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Simultaneous Quantitative Analysis of Anionic, Cationic, and Nonionic Surfactants in Water by Electrospray Ionization Mass Spectrometry with Flow Injection Analysis Mo´nica Barco,† Carles Planas,† Oscar Palacios,† Francesc Ventura,‡ Josep Rivera,† and Josep Caixach*,†
Mass Spectrometry Laboratory, Department of Ecotechnologies, IIQAB-CSIC. C/Jordi Girona 18-26, 08034-Barcelona, Spain, and AGBAR. Aigu¨es de Barcelona S.A. Passeig de Sant Joan 45, 08009-Barcelona, Spain
A rapid method is described for the quantitative analysis of anionic, cationic, and nonionic surfactants in water samples by flow injection analysis coupled to electrospray ionization mass spectrometry (FIA/ESI-MS). All surfactants were isolated by liquid-liquid extraction and quantified using labeled triethoxylated nonylphenol ([13C6]NP3EO) and sodium dibutylnaphthalenesulfonate as internal standards. FIA/ESI-MS was performed by alternating both positive and negative ionization modes, which allows simultaneous analysis of most common surfactants in a short time. Quality parameters of the method, such as linear range, repeatability, reproducibility, and limits of detection were studied. This method was applied to the analysis of wastewater treatment plant effluents from Catalonia (NE Spain). Several environmental problems reported in the literature are related to the pollution caused by detergents.1,2 Typical examples are foaming by poorly degradable surfactants (e.g., highly branched alkylbenzenesulfonated or ABS), eutrophication of natural waters by phosphates, and the presence of toxic metabolites such as nonylphenol, produced by the degradation of polyethoxylated nonylphenols. In the past decades, several actions have been taken in order to avoid these problems. For example, many surfactants have been replaced by other compounds environmentally more acceptable (e.g., phosphates by nitriloacetic acid, nonylphenols by polyethoxylated alcohols (AnEOs), ABS by linear alkylbenzene sulfonates (LAS), and the cationic surfactant dimethylditallowammonium chloride by esterquat). On the other hand, a better knowledge of the environmental occurrence of the most commonly used surfactants (i.e., NPnEOs, AnEOs, and LAS) and their metabolites is needed in order to reduce the pollution effects related to these compounds. There* Corresponding author. Tel: +34 93 400 61 00. Fax: +34 93 204 59 04. E-mail:
[email protected]. † IIQAB-CSIC. ‡ AGBAR. (1) Giger, W.; Alder, A. C.; Fernandez, P.; Molnar, E. EAWAG News 1994, 10, 24-27. (2) Giger, W. EAWAG News 1989, 28F, 8-11. 10.1021/ac020708r CCC: $25.00 Published on Web 08/30/2003
© 2003 American Chemical Society
fore, many efforts have been carried out to improve the accurate determination of surfactants in water and other environmental samples. Different extraction and preconcentration methods have been used for the determination of surfactants in environmental samples. Isolation techniques such as solvent sublation, liquidliquid extraction, and solid-phase extraction are the most described in the literature.3,4 Analytical methods involving GC/MS have been used for the determination of LAS, other anionics, AnEOs after derivatization, and NPnEOs with low degree of ethoxylation.3,4 Soft ionization techniques such as field desorption or fast atom bombardment (FAB) were used for qualitative characterization of surfactant mixtures in environmental samples.5-9 Matrix effects related to these techniques almost precluded their use for quantitative measurements. The increasing use of LC/MS methodssespecially in the electrospray ionization modeshas overcome many of the limitations of the GC/MS technique and has been applied to the analysis of surfactants and their polar metabolites.10-15 A review of LC/MS applications in this field has recently been published.16 (3) Lee, H. B. Water Qual. Res. J. Can. 1999, 34, 3-35. (4) Schmitt, T. M. Analysis of Surfactants; Marcel Dekker Inc.: Basel, Switzerland, 2001; Vol. 96. (5) Yasuhara, A.; Shiraishi, H.; Tsuji, M.; Okuno, T. Environ. Sci. Technol. 1981, 15, 570-573. (6) Ventura, F.; Caixach, J.; Figueras, A.; Espadaler, I.; Fraisse, D.; Rivera, J. Water Res. 1989, 23, 1191-1203. (7) Ventura, F.; Caixach, J.; Romero, J.; Espadaler, I.; Rivera, J. Water Sci. Technol. 1992, 25, 257-264. (8) Ventura, F.; Figueras, A.; Caixach, J.; Espadaler, I.; Romero, J.; Guardiola, J.; Rivera, J. Water Res. 1988, 22, 1211-1217. (9) Ventura, F.; Caixach, J.; Fraisse, D.; Rivera, J. Anal. Chem. 1991, 63, 20952099. (10) Riu, J.; Martı´nez, E.; Barcelo´, D.; Ginebreda, A.; Ll. Tirapu. Fresenius J. Anal. Chem. 2001, 371, 448-455. (11) Jonkers, N.; Knepper, T. P.; De Voogt, P. Environ. Sci. Technol. 2001, 35, 335-340. (12) Marcomini, A.; Zanette, M.; Pojana, G.; Suter, M. J. Environ. Toxicol. 2000, 19, 549-554. (13) Radke, M.; Behrends, T.; Fo¨rster, J.; Herman, R. Anal. Chem. 1999, 71, 5362-5366. (14) Schoro ¨der, H. F. J. Chromatogr. 1993, 647, 219-234. (15) Schoro ¨der, H. F. J. Chromatogr.. A 2001, 926, 127-150. (16) Schro ¨eder, H. Fr.; Ventura, F. In Sample Handling and Trace Analysis of Pollutants; Barcelo´, D., Ed.; Elsevier: Amsterdam,The Netherlands, 2000; Vol. 21, pp 827-933.
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Figure 1. Chemical structures and acronyms. (a) Esterquat; (b) linear alkylbenzenesulfonates (LAS); (c) polyethoxylated nonylphenols (NPnEOs, n ) ethoxylation degree); (d) Polyethoxylated alcohols (AnEOs, n ) ethoxylation degree).
Numerous studies have been published on the occurrence of surfactants in different environmental compartments. However, most analytical methods do not include simultaneous determination of the three major groups of surfactants (anionic, nonionic, and cationic). In addition, only qualitative analysis is usually carried out. The objective of this work was the simultaneous quantitative determination of the three main groups of surfactants in water samples. Figure 1 displays the chemical structures and acronyms used. The developed method is based on liquid-liquid extraction followed by flow injection analysis coupled to electrospray ionization mass spectrometry (FIA/ESI-MS) analysis. This procedure has been applied to the analysis of river water samples and wastewater treatment plant (WWTP) effluents from Catalonia (NE Spain). EXPERIMENTAL SECTION Chemicals. All reagents were of analytical or high-performance liquid chromatographic grade. Dichloromethane, methanol, and hydrochloric acid were purchased from Merck (Darmstadt, Germany). Isopropyl alcohol was from Carlo Erba (Rodano, Milan, Italy), and formic acid was from Panreac (Barcelona, Spain). Highpurity water produced with a Milli-Q Organex-Q System Millipore (Millipore Corp., Bedford, MA) was used. Linear alkylbenzenesulfonates and esterquat (dimethyldiestearylammonium chloride) were provided by Pulcra (Barcelona, Spain). Polyethoxylated decyl alcohol (AnEO-C10, n ) 2-13), a mixture of polyethoxylated lauryl, tridecyl, myristyl, and pentadecyl alcohols (n ) 2-13), and polyethoxylated nonylphenols (NPnEOs, n ) 2-11) were provided by KAO Corp. (Barcelona, Spain). The internal standards sodium 5130
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dibutylnaphthalenesulfonate and [13C6]-NP3EO were purchased from Chem Service (West Chester, PA) and Cambridge Isotope Laboratories (Andover, MA), respectively. Sample Collection and Preparation. Four real samples from WWTP effluents were collected in Pyrex borosilicate amber glass bottles. Samples were stored at 4 °C and analyzed within 48 h. From each sample, two 200-mL aliquots were prepared in parallel, one of them spiked with 10 µg of NPnEOs, AnEOs, esterquat, and LAS standards. Samples were acidified with 10% hydrochloric acid to pH 2. Each aliquot was extracted with 2 × 100 mL of dichloromethane/isopropyl alcohol (90:10 v/v). Extracts were filtered through a Whatman paper filter (Maidstone, U.K.). [13C6]NP3EO and sodium dibutylnaphthalenesulfonate were added as internal standards to the extracts, which were concentrated down to 500 µL at 45 °C under nitrogen. ESI-MS Equipment. ESI-MS experiments were conducted on a P 580 A LPG liquid chromatograph Gynkotek (Munich, Germany) coupled to a quadrupolar mass spectrometer Navigator (Finnigan, MassLab Group, Manchester, U.K.) with an electrospray source. Electrospray-MS Operating Conditions. Samples were introduced using flow injection analysis (loop volume of 5 µL). The mobile phase was methanol/water (80:20) at a flow rate of 50 µL/min. ESI-MS analyses were carried out in positive and negative electrospray ionization modes (ESI+ and ESI- analyses). In the positive mode, full-scan mass spectra were performed from 100 to 1200 m/z at 5.00 s/scan in continuum mode. The interscan time was 0.10 s. In the negative mode, scans from 100 to 500 m/z were acquired in continuum mode at 3.00 s/scan with an interscan time of 0.10 s. Positive and negative spectra were obtained alternately in one run. Identification and Quantitation. Surfactant identification was performed manually by comparing sample and standard mass spectra. Quantitative analyses were carried out by the standard addition procedure (successive analysis of spiked and nonspiked aliquots described in the Sample Collection and Preparation section) using [13C6]-NP3EO and sodium dibutylnaphthalenesulfonate as internal standards. NPnEOs, AnEOs, and esterquat were quantified in the positive mode related to [13C6]-NP3EO response (ion [M + Na]+, m/z ) 381), while LAS were quantified in the negative mode using the sodium dibutylnaphthalenesulfonate response (ion [M]-, m/z)319). The anionic surfactants alkyl sulfonates (AS) and polyethoxylated alkyl sulfates (AnEOS) were also quantified in the negative mode. Single charged ions [M + Na]+ are formed when standards of polyethoxylated aliphatic alcohols (AnEOs) are analyzed by ESIMS. However, [M + H]+ andsless frequentlys[M + Na]+ ions are observed in sample extracts. These compounds were quantified using the sum of single charged ion responses ([M + H]+ or [M + Na]+) and the response factor of polyethoxylated decyl alcohol (sum of [M + Na]+ ion responses from n ) 2 to n ) 13). For quantitation of polyethoxylated nonylphenols (NPnEOs), the sum of [M + Na]+ ion responses related to n ) 2-11 was used. Esterquat was quantified using the sum of ion responses related to the diester signals (see Table 2). Quantitation of AS was carried out using the [M]- ion responses, while AnEOS were quantified using the sum of [M]-
Table 1. Mobile Phases Studied mobile phase
signal/noisea
methanol acetonitrile water acetonitrile + 0.08% formic acid water + 0.16% formic acid methanol/water (80:20) methanol/water (60:40) methanol/acetonitrile (70:30) methanol/acetonitrile (50:50) acetonitrile/water (50:50) methanol/water (70:30) + 0.16% formic acid methanol/water (80:20) + 0.16% formic acid water/acetonitrile (50:50) + 0.08% formic acid methanol/acetonitrile/water (30:25:45)
2126 1847 1045 1650 493 2453 1415 1045 957 1184 348 842 1155 1820
a
Parameter obtained by analyzing 2.5 ng of NPnEOs (n ) 2-11).
Table 2. Ions Observed in the ESI Spectrum of Esterquata R1 McLafferty rearrangement monoester diester
triester
a
R2
R3
m/z
C16
283.2
C18 C18:1 C16 C18 C18:1 C16 C16 C16 C18 C18 C16 C16 C16 C16 C18 C18
309.2 311.2 402.3 428.3 430.2 640.6 666.5 668.5 694.6 696.4 878.7 904.8 930.6 932.9 958.6 960.9
H H H C16 C18:1 C18 C18:1 C18 C16 C16 C18:1 C18 C18:1 C18
H H H H H H H H C16 C18 C18:1 C18:1 C18:1 C18:1
C16, palmitic acid; C18, estearic acid; C18:1, oleic acid.
ion responses corresponding to the different degrees of ethoxylation. Finally, for LAS (C10-C13), the sum of [M]- ion responses related to the different species was used (m/z ) 297, 311, 325, 339). AS, AnEOS, and LAS were quantified using the response factor of LAS (C10-C13). Response factors were calculated daily. We have also studied the ion suppression effect when different surfactants are present in real samples. Results showed that there is no ion suppression for nonionic (N) and cationic (C) surfactant mixtures even when large amounts of one of them (100:1) are present. The analysis of mixtures of anionic (A) and nonionic (N) surfactants only showed ion suppression when the amounts of (A) were 100-fold higher than (N), which is not the case for the samples analyzed. When mixtures of (A) and (C) were tested, we did not observe ion suppression for large quantities (100:1) of (A) with respect to (C). However, this effect was significant (50 and 90%) for A/C (1:1) and (1:10) amounts. In this case, the use of internal standards allows correction of possible ion suppression effects due to ion pairing. RESULTS AND DISCUSSION Optimization of ESI Parameters. Different ESI parameters were studied in order to obtain the best sensitivity. The parameters
evaluated were the drying gas flow rate, source temperature, and capillary and cone voltages. Optimization of mobile-phase composition and flow rate was also carried out. Mobile Phase. Several polar mobile phases were studied in order to maximize sensitivity. The other ESI parameters were fixed according to the experience of the Mass Spectrometry Laboratory (mobile-phase flow rate, 50 µL/min; drying gas flow rate, 178 L/h; cone voltage, 50 V; and source temperature, 110 °C). As shown in Table 1, the best signal-to-noise ratio was obtained with methanol:water (80:20). This mobile phase has also the advantage of generating only the single charged ion [M + Na]+ for NPnEOs and AnEOs in the ESI+ analysis. On the contrary, a mixture of adducts ([M + Na]+ and [M + H]+) is formed when the mobile phases containing formic acid are used. Mobile-Phase Flow Rate. With the optimized mobile phase, the flow rate was evaluated in the range 30-200 µL/min using the same instrumental conditions. The best sensitivity was obtained at 50 µL/min. Drying Gas Flow Rate. The drying gas was nitrogen, and its flow rate was evaluated in the range 150-300 L/h. The maximum signal was obtained at 225 L/h. Source Temperature. The range between 90 and 190 °C was evaluated, the optimum temperature being 110 °C. Capillary Voltage. The respective optimum values were 3.5 and 3.0 kV in the positive and negative ionization modes, the studied range being 3.00-4.00 kV in both cases. Cone Voltage. Cone voltages of 30, 50, 70, and 90 V were evaluated giving 50 V the best sensitivity. The other ESI parameter values are indicated in parentheses: skimmer (1.6 and 1.5 V in ESI+ and ESI- modes, respectively), skimmer lens offset (5 V in both ESI( modes), rf lens (0.2 V in both ESI( modes), ion energy (1.1 and 1.0 V in ESI+ and ESImodes, respectively). Surfactant ESI Spectra. Figure 2 shows mass spectra of target compounds, which, in this case, are similar to those previously reported by FAB elsewhere.6 Cationic Surfactants. Esterquats are mixtures of mono-, di-, and triesters of triethanolamine with fatty acids (palmitic, estearic, and oleic acids). The mass spectrum of esterquat (Figure 2a) presents single charged ions [M]+ arising from the different combinations of alkyl chains with mono-, di-, and triesters. MacLafferty fragmentation ions are also formed. Table 2 shows the different ions and intensities observed in the mass spectrum.17 Anionic Surfactants. Single charged ions [M]- spaced 14 m/z units corresponding to the different C10-C13 homologues of LAS were obtained in the negative mode (Figure 2b). The base peak corresponded to m/z 325, related to LAS-C12. Nonionic Surfactants. Series of single charged ions [M + Na]+ spaced 44 m/z units corresponding to the different ethoxylation degrees are observed in the positive mode for both polyethoxylated alcohols (lauryl, tridecyl, myristyl, pentadecyl) and NPnEOs (Figure 2c and d). Optimization of the Analytical Methodology. To optimize the extraction recoveries three different solvents were evaluated. A 200-mL aliquot of Milli-Q water spiked with NPnEOs, AnEO(17) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Mill Valley, CA, 1993.
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Figure 2. Mass spectra of target compounds.
Table 3. Recoveries of Surfactants in Milli-Q Water and Surface Water Samples Using Dichloromethane/ Isopropyl Alcohol (90:10 v/v) as Extraction Solvent recoveries (%) surfactants
spiked Milli-Q water (n ) 3)
surface water samples (5 different samples)
NPnEOs (n ) 2-11) AnEO-C10 (n ) 2-13) LAS esterquat
105.2 ( 4.2 99.2 ( 3.3 79.8 ( 4.5 88.6 ( 4.3
76.6 ( 4.7 74.0 ( 6.4 63.4 ( 7.2 74.2 ( 10.2
C10, esterquat, and LAS standards (100 µg/L of each analyte) was extracted three times using 100 mL of ethyl acetate, dichloromethane, and dichloromethane/isopropyl alcohol (90:10 v/v), respectively. When ethyl acetate was used as the extraction solvent, poor recoveries were obtained for all the surfactants analyzed (ranging from 17% for LAS to 36% for AnEO-C10). With dichloromethane, recoveries of NPnEOs and AnEO-C10 were higher than 90%, but LAS and esterquat were recovered in low percentages (32 and 42%, respectively). Finally, the best results were obtained when extracting with 100 mL of dichloromethane/isopropyl alcohol (90:10 v/v), with recoveries higher than 79% for all the surfactants analyzed. Table 3 shows recoveries obtained for each group of surfactants in Milli-Q water and real samples (surface water samples analyzed by the standard addition procedure described in the Experimental Section) when dichloromethane/isopropyl alcohol (90:10 v/v) was used as the extraction solvent. The number of extractions was optimized with the following procedure. A 200-mL aliquot of Milli-Q water was spiked with NPnEOs, AnEOs, esterquat, and LAS standards (100 µg/L of each analyte) and extracted four times with 100 mL of dichloromethane/ isopropyl alcohol (90:10 v/v) each time. Two extractions were enough to extract all the analytes with high recoveries (100% for NPnEOs, 96% for AnEOs, 77% for LAS, and 79% for esterquat).
Quality Parameters. Table 4 displays the results concerning instrumental quality parameters. Relative standard deviation (RSD) values for repeatability (n ) 10) and reproducibility (n ) 3, 5 days) were evaluated at a low level near the low limit of the linear range for most surfactants and an intermediate level (23 and 100 ng of each compound injected, respectively). Repeatability ranged from 2.0 to 4.4% and from 2.2 to 4.6% for the low and intermediate levels, respectively. Reproducibility varied from 2.3 to 8.6% and from 3.4 to 5.9% for the low and intermediate levels, respectively. The linear range and calibration data for the different surfactants were examined over the range 2-456 ng (6 different standard concentrations) depending on the compound. Limits of detection (LODs), calculated by injecting 2.5 ng of each compound and using a signalto-noise ratio of 3, varied from 93 (LAS) to 797 pg (esterquat). Method precision and method detection limits (MDLs) were evaluated by analyzing a real sample (Igualada, April 2000) three times. Average concentrations and RSDs were calculated for each compound. Values for RSD ranged from 4.3 (NPnEOs) to 6.7% (esterquat), while MDLs (calculated using a signal-to-noise ratio of 3) varied from 0.34 (polyethoxylated stearyl alcohol) to 1.98 µg/L (esterquat). Analysis of Environmental Samples. The developed method based on FIA/ESI-MS was applied to the analysis of surfactants in real samples from WWTP effluents. The selected WWTPs received discharges mainly from industrial or urban origin. The results obtained are shown in Table 5. Nonionic surfactants such as polyethoxylated alcohols and nonylphenols; anionic surfactants such as AS, AnEOs, LAS and their metabolites, and cationic surfactants (esterquats) were identified in the WWTP effluents. Figure 3 shows, as an example, the reconstructed (() TIC profiles and the corresponding ESI ( spectra of the sample from the Martorell WWTP (30 km NW of Barcelona) collected in March 2000. In this sample, surfactants were detected at not very high concentrations, when compared to the other analyzed samples (see Table 5). However, a wide variety of compounds originating from industrial or domestic sources were identified. Regarding Analytical Chemistry, Vol. 75, No. 19, October 1, 2003
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Figure 3. Reconstructed (() TIC profiles of a real sample (Martorell, March 2000) and their corresponding ESI ( spectra.
Table 4. Instrumental Quality Parameters of FIA/ESI-MS with Simultaneous Acquisition in Both Positive and Negative Ionization Modes
surfactant
linear range (ng)
calibration data
r2
LOD (pg)a
NPnEOs (n ) 2-11) AnEO-C10 (n ) 2-13) esterquat LAS
23-455 22-456 2-433 20-402
y ) 0.965x + 0.141 y ) 0.818x + 0.151 y ) 0.357x - 0.015 y ) 2.291x - 0.026
0.9989 0.9993 0.9998 0.9983
255 362 797 93
RSD (%) repeatabilityb 23 ng 100 ng 3.6 4.4 3.8 2.0
2.2 3.2 4.6 2.4
RSD (%) reproducibilityc 23 ng 100 ng 4.0 8.6 3.9 2.3
3.5 4.3 5.9 3.4
a LOD, Limit of detection, calculated as the amount of surfactant injected in the instrument which produces a signal equal to 3 times the standard deviation of noise. b n ) 10. c n ) 3 (5 days).
Table 5. FIA/ESI-MS Analysis of Real Samples from Wastewater Treatment Plant Effluentsa
type
surfactant
Martorell (March 2000)
nonionic
NPnEOs AnEO-C10 AnEO-C11 AnEO-C12 AnEO-C13 AnEO-C14 AnEO-C15 AnEO-C16 AnEO-C17 AnEO-C18 esterquat LAS AS-C1O AS-C11 AS-C12 AnEOS-C12 AnEOS-C13 AnEOS-C14 AnEOS-C15 SPCs
10.8
