Separation and determination of fluorobenzoic acids using ion chromatography–electrospray mass spectrometry

Journal of Chromatography A, 1270 (2012) 96–103

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Separation and determination of fluorobenzoic acids using ion chromatography–electrospray mass spectrometry Karsten Müller, Andreas Seubert ∗ University of Marburg, Institute of Analytical Chemistry, D-35032 Marburg, Germany

a r t i c l e

i n f o

Article history: Received 3 September 2012 Received in revised form 4 October 2012 Accepted 15 October 2012 Available online 3 November 2012 Keywords: Fluorobenzoic acids Tracer Solid-phase extraction Mass spectrometry Ion exchange chromatography, Trace analysis

a b s t r a c t A method for the trace analysis of fluorobenzoic acids (FBAs) via IC–MS based on solid-phase extraction (SPE) prior to isocratic anion exchange chromatography is described. Up to 23 different fluorobenzoic acids were enriched and determined simultaneously. Solid-phase extraction on hydrophilic–lipophilicbalanced reversed-phase cartridges containing a poly(divinylbenzene-co-N-vinylpyrrolidone) polymer allowed a 500-fold enrichment of the acids if 100 mL sample was used with recoveries between 28% and 98%. The method enables the determination of FBAs down to the range of 16–210 ng L−1 , provides sample-preparation (pH-adjusting prior to SPE only) with no need of derivatization, uses low amounts of chemicals and is adaptable to larger or smaller sample volumes. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The separation and determination of polar organic compounds using ion chromatography coupled with mass spectrometry has the advantage over GC–MS in aqueous samples or when a derivatization should be avoided. The polar organic fluorobenzoic acids (FBAs) are used as water tracers in oil reservoirs, ground and soil water studies and are important intermediates in the synthesis of antibacterial drugs [1–7]. Currently FBAs are tested as indicators in leaching studies for carbon sequestration techniques also known as Carbon Capture and Storage (CCS). Conducting several simultaneous studies with different FBAs at the same reservoir or location without interference, no known toxicity and no naturally occurrence are the main advantages of FBA tracers. Due to large dilutions in reservoir studies and high cost for their application, a sensitive determination at trace levels is of great importance. In the past FBAs have been analyzed using HPLC-UV [8–12], tandem mass spectrometry [13,14], ion chromatography [15,16], capillary electrophoresis [17], and GC–MS [12,18–20]. In 1998 Galdiga and Greibrokk [18] observed detection limits down to 0.1 ␮g L−1 after extracting 1 L of aqueous reservoir samples on Isolute ENV+ and derivatization with diazomethane using GC–MS with electron impact ion source. In the same year both

∗ Corresponding author. Tel.: +49 6421 28 25661; fax: +49 6421 28 22124. E-mail address: [email protected] (A. Seubert). 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.10.046

authors also reported detection limits of 0.010 ␮g L−1 if pentafluorobenzyl ester were formed [19]. Solid-phase extraction and detection by negative ion chemical ionization mass spectrometry was used. Ion chromatography with conductivity detection was used in 1991 by Pearson and Inskeep [15] to separate three FBAs across a 0.25–25 mg L−1 calibration range. In 2005 Hu and Moran [16] used IC with conductivity detection under gradient conditions to separate seven FBAs from other common groundwater constituents with method detection limits around 20 ␮g L−1 . Nevertheless, enhancements in ion chromatography column development allow higher separation performance and the use of MS detection allows high sensitivity combined with the ability of separating coeluting analytes by their mass-to-charge ratio. The coupling of ion chromatography with electrospray mass spectrometry (IC–MS) is a known method for the separation and determination of ionic organic analytes in aqueous samples. Knepper et al. used IC–MS in 1999 for example for the determination of synthetic chelating agents in surface and waste water [21] and in 2005 for the analysis of polar organic micropollutants [22]. In this study a SPE using Oasis HLB Plus combined with high performance ion exchange columns and mass selective detection using negative electrospray ionization result in a selective, robust and reproducible method for the extraction, isocratic separation and determination of up to 23 fluorobenzoic acids at trace levels. Due to a multitude of sample matrices (e.g., ground water, surface water, rain water, and aqueous samples of oil reservoirs) and their poor and limited availability, tap water was used as sample matrix.

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Table 1 Abbreviations, suppliers, qualities, retention times and main fragments of the fluorobenzoic acids (n.o. = not observed). No.

2 12 10 9 6 7 4 18 17 15 13 8 11 5 22 19 14 16 3 20 21 1 23

Compound; purity (%, w/w)

Abbreviation

2-Fluorobenzoic acid; >98% 3-Fluorobenzoic acid; 97% 4-Fluorobenzoic acid; >99% 2,3-Difluorobenzoic acid; >97% 2,4-Difluorobenzoic acid; 99% 2,5-Difluorobenzoic acid; 98% 2,6-Difluorobenzoic acid; >98% 3,4-Difluorobenzoic acid; >98% 3,5-Difluorobenzoic acid; >98% 2,3,4-Trifluorobenzoic acid; >98% 2,3,5-Trifluorobenzoic acid; >98% 2,3,6-Trifluorobenzoic acid; >98% 2,4,5-Trifluorobenzoic acid; >97% 2,4,6-Trifluorobenzoic acid; >98% 3,4,5-Trifluorobenzoic acid; >98% 2,3,4,5-Tetrafluorobenzoic acid; >98% 2,3,5,6-Tetrafluorobenzoic acid; >97% 2,3,4,5,6-Pentafluorobenzoic acid; 99% 2-(Trifluoromethyl)-benzoic acid; 98% 3-(Trifluoromethyl)-benzoic acid; >98% 4-(Trifluoromethyl)-benzoic acid; 98% 2,6-di-(Trifluoromethyl)-benzoic acid; 98% 3,5-di-(Trifluoromethyl)-benzoic acid; 98%

2-FBA 3-FBA 4-FBA 2,3-DFBA 2,4-DFBA 2,5-DFBA 2,6-DFBA 3,4-DFBA 3,5-DFBA 2,3,4-TFBA 2,3,5-TFBA 2,3,6-TFBA 2,4,5-TFBA 2,4,6-TFBA 3,4,5-TFBA 2,3,4,5-TetraFBA 2,3,5,6-TetraFBA penta-FBA 2-TFMBA 3-TFMBA 4-TFMBA 2,6-BTFMBA 3,5-BTFMBA

CAS-No.

[445-29-4] [455-38-9] [456-22-4] [4519-39-5] [1583-58-0] [2991-28-8] [385-00-2] [455-86-7] [455-40-3] [61079-72-9] [654-87-5] [2358-29-4] [446-17-3] [28314-80-9] [121602-93-5] [1201-31-6] [652-18-6] [602-94-8] [433-97-6] [454-92-2] [455-24-3] [24821-22-5] [725-89-3]

2. Experimental 2.1. Equipment IC–MS analysis have been performed on an Agilent 1100 Series LC system (Palo Alto, CA, USA) interfaced to an Agilent G1946 single quadrupole mass spectrometer with electrospray ionization source (ESI). A chemical suppressor (Metrohm 761 SD Compact IC, Herisau, CH) that allows the use of ACN as organic modifier was used to remove sodium carbonate from the eluent. In addition a conductivity detector was attached between suppressor and ESI. As regenerant 50 mM sulfuric acid

Supplier

Merck Aldrich Merck TCI Europe Acros Organic Acros Organic Merck Merck TCI Europe TCI Europe TCI Europe TCI Europe TCI Europe TCI Europe TCI Europe TCI Europe TCI Europe Acros Organics Acros Organics Merck Acros Organics Aldrich Acros Organics

Retention time

fragments (m/z)

AS10 (min)

[M−H]−

[M−COOH]−

19.0 34.6 31.1 29.1 23.3 25.3 19.9 48.4 46.8 39.1 36.0 28.7 32.1 21.1 68.9 48.9 37.8 45.8 20.1 60.3 64.0 18.4 115.4

139 139 139 157 157 157 157 157 157 175 175 175 175 175 175 193 193 (211) n.o. 189 189 189 257 257

95 95 95 113 113 113 113 113 113 131 131 131 131 131 131 149 149 167 145 145 145 213 213

was used. The separation of all 23 FBAs was achieved on the latex type anion exchange column AS10 (250 mm × 4 mm, Dionex, Sunnyvale, CA, USA) under isocratic conditions. For RR, LOD and LOQ determination the anion exchange column A Supp 10 (100 mm × 2 mm, Metrohm, Herisau, Switzerland) was used. Agilent ChemStation (Version B 04.01 SP1 [650], Agilent Technologies) was used for data handling and QtiPlot for data visualization [23]. For solid-phase extraction a CHROMABOND vacuum manifold for 12 cartridges (Ref. No. 730150, Macherey-Nagel, Düren, Germany) with a membrane pump (Type MZ 2C, Vacuubrand, Wertheim, Germany) was used. Solid-phase extractions were

Table 2 Instrument detection limits, limits of quantification, method detection limits, recovery rates and fragmentor voltages of fluorobenzoic acids. BA (m/z) (␮g L−1 )

IDLa

LOQa (ng L−1 )

MDLb (ng L−1 )

RRc (%)

RSDc (%)

Contained in

2-FBA (139) 3-FBA (139) 4-FBA (139) 2,3-DFBA (157) 2,4-DFBA (157) 2,5-DFBA (157) 2,6-DFBA (157) 3,4-DFBA (157) 3,5-DFBA (157) 2,3,4-TFBA (175) 2,3,5-TFBA (175) 2,3,6-TFBA (131) 2,4,5-TFBA (175) 2,4,6-TFBA (131) 3,4,5-TFBA (175) 2,3,4,5-TFBA (149) 2,3,5,6-TFBA (149) PentaFBA (167) 2-TFMBA (189) 3-TFMBA (189) 4-TFMBA (189) 2,6-BTFMBA (213) 3,5-BTFMBA (257)

74 67 78 48 47 56 80 30 37 114 92 89 79 88 95 45 45 n.d. 36 31 29 23 13

233 153 244 108 103 108 177 68 72 303 213 205 224 194 295 51 52 n.d. 49 57 48 35 28

132 154 157 104 137 210 84 54 80 68 40 101 74 81 37 35 34 n.d. 33 46 35 23 16

93 80 87 81 90 86 89 91 74 86 94 84 90 89 67 98 93 n.d. 86 91 92 97 28

0.9 8.8 2.2 3.1 0.7 3.1 2.4 0.5 13.7 4.5 0.3 1.7 5.6 0.3 13.5 2.3 1.4 n.d. 9.0 2.5 0.7 2.7 9.1

Mix 1 Mix 2 Mix 1 Mix 2 Mix 2 Mix 1 Mix 1 Mix 1 Mix 2 Mix 2 Mix 1 Mix 1 Mix 2 Mix 1 Mix 2 Mix 1 Mix 2 Mix 2 Mix 2 (IS in Mix 1) Mix 2 Mix 1 Mix 1 (IS in Mix 2) Mix 2

Average

59

138

79

85

4.0

a b c

Data refers to measurements of FBAs in ACN (150 ␮g L−1 , n = 2, inj. 5 ␮L, IDL: SNR = 3, LOQ: SNR = 10). Data refers to a SPE of 100 mL tap water including FBAs (500 ng L−1 , n = 2, inj. 5 ␮L, SNR = 3). Data refers to a SPE of 100 mL tap water including 5 ␮g L−1 FBAs (n = 2), n.d. = not determined.

Fragmentor voltage (V) 60 60 60 60 60 60 60 60 60 60 60 80 60 80 60 100 100 120 70 70 70 140 95

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Fig. 1. SIM chromatograms of Mix 1 after SPE (500 ng L−1 in tap water, Metrohm A Supp 10 100 mm × 2 mm, 75% 0.63 mM Na2 CO3 25% ACN, 0.25 mL min−1 isocratic, inj. 5 ␮L, numbering refers to Table 1).

performed using Oasis HLB Plus cartridges (186000132, Waters Corporation, Milford, MA, USA) and Isolute ENV+ cartridges (Part No. 915-0020-C, Biotage, Uppsala, Sweden). For evaporation of the extracts a nitrogen stream manifold was used.

2.2. Chemicals The fluorobenzoic acids were purchased as shown in Table 1. All acids had at least purum quality (≥97%). The analytes were eluted

Fig. 2. SIM chromatograms of Mix 2 after SPE (500 ng L−1 in tap water, Metrohm A Supp 10 100 mm × 2 mm, 75% 0.63 mM Na2 CO3 25% ACN, 0.25 mL min−1 isocratic, inj. 5 ␮L, numbering refers to Table 1).

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and dissolved with acetonitrile (ACN, p.a.), purchased by Merck, Darmstadt, Germany. Nitric acid (65%, p.a.) and hydrochloric acid (37%, p.a.), used for pH adjustment, was supplied from Riedel de Haën, Seelze, Germany. The methanol (MeOH, p.a.) used in the conditioning step was purchased by Merck, Darmstadt, Germany. Ultrapure water used in the experiments was generated with a Milli-Q gradient Water System (Millipore Corp., Bedford, MA, USA). Single standard stock solutions were prepared by solving each FBA in ACN to give a concentration of 1000 mg L−1 . Stock solutions were combined and diluted with ultrapure water to obtain Mix 1 and Mix 2 as multi standard working solutions with a concentration of 1 mg L−1 and 10 mg L−1 of each FBA. 2.3. Chromatographic and MS detection conditions Three columns were used: A Supp 10 (Metrohm, Herisau, Switzerland) in the dimensions 250 mm × 2 mm and 100 mm × 2 mm and Dionex AS10 (Dionex, Sunnyvale, CA, USA) in the dimension 250 mm × 4 mm. The following isocratic conditions were used: in case of the AS10 82% 0.63 mM Na2 CO3 with 18% ACN at a flow rate of 1.0 mL min−1 and in case of the A Supp 10 75% 0.63 mM Na2 CO3 with 25% ACN at a flow rate of 0.25 mL min−1 . The injection volume was 5 ␮L and the temperature of the separation column and eluent was 45 ◦ C. Chromatograms for mass spectra were acquired in “scan” mode scanning the quadrupole from m/z 50 to m/z 500. Used fragments in selected ion mode (SIM) are listed in Table 1. The fluorobenzoic acids were detected as anions in negative mode. The spray chamber settings were capillary voltage 3250 V, drying gas flow 7.0 L min−1 , nebulizer pressure 30 psig and a drying gas temperature of 350 ◦ C. The values for the fragmentor voltage can be found in Table 2. 2.4. Sample preparation and solid-phase extraction First the pH of the sample was adjusted to pH 3.4 (Oasis) with nitric acid or pH 1.5 (Isolute) with hydrochloric acid and internal standard (IS) was added. Cartridges were conditioned with 5 mL MeOH and 10 mL ACN. For pH-equilibration 25 mL of pHadjusted ultrapure water was used. Then the sample was extracted by vacuum aspiration through the cartridge at a flow rate of approximately 10–15 mL min−1 . The FBAs were eluted with 15 mL ACN into 25 mL pear shaped flasks. The solvent was evaporated under a stream of nitrogen. The residue was finally reconstituted with 0.2 mL ACN. Drying the sorbent material before elution and the use of dried ACN is not necessary. For routine analysis of unknown analyte concentrations an internal standard was added prior and after sample processing. The relative standard deviation was calculated using formula (1):



RSD =

n (x i=1 i

n

− x)

2

× 100

(1)

3. Results and discussion 3.1. Sample preparation and solid-phase extraction For a successful enrichment of the fluorobenzoic acids with a simultaneous separation from the sample matrix, choosing the right sorbent material is a very important step. As the fluorobenzoic acids are organic compounds and appear in the normal pH range as carboxylate anions (pKa < 4), theoretically the protonated and the deprotonated molecules can be used for extraction. However, the two sorbent materials in Oasis HLB, a poly(divinylbenzene-co-N-vinylpyrrolidone) polymer, and Isolute

Fig. 3. Simultaneous detection of 23 fluorobenzoic acids with IC-ESI-MS (10 mg L−1 in ultrapure water) on the AS10 (250 mm × 4 mm) column under isocratic conditions (0.63 mM Na2 CO3 , 18% ACN, 1 mL min−1 , inj. 5 ␮L). The ion count at the given m/z is shown as a function of retention time (numbering refers to Table 1).

ENV+, a hydroxylated polystyrene-divinylbenzene copolymer, were found to have stability at low pH combined with high capacity and selectivity for the FBAs if only the samples pH is adjusted to 3.4 in case of Oasis HLB [20] or pH 1.5 in case of Isolute ENV+ [18]. Because the A Supp 10 was not able to separate all FBAs but a 2 mm column was necessary to achieve best results in mass selective detection, the acids were divided into two groups (Mix 1 and Mix 2, see Table 2). A comparison between the two cartridges based on SPE of 1 ␮g L−1 Mix 1 in tap water (n = 3) showed, that both cartridges are almost equivalent with respect to selectivity and recovery. Both cartridges reached an average recovery rate (RR, see 3.4.2) of 84% ranging between 37% (2,5-DFBA) and 94% (2,6-BTFMBA) for Isolute ENV+ and between 67% (2,5-DFBA) and 94% (3,4-DFBA) for Oasis HLB (see Table 3). Because of easier handling and better reproducibility, all further extractions were carried out using Oasis HLB. The RR of 22 FBAs divided into Mix 1 and Mix 2 with use of two different internal standards (2-TFMBA as IS in Mix 1 and 2,6BTFMBA as IS in Mix 2) at a concentration of 500 ng L−1 in tap water were determined (n = 3, see Figs. 1 and 2 and Table 2). The standardized peak areas after enrichment from 100 mL lie in the range between 713,140 (2-FBA) and 3,681,920 (2,3,4,5-TetraFBA) with RSD between 0.3% (2,4,6-TFBA) and 13.7% (3,5-DFBA). For standardization all samples were reconstituted in ACN containing 1 mg L−1 internal standard (IS). The internal standard peak Table 3 Comparison of recovery rates and standard deviations of the standardized peak areas after SPE of 1 ␮g L−1 FBAs from Mix 1 in 100 mL tap water using Oasis HLB and Isolute ENV+ (n = 3, IS = internal standard). FBA (m/z)

RR (Oasis)

RSD % (Oasis)

RR (Isolute)

RSD % (Isolute)

2-FBA (139) 4-FBA (139) 2,5-DFBA (157) 2,6-DFBA (157) 3,4-DFBA (157) 2,3,6-TFBA (131) 2,4,6-TFBA (131) 2,3,5-TFBA (131) 2,3,4,5-TetraFBA (149) 2-TFMBA 4-TFMBA (145) 2,6-BTFMBA

89 81 93 67 94 88 82 83 86 IS 82 83

2.2 2.2 3.4 3.4 1.5 2.9 2.5 1.0 3.6 IS 0.9 1.7

89 81 93 37 87 91 90 85 86 IS 88 94

3.7 5.8 3.7 9.0 3.4 5.4 4.3 3.8 8.6 IS 1.3 4.2

Average

84

2.3

84

4.8

100

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Fig. 4. Selected ion monitoring (SIM) chromatograms showing the separation of 23 FBAs under isocratic conditions on the AS10 (10 mg L−1 , Dionex AS10 (250 mm × 4 mm), 82% 0.63 mM Na2 CO3 , 18% ACN, 1 mL min−1 , inj. 5 ␮L, numbering refers to Table 1).

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area of one selected measurement was chosen as benchmark, to which all other measurements were adjusted. 3.2. IC-MS analysis For quantification all FBAs are measured by their [M−H]− or [M−CO2 ]− fragment and for qualification by the main fragment(s) in combination with the retention time. As it is common to compounds within each group it is not possible to distinguish between the isomers. If two or more isomers of the same group are used simultaneously, chromatographic separation for qualification and quantification is necessary. Due to equal fragmentation, SIM mode shows 23 acids by selecting 14 fragments. The monitored fragments are listed in Table 1. Fragments used for quantification are in bold. 3.2.1. Electrospray ionization Due to the electron-withdrawing effect of fluorine, most FBAs show decarboxylation to the [M−CO2 ]− fragment after deprotonation as well as the [M−H]− fragment. It was found that the FBAs at least substituted in both ortho-positions show greater tendency to cleave CO2 . This property is most pronounced in 2,3,6-TFBA, 2,4,6-TFBA, 2,3,4,5-TetraFBA, 2,3,5,6-TetraFBA, penta-FBA and 2,6BTFMBA. In case of penta-FBA no [M−H]− signal could be found under the mentioned conditions. For quantification the fragment with the better SNR (printed in bold in Table 1) was chosen. To achieve the highest possible ionization, FIA series of each FBA with respect to capillary voltage (1500–5000 V) and fragmentor voltage (20–200 V) were performed (data not shown). The best compromise between negative ionization and protecting the spray chamber from damage turned out to be 3250–4000 V. Because only one value can be set for a method, 3250 V has been selected. The best values with respect to the fragmentor voltage are listed in Table 2. In addition to the ionization, decomposition of penta- and 2,3,5,6TetraFBA in solution was found, if only ACN was used as solvent. Within days the two FBAs decompose quantitatively in the aprotic solvent even in concentrations >1000 mg L−1 , probably due to the high electron-withdrawing effect and the resulting high acidity in combination with an aprotic solvent. This behavior was not observed for the other FBAs or if water or an ACN/water mixture was used as solvent. It has proven to be useful to first create a solution in ACN and then dilute it immediately with water. 3.2.2. Separation It was found, that anion exchange columns of the latex type (A Supp 10, AS10) are best suited for the separation of fluorobenzoic acids (see Figs. 3 and 4). The use of functionalized, high capacity, non-latex type PS/DVB columns (A Supp 15 or A Supp 16) showed very high retention times possibly due to a high reversed phase share (pi-stacking) in the present mixed mode retention process. The addition of ACN as modifier reduced the retention dramatically depending on the FBA. In case of A Supp 15 or 16 higher contents of ACN (>5%) led to high back pressures and therefore these columns were not applicable for the separation. In the final method the isocratic use of 0.63 mM sodium carbonate with 18–25% ACN has several advantages. The low amount of carbonate allows a long operational time of the suppressor (up to 6 h) between stepping. The amount of 15–25% ACN is equally the right value to achieve the best separation and highest sensitivity of the spray as well. A correlation between modifier content and signal area shows, that 15–25% ACN represents the best values with respect to the signal area (see Fig. 5). To exclude influences within the spray caused by inorganic anions, runs with an inorganic anion mix (10 mg L−1 of fluoride, chloride, nitrite, bromide, sulfate, nitrate and phosphate) on the A Supp 10 (250 mm × 2 mm) with a conductivity detector prior to the spray chamber were performed. Fig. 6 shows an overlay of the ESI signal from a blank (dotted) and a run with 22 FBAs

Fig. 5. Comparison between peak area and modifier content on the example of the TFMBAs (FIA, 10 mg L−1 , 1.25 mM Na2 CO3 , suppressed, 0.2 mL min−1 , inj. 5 ␮L).

(dashed) with the conductivity signal (black). Three anions (chloride, nitrite and bromide) elute within the retention time of five FBAs. The inorganic anions occur as small signal depressions on the blank chromatogram. 3.3. Method performance 3.3.1. Detection limits The signal-to-noise ratio is the most frequent parameter used to report the instrument detection limit (IDL), limit of quantification (LOQ) and method detection limit (MDL). In this study, the IDL and MDL were defined as the FBA concentration required to produce a signal-to-noise ratio of three and the LOQ to produce a signal-tonoise ratio of ten. ChemStation was used for the determination of the signal-to-noise ratio. To be able to determine minimum quantities of FBAs the column dimension plays an important role. The best separation was achieved on the AS10 (250 mm × 4 mm), but the flow rate of 1 mL min−1 is too high to give best results with respect to LOD and LOQ. Therefore a new A Supp 10 with a diameter of 2 mm at a flow rate of 0.25 mL min−1 was used.

Fig. 6. Overlay of ESI signal (blank and 22 FBAs) and conductivity signal to evaluate the influence of inorganic ions on the separation, ionization and mass selective detection (A Supp 10 250 mm × 2 mm, 75% 1.25 mM Na2 CO3 20% ACN, 0.2 mL min−1 , inj. 5 ␮L, Conc. 10 mg L−1 , numbering refers to Table 1).

102

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Fig. 7. Selected ion monitoring (SIM) chromatograms of FBA Mix 1 at 150 ␮g L−1 to determine the IDL (Metrohm A Supp 10 (250 mm × 2 mm), 75% 0.63 mM Na2 CO3 , 25% ACN, isocratic, 0.25 mL min−1 , inj. 5 ␮L, numbering refers to Table 1).

The average IDL for the fluorobenzoic acids was found to be approximately 59 ␮g L−1 . In very general terms, the fragments with higher masses result in better IDL. Fig. 7 shows the SIM chromatograms for selected FBAs at concentration closer to IDL obtained from ACN spiked with FBAs at 150 ␮g L−1 . The detection limits as described before discounts the solid-phase extraction step. After SPE the average MDL was found to be approximately 79 ng L−1 ranging between 16 ng L−1 (3,5-BTFMBA) and 210 ng L−1 (2,5-DFBA). The discrete values (MDL and LOQ) are listed in Table 2. An enhancement of the limits of detection is still possible, for example, by increasing the injection volume.

3.3.2. Recovery rate To proof the capability and reproducibility of the method, recovery rates (RR) of the FBAs were determined. The ratio between sample volume before and after solid-phase extraction equals to the enrichment factor. If the concentrations of the FBAs prior to SPE are known, the concentration after SPE are determined by the product oft the concentration prior to SPE with the enrichment factor and the recovery rate. In this study, FBAs were enriched out of 100 mL tap water samples and eluted in 200 ␮L ACN. The volume ratio result into an enrichment by factor 500. Recovery rates of all FBAs were determined by using a tap water sample containing 5 ␮g L−1 of each FBA (n = 2) compared to the average peak areas from a FBA mix

K. Müller, A. Seubert / J. Chromatogr. A 1270 (2012) 96–103

in ACN containing 2500 ␮g L−1 (n = 2). After standardization recovery rates between 28% (3,5-BTFMBA) and 98% (2,3,4,5-TetraFBA) were achieved (discrete values shown in Table 2). Except of 3,5BTFMBA (possibly due to its amphiphilic properties) all other FBAs show recovery rates >67%. 4. Conclusions In this work, a sensitive and rapid IC/MS method for determination and quantification of up to 23 FBAs in aqueous samples was developed. Compared to [14] the present paper describes similar results with a less advanced intrumentation, however a preconcentration is necessary. The sorbent material in Oasis HLB was found to be highly selective and to provide high recovery rates. The method presents low detection limits and good reproducibility. In comparison to previously published work the method shows higher separation performance, good recovery rates, a sample preparation suitable for all commercially available fluorobenzoic acids with an enrichment by factor 500 if 100 mL sample volume is used and conditioning steps with low consumption of chemicals. By using anion exchange chromatography with electrospray ionization, no derivatization steps after SPE are necessary. Acknowledgement The authors gratefully acknowledge financial support by the Hans-Böckler-Stiftung.

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