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Determination of trace levels of sulphate in water using flow-injection and in-line preconcentration
109
Analyfica Chimica Acta, 244 (1991) 109-113 Elsevier Science Publishers B.V., Amsterdam
Determination of trace levels of sulphate in water using flow-injection and in-line preconcentration M. Karlsson
*, J.-A. Persson
and J. Moller
Tecator AB, Box 70, S-263 21 Hijganiis (Sweden) (Received 8th March 1990)
Abstract Sulphate is preconcentrated on a strong anion-exchange resin and determined using the effect of sulphate ions on the complexation of methylthymol blue and barium. A computer-controlled flow-injection analyser is used to automate the whole procedure. The resin has two functions: it preconcentrates sulphate and also separates sulphate from divalent cations that may interfere in the determination step. The system can handle 30 samples per hour and has a working range from 25 to 1000 ng 1-l of sulphate. Lower detection limits can be obtained by changing the preconcentration conditions. The effect of both anionic and cationic interferents was studied. Keywords:
Flow system; Spectrophotometry;
Sulphate; Waters
Determination of sulphate ions is important in industrial, environmental and biological systems and the need for sulphate measurements is continuously increasing. In comparison with other analytes, there are few analytical procedures for sulphate. The most commonly used procedures are based on turbidimetric measurements of barium sulphate [l] or on spectrophotometric measurements using methylthymyl blue [3], dimethylsulphonazo-3 [2] or 2-aminoperimidine [4,5]. All these procedure have limitations at low sulphate concentrations. With manual methods, concentrations down to about 5 mg 1-l can be detected [6] and with automated procedures about 0.5 mg 1-i can be detected [7]. Further, the spectrophotometric procedures are non-specific owing to the reagents used. Severe interferences from divalent cations occur [3,7-91. During the last decade, ion chromatography (IC) has replaced turbidimetric and spectrophotometric techniques in many application areas. IC offers improved sensitivity (1 mg 1-l can be directly determined) and also superior selectivity 0003-2670/91/$03.50
0 1991 - Elsevier Science Publishers B.V.
[lo]. However, the sample throughput using IC is not sufficient if the number of samples is large. To be able to determine sulphate at low concentration levels all of the above-mentioned methods have to be combined with a preconcentration step. Using IC, anionic preconcentration columns can be incorporated into automated procedures [lo-131. This technique can, however, be of limited value when large amounts of other anions are present in the sample owing to overloading of the analytical column and insufficient peak separation. Tailing of peaks may also occur. Flow-injection analysis (FIA) offers a convenient way of automating analytical procedures. Although more than 2000 papers have been published, only about 5% have dealt with the incorporation of preconcentration steps into FIA procedures [14]. Using FIA, total automation of the analytical procedure including automated regeneration of the preconcentration column can be accomplished. Other advantages of FIA are that low sample volumes are required, the reagent consumption is low and the sample throughput is
M. KARLSSON
110
high. The aim of this work was to explore these features by developing a method for low levels of sulphate using a FIA system.
EXPERIMENTAL
Apparatus A Tecator FIAstar 5010 flow injection analyser with a Model 5023 spectrophotometer (flow cell path length 10 mm and volume 0.018 ml) and a Model 5007 sampler were used (Tecator, Hogan%, Sweden). The manifold consisted of a Tecator Chemifold III. A Plexiglas column (11 mm X 2 mm i.d.) and two PTFE filters with a pore size of 0.5 pm were used. The system was connected to an IBM P/S2 Model 30 computer and Tecator SuperFlow II software was installed to control the instrument functions and for recording, calibration and storage of the results. Reagents and materials All chemicals were of analytical-reagent grade. For calibration potassium sulphate was used. All solutions was prepared using deionized water. For the carrier flow lines sodium chloride solution was used. Methylthymol blue (MTB) {3,3-bis[N,N-bis(carboxymethyl)aminomethyl]thymolsulphonphthalein, sodium salt} (Riedel-de Ha&) was prepared by dissolving 0.1500 g of MTB (purity 64% see below) in a solution which contained 6 ml of 1 M HCl, 80 ml of deionized water and 14 ml of 1.9822 g BaCl, * 2H,O per litre. The final dilution was made with 95% ethanol to a total volume of 1000 ml [7,8]. For optimum response with this reagent, the barium concentration should be equal to the MTB concentration. Because of impurities in the MTB salt, an exact purity determination must be made. In this study the purity measurements were made according to Colovos et al. [7] and resulted in a purity of 64% (w/w). This low purity of the MTB is the main reason for a nonlinear calibration graph [7]. The second reagent was sodium hydroxide. Chemicals employed for the interference studies
ET AL.
were calcium chloride, potassium dihydrogenphosphate, potassium chloride and potassium nitrate. For the anion-exchange column (11 mm X 2 mm i.d.), a strong anion exchanger (Bio-Rad AGl-X8,100-200 mesh) was used (Bio-Rad Labs., Richmond, CA). Column packing and conditioning The column was filled with an aqueous slurry of the resin. The slurry was poured into the column at the top, excess of water was drained carefully by sucking with a syringe at the outlet of the column. When the column was completely filled, care was taken to remove all air bubbles. When the column was taken for use, the only conditioning required was a 5-min wash with the eluent, 0.3 M NaCl. Detection system The sulphate determination is based on the competitive reaction of sulphate and MTB with barium in solution. The absorbance measurement can be performed either on the uncomplexed MTB or on the MTB-barium complex. The absorbance at 460 nm, due to the uncomplexed MTB, will increase, whereas the absorbance at 620 nm, due to the MTB-barium complex, will decrease as the sulphate concentration increases [8]. The relative absorbance decrease at 620 nm is greater than the increase at 460 nm under the same conditions and the baseline stability is the same at both wavelengths [8]. In this study a wavelength of 620 nm was used. Figure 1 shows the manifold for the conventional determination of sulphate without preconcentration. After injection of the sample, an initial reaction at pH 2.5 takes place between sulphate ions and the barium ions in the MTB reagent. Sample
I
1
Fig. 1. Standard preconcentration. = 0.05 M NaOH.
620 “In
manifold for detection of sulphate without C = water; Rl = MTB reagent (pH 2.5); R2
FIA OF SULPHATE
TRACES
111
IN WATER
This reaction is time dependent and it limits the sample throughput [3,7,8]. The secondary reaction is the complexation of barium with MTB occurring at a pH between 12.5 and 13.0 [3,7,8]. This reaction is used for the detection step. Close pH control is needed, as pH variations will give an unstable baseline owing to changes in the background absorbance from the unreacted reagent. Hence large pH differences in the injected samples cannot be tolerated. The detection limit of this system is 0.5 mg 1-l sulphate and a linear calibration graph is obtained 171. Sulphate determination with preconcentration When a system for preconcentration of sulphate is designed, an appropriate ion exchanger for the concentration step must be selected. Also, an eluent system which minimizes possible problems due to pH changes during elution must be selected with care. In a dynamic flow system, changes in the total ionic strength in the vicinity of the eluted sample plug can give rise to spectral problems caused by variations in the refractive index of the liquid. The baseline stability will be poor and low detection. limits cannot be achieved. Choice of ion-exchange resin Bio-Rad AGl-X8 (100-200 mesh) resin was used. The particle size chosen was between 5 and 10% of the column diameter. For the enrichment cycle it was decided that the column should trap at least 1 mg 1-l sulphate without breakthrough. The minimum size of the resin will then be 0.34 ~1 if a flow-rate of 2.6 ml min-’ and an enrichment time of 90 s are used. To ensure that the sample matrix will not elute sulphate during the enrichment cycle, the actual column size chosen was 100 times larger.
ing the elution step could not be controlled. This was probably caused by the large pH difference between the eluent and the sample matrix left in the dead volume of the column. Consequently, sodium chloride was chosen as a neutral eluent, and an ion exchanger in the chloride form was used for the preconcentration step. In this way close control over pH could be obtained; however, severe refractive index problems, caused by salt concentration gradients between the eluent and water in the column volume, still resulted. The minimize these effects, a second carrier was introduced for dilution to level out these effects. To determine the optimum composition of the eluent and the second carrier, various concentrations of sodium chloride were investigated. A large baseline shift was obtained by increasing the ionic strength up to about 0.1 M NaCl and a more or less constant baseline was obtained at the higher concentrations. This means that possible salt gradient problems can be minimized by using a high ionic strength in the second carrier. Any variations in the eluent line will then have a negligible effect. The manifold depicted in Fig. 2 was used to verify that complete elution of sulphate could be obtained from the resin used for preconcentration. The concentration of NaCl in the second carrier
Enrichment column
Sample
S
Cl
RESULTS
AND DISCUSSION
Initially an ion exchanger in the hydroxide form was used for the preconcentration step and sodium hydroxide as the eluent. A buffer was incorporated in the flow system to minimize pH changes. However, the pH changes occurring dur-
Fig. 2. Manifold used for preconcentration and detection of low levels of sulphate. S = sample (2.6 ml min-‘); Cl and C2 = 0.3 M NaCl (both 0.6 ml min-‘); Rl= MTB reagent (pH 2.5) (1.4 ml min-‘); R2 = 0.05 M NaCl (0.6 ml min-‘).
112
was 0.3 M and different concentrations of NaCl were tested as the eluent (carrier 1). At low NaCl concentrations poor sensitivity was obtained. When the concentration was increased the sensitivity also increased. Optimum sensitivity was obtained at NaCl concentrations > 0.2 M. The eluent concentration finally chosen was 0.3 M. Using this set-up, a stable baseline and good sensitivity were obtained. Preferred flow system for preconcentration of sulphate A schematic diagram of the flow system used to preconcentrate and determine sulphate is shown in Fig. 2. The enrichment column is used as the sample loop in the injector, which means that the injection valve can be used for automatic change between sample uptake (preconcentration) and the elution cycle (carrier 1). Initially, sulphate was preconcentrated on the column by aspirating sample for a preset time. Then the injection valve was switched and 0.3 M sodium chloride was pumped through the column to elute sulphate into the system. For the detection the same basic chemistry as shown in Fig. 1 was used. Different working ranges can easily be obtained by changing the enrichment time. In the ion-exchange procedure sulphate ions are retained on the resin during the enrichment step and cations will pass through unaffected. However, at high concentrations of divalent cations interferences can occur from those cations left in the column dead volume. This problem can be eliminated by incorporating a wash cycle using distilled water prior to switching the valve. Any cations in the column will then be washed away. This step can be automated and the wash time can easily be controlled with the software used. The software controls the peristaltic pumps, injection valve and autosampler, which makes it possible to automate completely the whole em-ichment, wash and elution procedure. The timing of these cycles is easily defined or changed to desired values when setting up the method in the software. To avoid transport of air into the system when the sample line is moved from the sample to wash position, pump 2 is stopped for a short period. When the injection valve turns to the inject posi-
M. KARLSSON
ET AL.
tion, the carrier will elute sulphate into the system. The carrier will also regenerate the column at the same time, which means that the column can be used continuously. Calibration In the method used, the working range and detection limit depend on two parameters, flowrate and enrichment time. A nearly linear relationship between enrichment time and the analytical signal is obtained, which makes it easy to adapt the method to the analytical requirements. As an example, an enrichment time of 90 s gives a detection limit of 25 pg 1-l and a cycle time of 180 s for one measurement. The calibration graph is linear between 25 and 1000 pg 1-i sulphate and the repeatibility for consecutive measurements is good, the r.s.d. being less than 1.5%. Interference studies In the direct measurement of sulphate without preconcentration, the main interference is from divalent cations competing with barium in binding to the MTB reagent. To minimize this interference, a cation exchanger can be used to trap the ions. The cation exchanger used can be placed in the sample line or in the manifold. The drawback with this technique is that the column has to be regenerated regularly [ 81. Using the described preconcentration scheme, cationic interferents can be totally removed by a proper wash cycle. In Table 1, the effect of adding various amounts of calcium to the sample is shown. A wash period of 10 s was used and it can be seen that calcium levels up to 200 mg 1-l have no influence on the sulphate determination. For higher levels of divalent ions a longer wash time can be used. Using this preconcentration method, the main interference will be from anions that compete with sulphate in binding to the active sites of the column. The ions that are sorbed to the column can be desorbed by introducing an ion with a higher affinity for the resin or an ion with a lower affinity but a higher concentration. A general idea of possible anionic interferences can be obtained from the data sheet for the specific ion exchanger. The affinity for sulphate to the column used is
FIA OF SULPHATE
TABLE
TRACES
113
IN WATER
1
Determination of 500 pg 1-i sulphate amount of calcium and phosphate Additive
Calcium
Phosphate (PH 7)
Phosphate (PH 2)
containing
Amount added (rng 1-1)
(pg I-‘)
5 50 200 500 0.5 1.0 5.0 50.0 0.5 1.0 5.0 10.0 50.0
490 505 510 305 490 500 650 2300 500 499 505 580 989
different
Sulphate found
centration of HPO:will be reduced by a decrease in pH. At low pH H,PO,/H,PO;will be the major ions in solution; these ions will not be bound as strongly to the resin as the divalent form. The interference from phosphate ions in samples acidified to pH 2 is shown in Table 1. The level that can be tolerated has now increased to between 5 to 10 mg 1-l. Higher concentrations of anions can be tolerated by increasing the size of the ion-exchange resin. The limiting factor will then be for those concentrations of anions where the eluting power of the ions in the sample is so high that sulphate is eluted from the resin during the preconcentration step.
REFERENCES
higher than that for all other common anions occuring in natural samples. In this work the effects of various amounts of chloride, nitrate and phosphate were investigated using the manifold in Fig. 2. The sulphate concentration for these experiments was 500 pg 1-l. The relative selectivities for HSO;, NO, and Cl- to the AG l-X8 resin are 85, 65 and 22, respectively, which means that higher concentrations of chloride than nitrate should be tolerated. For chloride, concentrations up to about 2000 mg 1-l had no effect on the sulphate sensitivity whereas for nitrate only about 200 mg 1-l could be tolerated. Phosphate ions were found to be the most severe interferent. Table 1 gives the sensitivity for sulphate in the presence of various concentrations of phosphate. As can be seen, the tolerance limit for phosphate is between 1 and 5 mg 1-l. This interference can be controlled to some extent by decreasing the pH in the samples prior to the preconcentration. The pK, values for H,PO, are 2.15, 7.21 and 12.36, which means that the con-
1 F.J. Krug, H.B. Filho, E.A.G. Zagatto and S.S. Jorgensen, Analyst, 102 (1977) 503. 2 0. Kundo, H. Miyata and K. Toei, Anal. Chim. Acta, 134 (1982) 353. 3 B. Koch, Fresenius’ 2. Anal. Chem., 329 (1988) 707. 4 A.W. Archer, Analyst, 100 (1975) 755. 5 T. Kamara, K. Suzuki and K. Ohzeki, Anal. Chim. Acta, 136 (1982) 435. 6 Sulphate in Waters, Effluents and Solids, Methods for the Examination of Waters and Associated Materials, H.M. Stationery Office, London, 1979. 7 G. Colovos, M.R. Panesar and E.P. Parry, Anal. Chem., 48 (1976) 1693. 8 B.C. Madson and R.J. Murphy, Anal. Chem., 53 (1981) 1924. 9 J.F. Van Staden, Fresenius’ Z. Anal. Chem., 312 (1982) 438. 10 E. Hoffmann, G. Marko-Varga, I. Csiky and J.A. Jiinsson, Anal. Chem., 25 (1986) 161. 11 G. Marko-Varga, I. Csiky and J.A. Jiinsson, Anal. Chem., 56 (1984) 2066. 12 C.A. Hordjik and T.E. Cappenburg, J. Microbial Methods, 3 (1985) 205. 13 L. Balconi, R. Pascah and F. Signon, Anal. Chim. Acta, 179 (1986) 419. 14 FIAstar, Flow Injection Analysis Bibliography 1974-1988, Tecator, Hogan&, Sweden.
Analyfica Chimica Acta, 244 (1991) 109-113 Elsevier Science Publishers B.V., Amsterdam
Determination of trace levels of sulphate in water using flow-injection and in-line preconcentration M. Karlsson
*, J.-A. Persson
and J. Moller
Tecator AB, Box 70, S-263 21 Hijganiis (Sweden) (Received 8th March 1990)
Abstract Sulphate is preconcentrated on a strong anion-exchange resin and determined using the effect of sulphate ions on the complexation of methylthymol blue and barium. A computer-controlled flow-injection analyser is used to automate the whole procedure. The resin has two functions: it preconcentrates sulphate and also separates sulphate from divalent cations that may interfere in the determination step. The system can handle 30 samples per hour and has a working range from 25 to 1000 ng 1-l of sulphate. Lower detection limits can be obtained by changing the preconcentration conditions. The effect of both anionic and cationic interferents was studied. Keywords:
Flow system; Spectrophotometry;
Sulphate; Waters
Determination of sulphate ions is important in industrial, environmental and biological systems and the need for sulphate measurements is continuously increasing. In comparison with other analytes, there are few analytical procedures for sulphate. The most commonly used procedures are based on turbidimetric measurements of barium sulphate [l] or on spectrophotometric measurements using methylthymyl blue [3], dimethylsulphonazo-3 [2] or 2-aminoperimidine [4,5]. All these procedure have limitations at low sulphate concentrations. With manual methods, concentrations down to about 5 mg 1-l can be detected [6] and with automated procedures about 0.5 mg 1-i can be detected [7]. Further, the spectrophotometric procedures are non-specific owing to the reagents used. Severe interferences from divalent cations occur [3,7-91. During the last decade, ion chromatography (IC) has replaced turbidimetric and spectrophotometric techniques in many application areas. IC offers improved sensitivity (1 mg 1-l can be directly determined) and also superior selectivity 0003-2670/91/$03.50
0 1991 - Elsevier Science Publishers B.V.
[lo]. However, the sample throughput using IC is not sufficient if the number of samples is large. To be able to determine sulphate at low concentration levels all of the above-mentioned methods have to be combined with a preconcentration step. Using IC, anionic preconcentration columns can be incorporated into automated procedures [lo-131. This technique can, however, be of limited value when large amounts of other anions are present in the sample owing to overloading of the analytical column and insufficient peak separation. Tailing of peaks may also occur. Flow-injection analysis (FIA) offers a convenient way of automating analytical procedures. Although more than 2000 papers have been published, only about 5% have dealt with the incorporation of preconcentration steps into FIA procedures [14]. Using FIA, total automation of the analytical procedure including automated regeneration of the preconcentration column can be accomplished. Other advantages of FIA are that low sample volumes are required, the reagent consumption is low and the sample throughput is
M. KARLSSON
110
high. The aim of this work was to explore these features by developing a method for low levels of sulphate using a FIA system.
EXPERIMENTAL
Apparatus A Tecator FIAstar 5010 flow injection analyser with a Model 5023 spectrophotometer (flow cell path length 10 mm and volume 0.018 ml) and a Model 5007 sampler were used (Tecator, Hogan%, Sweden). The manifold consisted of a Tecator Chemifold III. A Plexiglas column (11 mm X 2 mm i.d.) and two PTFE filters with a pore size of 0.5 pm were used. The system was connected to an IBM P/S2 Model 30 computer and Tecator SuperFlow II software was installed to control the instrument functions and for recording, calibration and storage of the results. Reagents and materials All chemicals were of analytical-reagent grade. For calibration potassium sulphate was used. All solutions was prepared using deionized water. For the carrier flow lines sodium chloride solution was used. Methylthymol blue (MTB) {3,3-bis[N,N-bis(carboxymethyl)aminomethyl]thymolsulphonphthalein, sodium salt} (Riedel-de Ha&) was prepared by dissolving 0.1500 g of MTB (purity 64% see below) in a solution which contained 6 ml of 1 M HCl, 80 ml of deionized water and 14 ml of 1.9822 g BaCl, * 2H,O per litre. The final dilution was made with 95% ethanol to a total volume of 1000 ml [7,8]. For optimum response with this reagent, the barium concentration should be equal to the MTB concentration. Because of impurities in the MTB salt, an exact purity determination must be made. In this study the purity measurements were made according to Colovos et al. [7] and resulted in a purity of 64% (w/w). This low purity of the MTB is the main reason for a nonlinear calibration graph [7]. The second reagent was sodium hydroxide. Chemicals employed for the interference studies
ET AL.
were calcium chloride, potassium dihydrogenphosphate, potassium chloride and potassium nitrate. For the anion-exchange column (11 mm X 2 mm i.d.), a strong anion exchanger (Bio-Rad AGl-X8,100-200 mesh) was used (Bio-Rad Labs., Richmond, CA). Column packing and conditioning The column was filled with an aqueous slurry of the resin. The slurry was poured into the column at the top, excess of water was drained carefully by sucking with a syringe at the outlet of the column. When the column was completely filled, care was taken to remove all air bubbles. When the column was taken for use, the only conditioning required was a 5-min wash with the eluent, 0.3 M NaCl. Detection system The sulphate determination is based on the competitive reaction of sulphate and MTB with barium in solution. The absorbance measurement can be performed either on the uncomplexed MTB or on the MTB-barium complex. The absorbance at 460 nm, due to the uncomplexed MTB, will increase, whereas the absorbance at 620 nm, due to the MTB-barium complex, will decrease as the sulphate concentration increases [8]. The relative absorbance decrease at 620 nm is greater than the increase at 460 nm under the same conditions and the baseline stability is the same at both wavelengths [8]. In this study a wavelength of 620 nm was used. Figure 1 shows the manifold for the conventional determination of sulphate without preconcentration. After injection of the sample, an initial reaction at pH 2.5 takes place between sulphate ions and the barium ions in the MTB reagent. Sample
I
1
Fig. 1. Standard preconcentration. = 0.05 M NaOH.
620 “In
manifold for detection of sulphate without C = water; Rl = MTB reagent (pH 2.5); R2
FIA OF SULPHATE
TRACES
111
IN WATER
This reaction is time dependent and it limits the sample throughput [3,7,8]. The secondary reaction is the complexation of barium with MTB occurring at a pH between 12.5 and 13.0 [3,7,8]. This reaction is used for the detection step. Close pH control is needed, as pH variations will give an unstable baseline owing to changes in the background absorbance from the unreacted reagent. Hence large pH differences in the injected samples cannot be tolerated. The detection limit of this system is 0.5 mg 1-l sulphate and a linear calibration graph is obtained 171. Sulphate determination with preconcentration When a system for preconcentration of sulphate is designed, an appropriate ion exchanger for the concentration step must be selected. Also, an eluent system which minimizes possible problems due to pH changes during elution must be selected with care. In a dynamic flow system, changes in the total ionic strength in the vicinity of the eluted sample plug can give rise to spectral problems caused by variations in the refractive index of the liquid. The baseline stability will be poor and low detection. limits cannot be achieved. Choice of ion-exchange resin Bio-Rad AGl-X8 (100-200 mesh) resin was used. The particle size chosen was between 5 and 10% of the column diameter. For the enrichment cycle it was decided that the column should trap at least 1 mg 1-l sulphate without breakthrough. The minimum size of the resin will then be 0.34 ~1 if a flow-rate of 2.6 ml min-’ and an enrichment time of 90 s are used. To ensure that the sample matrix will not elute sulphate during the enrichment cycle, the actual column size chosen was 100 times larger.
ing the elution step could not be controlled. This was probably caused by the large pH difference between the eluent and the sample matrix left in the dead volume of the column. Consequently, sodium chloride was chosen as a neutral eluent, and an ion exchanger in the chloride form was used for the preconcentration step. In this way close control over pH could be obtained; however, severe refractive index problems, caused by salt concentration gradients between the eluent and water in the column volume, still resulted. The minimize these effects, a second carrier was introduced for dilution to level out these effects. To determine the optimum composition of the eluent and the second carrier, various concentrations of sodium chloride were investigated. A large baseline shift was obtained by increasing the ionic strength up to about 0.1 M NaCl and a more or less constant baseline was obtained at the higher concentrations. This means that possible salt gradient problems can be minimized by using a high ionic strength in the second carrier. Any variations in the eluent line will then have a negligible effect. The manifold depicted in Fig. 2 was used to verify that complete elution of sulphate could be obtained from the resin used for preconcentration. The concentration of NaCl in the second carrier
Enrichment column
Sample
S
Cl
RESULTS
AND DISCUSSION
Initially an ion exchanger in the hydroxide form was used for the preconcentration step and sodium hydroxide as the eluent. A buffer was incorporated in the flow system to minimize pH changes. However, the pH changes occurring dur-
Fig. 2. Manifold used for preconcentration and detection of low levels of sulphate. S = sample (2.6 ml min-‘); Cl and C2 = 0.3 M NaCl (both 0.6 ml min-‘); Rl= MTB reagent (pH 2.5) (1.4 ml min-‘); R2 = 0.05 M NaCl (0.6 ml min-‘).
112
was 0.3 M and different concentrations of NaCl were tested as the eluent (carrier 1). At low NaCl concentrations poor sensitivity was obtained. When the concentration was increased the sensitivity also increased. Optimum sensitivity was obtained at NaCl concentrations > 0.2 M. The eluent concentration finally chosen was 0.3 M. Using this set-up, a stable baseline and good sensitivity were obtained. Preferred flow system for preconcentration of sulphate A schematic diagram of the flow system used to preconcentrate and determine sulphate is shown in Fig. 2. The enrichment column is used as the sample loop in the injector, which means that the injection valve can be used for automatic change between sample uptake (preconcentration) and the elution cycle (carrier 1). Initially, sulphate was preconcentrated on the column by aspirating sample for a preset time. Then the injection valve was switched and 0.3 M sodium chloride was pumped through the column to elute sulphate into the system. For the detection the same basic chemistry as shown in Fig. 1 was used. Different working ranges can easily be obtained by changing the enrichment time. In the ion-exchange procedure sulphate ions are retained on the resin during the enrichment step and cations will pass through unaffected. However, at high concentrations of divalent cations interferences can occur from those cations left in the column dead volume. This problem can be eliminated by incorporating a wash cycle using distilled water prior to switching the valve. Any cations in the column will then be washed away. This step can be automated and the wash time can easily be controlled with the software used. The software controls the peristaltic pumps, injection valve and autosampler, which makes it possible to automate completely the whole em-ichment, wash and elution procedure. The timing of these cycles is easily defined or changed to desired values when setting up the method in the software. To avoid transport of air into the system when the sample line is moved from the sample to wash position, pump 2 is stopped for a short period. When the injection valve turns to the inject posi-
M. KARLSSON
ET AL.
tion, the carrier will elute sulphate into the system. The carrier will also regenerate the column at the same time, which means that the column can be used continuously. Calibration In the method used, the working range and detection limit depend on two parameters, flowrate and enrichment time. A nearly linear relationship between enrichment time and the analytical signal is obtained, which makes it easy to adapt the method to the analytical requirements. As an example, an enrichment time of 90 s gives a detection limit of 25 pg 1-l and a cycle time of 180 s for one measurement. The calibration graph is linear between 25 and 1000 pg 1-i sulphate and the repeatibility for consecutive measurements is good, the r.s.d. being less than 1.5%. Interference studies In the direct measurement of sulphate without preconcentration, the main interference is from divalent cations competing with barium in binding to the MTB reagent. To minimize this interference, a cation exchanger can be used to trap the ions. The cation exchanger used can be placed in the sample line or in the manifold. The drawback with this technique is that the column has to be regenerated regularly [ 81. Using the described preconcentration scheme, cationic interferents can be totally removed by a proper wash cycle. In Table 1, the effect of adding various amounts of calcium to the sample is shown. A wash period of 10 s was used and it can be seen that calcium levels up to 200 mg 1-l have no influence on the sulphate determination. For higher levels of divalent ions a longer wash time can be used. Using this preconcentration method, the main interference will be from anions that compete with sulphate in binding to the active sites of the column. The ions that are sorbed to the column can be desorbed by introducing an ion with a higher affinity for the resin or an ion with a lower affinity but a higher concentration. A general idea of possible anionic interferences can be obtained from the data sheet for the specific ion exchanger. The affinity for sulphate to the column used is
FIA OF SULPHATE
TABLE
TRACES
113
IN WATER
1
Determination of 500 pg 1-i sulphate amount of calcium and phosphate Additive
Calcium
Phosphate (PH 7)
Phosphate (PH 2)
containing
Amount added (rng 1-1)
(pg I-‘)
5 50 200 500 0.5 1.0 5.0 50.0 0.5 1.0 5.0 10.0 50.0
490 505 510 305 490 500 650 2300 500 499 505 580 989
different
Sulphate found
centration of HPO:will be reduced by a decrease in pH. At low pH H,PO,/H,PO;will be the major ions in solution; these ions will not be bound as strongly to the resin as the divalent form. The interference from phosphate ions in samples acidified to pH 2 is shown in Table 1. The level that can be tolerated has now increased to between 5 to 10 mg 1-l. Higher concentrations of anions can be tolerated by increasing the size of the ion-exchange resin. The limiting factor will then be for those concentrations of anions where the eluting power of the ions in the sample is so high that sulphate is eluted from the resin during the preconcentration step.
REFERENCES
higher than that for all other common anions occuring in natural samples. In this work the effects of various amounts of chloride, nitrate and phosphate were investigated using the manifold in Fig. 2. The sulphate concentration for these experiments was 500 pg 1-l. The relative selectivities for HSO;, NO, and Cl- to the AG l-X8 resin are 85, 65 and 22, respectively, which means that higher concentrations of chloride than nitrate should be tolerated. For chloride, concentrations up to about 2000 mg 1-l had no effect on the sulphate sensitivity whereas for nitrate only about 200 mg 1-l could be tolerated. Phosphate ions were found to be the most severe interferent. Table 1 gives the sensitivity for sulphate in the presence of various concentrations of phosphate. As can be seen, the tolerance limit for phosphate is between 1 and 5 mg 1-l. This interference can be controlled to some extent by decreasing the pH in the samples prior to the preconcentration. The pK, values for H,PO, are 2.15, 7.21 and 12.36, which means that the con-
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