Anal. Chem. 2008, 80, 3445-3449
Organochlorine Pesticides by LC-MS G. Famiglini, P. Palma, E. Pierini, H. Trufelli, and A. Cappiello*
Istituto di Scienze Chimiche “F. Bruner”, Universita` di Urbino “Carlo Bo”, Piazza Rinascimento 6, 61029 Urbino, Italy
Contamination of water resources by organochlorine pesticides (OCPs) continues to receive widespread attention because of the increasing concern regarding their high persistence and bioaccumulation. These organic pollutants are not amenable by liquid chromatography (LC) coupled to atmospheric pressure ionization-mass spectrometry, which represents the method of choice for the characterization of pesticide residues in water. Gas chromatography-mass spectrometry provides excellent response for OCPs, but it falls short when complex, multiresidue analyses are required. As recently demonstrated, an efficient EI-based LC-MS interface can generate very good spectra for an extremely wide range of small-medium molecular weight molecules of different polarity and can represent a valid tool in solving the analytical challenge of analyzing OCPs by LC-MS. Based on this assumption, we present a new approach for the determination of 12 OCPs in water samples. The method requires a solid-phase extraction preconcentration step followed by nanoscale liquid chromatography coupled to a direct-electron ionization direct interface (Direct-EI). Direct-EI is a miniaturized interface for efficiently coupling a liquid chromatograph with an EI mass spectrometer. The capability to acquire high-quality EI spectra in a wide range of concentrations, and to operate in selected ion monitoring mode during analyses, allowed a precise quantification of the OCPs. Without sample injection enrichment, limits of detection of the method span from 0.044 to 0.33 µg/L, corresponding to an instrumental detection limit of 120-850 pg. In addition, a careful evaluation of the matrix effect showed that the response of the Direct-EI interface was never affected by sample interferences. From our knowledge, the proposed method represents the first application of LC-MS in the analysis of organochlorine pesticides. Organochlorine pesticides (OCPs) have been extensively used throughout the world. They are very persistent in the environment due to their high chemical stability and their strong tendency to sorb to organic material in soil and sediments.1,2 They can cause acute and chronic effects, including cancer, neurological damages, and birth defects,3-5 and many of them act as endocrine disrup* To whom correspondence should be addressed. Tel: +390722303344. Fax: +390722303311. E-mail:
[email protected]. (1) Fiedler, H.; Lau, C. Ecotoxicology 1998, 317-370. (2) Jones, K. C.; De Voogt, P. Environ. Pollut. 1999, 100, 209-221. (3) Ritchie, J. M.; Vial, S. L.; Fuortes, L. J.; Guo, H.; Reedy, V. E.; Smith, E. M. J. Occup. Environ. Med. 2003, 45, 692-702. 10.1021/ac8000435 CCC: $40.75 Published on Web 03/18/2008
© 2008 American Chemical Society
tors.6 Most developed countries have banned their use since the late 1970s, in favor of more modern and readily degradable pesticide formulations. However, OCPs are still extensively used in developing regions because of their low cost and their effectiveness. They enter the food chain through contaminated water, fish, and shellfish.7-15 OCPs show low polarity, high thermal stability, and volatility. As a consequence, the analytical protocols proposed for their determination are traditionally based on gas chromatography (GC) mainly coupled to detection techniques such as electron capture detection and mass spectrometry (MS).16-19 However, this represents a significant constrain in multiresidue analysis, when high polar, poorly volatile compounds are present as well. Therefore, liquid chromatography-mass spectrometry (LC-MS) represents the method of choice for pesticide multiresidue applications.20,21 In a recent review,22 the applicability and the sensitivity obtained by GC-EI-MS and LC(4) Fenster, L.; Eskenazi, B.; Andreson, M.; Bradman, A.; Harley, K.; Hernandez, H.; Hubbard, A.; Barr, D. B. Environ. Health Perspect. 2006, 114, 597602. (5) Kaushik, P.; Kaushik, G. J. Hazard. Mater. 2007, 143, 102-111. (6) Gore, A. C. Mol. Cell. Endocrinol. 2002, 192, 157-170. (7) Nowell, L. H.; Capel, P. D; Dileanis, P. D. Pesticides in Stream Sediment and Aquatic Biota-Distribution, Trends, and Governing Factor; CRC Press: Boca Raton, FL, 1999. (8) Carvalho, F. P.; Gonzalez-Farias, F.; Villeneuve, J. P.; Cattini, C.; HernandezGarza, M.; Mee, L. D.; Flower, S. W. Environ. Technol. 2002, 23, 12571270. (9) Ming-Lone, L.; Shin-Cheng, Y.; Yong-Chien, L.; Chien-Min, C. Hum. Ecol. Risk Assess. 2006, 12, 390-401. (10) Environamental Protection Agency (EPA). Assessing human health risk from chemically contaminated fish and shellfish: a guidance manual/EPA-503/ 8-89-002, Appendix F; U.S. EPA: Cincinnati, OH, 1998. (11) MacIntosh, D. L.; Spengler, J. D.; Ozkaynak, H.; Tsai, L.; Ryan, P. B. Environ. Health Perspect. 1996, 104, 202-209. (12) Jiang, Q. T.; Lee, T. K. M.; Chen, K.; Wong, H. L.; Zheng, J. S.; Giesy, J. P.; Lo, K. K. W.; Yamashita, N.; Lam, P. K. S. Environ. Pollut. 2005, 136, 155165. (13) Scheyer, A.; Graeff, C.; Morville, S.; Mirabel, P.; Millet, M. Chemosphere 2005, 58, 1517-1524. (14) Connor, M. S.; Davis, J. A.; Leatherbarrow, J.; Greenfield, B. K.; Gunther, A.; Hardin, D.; Mumley, T.; Werme, C. Environ. Res. 2007, 105, 87-100. (15) Ferrante, M. C.; Cirillo, T.; Naso, B; Clausi, M. T.; Lucisano, A.; Cocchieri, R. A. J. Food Prot. 2007, 70, 706-715. (16) Mangani, F.; Maione, M.; Palma, P. In Handbook of Water Analysis; Nollet, L. M. L., Ed.; Marcel Dekker: New York, 2000; pp 517-536. (17) Barriada-Pereira, M.; Gonza´lez-Castro, M. J.; Muniategui-Lorenzo, S.; Lo´pezMahı´a, P.; Prada-Rodrı´guez, D.; Ferna´ndez-Ferna´ndez, E. Chemosphere 2005, 1571-1578. (18) Yenisoy-Karakas¸ , S. Anal. Chim. Acta 2006, 571, 298-307. (19) Guardia-Rubio, M.; Ruiz, Medina, A.; Pascual, Reguera, M. I.; Ferna´ndez, de Co´rdova, M. L. Microchem. J. 2007, 85, 257-264. (20) Reemtsma, T. J. Chromatogr., A 2003, 1000, 477-501. (21) Kuster, M.; Lo´pez, de Alda, M.; Barcelo´, D. Mass Spectrom. Rev. 2006, 25, 900-916. (22) Alder, L.; Greulich, K.; Kempe, G.; Vieth, B. Mass Spectrom. Rev. 2006, 25, 838-865.
Analytical Chemistry, Vol. 80, No. 9, May 1, 2008 3445
ESI-MS/MS for pesticides belonging to different classes was compared. The data shown in this review clearly indicate that OCPs represent a major challenge in the development of a “rapid and universal” multiresidue analytical method based on LC-MS. The fact that OCPs provide a very good EI spectrum make them perfect candidates for Direct-EI, LC-MS interfacing. This research group has devoted a considerable effort to improve the performance of LC-MS interfaces based on electron ionization.23 As discussed in a previous paper, Direct-EI allows the direct introduction of the effluent from a capillary HPLC column into the electron ionization ion source of a mass spectrometer.24-26 Thanks to a simple, reliable, and effective interfacing process, many typical HPLC amenable compounds can generate good quality, library matchable, readily interpretable EI spectra27-29 for an easy identification and detection. In a recent publication, we presented an improved version of the Direct-EI, in which the interface has been widely refined, and applied to modern instrumentation.26 The new interface shows superior performances in the analysis of small-medium molecular weight compounds when compared to its predecessor: (1) It delivers high-quality, fully library matchable spectra of most sub-1 kDa molecules amenable by HPLC; (2) it is a chemical ionization-free interface; (3) response is not influenced by matrix components in the sample or in the mobile phase (nonvolatile salts are also accepted). This paper describes the development of a sensitive and rapid method for the LC-EI-MS analysis of OCPs in water samples using the latest version of the Direct-EI interface.26 An off-line solidphase extraction (SPE) was included in the method for suitable sample preparation and preconcentration. To the best of our knowledge, this is the first analytical approach based on LC-MS for the detection of OCPs. MATERIALS AND METHODS Chemicals and Reagents. Methylene chloride, tetrahydrofuran, HPLC grade acetonitrile, and methanol were purchased from VWR (Milan, Italy). Water was purified in a Direct-Q 3 UV system (Millipore Corp., Bedford, MA). Aldrin, R-endosulfan, β-endosulfan, o,p′-dichlorodiphenyldichloroethane (o,p′-DDD), p,p′- dichlorodiphenyldichloroethane (p,p′-DDD), o,p′-dichlorodiphenylchloroethane (o,p′-DDE), o,p′-dichlorodiphenyltrichloroethane (o,p′-DDT), p,p′-dichlorodiphenyltrichloroethane (p,p′DDT), heptachlor, hexachlorbenzene (HCB), lindane, and methoxychlor were purchased from Sigma-Aldrich Chemie (Steinheim, Germany). Stock solutions of each pesticide at a concentration of 15000 mg/L were prepared in tetrahydrofuran. A mixed solution containing the investigated compounds at 1000 mg/L in tetrahydrofuran was diluted with tetrahydrofuran until appropriate working solutions were obtained. (23) Cappiello, A.; Famiglini, G.; Palma, P. Anal. Chem. 2003, 75, 497A-503A. (24) Cappiello, A.; Famiglini, G.; Mangani, F.; Palma, P. J. Am. Soc. Mass Spectrom. 2002, 13, 265-273. (25) Cappiello, A.; Famiglini, G.; Palma, P.; Siviero, A. Mass Spectrom. Rev. 2005, 24, 978-989. (26) Cappiello, A.; Famiglini, G.; Pierini, E.; Palma, P.; Trufelli, H. Anal. Chem. 2007, 79, 5364-5372. (27) Cappiello, A.; Famiglini, G.; Palma, P.; Pierini, P.; Trufelli, H.; Maggi, C.; Manfra, L.; Mannozzi, M. Chemosphere 2007, 69, 554-560. (28) Cappiello, A.; Famiglini, G.; Palma, P.; Mangani, F. Anal. Chem. 2002, 74, 3547-3554. (29) Famiglini, G.; Palma, P.; Siviero, A.; Attaran Rezai, M.; Cappiello, A. Anal. Chem. 2005, 77, 7654-7661.
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Analytical Chemistry, Vol. 80, No. 9, May 1, 2008
Table 1. List of the Selected Pesticides with Their Characteristic Ions and SIM Program time (min) 0-30 30-45
compound lindane methoxychlor β-endosulfan R-endosulfan o,p′-DDD p,p′-DDD heptachlor o,p′-DDT p,p′-DDT DDE HCB aldrin
characteristic ions 111-181-219 227-228 195-241 195-241 165-235-237 165-235-237 100-235-237272-274-339 165-235-237 165-235-237 246-248-318 249-284-286 263-265
dwell time (s) 450 70
monitored ions 111-181-219 100-165-195 227-228-235 237-241-246 248-249-263 265-272-274 284-286-318 339
Liquid Chromatography. Liquid chromatography was performed on an Agilent 1100 series nano HPLC system (Agilent Technologies Inc., Santa Clara, CA). For more flexibility, the autoinjector was replaced with a manual injector from Valco, equipped with a 500-, 60-, and 20-nL internal loops (Valco, Houston, TX). Chromatographic separations were performed using a Agilent Zorbax SB-C18 column (150 mm × 75 µm i.d., 3.5-µm particle size) at a flow rate of 300 nL/min. Mobile phase was composed of water (solvent A) and acetonitrile (solvent B). The gradient elution was from 0 to 100% B in 30 min. Direct-EI-MS. The Direct-EI interface was mounted on a Agilent 5975B Inert MSD single quadrupole mass spectrometer (Agilent Technologies Inc.). The interface nebulizer consisted of a square-shaped tip, with an orifice of 15 µm and an outside diameter of 1/32 in. The nebulizer tip protruded into the ion source so that the spray expansion was completely contained inside the ion source. Mass spectrometer tuning and calibration were performed automatically at 200 °C, using perfluorotributylamine as a reference compound and monitoring the ions at m/z 69, 219, and 502. No mobile phase was allowed into the ion source during calibration. Source temperature is an important factor in the mechanism of Direct-EI, and its effect must be evaluated carefully. For the selected pesticides, the best temperature to obtain the highest signal and the best quality spectrum was 280 °C. Scan times for full spectrum acquisition were adjusted in order to obtain a mean of 15 acquisition samples for each HPLC peak (1.5-1.8 scan/s). Data acquisition during the chromatographic separations was carried out in selected ion monitoring (SIM) using different retention time windows (Table 1); dwell times were chosen on the basis of the number of ions present in each ion program in order to collect a minimum of 20 acquisition samples for each HPLC peak (0.8-1 cycles/s). A detailed description of the characteristic and functioning of the direct-EI interface is reported elsewhere.26 Matrix Effect Evaluation. Tests were performed on seawater, river water, and Milli-Q water samples. River water and seawater samples were collected in glass bottles, stored in the dark at 4 °C, and extracted by SPE within 24 h. Sample enrichment took place always after the SPE procedure. All samples were filtered through a HA MF-Millipore membrane filter (0.45 µm nominal) (Millipore Corp., Bedford, MA) prior to SPE extraction. SPE was carried out using 6-mL cartridges packed with 500 mg of AccuBOND II ODS-C18 (Agilent) and following the procedure
Figure 1. Comparison between Direct-EI mass spectrum of heptachlor in a tetrahydrofuran solution (6 mg/L) (upper trace) and standard NIST EI library spectrum (lower trace).
developed by Guardia Rubio et al.19 The cartridges were placed on a 12-port Visiprep SPE vacuum manifold (Supelco, Bellefonte, PA). The cartridges were conditioned by passing two portions of 5 mL of methylene chloride, two portions of 5 mL of methanol, and finally two portions of 5 mL of Milli-Q water. Water samples (1 L) were forced through the traps at a flow rate ranging from 12 to 15 mL/min. Before extraction, the cartridges were dried under vacuum for 45 min. After that, the analytes were eluted with 4 mL of methylene choride. Prior to injection, the final SPE extract in methylene chloride was evaporated under a gentle stream of nitrogen until dryness and then reconstituted in 200 µL of a tetrahydrofuran standard solution containing the selected pesticides at the following concentration: lindane (1.15 mg/L), methoxychlor (1.49 mg/L), R-endosulfan (3.30 mg/L), β-endosulfan (1.70 mg/L), o,p-DDD (1.11 mg/L), p,p-DDD (0.98 mg/L), heptachlor (1.39 mg/L), o,p-DDD (1.07 mg/L), p,p-DDD (1.15 mg/L), o,p-DDE (1.45 mg/L), hexachlorobenzene (0.99 mg/L), and aldrin (1.04 mg/L). RESULTS AND DISCUSSION Twelve OCPs were selected and analyzed by Direct-EI-MS, using the latest interface version. This straightforward approach based on electron ionization has proven to be a valid tool in solving a wide range of analytical tasks. This new version has a similar interfacing mechanism compared to previous prototypes with a few modifications and improvements, which allow superior performances in the analysis of small-medium molecular weight compounds.26 The inertness of the new ion source allows for a quicker conversion of the analytes into the gas phase with a better sensitivity and mass spectral quality. The use of Direct-EI in the LC-MS analysis of the OCPs can offer several advantages: (1) The simple operating principle of EI can induce the ionization of any molecule present in the gas phase, regardless of its chemical background, as long as it can withstand the typical EI source conditions. This allows us to overcome several drawbacks typical of API-based interfaces such as signal suppression, influence of mobile-phase composition, need for desalting, or postcolumn solvent modifications. (2) Moreover, the typical EI spectrum is very informative, and its high reproducibility allows an easy comparison with thousands of spectra from commercially available source (NIST or Wiley). (3) Under EI conditions, a single chromatographic run generally allows the simultaneous analysis of substances with different chemical properties, reducing time consumption and complexity of the analytical procedure. Consequently, there is no need for different ionization polarities or MSMS detection. (4) EI detection can also benefit from sophisticated algorithms such as the one developed
by the National Institute of Standards and Technology (NIST) and called AMDIS (Automated MS Deconvolution and Identification System), which extracts the mass spectra of the analytes in complex chromatographic mixtures in the presence of overlapping peaks. Our previous publications demonstrated that a key property of the Direct-EI interface is to produce top quality EI spectra from the compound dissolved in the liquid phase. This feature represents one of the driving forces of the Direct-EI, making it very competitive in terms of specificity for compound characterization and confirmation with a single-stage MS. The first goal in the method development was to investigate the quality of the recorded EI spectra of the OCPs. When working with real samples, the capacity of generating highly informative and reproducible EI spectra represents an important step for the positive identification of the compounds of interest. In our case, the quality of each recorded organochlorine spectrum was expressed as the measure of the degree of overlap, when compared to the reference spectrum reported in the NIST electronic library. Spectrum acquisition was achieved on column, applying the conditions described in the Experimental Section. As an example, in Figure 1 the recorded spectrum of heptachlor is compared with the library spectrum showing a matching factor of 748, reversed matching factor of 778 and a probability factor of 95.8%. It is worth it to consider that the spectrum was collected close to the detection limit in scan mode and thus with a high background noise. The comparison between the recorded spectra and the reference is very satisfactory for all investigated OCPs. OCPs spectra offered many fragment ions, which were used for the development of the SIM program during the chromatographic run. In Figure 2, the extracted ion chromatograms relative to the SIM acquisition of a standard mixture are shown. The following parameters were taken into account to evaluate the performance of the interface in this specific application: sensitivity, linearity, and precision. The sensitivity of the nanoLCDirect-EI-MS was expressed by the instrumental limit of detection (LOD) and limit of quantification (LOQ) for each pesticide. The LOD, defined as the lowest analyte amount with a signal-to-noise ratio (S/N) of 3, and the LOQ, defined as the amount with S/N of 10, were evaluated by injecting on column 500 nL of diluted solutions of the standards. As reported in Table 2, instrumental LODs ranged from 120 to 850 pg, while LOQs were established between 400 and 2853 pg. The instrumental LODs were also calculated in flow injection analysis (FIA) mode, using a mobile phase of water and acetonitrile in equal proportions at a flow rate of 300 nL/min (injection volume of 60 nL). These experiments Analytical Chemistry, Vol. 80, No. 9, May 1, 2008
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Figure 2. Extracted ion chromatograms of a tetrahydrofuran standard solution containing the selected pesticides at the following concentration: lindane (1,15 mg/L), methoxychlor (1,49 mg/L), β -endosulfan (3,30 mg/L), R-endosulfan (1,70 mg/L), o,p-DDD (1,11 mg/L), p,p-DDD (0,98 mg/L), heptachlor (1,39 mg/L), o,p-DDD (1,07 mg/L), p,p-DDD (1,15 mg/L), o,p-DDE (1,45 mg/L), hexachlorobenzene (0,99 mg/L), and aldrin (1,04 mg/L).
demonstrated an instrumental sensitivity ranging from 13 to 90 pg. The instrumental LODs obtained on column were used for the evaluation of the sensitivity of the method. The LOD of the method was expressed as the minimum concentration of the analyte that can be detected in the actual sample. It takes into account the initial volume of the sample (1 L), the final volume of the SPE extract (200 µL), the percentage of recovery of each organochlorine, the volume that is introduced into the LC-MS system (500 nl), and the instrumental LOD. The recovery was calculated by the analysis of four blank Milli-Q water samples spiked with a pesticide tetrahydrofuran solution in order to obtain a final concentration of 1 µg/L for R- and β-endosulfan and 0.2 µg/L for the other investigated OCPs. The fortified water sample was processed by SPE, and the final extract was analyzed on-column, as reported in the Experimental Section. The percentages of recovery calculated on the average of four replicates are reported in Table 2. Based on these results, a method LOD ranging from 0.044 to 0.081 µg/L was obtained for the following pesticides: lindane, methoxychlor, o,p′-DDD, p,p′-DDD, heptachlor, o,p′-DDD, p,p′-DDD, o,p-DDE, hexachlorobenzene, and aldrin. These values allow the determination of this organochlorine at the levels established by the European Community Legislation for drinking water (0.1 µg/L).30 As can be deduced from the data in Table 2, the actual method cannot be applied for the quantification of Rand β-endosulfan at the levels required by the European normative. In light of this, the development of an on-line preconcentration (30) European Directive 98/83/EC.
3448 Analytical Chemistry, Vol. 80, No. 9, May 1, 2008
system or the use of a large-volume injection device, such as the Agilent Micro-Well Plate Autosampler, can easily increase method sensitivity. To assess linearity, six-point calibration curves were calculated over the linear range of the instrument response, determined for each pesticide. Diluted solutions of each compound were injected by FIA at a flow rate of 300 nL/min and using a mobile-phase composition of water and acetonitrile in equal proportions. The injection volume was of 60 nL. Linear regression lines were plotted using the peak area versus the compound concentration; five replicates were analyzed for each concentration level. Linearity was expressed as the squared correlation coefficient (R2). Calibration curves gave values of the R2 higher than 0.9950 for most of the investigated pesticides (Table 2). Precision was calculated in terms of intraday and interday repeatability as relative standard deviation (RSD) in FIA at two concentration level (1 and 10 mg/L), injecting 20 nL. In these conditions, the absolute amount injected is 20 and 200 pg. For the determination of the intraday repeatability, six replicates were analyzed in 1 day. The interday repeatability was evaluated repeating the analyses on five different days (n ) 30). Both intraday and interday repeatability of the method provided RSD values lower than 15%, showing an excellent precision at the two concentration levels. A major drawback in the analysis of pesticide residues in complex samples by LC-API-MS is represented by the occurrence of the so-called “matrix effect”:31,32 this phenomenon is considered to be an unexpected suppression or enhancement of the analyte response due to coeluting sample constituents. Matrix effect can occur in the API source through different mechanisms, such as competition for the available charges and for the access to the droplet surface for gas-phase emission.33 In the Direct-EI interface, the ionization occurs in the gas phase, thus eliminating most matrix interferences observed with API. This point of strength of the interface was tested previously in other environmental applications such as the analysis of diethylene glycol29 and pesticides belonging to the class of triazines and carbamates in water samples.26 The most straightforward way to evaluate the matrix effect is to compare the response of an analyte in a standard solution and in a post-extraction spiked sample (matrix-matched standard).34,35 Difference in response indicates ion suppression or enhancement. This spiking procedure prevents any recovery miscalculation and focuses the response only to the matrix effect. In this work, the role of the matrix in the response of the OCPs in the Direct-EI interface was carefully investigated using samples of Milli-Q, seawater and river water. Due to the presence of fulvic and humic acids, the river water represents a major source of matrix effect.36 The samples underwent off-line SPE.19 Each final extract was spiked with a known amount of the selected pesticides and (31) Taylor, P. J. Clin. Biochem. 2005, 38, 328-334. (32) Niessen, W. M. A.; Manini, P.; Andreoli, R. Mass Spectrom. Rev. 2006, 25, 881-899. (33) Antignac, J. P.; de Wasch, K.; Monteau, F.; De Brabander, H.; Francois, A.; Le Bizec, B. Anal. Chim. Acta 2005, 529, 129-136. (34) Buhrman, D.; Price, P.; Rudewicz, P. J. Am. Soc. Mass Spectrom. 1996, 7, 1099-1105. (35) Matuszewsky, B. K.; Constazer, M. L.; Chavez-Eng, C. M. Anal. Chem. 2003, 75, 3019-3030. (36) Steen, R. J. C. A.; Hogeboom, A. C.; Leonards, P. E. G.; Peerboom, R. A. L.; Cofino, W. P.; Brinkman, U. A. Th. J. Chromatogr., A 1999, 857, 157-166.
Table 2. Calibration Data and Recoveries Obtained for the Selected Pesticides compound lindane methoxychlor β-endosulfan R-endosulfan o,p′-DDD p,p′-DDD heptachlor o,p′-DDT p,p′-DDT DDE Hexachlorobenzene Aldrin
LOD (pg)
LOQ (pg)
linear regression equation
R2
mean recovery ( RSD (%)
method LOD (µg/L)
130 160 850 850 120 120 180 120 120 150 130 130
435 500 2833 2833 400 400 500 400 400 500 435 435
y ) 6235.9x - 16105 y ) 15140x + 6425.4 y ) 568.93x + 1649.5 y ) 568.93x + 1649.5 y ) 23406x - 47253 y - 9837.8x + 29638 y ) 6874.2x - 43475 y ) 18952x - 132910 y ) 17887x + 47577 y ) 11168x + 373269 y ) 5686.3x + 787.3 y ) 4214.3x - 8125.2
0.9996 1 0.9999 0.9999 0.9986 0.9997 0.9986 0.9965 0.9957 0.9953 0.9990 0.9993
95 ( 15 108 ( 4 108 ( 19 102 ( 27 110 ( 10 91 ( 16 113 ( 6 109 ( 9 109 ( 13 74 ( 32 66 ( 24 109 ( 8
0.055 0.059 0.32 0.33 0.044 0.053 0.064 0.044 0.044 0.081 0.079 0.048
Table 3. Matrix Effect Evaluation compound
Milli-Q watera
seawatera
river watera
lindane methoxychlor β-endosulfan R-endosulfan o,p′ and-p,p′ DDD heptachlor o,p-DDT p,p-DDT DDE hexachlorobenzene aldrin
102 ( 3 104 ( 7 104 ( 12 100 ( 9 108 ( 5 107 ( 7 113 ( 8 108 ( 11 109 ( 7 106 ( 13 112 ( 10
104 ( 16 105 ( 14 107 ( 24 105 ( 12 106 ( 13 99 ( 13 106 ( 16 104 ( 12 104 ( 16 97 ( 12 100 ( 9
96 ( 11 115 ( 16 95 ( 7 100 ( 11 102 ( 2 95 ( 6 106 ( 13 99 ( 6 100 ( 6 98 ( 10 102 ( 9
a
Average matrix effect (n ) 5) ( RSD (%).
analyzed as reported in the Experimental Section. The experiments were repeated five times and compared with the injections of standards at the same concentration level. The matrix effect was calculated in percent, as the ratio between the average peak area of the neat standard solution and the average peak area of the sample spiked after extraction, multiplied by 100. In this context, a value of >100% indicates ionization enhancement, while a value of
