2
Anal. Chem. 1983, 55, 2-4
Separation of Sulfite, Sulfate, and Thiosulfate by Ion Chromatography with Gradient Elution Thomas Sunden, * Mats Llndgren, and Anders Cedergren Department of Analytical Chemistry, University of Ume a, S-90 1 87 Ume A, Sweden
Darryl D. Slemer Exxon Nuclear Idaho Company, Inc., P.O. Box 2800, Idaho Falls, Idaho 83401
A simple gradient apparatus, consisting of a perlstaltic pump in addltlon to a standard hlgh-pressure pump, Is descrlbed. The device is used to make a slngle-run ion chromatographic separation of sulfite, sulfate, and thiosulfate In less than 15 min. This separatlon requlred a step gradlent with 4.8 mM NaHC0,/4.7 mM Na,CO, as start eluent and 6.9 mM NaHCOJ8.6 mM Na,CO, as final eluent when two (4 X 50) mm Dionex anion precolumns In serles were used as separator. The eluent compositions were slmplex optlmlzed.
Ion chromatography (IC) (1)has been shown to be a suitable analytical technique for determination of sulfur anions such as sulfite, sulfate, and thiosulfate. One of the major problems concerned with these analyses is that the retention time for thiosulfate is unacceptably long when determined under the same ion chromatography (IC) conditions as for sulfite/sulfate. Holcombe e t al. (2) have reported IC determinations of these sulfoxy ions, but two separate IC runs with different eluents for sulfite/sulfate and thiosulfate, respectively, were required. Trujillo et al. (3) reported on the use of a very short column with high eluent flow rate to elute thiosulfate in about 4 min. However, this short retention time for Sz032-caused the other anions present, such as F-, C1-, NO;, NO3-, PO?-, and Sod2to elute as a single peak. As a consequence, two separate IC runs are required for the sulfite, sulfate, and thiosulfate analyses. Gjerde et al. (4, 5 ) have shown chromatograms with thiosulfate and sulfate well separated, with a retention time for thiosulfate of about 12 min. However, their tabulated retention times for sulfite and sulfate indicate poor separation of these ions. The purpose of this paper was to investigate the possibility of separating sulfite, sulfate, and thiosulfate in an acceptable time in a single IC run.
EXPERIMENTAL SECTION Instrumentation. A diagram of the complete ion chromatograph is presented in Figure 1. The system consisted of the following parts: (1)a Constametric I11 Pump (Laboratory Data Control) which delivers the eluent at a constant flow rate through the sample loop, the analytical column, the suppressor column and the conductivity cell; (2) a stepper motor driven peristaltic pump P-1 (Pharmacia Fine Chemicals) used to generate gradients in eluent strength, the pump speed was controlled by a homemade variable frequency sweep generator (F.S.G.);(3) a sample injection valve (Rheodyne 70-10) with 50-, loo-, or 200-pL sample loops; (4) two plastic (4 X 50) mm anion precolumns (Dionex Corp., catalog no. 030825) were coupled in series t o serve both as precolumn and analytical column; ( 5 ) a laboratory made (5.7 X 300) mm glass column packed with Amberlite AG, 100-200 mesh (Serva AG), in the hydrogen form serves as suppressor column, the suppressor was regenerated with 0.5 M H2S04;(6) a Conducto Cell with Conducto Monitor (LDC) to serve as the detector; (7)
a Vitatron two-channel recorder to trace the chromatograms. Chemicals. All solutions were prepared in doubly deionized water using reagent grade chemicals. The sulfite standard solutions were prepared from HOCH2S03Na(98% Aldrich) in order t o avoid oxidation to sulfate (6). The sodium hydrogen carbonate and disodium carbonate solutions, of equal molarity (e.g., 5 mM), for the gradient elution experiments were prepared in the following manner: (1)1000 mL of 5 mM NaHC03was made by using the dried salt (Merck p.a.). (2) This solution was divided in two equal parts. One was set aside and the other was converted to carbonate by replacing 500 p L of the solution with a 500-pL aliquot of 5.00 M NaOH. The Gradient System. There are two ways of effecting gradient elution in liquid chromatography, viz., (i) high-pressure mixing and (ii) low pressure mixing. The former, conventional approach requires two high-pressure pumps and a solvent programmer. An alternate, less expensive, approach requires only one high-pressure pump. To create the gradient, this system uses a peristaltic pump in addition to the high-pressure liquid chromatography (HPLC) pump. This peristaltic pump (P-1) is connected to the suction side of the HPLC pump. When P-1 is off, only eluent A (see Figure 1)is pumped through the chromatograph. But when P-1 is on, it will force eluent B into a mixing “tee” at the suction side of the HPLC pump. Therefore, the rate of the two eluents (A/B) reaching the column will vary depending on the pumping rate of P-1. Since P-1 is driven by a stepper motor its speed depends only of the driving frequency of the motor. The frequency sweep generator makes it possible to increase and decrease the motor speed as a function of time. The frequency sweep generator consists of two main parts, a ramp generator and a voltage-to-frequencyconverter. The ramp generator produces a voltage ramp with adjustable, positive and negative, slopes. There is also a possibility to hold the output (and thus the output frequency) at any desired value between 0 and +5 V dc. The circuit diagram of the frequency sweep generator is shown in Figure 2.
RESULTS AND DISCUSSION The Gradient System. The initial aim of this work was t o make a gradient elution with no base line shift. This is achieved by starting the gradient with hydrogen carbonate and finishing it with carbonate of the same molar concentration. By this procedure the carbonic acid concentration in the detector should be constant during the whole gradient run. Figure 3 shows a gradient, with 5 mM NaHC03 as eluent A (see Figure 1)and 5 mM Na2C03as eluent B, as it appears on top of the separator column. The line below is the detector base line. The disturbances in the base line, which originate from the separator column, are positive and negative carbonate peaks (humps). These peaks arise from the variations in the ionic strength, for example, when the eluent is changed from 5 mM hydrogen carbonate to 5 mM carbonate. The double negatively charged carbonate ion causes a temporary increase in the carbonic acid concentration in the detector when two hydrogen carbonate ions are replaced by one carbonate ion
0003-2700/83/0355-0002$01.50/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
3
'1" ci
Smin COLUMN SUPPRESSOR COLUMN CONDUCTIVITY DETECTOR
Flgure 1. Schematic diagram of the ion chromatograph.
Figure 4. S O : eluted with (a) 5 mM NaHCO, and (b) 5 mM Na,CO,: flow, 1.4 mL min-'; columns, (4 X 50) -t(4 X 250) mm Dionex anion separators (catalog no. 030825 and 030827) and (5.7 X 300) mm Amberlite AG suppressor.
-5b
V
Flgure 2. Circuit diagrarn of the frequency eweep generator: V/F, Date1 VFQ-1.
;5
1
0
5
'/mi"
10
~A-&------B-/~---A
Flgure 5. Chromatogram of SO3'-, SO4'-, and S,03,- with step gradient and the best eluent composition found by simplex optimization: eluent A, 4.8 mM NaHC0,/4.7 mM Na,CO,; eluent B, 7.2 mM NaHco,/9.1 mM Na2C03.
Table I. Simplex Optimization Facts variables
I/ Figure 3. (a) Typical gradient as it appears on top of the separator column and (b) detector response during the gradlent: eluent A, 5 mM NaHCO,; eluent B, 5 mM Na,CO,.
at the amino groups in the separator resin. In order to minimize base line shift, due to small differences in HCO3-/C0?concentrations, special care in making the eluents was necessary. The recommended procedure is described in the Experimental Section, The higher eluting strength of carbonate compared with hydrogen carbonate gives rise to a decrease in retention time of about 25% for, e.g., sulfate (Figure 4). This difference in retention times might be sufficient for certain applications, but the effect is not adequate in the case of strongly retarded anions such as thiosulfate. The increase in p H when going from hydrogen carbonate to carbonate makes it possible to control the charge of, e.g., phosphate and thus its retention time. If, for example, nitrate and phosphate coelute with an eluent of a pH where HP042exists, you are able to change the pH of the eluent so Pod3elutes with a longer retention time than nitrate. This is simply and quickly done with the equipment and the type of eluents described herein. The fact that these eluents give the same carbonic acid concentration in the detector causes no base line shift when changing eluent pH. Moreover, the possibility of altering the pH without long waiting times is useful in method development work. In order to resolve sulfite, sulfate, and thiosulfate in a reasonable time, we had to abandon this initial approach. A much greater change in eluent ionic strength during the IC
total eluent flow eluent B flowa separator column
3.0 mL min-' 2 Dionex precolumns ( 4 X 50) mm in
series suppressor column (5.7 X 300) mm Amberlite AG, 100-200 mesh 200 /.lL sample loop 50 ppm SO,", 25 ppm SO,:' sample 100 ppm S,O,2a Peristaltic pump was turned on 3 min after injection.
run apparently was necessary, to shorten the time between the elution of sulfate and thiosulfate. Studies showed that steep gradients were required to fulfill the above mentioned requirements, and in practice, a limiting step gradient, controlled only by the mixing of the two eluents, was employed. However, to find the best conditions for this eluent change, a simplex optimization of the eluent composition was done. When we switch on the peristaltic pump a t a certain time after the injection, an eluent change will occur after approximately 2 min of delay as shown as a base line shift on the recorder. Simplex Optimization. A function minimization with four variables was done with the simplex method (7). The variables were NaHCO, and Na2C03concentrations in eluent A and eluent B, respectively. The function to be minimized was the sum of three terms, namely, the thiosulfate retention time, an estimate of the separation between sulfite and sulfate, taken as the distance from the base line to the valley between the peaks, and an estimate of the separation of the sulfate peak from the base line shift due to eluent change. The chromatographic specifications are given in Table I. The best result obtained with this formulated function is shown in Figure 5.
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Anal. Chem. 1983, 55, 4-7
Although the retention time for thiosulfate is 10 min, the total time for a full cycle, including restabilization of the base line with eluent A, is about 15 min. This is mainly due to the tailing of the thiosulfate peak and to a lesser extent depending on the restabilization of the base line, which requires approximately 1 min. In order to decrease the severe tailing of the thiosulfate peak, experiments were performed with B40$-in both eluents as a modifier. However, the extent of tailing was not reduced and the sulfite/sulfate separation deteriorated. It is possible that the severe tailing of this peak not only is due to adsorption effects in the separator column but also is caused by decomposition of thiosulfate when it is acidified in the suppressor column. This, however, is not confirmed. The use of eluents with concentrations high enough to elute thiosulfate in about 10-12 min shortens the lifetime of the suppressor column. T o cope with this problem, it is possible to use two suppressors in parallel, alternatively using one while regenerating the other. Another possibility is the use of the hollow fiber suppressor, with continuous regeneration, recently described by Stevens et al. (8). Note: Eluent suppression ion chromatography is covered
by patents in several countries.
ACKNOWLEDGMENT The authors wish to thank Lars Lundmark and Svante Jonsson for the electronic and mechanical constructions. LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. (2) Holcombe, L. J.; Jones, B. F.; Ellsworth, E. E.; Meserole, F. B. I n "Ion Chromatographic Analysis of Environmental Pollutants"; Ann Arbor Science: Ann Arbor, MI, 1979; Vol. 2, pp 401-412. (3) Trujillo, F. J.; Miller, M. M.; Skogerboe, R. K.; Taylor, H. E.; Grant, C. L. Anal. Chem. 1981, 53, 1944-1946. (4) Gjerde, D. T.; Fritz, J. S.;Schmuckler, G. J . Chromafogr. 1979, 186, 509-519. (5) Gjerde, D. T.: Schmuckler, G.; Fritz, J. S.J . Chromatogr. 1980, 187, 35-45. (6) Llndgren, M.; Cedergren, A.; Lindberg, J. Anal. Chim. Acta 1982, 14 I , 279-286. (7) Nelder, J. A.; Mead, R. Computer J . 1965, 7 , 308-313. (8) Stevens, T. S.; Davls, J. C.; Small, H. Anal. Chem. 1981, 53, 1408-1 492.
RECEIVED for review July 26, 1982. Accepted September 29, 1982.
Determination of Cyanide, Sulfide, Iodide, and Bromide by Ion Chromatography with Electrochemical Detection Roy D. Rockfln" and Edward L. Johnson Dionex Corp., 7228 Titan Way, Sunnyvale, California 94086
Cyanlde, sulfide, iodlde, and bromide are separated and detected by uslng ion chromatography (IC) and electrochemical detection via a sllver working electrode. The detection limlts are 2 ppb, 30 ppb, 10 ppb, and 10 ppb, respectlvely. Cyanide and sulflde can be determined simultaneously, as well as with other anions commonly determtned by IC. Cyanlde contained in Cd and Zn complexes Is quantitatlvely determined as total "free" cyanide, whlle cyanide contained In NI and Cu complexes Is only partlally determlned as "free" cyanlde. The strongly bound cyanlde in Au, Fe, or Co complexes Is not detected.
Although ion exchange techniques can easily separate cyanide or sulfide from a host of common anions, their detection via common methods such as conductivity is very poor. During an IC analysis, the weakly acidic species HCN and H2S are formed in the anion suppressor column. Unlike the halogen acids, they are not detected by the conductivity detector due to their low dissociation and, therefore, low conductivity. This inability to detect cyanide and sulfide has prevented the exploitation of the separating power of ion chromatography (I) for the determination of these ions. In all the analytical methods so far developed for cyanide and sulfide, removing interferences is a necessary first step when analyzing most samples. In addition to interfering with each other, other species interfering with cyanide and sulfide determination include the halogens, thiocyanate, and thiosulfate. The traditional wet chemical analytical method for cyanide, including the removal of interferences, involves precipitating sulfide with cadmium ion, filtering, acidifying,
and distilling the sample (2). The cyanide is trapped in a sodium hydroxide solution, which is usually assayed by argentometric titration, by spectrophotometry, or by an ion selective electrode. The entire process takes approximately 2 h. Samples which can be analyzed directly without distillation are those which are known not to contain significant quantities of interfering species. Sulfide is usually determined by precipitating sulfide with zinc ion, filtering, and then acidifying the precipitate. This solution can be assayed by iodometric titration, spectrophotometry (methylene blue method) or by an ion selective electrode. This procedure takes approximatley h per sample. Electrochemical methods for cyanide determination include amperometry (3-5) and polarography (6). Sulfide can be determined by cathodic stripping voltammetry (7). The polarographic method (6) can determine cyanide or sulfide when in the presence of the other; however, iodide and thiosulfate interfere. Recently Pihlar and Kosta (8, 9) developed an electrochemical method for cyanide analysis by using flow injection analysis (FIA). The method is based on the ability of a silver working electrode in an amperometric electrochemical flowthrough cell to produce a current. The reaction for cyanide is Ag
+ 2CN-
-
Ag(CN)2-
+ e-
The main conclusions from the work of Philar and Kosta are as follows: (1)Current is directly proportional to cyanide ion concentration. (2) The electrode maintains the same sensitivity over long periods of time; i.e., it is not poisoned.
0003-2700/83/0355-0004$01.50/00 1982 American Chemical Society
