Solid-phase extraction and field-amplified sample injection–capillary zone electrophoresis for the analysis of benzophenone UV filters in environmental water samples

A field-amplified sample injection–capillary zone electrophoresis (FASI-CZE) method for the analysis of benzophenone (BP) UV filters in environmental water samples was developed, allowing the separation of all compounds in less than 8 min. A 9- to 25-fold sensitivity enhancement was obtained with FASI-CZE, achieving limits of detection down to 21–59 μg/L for most of the analyzed BPs, with acceptable run-to-run and day-to-day precisions (relative standard deviations lower than 17 %). In order to remove water sample salinity and to enhance FASI sensitivity, an off-line solid-phase extraction (SPE) procedure using a Strata X polymeric reversed-phase sorbent was used and afforded recoveries up to 72–90 % for most BPs. With the combination of off-line SPE and FASI-CZE, limits of detection in the range 0.06–0.6 μg/L in a river water matrix, representing a 2,400- to 6,500-fold enhancement, were obtained. Method performance was evaluated by quantifying a blank river water sample spiked at 1 μg/L. For a 95 % confidence level, no statistical differences were observed between found concentrations and spiked concentrations (probability at the confidence level, p value, of 0.60), showing that the proposed off-line SPE-FASI-CZE method is suitable for the analysis of BP UV filters in environmental water samples at low microgram per liter levels. The method was successfully applied to the analysis of BPs in river water samples collected up- and downstream of industrialized and urban areas, and in some drinking water samples.

cosmetics are described in the literature [5,[18][19][20][21]. To the best of our knowledge, there 94 is only one publication in the literature describing the use of a capillary electrophoresis-95 mass spectrometry (CE-MS) method for the analysis of several UV-filters, including 96 some BPs, in river water samples [22]. Despite the high efficiency of CE methods they 97 present relatively low sensitivity because of the small volume of sample injected (2-10 98 nL) and the short optical-path length (25-100 µm). This problem can be overcome by 99 on-line preconcentration techniques such as field-amplified sample injection (FASI), 100 stacking, and sweeping [23]. Among these techniques, FASI is very popular since it is 101 quite simple only requiring the electrokinetical injection of the sample after the 102 introduction of a short plug of a high-resistivity solvent such as methanol or water [24]. 103 FASI is taking advantage of the higher amount of analytes introduced into the capillary 104 when electrokinetic injections are used. The pre-injection of a short plug of a high-105 resistivity solvent such as water allow the enhancement of the sample electrokinetic 106 injection because of the conductivity differences between the sample and the water 107 plug. Once the analytes enter into the capillary they will stack-up in the boundary region 108 between the high-resistivity solvent and the background electrolyte (BGE) used, and 109 separation will take place. The benzophenone UV-filters studied, which are shown in Table 1  6.81 to 8.14, Table 1), but because of the high pH value used, the electrophoretic to preconcentrate the analyte. In this study, the electrolyte previously optimized for the 245 conventional CZE separation (35 mM sodium tetraborate buffer at pH 9.2) was used as 246 BGE for the FASI-CZE procedure, and water was used as the high resistivity solvent.

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Other solvents such as methanol were also tested but the electrophoretic voltage 248 frequently failed, probably due to the formation of bubbles into the capillary.

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Additionally, sample matrix will also play an important role during FASI 250 application and even more with low acidic compounds such as BPs due to the 251 requirement of using a matrix with a pH higher than BPs pka values in order to 252 guarantee the presence of ionic compounds and, consequently, a good introduction of 253 the analytes into the capillary when electrokinetic injection is used. For this purpose, 254 sodium tetraborate buffer solutions were used as sample matrix and the effect of its 255 concentration (from 1 to 10 mM) was evaluated when FASI was applied under some 256 preliminary conditions, i.e. hydrodynamic injection of a water plug for 10 s (3.5 kPa) 257 and sample electrokinetic injection at -10 kV for 10 s. Milli-Q water was also evaluated 258 as sample matrix. The electropherograms obtained in this study are shown in Figure 2a.

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As can be seen, when only water was used no effective FASI injection was observed  Run-to-run and day-to-day precisions for BP quantification were calculated at 311 two concentration levels, a low level (LOQ) and a medium level (~20 mg/L for CZE-312 UV, and ~1 mg/L for FASI-CZE). In order to obtain the run-to-run precision, five RSDs) obtained with conventional CZE-UV at medium-concentration level were 318 between 0.8 and 5.6% and between 2.9 and 11.5% for run-to-run and day-to-day 319 precisions, respectively. The values were slightly higher for the low-concentration level, 320 as it can be expected, but always RSD values lower than 13.0 and 14.5% for the run-to-321 run and the day-to-day, respectively, were obtained. Regarding FASI precision at   respectively, were obtained (Table 3), which were similar to those previously obtained  Table 3).

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For method validation, a blank river water sample was spiked at around 1 µg/L 419 of each benzophenone and quantified by external calibration following the proposed 420 off-line SPE-FASI-CZE method, and the found concentrations, as well as the accuracies 421 in terms of relative errors (%), are also summarized in Table 3. As can be seen, good  Table 4.

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As can be seen, none of the analyzed BPs was detected in the mineral water BPs at quantified levels (see Table 4) was observed when the sample was collected after values lower than 17%), and accuracy (relative errors lower than 8%).

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In order to remove sample salinity from environmental waters which can  The good results obtained in this study shown that the proposed off-line SPE-495 water samples at low µg/L levels.

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The authors gratefully acknowledge the financial support received from Spanish      With more than forty scientific papers and book chapters to his name, he has been working for 122 several years on the development of capillary electrophoresis, liquid chromatography, mass 123 spectrometry and high resolution mass spectrometry methods in the analysis of environmental 124 and food samples. 125