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Determination of trace inorganic anions in anionic surfactants by single-pump column-switching ion chromatography
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Chinese Chemical Letters 23 (2012) 835–838 www.elsevier.com/locate/cclet
Determination of trace inorganic anions in anionic surfactants by single-pump column-switching ion chromatography Jia Jie Zhang a,*, Hai Bao Zhu b, Yan Zhu a a
Department of Chemistry, Zhejiang University, Hangzhou 310028, China b Zhejiang Academy of Medical Sciences, Hangzhou 310013, China Received 5 March 2012 Available online 9 June 2012
Abstract An ion chromatography method has been proposed for the determination of three common inorganic anions (chloride, nitrate and sulfate) in anionic surfactants using a single pump system. The new system consists of an ion exclusion column, a concentrator column, and an anion exchange column connected in series via two 6-ports valves in a Dionex ICS-2000 ion chromatograph. The valves were switched several times for removing surfactants, concentrating and separating the three anions. The chromatographic conditions were optimized. Detection limits (S/N = 3) were in the range of 0.10–0.68 mg/L. The relative standard deviations (RSDs) of peak area were less than 4.6%. The recoveries were in the range of 84.1–112.6%. # 2012 Jia Jie Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Anionic surfactants; Single pump; Column-switching
Ion chromatography is regarded as a versatile analytical technique for separating and quantifying ions. With the development of column, detection and data analysis technology, there has been considerable interest in the determination of ions at trace levels in highly ionic matrices by ion chromatography. For weakly ionized acids, an approach which uses ion-exclusion chromatography (ICE) as a pretreatment step for isolating ions from the matrix acid has been developed [1]. In the past, several ICE methods coupled with ion-exchange chromatography (IC) have been developed for anion analysis in concentrated acids [2–6]. The methods described in the literatures usually consisted of two eluent systems: potassium hydroxide for ion exchange and deionized water for ion exclusion. Two pumps were necessary to provide the two different eluents. Another method with only one pump, a 6-port valve and a 10-port valve has been proposed in our recent paper [7]. And the eluent from the outlet of conductivity cell was used as mobile phase for ion exclusion. The goal of this investigation is to expand upon the previous work and document a procedure that reliably determines trace anions in anionic surfactants. It is also the first paper on the determination of the trace inorganic anions in anionic surfactants. In this paper, two 6-port valves have been applied instead of one 6-port and one 10-port valve. The following anionic surfactants were evaluated: p-toluenesulfonic acid, dodecylbenzenesulfonic acid and octanesulfonic acid. The method is reliable and useful. No additional sample pretreatment was required.
* Corresponding author. E-mail address: [email protected] (J.J. Zhang). 1001-8417/$ – see front matter # 2012 Jia Jie Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.04.026
836
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
1. Experimental All the reagents used in this study were of analytical-reagent grade and water was purified using a Milli-Q system (Millipore, Bedford, MA, USA). Three samples including p-toluenesulfonic acid, dodecylbenzenesulfonic acid and octanesulfonic acid were obtained from Changzheng Reagent Co. (Hangzhou, China). Anion standards (200 mg/L) for chloride, nitrate and sulfate were prepared from analytical-reagent grade salts. Standard solutions were prepared by further diluting the 200 mg/L standards to the range expected by deionized water. And the sample solutions (1000 mg/L) were also dissolved by deionized water and filtered through 0.45 mm filters before injection. All preservation solutions were made daily or refrigerated at 4 8C for a maximum of 14 days. Chromatographic analysis was performed on an ICS-2000 (Dionex, Sunnyvale, CA, USA) liquid chromatograph equipped with a pump, an eluent generator (ED40) with potassium hydroxide (KOH) cartridge, a 6-port valve (A) and a CD20 conductivity detector. An anion self-regenerating suppressor from Xiamen University was applied to avoid the leakage of suppressor caused by the high pressure of the system. An IonPac ICE-AS6 column (250 mm 9 mm) was used for the ion-exclusion separation followed by an AG11-HC column (50 mm 4 mm) to concentrate anions prior to injection into the anion-exchange system. An AG11-HC guard column (50 mm 2 mm) and AS11-HC separation column (250 mm 2 mm) were used for the final separation. Another 6-port valve (B) was utilized for the columnswitching technique where the interested anions were ‘‘cut’’ into the concentrator column. The configuration of system is illustrated in Fig. 1a. Polyether ether ketone tubings (2 mm i.d.) were adopted for all connections, which were kept as short as possible to minimize the dead volume. Separations were isocratic by using 10 mmol/L KOH as the eluent at a flow-rate of 0.25 mL/min. Data collection and handling were carried out by Dionex Chromeleon 6.5 software. The sample loop is 200 mL. At the beginning of each day, the system was cleaned by passing a solution of 100 mmol/L KOH to remove retained analytes. The trace anion analysis in anionic surfactants was accomplished in four steps. First, the valve B was switched to the loading position as shown in Fig. 1a, and the sample was manually loaded into the sample loop while valve A was in the injecting position. Next, the sample was delivered from the sample loop to the ion-exclusion column with the eluent from the outlet of conductivity cell (deionized water) by switching the valve B to the injection position (Fig. 1b) while the analyte anions were separated from the weak acid matrix by ICE. Then the anions were concentrated on the concentrator column by switching the valve A (Fig. 1c). Finally the analytical columns were placed after concentrator column (Fig. 1a). The anions were eluted from the concentrator column and separated. 2. Results and discussion A series of experiments were performed to determine suitable conditions for the separation of the anions and the matrix on the IonPac ICE-AS6 (250 mm 9 mm). It was found in the experiment that the flow rate of eluent was the
Fig. 1. Chromatographic construction for anion analysis in anionic surfactants.
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
837
main factor that influenced the resolution of ICE. For the analysis of trace anions in alkyl sulfonic acid, an ICE flowrate of 0.25 mL/min was selected. The ion-exchange column in the 250 mm 2 mm format was used instead of 250 mm 4 mm in our previous paper [7] because of its ability to separate the analyte anions and relatively lower capacity for rapid analysis. On other hand, it can be seen that the concentration of KOH played a major role in the separation of three anions by IC. Instead of 30 mmol/L, 10 mmol/L KOH was chosen as a washing solution. The efficiency of the column may be slightly changed due to the usage of the column. However under the current separation conditions, the separation efficiency and peak shapes appear relatively good (Fig. 3). The concentrator column AG11-HC column (50 mm 4 mm) is considered a high capacity column that allows more concentrated samples to be analyzed without overloading the column or creating peak broadening. According to the IC literature [8], the column must be flushed with a higher concentration of eluent than used for separation to remove retained analytes. Therefore, the column was flushed with 100 mmol/L KOH eluent at the beginning of each day. Incorporating the 100 mmol/L KOH flush eliminated both retention time and non-reproducible peak problems. It is well known that the accurate column-switching program is the most important in the study. The valve (B)switching should be planed to ensure a complete capture of the anions of interest and avoid the excessive matrix. To simplify the procedures for determining the switching time, a relatively high concentration standard (1000 mg/L) was used to determine the switching time. Because in the wide time window determined by high concentration sample solution, the analyte anions can be introduced to concentrator column completely while the interference of matrix can be minimized. So another conductivity cell was placed after ICE-AS6 column. And 1000 mg/L octanesulfonic acid spiked each anion at 10 mg/L, respectively was loaded into the sample loop and analyzed. Fig. 2 shows the signals from the added conductivity. This chromatogram is a measurement of the unsuppressed conductivity response for the ICE separation. According to the result of experiment, the pre-concentration time was selected between 16 and 22 min. Then the above sample was injected into the system and the column switch was performed as shown in Fig. 1. And it took about 65 min to finish the whole analysis including washing the recirculation of eluted inorganic anions from ICE. Under the optimized experimental conditions, the recovery results (Table 2) show that the presence of relatively high concentration (1000 mg/L) cannot affect the extraction efficiency of all three anions. When the concentration was 1500 mg/L a baseline drift was caused by the sample matrix. A blank was determined by performing all steps of the analysis with deionized water as the sample. This result was then applied as a correction to all subsequent sample measurements. The blank value established baseline of the anions from such sources as the sample container, tubing and eluent. Under optimized experimental conditions, all three anions showed good linearities between the concentrations and peak area responses. The detection limits, defined as the signals three times the noise levels, were also calculated. The precisions were evaluated by performing 8 replicate analysis of a standard solution where the concentrations of the three anions were 2 mg/L. All the results are listed in Table 1. To illustrate an application of the developed method, three anionic surfactants were collected for determination. Fig. 3 is the chromatogram of p-toluenesulfonic acid. It can be seen that two anions were found in the sample. The concentrations of the anions in real samples were determined with the external calibration method. Suitable amounts of anion standards were added to the real samples of known concentration, the mixtures were analyzed using the
Fig. 2. The chromatogram of ICE separation of 10 mg/L of each anion from 1000 mg/L octanesulfonic acid; peak 1 = three inorganic anions (10 mg/L each); peak 2 = octanesulfonic acid (1000 mg/L).
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
838
Table 1 Analytical performance of the proposed method. Anion
Linear equation
Linear range (mg/L)
Correlation coefficient
Detection limit (mg/L) (S/N = 3)
Peak area RSD (%) (n = 8)
Cl NO3 SO42
Y = 8.3334X + 0.8708 Y = 4.7750X 0.4286 Y = 6.4399X + 2.3162
0.02–20 0.02–20 0.02–20
0.9998 0.9996 0.9998
0.11 0.68 0.10
2.9 4.6 3.5
Table 2 Results of determination in real samples. Analyte Anion
p-Toluenesulfonic acid Concentration (mg/L)/ added (mg/L)/recovery (%)
Dodecylbenzenesulfonic acid Concentration (mg/L)/added (mg/L)/recovery (%)
Octanesulfonic acid Concentration (mg/L)/added (mg/L)/recovery (%)
Cl NO3 SO42
0.39/0.50/88.8 ND/0.25/86.7 0.30/0.50/84.1
0.36/0.50/88.1 0.22/0.25/110.4 0.13/0.25/112.6
0.12/0.25/95.2 ND/0.25/96.6 0.21/0.25/103.0
ND, not detectable.
Fig. 3. Chromatograms of p-toluenesulfonic acid: (a) 1000 mg/L p-toluenesulfonic acid; and (b) 1000 mg/L p-toluenesulfonic acid spiked with chloride, nitrate and sulfate at 1.00 mg/L.
proposed procedure. Recovery was expressed for each component as the mean percentage ratio between the measured amounts and added ones. The results are shown in Table 2. 3. Conclusion This method provides for the accurate and precise determination of chloride, nitrate and sulfate at low concentrations in anionic surfactants with single pump and two 6-port valves. It is relatively simple and at low cost without pretreatment. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos. 20775070 and J0830413), Zhejiang Provincial Natural Science Foundation of China (Nos. R4080124 and J20091495). References [1] [2] [3] [4] [5] [6] [7] [8]
E. Kaiser, J.S. Rohrer, K. Watanabe, J. Chromatogr. A 850 (1999) 167. F.S. Stover, J. Chromatogr. A 956 (2002) 121. L.E. Vanatta, D.E. Coleman, J. Chromatogr. A 804 (1998) 161. S. Liu, N. Shinkai, I. Kakubari, et al. Biomed. Chromatogr. 22 (2008) 1442. V. Ruiz-Calero, L. Puignou, M. Diez, Analyst 126 (2001) 169. K. Wang, Y. Lei, M. Eitel, et al. J. Chromatogr. A 956 (2002) 109. H.B. Zhu, H.D. Chen, Y.Y. Zhong, J. Chromatogr. Sci. 48 (2010) 553. http://www.dionex.com/en-us/webdocs/4350-Man-031333-08-IonPac-AS11HC-Nov08.pdf, p. 44.
Chinese Chemical Letters 23 (2012) 835–838 www.elsevier.com/locate/cclet
Determination of trace inorganic anions in anionic surfactants by single-pump column-switching ion chromatography Jia Jie Zhang a,*, Hai Bao Zhu b, Yan Zhu a a
Department of Chemistry, Zhejiang University, Hangzhou 310028, China b Zhejiang Academy of Medical Sciences, Hangzhou 310013, China Received 5 March 2012 Available online 9 June 2012
Abstract An ion chromatography method has been proposed for the determination of three common inorganic anions (chloride, nitrate and sulfate) in anionic surfactants using a single pump system. The new system consists of an ion exclusion column, a concentrator column, and an anion exchange column connected in series via two 6-ports valves in a Dionex ICS-2000 ion chromatograph. The valves were switched several times for removing surfactants, concentrating and separating the three anions. The chromatographic conditions were optimized. Detection limits (S/N = 3) were in the range of 0.10–0.68 mg/L. The relative standard deviations (RSDs) of peak area were less than 4.6%. The recoveries were in the range of 84.1–112.6%. # 2012 Jia Jie Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Anionic surfactants; Single pump; Column-switching
Ion chromatography is regarded as a versatile analytical technique for separating and quantifying ions. With the development of column, detection and data analysis technology, there has been considerable interest in the determination of ions at trace levels in highly ionic matrices by ion chromatography. For weakly ionized acids, an approach which uses ion-exclusion chromatography (ICE) as a pretreatment step for isolating ions from the matrix acid has been developed [1]. In the past, several ICE methods coupled with ion-exchange chromatography (IC) have been developed for anion analysis in concentrated acids [2–6]. The methods described in the literatures usually consisted of two eluent systems: potassium hydroxide for ion exchange and deionized water for ion exclusion. Two pumps were necessary to provide the two different eluents. Another method with only one pump, a 6-port valve and a 10-port valve has been proposed in our recent paper [7]. And the eluent from the outlet of conductivity cell was used as mobile phase for ion exclusion. The goal of this investigation is to expand upon the previous work and document a procedure that reliably determines trace anions in anionic surfactants. It is also the first paper on the determination of the trace inorganic anions in anionic surfactants. In this paper, two 6-port valves have been applied instead of one 6-port and one 10-port valve. The following anionic surfactants were evaluated: p-toluenesulfonic acid, dodecylbenzenesulfonic acid and octanesulfonic acid. The method is reliable and useful. No additional sample pretreatment was required.
* Corresponding author. E-mail address: [email protected] (J.J. Zhang). 1001-8417/$ – see front matter # 2012 Jia Jie Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.04.026
836
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
1. Experimental All the reagents used in this study were of analytical-reagent grade and water was purified using a Milli-Q system (Millipore, Bedford, MA, USA). Three samples including p-toluenesulfonic acid, dodecylbenzenesulfonic acid and octanesulfonic acid were obtained from Changzheng Reagent Co. (Hangzhou, China). Anion standards (200 mg/L) for chloride, nitrate and sulfate were prepared from analytical-reagent grade salts. Standard solutions were prepared by further diluting the 200 mg/L standards to the range expected by deionized water. And the sample solutions (1000 mg/L) were also dissolved by deionized water and filtered through 0.45 mm filters before injection. All preservation solutions were made daily or refrigerated at 4 8C for a maximum of 14 days. Chromatographic analysis was performed on an ICS-2000 (Dionex, Sunnyvale, CA, USA) liquid chromatograph equipped with a pump, an eluent generator (ED40) with potassium hydroxide (KOH) cartridge, a 6-port valve (A) and a CD20 conductivity detector. An anion self-regenerating suppressor from Xiamen University was applied to avoid the leakage of suppressor caused by the high pressure of the system. An IonPac ICE-AS6 column (250 mm 9 mm) was used for the ion-exclusion separation followed by an AG11-HC column (50 mm 4 mm) to concentrate anions prior to injection into the anion-exchange system. An AG11-HC guard column (50 mm 2 mm) and AS11-HC separation column (250 mm 2 mm) were used for the final separation. Another 6-port valve (B) was utilized for the columnswitching technique where the interested anions were ‘‘cut’’ into the concentrator column. The configuration of system is illustrated in Fig. 1a. Polyether ether ketone tubings (2 mm i.d.) were adopted for all connections, which were kept as short as possible to minimize the dead volume. Separations were isocratic by using 10 mmol/L KOH as the eluent at a flow-rate of 0.25 mL/min. Data collection and handling were carried out by Dionex Chromeleon 6.5 software. The sample loop is 200 mL. At the beginning of each day, the system was cleaned by passing a solution of 100 mmol/L KOH to remove retained analytes. The trace anion analysis in anionic surfactants was accomplished in four steps. First, the valve B was switched to the loading position as shown in Fig. 1a, and the sample was manually loaded into the sample loop while valve A was in the injecting position. Next, the sample was delivered from the sample loop to the ion-exclusion column with the eluent from the outlet of conductivity cell (deionized water) by switching the valve B to the injection position (Fig. 1b) while the analyte anions were separated from the weak acid matrix by ICE. Then the anions were concentrated on the concentrator column by switching the valve A (Fig. 1c). Finally the analytical columns were placed after concentrator column (Fig. 1a). The anions were eluted from the concentrator column and separated. 2. Results and discussion A series of experiments were performed to determine suitable conditions for the separation of the anions and the matrix on the IonPac ICE-AS6 (250 mm 9 mm). It was found in the experiment that the flow rate of eluent was the
Fig. 1. Chromatographic construction for anion analysis in anionic surfactants.
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
837
main factor that influenced the resolution of ICE. For the analysis of trace anions in alkyl sulfonic acid, an ICE flowrate of 0.25 mL/min was selected. The ion-exchange column in the 250 mm 2 mm format was used instead of 250 mm 4 mm in our previous paper [7] because of its ability to separate the analyte anions and relatively lower capacity for rapid analysis. On other hand, it can be seen that the concentration of KOH played a major role in the separation of three anions by IC. Instead of 30 mmol/L, 10 mmol/L KOH was chosen as a washing solution. The efficiency of the column may be slightly changed due to the usage of the column. However under the current separation conditions, the separation efficiency and peak shapes appear relatively good (Fig. 3). The concentrator column AG11-HC column (50 mm 4 mm) is considered a high capacity column that allows more concentrated samples to be analyzed without overloading the column or creating peak broadening. According to the IC literature [8], the column must be flushed with a higher concentration of eluent than used for separation to remove retained analytes. Therefore, the column was flushed with 100 mmol/L KOH eluent at the beginning of each day. Incorporating the 100 mmol/L KOH flush eliminated both retention time and non-reproducible peak problems. It is well known that the accurate column-switching program is the most important in the study. The valve (B)switching should be planed to ensure a complete capture of the anions of interest and avoid the excessive matrix. To simplify the procedures for determining the switching time, a relatively high concentration standard (1000 mg/L) was used to determine the switching time. Because in the wide time window determined by high concentration sample solution, the analyte anions can be introduced to concentrator column completely while the interference of matrix can be minimized. So another conductivity cell was placed after ICE-AS6 column. And 1000 mg/L octanesulfonic acid spiked each anion at 10 mg/L, respectively was loaded into the sample loop and analyzed. Fig. 2 shows the signals from the added conductivity. This chromatogram is a measurement of the unsuppressed conductivity response for the ICE separation. According to the result of experiment, the pre-concentration time was selected between 16 and 22 min. Then the above sample was injected into the system and the column switch was performed as shown in Fig. 1. And it took about 65 min to finish the whole analysis including washing the recirculation of eluted inorganic anions from ICE. Under the optimized experimental conditions, the recovery results (Table 2) show that the presence of relatively high concentration (1000 mg/L) cannot affect the extraction efficiency of all three anions. When the concentration was 1500 mg/L a baseline drift was caused by the sample matrix. A blank was determined by performing all steps of the analysis with deionized water as the sample. This result was then applied as a correction to all subsequent sample measurements. The blank value established baseline of the anions from such sources as the sample container, tubing and eluent. Under optimized experimental conditions, all three anions showed good linearities between the concentrations and peak area responses. The detection limits, defined as the signals three times the noise levels, were also calculated. The precisions were evaluated by performing 8 replicate analysis of a standard solution where the concentrations of the three anions were 2 mg/L. All the results are listed in Table 1. To illustrate an application of the developed method, three anionic surfactants were collected for determination. Fig. 3 is the chromatogram of p-toluenesulfonic acid. It can be seen that two anions were found in the sample. The concentrations of the anions in real samples were determined with the external calibration method. Suitable amounts of anion standards were added to the real samples of known concentration, the mixtures were analyzed using the
Fig. 2. The chromatogram of ICE separation of 10 mg/L of each anion from 1000 mg/L octanesulfonic acid; peak 1 = three inorganic anions (10 mg/L each); peak 2 = octanesulfonic acid (1000 mg/L).
J.J. Zhang et al. / Chinese Chemical Letters 23 (2012) 835–838
838
Table 1 Analytical performance of the proposed method. Anion
Linear equation
Linear range (mg/L)
Correlation coefficient
Detection limit (mg/L) (S/N = 3)
Peak area RSD (%) (n = 8)
Cl NO3 SO42
Y = 8.3334X + 0.8708 Y = 4.7750X 0.4286 Y = 6.4399X + 2.3162
0.02–20 0.02–20 0.02–20
0.9998 0.9996 0.9998
0.11 0.68 0.10
2.9 4.6 3.5
Table 2 Results of determination in real samples. Analyte Anion
p-Toluenesulfonic acid Concentration (mg/L)/ added (mg/L)/recovery (%)
Dodecylbenzenesulfonic acid Concentration (mg/L)/added (mg/L)/recovery (%)
Octanesulfonic acid Concentration (mg/L)/added (mg/L)/recovery (%)
Cl NO3 SO42
0.39/0.50/88.8 ND/0.25/86.7 0.30/0.50/84.1
0.36/0.50/88.1 0.22/0.25/110.4 0.13/0.25/112.6
0.12/0.25/95.2 ND/0.25/96.6 0.21/0.25/103.0
ND, not detectable.
Fig. 3. Chromatograms of p-toluenesulfonic acid: (a) 1000 mg/L p-toluenesulfonic acid; and (b) 1000 mg/L p-toluenesulfonic acid spiked with chloride, nitrate and sulfate at 1.00 mg/L.
proposed procedure. Recovery was expressed for each component as the mean percentage ratio between the measured amounts and added ones. The results are shown in Table 2. 3. Conclusion This method provides for the accurate and precise determination of chloride, nitrate and sulfate at low concentrations in anionic surfactants with single pump and two 6-port valves. It is relatively simple and at low cost without pretreatment. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos. 20775070 and J0830413), Zhejiang Provincial Natural Science Foundation of China (Nos. R4080124 and J20091495). References [1] [2] [3] [4] [5] [6] [7] [8]
E. Kaiser, J.S. Rohrer, K. Watanabe, J. Chromatogr. A 850 (1999) 167. F.S. Stover, J. Chromatogr. A 956 (2002) 121. L.E. Vanatta, D.E. Coleman, J. Chromatogr. A 804 (1998) 161. S. Liu, N. Shinkai, I. Kakubari, et al. Biomed. Chromatogr. 22 (2008) 1442. V. Ruiz-Calero, L. Puignou, M. Diez, Analyst 126 (2001) 169. K. Wang, Y. Lei, M. Eitel, et al. J. Chromatogr. A 956 (2002) 109. H.B. Zhu, H.D. Chen, Y.Y. Zhong, J. Chromatogr. Sci. 48 (2010) 553. http://www.dionex.com/en-us/webdocs/4350-Man-031333-08-IonPac-AS11HC-Nov08.pdf, p. 44.