential sulfide peak height, mm eluent composition NaH2B03,
a
20 ng/mL
EDA,
mmol/L
Na2C03, mmol/L
mmol/L
10 10 10 10 10 20 30 40
10 10 10 10 10 20 30 40
0.0 1.4 14.8 22.2 29.6 14.8 14.8 14.8
1 ng/mL
PH
0.06 V" 100 nAIVb
0.06 V" 10 kA/Vb
0.02 V" 10 kA/Vb
10.5 10.9 11.0 11.1 11.2 11.1 11.1 11.2
68 246 245 233 226 312 355 385
77.0 97.0 98.5 96.3 93.8 175 204 229
71.7 90.0 93.1 91.0 90.0 165 201 223
Applied potential of working electrode vs. Ag/AgCl. Sensitivity setting of detector. HS -
400
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Figure 2. Chromatogramsof 20 ng/mL sulfide and 20 ng/mL cyankle: (a) eluent E(I), (b) eluent E(I1).
4.0 ng/L. The recommendation with respect to EDA is to use only freshly opened bottles of EDA or recently redistilled EDA. The preferred concentration of EDA in the eluent is 14.8 mmol/L. In a side experiment, ammonia was substituted for the EDA. However, with this eluent no response to sulfide was obtained. With the concentration of EDA at 14.8 mmol/L, the peak height of sulfide increased as the concentrations of NaH2B03 and Na2C03increased (Figure 3b). Hence, high concentrations of these components are advantageous from the standpoint of sensitivity, speed of elution, and minimization of absorption of sulfide on the columns (see section below). However, one must bear in mind that high concentrations of eluent also mean increased levels of contaminants from the reagents. Furthermore, additional experiments have shown that high concentrations of eluent were not suitable for the determination of sulfide below 1ng/mL because of unstable base lines and poor reproducibility. An admixture of 10 mmol/L NaH2B03, 1.0 mmol/L Na2C03, and 14.8 mmol/L EDA (designated herein as E(1)) is recommended for sulfide determinations in the range of 0.1-1.0 ng/mL. An admixture of 20 mmol/L NaH2B03, 2.0 mmol/L Na2C03, and 14.8 mmol/L EDA (herein designated as E(I1)) is recommended for sulfide determinations in the range of 1-1000 ng/mL. Stability of Low Concentration Solution of Sulfide. Before problems with the chromatography were looked a t to explain the difficulty in determining sulfide below 20 ng/mL, it first had to be established that solutions of sulfide at that level were stable. A 20 ng/mL solution of sulfide was prepared
40 0
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(a) Concentration of EDA (rnrnol/L) 1 1
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8.4 13.2 17.6 22.0 26.4 30.8 35.2 39.6 44.0
(b) Concentration of NaH2BO3(rnrnollL)
Figure 3. Effect of eluent composition, 20 ng/mL sulfide, 0.06 V vs. Ag/AgCI, 100 nA/V: (a) 0 , effect of concentration of EDA, other components in eluent are 10 mmol/L NaH,CO, and 1.0 mmol/L Na,CO,; (b) A, effect of varying concentrations of borate and carbonate at a fixed 1O:l ratio of the two components, and at a fixed concentration of EDA (14.8 mmollL).
by using freshly made eluent (E(II),pH 11). The peak height of this solution was established immediately. The solution was stored in a borosilicate glass volumetric with a groundglass stopper. Over the subsequent 4 h, the stopper was removed and replaced several times to expose the solution to air. After 4 h the peak height was again determined and compared with a freshly prepared 20 ng/mL sulfide solution. No significant change in peak height was observed indicating that air oxidation of sulfide is not a major contributing factor to the nonlinearity observed for sulfide at these levels over a period of 4 h. Adsorption of Sulfide on the Ion Exchange Columns. Three interesting phenomena were observed in the course of this study which led us to believe that adsorption of sulfide on the columns was a critical factor. First, at the beginning of each day's experiments, the first few injections of sulfide solutions always gave low peak heights, even if the silver
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
working electrode had been already conditioned by the FIA method. Second, if more than 10 min elapsed between injections of low level sulfide solutions, the reproducibility of peak heights was very poor, and the base line would gradually drift downward. Third, the response factor for sulfide determinations below l ng/mL was always less than that for higher levels. The following experiment was conducted to confirm that adsorption was the cause of irreproducible sulfide values. (The problem with metal complexation of the sulfide has been alleviated by removing metal impurities in the eluent with a precolumn prior to the injection loop, as will be discussed in a later section.) With the detector disconnected from the eluent stream, 25 injections of 1 wg/mL sulfide were made, eluting with the weaker eluent, E(I), in order to equilibrate the columns with sulfide. The detector was then reconnected, and for the next five injections, the peak heights for sulfide were measured. After the last injection, the eluent was allowed to flow through the system for 8 min to establish a stable base line. It is usually assumed that at this point all the injected sulfide has been eluted. The detector was again disconnected and the eluent was changed to a very strong one, twice the concentration of E(I1). If any sulfide remained on the columns, this eluent should remove it, or at least more of it than the weak eluent. The eluate was collected in a 50-mL volumetric flask and diluted with deionized water. Analysis of this solution revealed that about 40 ng of sulfide was recovered. Verification of this result was obtained by a second identical experiment. It can therefore be concluded that in addition to the ion exchange retention mechanism for sulfide on these columns, which results in a nicely eluted sulfide peak, there is also an absorptive retention mechanism, albeit at much lower capacity, which holds the sulfide much more tightly. The first several injections of sulfide at the start of an experiment serve to saturate the absorption sites, resulting in lower peak heights for sulfide. Once the absorption sites are filled (although remembering that it is a dynamic equilibrium), then the ion exchange processes dominate the retention mechanism, and good reproducibility of peak heights results. The absorbed sulfide is slowly eluted, even with the weak eluent, although the elution rate is very slow causing very little base line disturbance except at the high sensitivities required for subnanogram-per-milliliter determinations. For high accuracy work, it is imperative that the absorption sites remain saturated. This becomes increasingly difficult as the levels of sulfide to be determined decrease to below the level needed to replenish the absorption sites. Consequently, extra care must be taken when analyzing low sulfide samples. A similar effect seems to influence the determination of cyanide, but this was not fully investigated. There is evidence from the studies of the system in the FIA mode that the tubing used in the system also absorbs some sulfide. However, the amount absorbed by the tubing is negligible compared to that absorbed by the columns. In addition to the problems associated with the columns, the analyst must be vigilant to assure that the working electrode remains in a conditioned mode. This analytical scheme for sulfide has several interdependent variables, all of which must be controlled to obtain accurate and reproducible results. Column Arrangement. It appears that the two primary causes for the decrease in sensitivity and the nonlinearity of sulfide at the nanogram-per-milliliterlevels are contamination in the eluent and absorption of sulfide on the columns. Both of these problems can be minimized by judicious choice and arrangement of the columns. First, a guard column (Dionex AG2) is placed just before the sample injection loop. This serves to trap impurities, thereby cleaning the eluent of
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Concentration of Sulfide (nglmL) Figure 4. Calibration curves for sulfide, 1-20 ng/mL, 0.06 V vs. Ag/AgCI, 100 nA/V: (a) 0 , with guard column before injection loop, eluent E(I1); (b) 0 , with guard column before injection loop, eluent E(1); (c), A, without guard column before injection loop, eluent E(I1); (d) W, without guard column before injection loop, eluent E(1).
contaminants just before the sample enters the eluent stream. This column must be cleaned at regular intervals by the procedure described above. Second, the amount of absorption of sulfide on the columns is minimized by using as short a column as possible without compromising separation and resolution. The use of the separator column (Dionex AS2) alone, without a guard column after the sample injection loop, is recommended. Chromatograms of sulfide are shown in Figure 2, using eluents E(1) and E(I1). The retention time for sulfide varies with the strength of the eluent and is approximately 1.1 min for E(I1) and 1.7 min for E(1). Dramatic improvements in results are obtained after incorporating the above two changes, as shown in Figures 4 and 5. Calibration curves 4c, 4d, and 5c in these figures are based on data using the customary column arrangement with the guard and separator columns in series after the injection loop. Calibration curves 4a, 4b, 5a, and 5b are based on data using the new arrangement of columns, i.e., guard column before the loop and separator column after the loop. The linearity in response, especially below 4.0 ng/mL, is very good with the new arrangement of columns, permitting quantitation at much lower levels. The correlation coefficients for three ranges (1.0-1000 ng/mL (not shown), 1.0-20 ng/mL, and 0.1-1.0 ng/mL) are 0.9995, 0.9998, and 0.9997, respectively. The detection limits have been improved by 2 orders of magnitude over previous methods ( 3 , 4 ) . The choice of eluent depends on the range of sulfide to be determined. The useful working range of E(I1) is 1 to 1000 ng/mL sulfide. The main advantage of using E(I1) is that it elutes sulfide faster, yielding sharper peaks. Hence, the response factor is higher. The peak height response of a 20 ng/mL sulfide solution using the 300 nA/V sensitivity setting was 129.2 mm with a standard deviation of 0.564 mm (relative standard deviation 0.4%, n = 11). A further advantage of using E(I1) is that fewer injections of sulfide solutions are needed to equilibrate the column, because the more concentrated eluent reduces the absorption effect. However, the base line is not as stable when using E(II), with a tendency to drift
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987 I
l
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Table 11. Comparison between Peak Area and Peak Height
Measurements peak height peak area measurements measurements sulfide, response RSD," response RSD," 70 height factor % ng/mL area factor
46.8 r 41.6 k-
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36-4 31.2
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Without a Guard Column before Injection Loop; AG2 and AS2 Column after Injection Loop 18.0 13.5 9.0 4.5 0.9
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19.4
47.7 36.4 23.7 11.0 1.7
2.650 2.696 2.633 2.444 1.888
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With a Guard Column before Injection Loop; AS2 Column after Injection Loop
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4.361 4.407 4.311 4.311 4.444
1.4
Relative standard deviation.
1.0
Concentration of Sulfide (ng/mL)
Flgure 5. Calibration curves for sulfide, 0.1-1.0 ng/mL, 0.06 V vs. Ag/AgCI, 30 nA/V: (a) a, with guard column before injection loop, eluent E(I1); (b) A,with guard column before injection loop, eluent E(1); (c) 0 , without guard column before injection loop, eluent E(1).
downward. E(I1) also has twice the impurities as E(I), with their concomitant problems. The useful working range of E(1) is 0.1-400 ng/mL. This eluent gives a more stable base line at the more sensitive detector settings. However, the response factor is slightly lower. The reduced levels of impurities in E(I) are a very important consideration when analyzing samples with less than 1ng/mL sulfide. The reproducibility of measurement, as evidenced by the relative standard deviation of replicate injections (n = 5 ) , ranged from about 4% at 1ng/mL sulfide to 12% at 0.2 ng/mL sulfide. At 0.1 ng/mL sulfide, the relative standard deviation was 25 % , and thus, this level is considered to be the limit of quantitation. Mode of Quantitation. A comparison was made between using peak height and peak area for quantitation, to determine which mode was preferable. Further, we wanted to check if the nonlinearity below 4 ng/mL sulfide may be caused by using peak height rather than the theoretically correct peak area. The results are tabulated in Table 11. In all cases, the peak height mode gave better response factors than peak area, as indicated by the relative standard deviation. The nonlinearity below 4 ng/mL, when using the customary column arrangement, was just as evident in the peak area mode as the peak height mode. Since peak height is easier to measure and gives better response factors, it is the mode of choice. Effect of Temperature. The effect of temperature on the determination of sulfide by IC-EC was determined by immersing the eluent reservoir, the separator column, and the sample solutions in a constant temperature bath. The temperature was varied from 11.0 to 35.0 "C. The results are shown in Figure 6. Under the experimental conditions, the peak height increases linearly with temperature from 11 to 25 "C, the slope being 9.15 mm/"C. Above 25 "C, the effect becomes nonlinear. At room temperature, 23 "C, the temperature coefficient is 2.9% per degree. If high precision is needed in an analysis, then temperature control must be established.
10
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Temoerature ('C)
Figure 6. Effect of temperature on peak height: 18 ng/mL sulfide: applied potential, 0.06 V vs. Ag/AgCI; sensitivity, 300 nA/V.
ACKNOWLEDGMENT We acknowledge the analytical work of Paul J. Paulsen, who performed the mass spectrometric determination of the metals on the column. Registry No. Sulfide, 18496-25-8. LITERATURE CITED Bond, A. M.; Heritage, 1. D.: Wallace, G. G. Anal. Chem. 1982, 5 4 , 582-585.
Koch, W. F. J . Res. Natl. Bur. Stand. ( U . S . ) 1983, 88, 157-161. Rocklin, R. D.; Johnson, E. L. Anal. Chem. 1983, 5 5 , 4-7. Mu Shifen, Han Kai, Luo Yuanzhang, Hou Xiaoping FenXl HauXue 1885, 6 , 457-460. Moody, J. R.; Beary, E. S. Talanta 1982, 29, 1003-1010. Standard Methods for the Examination of Water and Wastewater, 13th ed. 1971; pp 552-553. APHA. AWWA. WPCF. Dionex Corp., Technical Note 2R, 1985; p 6. Ion Chromatcgraphy: Mutou, Guchi, Kawanori, Hisho. Eds.; Kodansha: Tokyo, 1983; p 75.
RECEIVED for review September 8, 1986. Accepted December 1, 1986. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment is necessarily the best available for the purpose.
