- Email: [email protected]
Simultaneous Determination of Trace Monochlorophenols in Water by Ion Chromatography Atmospheric Pressure Chemical Ionization Mass Spectrometry
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 34, Issue 9, September 2006 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2006, 34(9), 1223–1226.
RESEARCH PAPER
Simultaneous Determination of Trace Monochlorophenols in Water by Ion Chromatography Atmospheric Pressure Chemical Ionization Mass Spectrometry Jin Micong1,*, Yan Yongqing2, Chen Xiaohong1, Shi Jiawei1 1
Ningbo Municipal Center for Disease Control and Prevention, Ningbo 315010, China School of Basic Medical Science, Peking University, Beijing 100083, China
2
Abstract: A novel, sensitive, and accurate method has been successfully developed for the simultaneous determination of 3 monochlorophenols (MCPs) in drinking water samples by ion chromatography (IC) coupled with atmospheric pressure chemical ionization mass spectrometry in the negative mode. Drinking water was preconcentrated by solid-phase extraction (SPE). The IC separation was carried out with an IonPac® AS18 analytical column (250 mm × 4.0 mm) and an IonPac® AG18 guard column (50 mm × 4.0 mm) using 15 mmol l–1 KOH containing 10% acetonitrile as mobile phase in the isocratic mode at a constant flow rate of 1.0 ml min–1. The molecular ion m/z 127 [M-H] was selected for the quantification in the selected ion monitoring (SIM) mode. Within-the-day and day-to-day relative standard deviations were less than 8.6 and 10.5%, respectively. The limits of quantification (LOQs) in water samples were 0.02, 0.01, and 0.01 µg l–1 for 2-MCP, 3-MCP, and 4-MCP, respectively. The method allows the three target compounds in water samples to be determined at a level of ng ml–1, and it has been successfully applied to the routine analyses. Key Words:
1
Ion chromatography-mass spectrometry (IC-MS); 2-Chlorophenol; 3-Chlorophenol; 4-Chlorophenol; Drinking water
Introduction
Monochlorophenols (MCPs) are some of the most important industrial chemical materials, which cause considerable contamination in the environment such as in water and soil[1]. Also it has been found that MCPs could be produced from treating drinking water with chlorine[2]. Recently, a variety of techniques have been reported, which state that MCPs are determined by gas chromatography after derivatization with electronic capture detector or mass spectrometry[3,4] and by high-performance liquid chromatography (HPLC) with various detectors such as ultraviolet, fluorescence, electrochemical, and mass spectrometry[5–11]. Unfortunately, as it is well known, there are no literatures for the simultaneous determination of the three MCPs by IC coupled with atmospheric pressure chemical ionization mass spectrometry (IC-APCI-MS). The aim of this study is to
develop a new IC method for the simultaneous determination of 2-MCP, 3-MCP, and 4-MCP in drinking water samples, using 15 mM KOH containing 10% acetonitrile as the mobile phase in the isocratic mode and to separate on an IonPac® AS18 analytical column (250 mm × 4.0 mm), and to detect by an APCI-MS in the negative mode, in selected ion monitoring (SIM) mode, using their molecular ions m/z 127 [M－H]–. This method could be used for the routine analysis of these MCPs in water samples.
2 2.1
Experimental Instrument and reagents
IC separations for method development and validation were carried out on an ICS2000 Ion Chromatography System (DIONEX, Sunnyvale, CA, USA), consisting of a single pump,
Received 21 July 2005; accepted 18 November 2005 * Corresponding author. Email: [email protected]; Tel: +86 574-87274559; Fax: +86 574-87361764 This work was supported by the Ningbo City Medicine Science and Technology Plan (No. 2003056). Copyright © 2006, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
a column heat exchanger, an AS40 automated sampler, and an ASRS®-ULTRA 4 mm suppressor. Detection was performed on an Agilent LC/MSD Trap SL mass spectrometer with an APCI interface (Agilent Technologies, Germany). The separation system was controlled on a computer equipped with DIONEX Chromeleon 6.5 Software. The APCI-MS system was controlled, and data were analyzed on a computer equipped with LC/MSD Trap Software 4.2 (Bruker). All tubings used for connections were PEEK (polyetheretherketo-
ne) (0.25 mm I.D., Agilent Technologies, Germany). The IC-MS scheme is shown in Fig.1. Acetonitrile (MeCN), methyl tert-butyl ether (MTBE), and methanol (MeOH) (Merck, Darmstadt, Germany) were HPLC grade. Purified water was supplied by a Milli-Q water purification system (Millipore, Molsheim, France). 2-CP (> 98%), 3-CP (> 97%) and 4-CP (> 99%) were obtained from Merck (Darmstadt, Germany).
Fig.1 IC-MS scheme
2 .2 2.2.1
Experiment method Preparation for MCPs stock solution
Each of the MCPs standard (100.0 mg) was accurately weighted into a 100 ml volumetric flask, and diluted with MeOH. Working standard solutions were prepared weekly in MeOH. Stock, and the working standards were stored at 4ºC in a refrigerator. The aqueous solutions were prepared daily by diluting the working solution with purified water. 2.2.2
Preparation of samples
Real sample (100 ml) was acidified to pH 2.5 by 0.10 M phosphoric acid, and then filtered through a 0.45-μm nylon membrane filter (Agilent Technologies, Germany) before the analysis. After that, the sample was extracted using an Oasis® HLB (3 ml per 60 mg, Waters) cartridge that was first conditioned with 5.0 ml MeOH/MTBE (10/90, v/v), 5.0 ml methanol, and 5.0 ml water. The flow rate of the water sample was 4.0 ml min–1. MCPs were eluted with 2.0 ml MeOH/MTBE (10/9， v/v) twice. The elution was evaporated to dryness under a gentle stream of nitrogen and the residue
was reconstituted with 0.5 ml of MeOH and filtered through a 0.45-μm nylon syringe filter (Agilent Technologies, Germany). Finally, an aliquot of 20.0 μl of the resulting solution was injected into the IC-MS system. 2.2.3
IC condition
The IC method utilized an IonPac® AS18 analytical column (250 mm × 4.0 mm, DIONEX) and an IonPac® AG18 guard column (50 mm × 4.0 mm, DIONEX). Injection volume was 20.0 μl. The mobile phase was 15 mM KOH, containing 10% MeCN modifier at a constant flow rate of 1.0 ml min–1. Column temperature was held constant at 35ºC. The ASRS®-ULTRA 4 mm suppressor was operated at 40.0 mA in the external water mode. 2.2.4
MS condition
The ion trap mass spectrometer was used in the negative mode with a full scan mass spectra over the m/z range of 75–200 amu using a cycle time of 1 s and a peak width of 0.1 s, a corona current of 4 μA, a capillary voltage of 2.5 kV, a capillary exit voltage of –85 V, a dry temperature of 325ºC, a
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
vaporizer temperature of 450ºC, a high purity (99.999%) dry nitrogen gas of 5.0 l min–1, a nitrogen nebulizer pressure of 60.0 psi, and a dwell time of 200 ms. The APCI interface and mass spectrometer parameters were optimized to obtain maximum sensitivity. Analytes were detected with an APCI interface in the negative SIM mode. The quantification ions were selected as the molecular ions [M–H]– m/z 127 for the MCPs. The SIM peak areas were integrated for quantification.
3 3.1
compatible with the organic solvents and could obtain good separation for 2-MCP, 3-MCP, and 4-MCP with IonPac® AS 18 column, and could also be co-eluent for the three compounds with OmniPac® PAX 500. Fig.2 shows MCP’s total ion chromatogram (TIC) and Fig.3 shows MCP′s mass spectra. As can be seen, the three MCP isomers had obtained baseline separation when using IonPac® AS18 and the chlorine isotopes (35Cl and 37Cl) were also obviously obtained with abundance ratio in nature.
Results and discussion Anion-exchange column
Initially, an OmniPac® PAX 500 column (250 mm × 4.0 mm, DIONEX) coupled with an OmniPac® PAX 500 guard column (50 mm × 4.0 mm, DIONEX) and an IonPac® AS 18 column (250 mm × 4.0 mm, DIONEX) coupled with an IonPac® AG18 guard column (50 mm × 4.0 mm, DIONEX) were considered for the separation of 2-MCP, 3-MCP, and 4-MCP. It was found that all these columns were totally
Fig.2 Total ion chromatogram of MCPs Peak identified: (1) 2-MCP; (2) 4-MCP; (3) 3-MCP.
Fig. 3 MS spectra of 2-MCP (A), 4-MCP (B) and 3-MCP (C)
3.2
Mobile phase
Initially, KOH, Na2CO3, and NaHCO3 with different concentrations were examined to investigate whether they all could achieve a better separation for 2-MCP, 4-MCP, and 3-MCP. It was found that KOH was the best, and as the KOH concentration increased, the peak could become narrow and the run time would decrease, when the KOH concentration achieved 15 mM, hence the 2-MCP, 4-MCP, and 3-MCP were completely separated within only 5 min. It could be explained that the MCPs had the characteristics of weak organic acid; they could be deprotonated in a basic medium with different degrees and form monochlorophenolic ions (Fig.4), because of the different pK′s for the three target compounds. This provided a possibility to retain MCPs onto an anion-exchange column owing to the differential adsorption and desorption on the ion-exchange resin. It was also found that the eluent order of the three isomers was closely correlated with their pK. The higher the pKa, the longer the retention time; for example if pKa = 8.52 for 2-MCP, tR = 3.41 min; pKa = 8.97 for 4-MCP, tR = 4.12 min; pKa = 9.37 for 3-MCP, tR = 4.64 min.
Fig. 4 Chemical structures of (A) MCPs and (A') anionic MCPs
3.3
Organic modifier
In the preliminary experiments, a peak broadening, lower sensitivity, and longer run time were observed when using IonPac® AS18 column for the separation of the three MCP isomers. When 10% (v/v) MeCN was added to the eluent as an organic modifier, to modify the polarity of the mobile phase, it was found that the peak broadening had greatly decreased and the sensitivity had obtained a significant increase, and the peaks had become more sharp and symmetrical (Fig.2).
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
3.4 Compatibility between IC and MS
compatibility between IC and MS. The ion movement scheme in ASRS®-ULTRA anion suppressor is shown in Fig.5. In addition, an almost neutral solution, an eluent of least conductivity, was obtained at 40.0 mA in the external water mode for the ASRS®-ULTRA 4 mm suppressor. This solution achieved the same order magnitude of signals for the three MCPs within a single IC run with the exclusive use of APCI interface. 3.5
Fig.5 Ion movement scheme in ASRS®-ULTRA anion suppressor
In this study, the eluent was KOH. If the KOH directly entered into the ionization chamber, it would not only cause an inorganic salt deposition and erosion on the surface of the corona needle and the circle of the electrode, but also cause instability, inhibition, and quenching of the mass signal, and result in the obstruction of the capillary. Furthermore no mass signal was observed when the KOH concentration reached 15 mM, and the monochlorophenolic ions in the ionization chamber were completely suppressed. As the interference of the KOH was visible, the eluent prior to the vaporization chamber would be modified; otherwise, the combination of IC and MS could not be achieved. Luckily, the ion suppressor was used well, to solve the problem by reducing the concentration of the potassium ion in the eluent, to gain the
Repeatability and precision
The repeatability and precision of MCPs were evaluated by performing five replicates of three spiked drinking water samples (0.05, 0.50, and 5.00 μg l–1) as quality control samples (QC), including SPE procedures, for three continuous days. The typical TIC is shown in Fig.6 and the repeatability and precision are shown in Table 1. Within-the-day precision (RSD) on the basis of MCPs content was less than 8.6% and the day-to-day precision (RSD) was less than 10.5% for the three objective compounds.
Fig. 6 Total ion chromatogram for spiked water sample (0.50 μg l–1) Peak identified: (1) 2-MCP; (2) 4-MCP; (3) 3-MCP.
Table 1 The test of repeatability and precision –1
Concentration (μg l )
RSD (%)
Relative error (%)b
Compound
2-MCP
3-MCP
4-MCP
Added
Found a
First day
Second day
Third day
0.05
0.046 ± 0.0031
6.7
7.2
8.4
–8.0
0.50
0.523 ± 0.044
8.4
9.3
9.5
4.6
5.00
4.82 ± 0.15
3.1
4.2
6.3
–3.6
0.05
0.044 ± 0.0022
5.0
5.2
7.4
–12.0
0.50
0.474 ± 0.041
8.6
9.1
9.9
–5.2
5.00
4.51 ± 0.23
4.6
5.5
6.7
–9.8
0.05
0.042 ± 0.0021
5.0
6.6
7.6
–16.0
0.50
0.483 ± 0.032
6.6
8.3
10.5
–3.4
5.00
4.72 ± 0.24
5.1
5.2
7.3
–5.6
Note: a. the mean value determined six times on the first day; b. the relative error of the value determined six times on the first day.
3.6
Calibration curves and LOQs
Calibration curves were obtained for 2-MCP, 3-MCP, and
4-MCP using a series of standard solutions ranging from 0.005 to 5.0 μg l–1 with an SPE step. Three repeated injections of the standard solution at each concentration were performed.
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
Table 2 lists the method and quality parameters for all the objective compounds. It shows that all the calibration curves are linear over a standard concentration range of 0.02 to 5.0 μg l–1 with a coefficient of determination r2 > 0.993. The limits of quantification (LOQs) were determined using spiked drinking water samples which were spiked with MCPs at 0.05 and 0.20 μg l–1 with an SPE step, detected in SIM mode, and evaluated by the criterion that the signal-to-noise ratio should be greater than10 for quantification purposes. The LOQs for 2-MCP, 3-MCP, and 4-MCP were 0.02, 0.01, and 0.01 μg l–1, respectively.
[1] Ohlenbusch G, Kumke M U, Frimmel F H, Sci. Total Environ., 2000, 253: 63–69 [2] Simpson K L, Hayes K P. Water Res., 1998, 32(5): 1522–1528 [3] Turnes M I, Rodriguez I, Mejuto M C, Cela R. J. Chromatogr. A, 1994, 683: 21–29 [4]
Rodriguez I, Cela R. Trends in Analytical Chemistry, 1997, 16(8): 463–475
[5] Jing F L, Xia L, Yu G C, Gui B J, Ya Q C. Anal. Chim. Acta, 2003, 487: 129–135 [6] Penalver A, Pocurull E, Borrull F, Marce R M. J. Chromatogr. A, 2002, 953: 79–87
4
Conclusions
[7] Sarrion M N, Santos F J, Galceran M T. J. Chromatogr. A, 2002, 947: 155–165
The analytical IC-APCI-MS method using Oasis® HLB to preconcentrate and clean, for the simultaneous determination of 2-MCP, 3-MCP, and 4-MCP in drinking water samples was developed. The method was demonstrated with the advantages of simplicity, rapidity, and sensitivity, and was applied to the determination of the three MCPs isomers in the drinking water samples.
[8] Jauregui O, Moyano E, Galceran M T. J. Chromatogr. A, 1998, 823: 241–248 [9] Jauregui O, Moyano E, Galceran M T. J. Chromatogr. A, 1997, 787: 79–89 [10] Jin M C, Wang B. J. Donghua University (English Edition), 2004, 21(6): 146–148 [11] Jin M C, Chen X H, Li X P, Chen H P. Chinese Journal of Health Laboratory Technology, 2005, 15(3): 280–283
References
Cite this article as: Chin J Anal Chem, 2006, 34(9), 1223–1226.
RESEARCH PAPER
Simultaneous Determination of Trace Monochlorophenols in Water by Ion Chromatography Atmospheric Pressure Chemical Ionization Mass Spectrometry Jin Micong1,*, Yan Yongqing2, Chen Xiaohong1, Shi Jiawei1 1
Ningbo Municipal Center for Disease Control and Prevention, Ningbo 315010, China School of Basic Medical Science, Peking University, Beijing 100083, China
2
Abstract: A novel, sensitive, and accurate method has been successfully developed for the simultaneous determination of 3 monochlorophenols (MCPs) in drinking water samples by ion chromatography (IC) coupled with atmospheric pressure chemical ionization mass spectrometry in the negative mode. Drinking water was preconcentrated by solid-phase extraction (SPE). The IC separation was carried out with an IonPac® AS18 analytical column (250 mm × 4.0 mm) and an IonPac® AG18 guard column (50 mm × 4.0 mm) using 15 mmol l–1 KOH containing 10% acetonitrile as mobile phase in the isocratic mode at a constant flow rate of 1.0 ml min–1. The molecular ion m/z 127 [M-H] was selected for the quantification in the selected ion monitoring (SIM) mode. Within-the-day and day-to-day relative standard deviations were less than 8.6 and 10.5%, respectively. The limits of quantification (LOQs) in water samples were 0.02, 0.01, and 0.01 µg l–1 for 2-MCP, 3-MCP, and 4-MCP, respectively. The method allows the three target compounds in water samples to be determined at a level of ng ml–1, and it has been successfully applied to the routine analyses. Key Words:
1
Ion chromatography-mass spectrometry (IC-MS); 2-Chlorophenol; 3-Chlorophenol; 4-Chlorophenol; Drinking water
Introduction
Monochlorophenols (MCPs) are some of the most important industrial chemical materials, which cause considerable contamination in the environment such as in water and soil[1]. Also it has been found that MCPs could be produced from treating drinking water with chlorine[2]. Recently, a variety of techniques have been reported, which state that MCPs are determined by gas chromatography after derivatization with electronic capture detector or mass spectrometry[3,4] and by high-performance liquid chromatography (HPLC) with various detectors such as ultraviolet, fluorescence, electrochemical, and mass spectrometry[5–11]. Unfortunately, as it is well known, there are no literatures for the simultaneous determination of the three MCPs by IC coupled with atmospheric pressure chemical ionization mass spectrometry (IC-APCI-MS). The aim of this study is to
develop a new IC method for the simultaneous determination of 2-MCP, 3-MCP, and 4-MCP in drinking water samples, using 15 mM KOH containing 10% acetonitrile as the mobile phase in the isocratic mode and to separate on an IonPac® AS18 analytical column (250 mm × 4.0 mm), and to detect by an APCI-MS in the negative mode, in selected ion monitoring (SIM) mode, using their molecular ions m/z 127 [M－H]–. This method could be used for the routine analysis of these MCPs in water samples.
2 2.1
Experimental Instrument and reagents
IC separations for method development and validation were carried out on an ICS2000 Ion Chromatography System (DIONEX, Sunnyvale, CA, USA), consisting of a single pump,
Received 21 July 2005; accepted 18 November 2005 * Corresponding author. Email: [email protected]; Tel: +86 574-87274559; Fax: +86 574-87361764 This work was supported by the Ningbo City Medicine Science and Technology Plan (No. 2003056). Copyright © 2006, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
a column heat exchanger, an AS40 automated sampler, and an ASRS®-ULTRA 4 mm suppressor. Detection was performed on an Agilent LC/MSD Trap SL mass spectrometer with an APCI interface (Agilent Technologies, Germany). The separation system was controlled on a computer equipped with DIONEX Chromeleon 6.5 Software. The APCI-MS system was controlled, and data were analyzed on a computer equipped with LC/MSD Trap Software 4.2 (Bruker). All tubings used for connections were PEEK (polyetheretherketo-
ne) (0.25 mm I.D., Agilent Technologies, Germany). The IC-MS scheme is shown in Fig.1. Acetonitrile (MeCN), methyl tert-butyl ether (MTBE), and methanol (MeOH) (Merck, Darmstadt, Germany) were HPLC grade. Purified water was supplied by a Milli-Q water purification system (Millipore, Molsheim, France). 2-CP (> 98%), 3-CP (> 97%) and 4-CP (> 99%) were obtained from Merck (Darmstadt, Germany).
Fig.1 IC-MS scheme
2 .2 2.2.1
Experiment method Preparation for MCPs stock solution
Each of the MCPs standard (100.0 mg) was accurately weighted into a 100 ml volumetric flask, and diluted with MeOH. Working standard solutions were prepared weekly in MeOH. Stock, and the working standards were stored at 4ºC in a refrigerator. The aqueous solutions were prepared daily by diluting the working solution with purified water. 2.2.2
Preparation of samples
Real sample (100 ml) was acidified to pH 2.5 by 0.10 M phosphoric acid, and then filtered through a 0.45-μm nylon membrane filter (Agilent Technologies, Germany) before the analysis. After that, the sample was extracted using an Oasis® HLB (3 ml per 60 mg, Waters) cartridge that was first conditioned with 5.0 ml MeOH/MTBE (10/90, v/v), 5.0 ml methanol, and 5.0 ml water. The flow rate of the water sample was 4.0 ml min–1. MCPs were eluted with 2.0 ml MeOH/MTBE (10/9， v/v) twice. The elution was evaporated to dryness under a gentle stream of nitrogen and the residue
was reconstituted with 0.5 ml of MeOH and filtered through a 0.45-μm nylon syringe filter (Agilent Technologies, Germany). Finally, an aliquot of 20.0 μl of the resulting solution was injected into the IC-MS system. 2.2.3
IC condition
The IC method utilized an IonPac® AS18 analytical column (250 mm × 4.0 mm, DIONEX) and an IonPac® AG18 guard column (50 mm × 4.0 mm, DIONEX). Injection volume was 20.0 μl. The mobile phase was 15 mM KOH, containing 10% MeCN modifier at a constant flow rate of 1.0 ml min–1. Column temperature was held constant at 35ºC. The ASRS®-ULTRA 4 mm suppressor was operated at 40.0 mA in the external water mode. 2.2.4
MS condition
The ion trap mass spectrometer was used in the negative mode with a full scan mass spectra over the m/z range of 75–200 amu using a cycle time of 1 s and a peak width of 0.1 s, a corona current of 4 μA, a capillary voltage of 2.5 kV, a capillary exit voltage of –85 V, a dry temperature of 325ºC, a
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
vaporizer temperature of 450ºC, a high purity (99.999%) dry nitrogen gas of 5.0 l min–1, a nitrogen nebulizer pressure of 60.0 psi, and a dwell time of 200 ms. The APCI interface and mass spectrometer parameters were optimized to obtain maximum sensitivity. Analytes were detected with an APCI interface in the negative SIM mode. The quantification ions were selected as the molecular ions [M–H]– m/z 127 for the MCPs. The SIM peak areas were integrated for quantification.
3 3.1
compatible with the organic solvents and could obtain good separation for 2-MCP, 3-MCP, and 4-MCP with IonPac® AS 18 column, and could also be co-eluent for the three compounds with OmniPac® PAX 500. Fig.2 shows MCP’s total ion chromatogram (TIC) and Fig.3 shows MCP′s mass spectra. As can be seen, the three MCP isomers had obtained baseline separation when using IonPac® AS18 and the chlorine isotopes (35Cl and 37Cl) were also obviously obtained with abundance ratio in nature.
Results and discussion Anion-exchange column
Initially, an OmniPac® PAX 500 column (250 mm × 4.0 mm, DIONEX) coupled with an OmniPac® PAX 500 guard column (50 mm × 4.0 mm, DIONEX) and an IonPac® AS 18 column (250 mm × 4.0 mm, DIONEX) coupled with an IonPac® AG18 guard column (50 mm × 4.0 mm, DIONEX) were considered for the separation of 2-MCP, 3-MCP, and 4-MCP. It was found that all these columns were totally
Fig.2 Total ion chromatogram of MCPs Peak identified: (1) 2-MCP; (2) 4-MCP; (3) 3-MCP.
Fig. 3 MS spectra of 2-MCP (A), 4-MCP (B) and 3-MCP (C)
3.2
Mobile phase
Initially, KOH, Na2CO3, and NaHCO3 with different concentrations were examined to investigate whether they all could achieve a better separation for 2-MCP, 4-MCP, and 3-MCP. It was found that KOH was the best, and as the KOH concentration increased, the peak could become narrow and the run time would decrease, when the KOH concentration achieved 15 mM, hence the 2-MCP, 4-MCP, and 3-MCP were completely separated within only 5 min. It could be explained that the MCPs had the characteristics of weak organic acid; they could be deprotonated in a basic medium with different degrees and form monochlorophenolic ions (Fig.4), because of the different pK′s for the three target compounds. This provided a possibility to retain MCPs onto an anion-exchange column owing to the differential adsorption and desorption on the ion-exchange resin. It was also found that the eluent order of the three isomers was closely correlated with their pK. The higher the pKa, the longer the retention time; for example if pKa = 8.52 for 2-MCP, tR = 3.41 min; pKa = 8.97 for 4-MCP, tR = 4.12 min; pKa = 9.37 for 3-MCP, tR = 4.64 min.
Fig. 4 Chemical structures of (A) MCPs and (A') anionic MCPs
3.3
Organic modifier
In the preliminary experiments, a peak broadening, lower sensitivity, and longer run time were observed when using IonPac® AS18 column for the separation of the three MCP isomers. When 10% (v/v) MeCN was added to the eluent as an organic modifier, to modify the polarity of the mobile phase, it was found that the peak broadening had greatly decreased and the sensitivity had obtained a significant increase, and the peaks had become more sharp and symmetrical (Fig.2).
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
3.4 Compatibility between IC and MS
compatibility between IC and MS. The ion movement scheme in ASRS®-ULTRA anion suppressor is shown in Fig.5. In addition, an almost neutral solution, an eluent of least conductivity, was obtained at 40.0 mA in the external water mode for the ASRS®-ULTRA 4 mm suppressor. This solution achieved the same order magnitude of signals for the three MCPs within a single IC run with the exclusive use of APCI interface. 3.5
Fig.5 Ion movement scheme in ASRS®-ULTRA anion suppressor
In this study, the eluent was KOH. If the KOH directly entered into the ionization chamber, it would not only cause an inorganic salt deposition and erosion on the surface of the corona needle and the circle of the electrode, but also cause instability, inhibition, and quenching of the mass signal, and result in the obstruction of the capillary. Furthermore no mass signal was observed when the KOH concentration reached 15 mM, and the monochlorophenolic ions in the ionization chamber were completely suppressed. As the interference of the KOH was visible, the eluent prior to the vaporization chamber would be modified; otherwise, the combination of IC and MS could not be achieved. Luckily, the ion suppressor was used well, to solve the problem by reducing the concentration of the potassium ion in the eluent, to gain the
Repeatability and precision
The repeatability and precision of MCPs were evaluated by performing five replicates of three spiked drinking water samples (0.05, 0.50, and 5.00 μg l–1) as quality control samples (QC), including SPE procedures, for three continuous days. The typical TIC is shown in Fig.6 and the repeatability and precision are shown in Table 1. Within-the-day precision (RSD) on the basis of MCPs content was less than 8.6% and the day-to-day precision (RSD) was less than 10.5% for the three objective compounds.
Fig. 6 Total ion chromatogram for spiked water sample (0.50 μg l–1) Peak identified: (1) 2-MCP; (2) 4-MCP; (3) 3-MCP.
Table 1 The test of repeatability and precision –1
Concentration (μg l )
RSD (%)
Relative error (%)b
Compound
2-MCP
3-MCP
4-MCP
Added
Found a
First day
Second day
Third day
0.05
0.046 ± 0.0031
6.7
7.2
8.4
–8.0
0.50
0.523 ± 0.044
8.4
9.3
9.5
4.6
5.00
4.82 ± 0.15
3.1
4.2
6.3
–3.6
0.05
0.044 ± 0.0022
5.0
5.2
7.4
–12.0
0.50
0.474 ± 0.041
8.6
9.1
9.9
–5.2
5.00
4.51 ± 0.23
4.6
5.5
6.7
–9.8
0.05
0.042 ± 0.0021
5.0
6.6
7.6
–16.0
0.50
0.483 ± 0.032
6.6
8.3
10.5
–3.4
5.00
4.72 ± 0.24
5.1
5.2
7.3
–5.6
Note: a. the mean value determined six times on the first day; b. the relative error of the value determined six times on the first day.
3.6
Calibration curves and LOQs
Calibration curves were obtained for 2-MCP, 3-MCP, and
4-MCP using a series of standard solutions ranging from 0.005 to 5.0 μg l–1 with an SPE step. Three repeated injections of the standard solution at each concentration were performed.
JIN Micong et al. / Chinese Journal of Analytical Chemistry, 2006, 34(9): 1223–1226
Table 2 lists the method and quality parameters for all the objective compounds. It shows that all the calibration curves are linear over a standard concentration range of 0.02 to 5.0 μg l–1 with a coefficient of determination r2 > 0.993. The limits of quantification (LOQs) were determined using spiked drinking water samples which were spiked with MCPs at 0.05 and 0.20 μg l–1 with an SPE step, detected in SIM mode, and evaluated by the criterion that the signal-to-noise ratio should be greater than10 for quantification purposes. The LOQs for 2-MCP, 3-MCP, and 4-MCP were 0.02, 0.01, and 0.01 μg l–1, respectively.
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Conclusions
[7] Sarrion M N, Santos F J, Galceran M T. J. Chromatogr. A, 2002, 947: 155–165
The analytical IC-APCI-MS method using Oasis® HLB to preconcentrate and clean, for the simultaneous determination of 2-MCP, 3-MCP, and 4-MCP in drinking water samples was developed. The method was demonstrated with the advantages of simplicity, rapidity, and sensitivity, and was applied to the determination of the three MCPs isomers in the drinking water samples.
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