Analytica Chimica Acta 807 (2014) 51–60

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Development of solid-phase microextraction to study dissolved organic matter—Polycyclic aromatic hydrocarbon interactions in aquatic environment Chloé de Perre a,b , Karyn Le Ménach a,b , Fabienne Ibalot a,b , Edith Parlanti a,b , Hélène Budzinski a,b,∗ a b

Université de Bordeaux, UMR 5805, EPOC-LPTC, 351 Cours de la Libération, Talence Cedex F-33405, France CNRS, UMR 5805, EPOC-LPTC, F-33405 Talence Cedex, France

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Deuterated

naphthalene used as internal standard provides better quantification of freely dissolved PAHs than regular external calibration. • KDOC values of 18 PAHs were calculated thanks to SPME–GC–MS. • Competition between PAHs, deuterated PAHs and DOM was demonstrated, pointing out the non-linearity of PAH–DOM interactions. • Interactions of light molecular weight PAHs are stronger (higher KDOC values) in absence of high molecular weight PAHs.

a r t i c l e

i n f o

Article history: Received 17 July 2013 Received in revised form 18 October 2013 Accepted 13 November 2013 Available online 19 November 2013 Keywords: Solid-phase microextraction Gas chromatography–mass spectrometry Polycyclic aromatic hydrocarbons Dissolved organic matter Interactions Aquatic environment

a b s t r a c t Solid-phase microextraction coupled with gas chromatography and mass spectrometry (SPME–GC–MS) was developed for the study of interactions between polycyclic aromatic hydrocarbons (PAHs) and dissolved organic matter (DOM). After the determination of the best conditions of extraction, the tool was applied to spiked water to calculate the dissolved organic carbon water distribution coefficient (KDOC ) in presence of different mixtures of PAHs and Aldrich humic acid. The use of deuterated naphthalene as internal standard for freely dissolved PAH quantification was shown to provide more accuracy than regular external calibration. For the first time, KDOC values of 18 PAHs were calculated using data from SPME–GC–MS and fluorescence quenching; they were in agreement with the results of previous studies. Competition between PAHs, deuterated PAHs and DOM was demonstrated, pointing out the non-linearity of PAH–DOM interactions and the stronger interactions of light molecular weight PAHs (higher KDOC values) in absence of high molecular weight PAHs. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel.: +33 5 40 00 69 98; fax: +33 5 40 00 22 67. E-mail address: [email protected] (H. Budzinski). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.11.026

Natural organic matter is a complex mixture of macromolecules originating from the biological and chemical degradation of plants or animals. In aquatic systems, dissolved organic matter (DOM)

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consists of organic macromolecules smaller than 0.45 ␮m. This fraction is well known for being able to modify the distribution [1,2], the bioavailability [3,4], the degradation [5,6] or the toxicity [1,7] of organic compounds, and particularly of Polycyclic Aromatic Hydrocarbons (PAHs). Because of these modifications, the presence of DOM in samples may also affect the analysis of PAHs [8]. PAHs are hydrophobic organic compounds coming mainly from petroleum and the incomplete combustion of organic matter. They are introduced into surface waters via atmospheric fallout, municipal effluents, leaching or oil spills, and they are environmentally problematic since some of them are mutagenic or carcinogenic [9]. To quantify the impact of DOM on the PAH fate, it is necessary to study the strength of interactions occurring between them. A few techniques can be used to study the interactions between DOM and PAHs, each of them with advantages and drawbacks. One of them, the fluorescence quenching, does not require the separation of free compounds from those associated with DOM. However, because DOM needs to be added to a DOM-free sample containing PAHs, this technique seems to be hardly applicable to samples collected in the natural environment containing PAHs and DOM [10]. Other techniques, particularly extraction or separation methods, can be used in association with analytical techniques, including dialysis, ultrafiltration, reverse phase chromatography or more recently Solid-phase microextraction (SPME). However, reverse phase chromatography was shown to disturb the equilibrium of the sample [11], and ultrafiltration and dialysis could raise sorption problems for hydrophobic compounds due to adsorption on the membrane that complicates free analyte analysis [12,13]. SPME seems to be the most promising technique since it appears to be faster, more sensitive and simpler than other techniques and since it is solvent free [13,14]. SPME, developed by Pawliszyn in the early 1990s [15], consists of a fiber coated with a phase that adsorbs (for sorbents) or absorbs (for polymers) analytes from a liquid, solid or gas sample. As a result of solvent or thermal desorption, analytes are then transferred into a liquid or gas chromatograph before being analyzed. One aim of this study was to develop SPME coupled to GC–MS to achieve the best sensitivity that would allow for the accurate quantification of all PAHs and their interaction with DOM, at environmental trace concentrations. Particular attention was given to SPME parameters that are usually not greatly detailed in the literature, i.e. which liner is the most efficient, is the fiber gauge an important parameter, metal alloy vs. silica core fibers, etc. This detailed method development was intent to set up a basis for any interested teams that would want to use the same approach, and to give potential explanations of differences observed in papers using the same SPME fibers. A second goal of our study was to compare SPME–GC–MS with dialysis and fluorescence quenching methodologies to define the applicability domain of these three techniques and to characterize interactions between DOM and PAHs.

2. Materials and methods 2.1. Organic compounds Four PAHs (phenanthrene, fluoranthene, chrysene and benzo[a]pyrene) were used as model compounds for the development of the analytical method, and their corresponding perdeuterated PAHs were used as internal standards for quantitative calibration in SPME (Table 1). After development, interactions were studied for the 19 PAHs listed in Table 1. Solutions of PAHs were prepared in ethanol and then 50 ␮L of a solution of 25 ␮g g−1 were added to 1L of ultrapure water (Milli-Q, Millipore, Molsheim, France) to obtain an aqueous solution of 1 ␮g L−1 of each PAH. This aqueous solution

was sonicated for 5 min to get a good homogenization, but, to avoid PAH adsorption on the stirring bar (which could reach 20% for benzo[a]pyrene, results not shown), no magnetic stirrer agitation was performed. These aqueous solutions were experimentally characterized using liquid–liquid extraction (LLE) with dichloromethane followed by GC–MS analysis. Deuterated PAHs were dissolved in ethanol at a concentration of 250 ng g−1 and diluted in ethanol if necessary. The solvents used were ethanol absolute HPLC grade (ScharlauChemie S.A, Sentmenat, Spain) and dichloromethane for residue and pesticide analysis (Acros Organics, Geel, Belgium). The DOM used for this study was Aldrich humic acid (SigmaAldrich Chemie GmbH, Steinheim, Germany). This humic acid was added to 500 mL of pure water to obtain an aqueous solution of DOM and the latter was then filtered on a GF/F (Whatman, Maidstone, England) glass fiber filter (0.7 ␮m). The concentrations of Dissolved Organic Carbon (DOC) were measured using a Shimadzu TOC-V CSN (Shimadzu, Duisburg, Germany). Several aqueous solutions of Aldrich humic acid were prepared for the experiment; all were within 11.0 ± 0.5 mg L−1 and were characterized precisely. 2.2. Experimental precautions All the glassware, after being carefully cleaned with detergent (TFD 7, Franklab, France) and rinsed with ultrapure water, was heated at 450 ◦ C overnight before using it for PAH analyses and it was also rinsed with ultrapure water before DOC and spectrofluorometry analyses. For SPME and LLE experiments, blanks were made to check PAH ambient pollution and to be aware of any possible contamination of the samples. In SPME, blanks were performed by extracting an empty flask for 10 s to check the good desorption of the previous analysis. Generally, blanks represented less than 1% of the PAH areas of samples. For spectrofluorometry experiments, blanks of ultrapure water were made to test the cleanliness of cuvette and to correct spectra for the Rayleigh and Raman scattering bands. 2.3. Experimental protocols and analytical tools 2.3.1. SPME–GC–MS SPME analyses of the 10 mL spiked water samples were performed with commercially available PDMS (polydimethylsiloxane) and PDMS-DVB (divinylbenzene) coated fibers from Supelco (Bellefonte, USA). Different sizes of the PDMS coating were compared (7 ␮m and 100 ␮m) and various parameters, (including extraction and desorption times, temperature, liner type), had to be optimized. After the immersion of the fiber in the sample, it was immediately desorbed into the GC–MS injection port. Analyses were performed in automated mode using a Combipal (CTC Analytics, Zwingen, Switzerland). The GC was an Agilent 6890 model (Agilent Technologies, Massy, France) equipped with a 5972 mass selective detector, operated with an energy of ionization of 70 eV (electronic impact). The column used was an HP-5MS ((5%-phenyl)-methylpolysiloxane; 30 m × 0.25 mm i.d.; 0.25 ␮m film; Agilent Technologies, Chromoptic, Courtaboeuf, France). Both SPME and direct liquid injection (after LLE) were performed with the inlet temperature at 250 ◦ C and in the pulsed splitless mode: A pulse pressure of 30 psi was maintained for 1 min, the purge flow to the split vent was 55 mL min−1 after 2 min, and the gas saver was set at 20 mL min−1 after 15 min. The carrier gas was helium (purity 5.6, Linde Gas, Toulouse, France) with a constant flow rate of 1.3 mL min−1 and linear velocity of 42 cm s−1 . The column temperature was initially held at 60 ◦ C for 2 min, and was then increased to 150 ◦ C at 20 ◦ C min−1 , to 250 ◦ C at 15 ◦ C min−1 and to 310 ◦ C at 10 ◦ C min−1 , where it was held for 3 min. For the determination of PAHs, the mass spectrometer was

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Table 1 Model compounds and internal standards. PAHs

Purity (%)

Supplier

(m/z)

Deuterated PAHs

Isotopic purity (%)

Supplier

(m/z)

Naphthalene (N) Acenaphthylene (Acy) Acenaphthene (Ace) Fluorene (Flu) Dibenzothiophene (DBT) Phenanthrene (Phe) Anthracene (An) Fluoranthene (Fluo) Pyrene (Pyr) Benz[a]anthracene (BaA) Chrysene (Chrys) Benzo[b]fluoranthene (BbF) Benzo[k]fluoranthene (BkF) Benzo[e]pyrene (BeP) Benzo[a]pyrene (BaP) Perylene (Per) Indeno[1,2,3-cd]pyrene (IP) Benzo[g,h,i]perylene (BP) Dibenz[a,h]anthracene (DahA)

99+ 99+ 99 99+ 99 99 98 99 99+ 99 99+ 99 98 99 97 99+ 99 99.7 97

Aldrich Aldrich Aldrich Aldrich Acros Aldrich Labosi Aldrich Fluka Aldrich Fluka Aldrich Aldrich Aldrich Aldrich Aldrich Promochem Promochem Aldrich

128 152 154 166 184 178 178 202 202 228 228 252 252 252 252 252 276 276 278

N d8 Acy d8 Ace d10 Flu d10 DBT d8 Phe d10 An d10 Fluo d10 Pyr d10 BaA d12 Chrys d12 BbF d12 BkF d12 BeP d12 BaP d12 Per d12 IP d12 BP d12 DahA d14

98 98 99 98 99 98 98 99.2 98 98 98 98 98 98 98 99.5 98+ 98 98

EGA-Chemie Promochem (CIL) Promochem (CIL) Promochem (CIL) MSD Isotopes CIL CIL MSD Isotopes CIL Promochem (CIL) MSD Isotopes CIL Promochem (CIL) CIL CIL MSD Isotopes Promochem (CIL) CIL Promochem (CIL)

136 160 164 176 192 188 188 212 212 240 240 264 264 264 264 264 288 288 292

operated in the selected ion monitoring (SIM) acquisition mode and only the molecular ions were sought (Table 1). A dwell time of 80 ms was used for each ion and the scan rate was 1.31 cycle s−1 . Interactions between PAHs and the dissolved Aldrich humic acid were first studied by changing the concentration of DOM (ranging from 0 to 5.5 mg L−1 of DOC) and keeping the total concentration of PAHs constant (1 ␮g L−1 of each) by both SPME and fluorescence quenching. Natural waters can be more concentrated in DOC since concentrations usually vary from 0.5 to 50 mg L−1 , depending on the source of DOM [1]. However, commercial humic acid was shown to have stronger interactions with PAHs than natural waters [16], so a high concentration of Aldrich humic acid was not necessary, and not used, to avoid “inner filter effect”, which is a fluorescence signal attenuation caused by an excess concentration of fluorophore or absorbing species in solution [10]. All samples contained the same volume of water and of solvent, as well as the same concentration of PAHs; only the concentration of DOM varied. In a second set of experiments, the concentration of DOM was kept constant (3 mg L−1 of DOC) whereas the concentrations of PAHs varied from 0.01 to 1 ␮g L−1 (by SPME only). To quantify the free and total concentrations, standards were made with spiked waters at known concentrations of PAHs and deuterated PAHs to calculate the response factors using the SPME–GC–MS for the various studied PAHs. As these aqueous standards provided the same response factors on the range 0.1–1 ␮g L−1 , they were eventually made at only one concentration (50–100 ng L−1 ). Two aqueous standards were extracted at the beginning and at the end of each SPME–GC–MS sequence and the mean value of the four response factors was used. Moreover, before each sequence, the sensitivity of the GC–MS system was checked using manual direct liquid injection of deuterated PAHs. Because pH can play an important part in the nature and the forces of interactions between DOM and PAHs [17], it was controlled for all experiments and it slightly varied from 6.8 to 7.1 between the lowest and the highest Aldrich humic acid concentrations.

2.3.2. Fluorescence analysis Interactions between DOM and PAHs were also studied by PAH fluorescence quenching thanks to the addition of DOM to the samples. Here, the fluorescence quenching technique was based on the decrease of PAH fluorescence when DOM was added, due to the formation of a non-fluorescent PAH–DOM complex (static quenching) [10].

Absorption measurements were recorded with a Jasco V-560 spectrophotometer (Jasco, France) equipped with deuterium and tungsten iodine lamps and fluorescence spectra with a Fluorolog FL3-22 spectrofluorometer (JobinYvon, France) equipped with a xenon lamp (450 W). Analyses were made with 1 cm quartz cuvettes (Hellma, France). All the fluorescence spectra were obtained at a constant temperature of 20 ◦ C, with an integration time of 0.5 s, an increment of 1 nm, excitation and emission band paths of 4 nm and in the Sc/R mode [the sample signal (S) was automatically multiplied by the emission correction factors (c) and normalized by the lamp reference signal (R)]. The samples used were at the same concentrations of PAHs and DOM as the SPME ones, but this time PAHs were studied individually because of the overlap of the fluorescence emission spectra in the mixture. Moreover, fluoranthene was replaced by pyrene for several reasons: the fluorescence intensity of fluoranthene is low, it overlaps with Aldrich humic acid fluorescence, and SPME showed that these two compounds have a similar behavior in terms of interactions. The fluorescence spectrum was recorded for both the sample containing DOM only and DOM in the presence of the PAH These two samples were strictly identical in their composition and concentration of DOM but one was spiked with 15 ␮L of ethanol whereas the other was spiked with 15 ␮L of one PAH dissolved in ethanol (the latter was weighed out to get the exact PAH concentrations). After that, the two spectra were subtracted to eliminate DOM fluorescence background and recover only the PAH spectrum. Finally, the spectra of PAHs were normalized to a PAH concentration of 1 ␮g L−1 to be able to compare spectra for exactly the same PAH concentrations. Before the interaction study, the best excitation and emission wavelengths were determined for each PAH: excitation/emission couples were 251/347 nm for phenanthrene, 230/373 nm for pyrene, 269/400 nm for chrysene and 296/405 nm for benzo[a]pyrene. These wavelengths were chosen to maximize PAH fluorescence intensities and to minimize the overlap with Aldrich humic acid fluorescence. In addition to the fluorescence analysis, the UV–visible absorption of the samples was acquired to know the absorbance values at excitation and emission wavelengths to correct the “inner filter effects,” as described by MacDonald et al. [18] in their mathematical correction. The correction factors varied from 1 to 2.3 depending on the compound and the DOM concentration and were thus less than the maximum factor of 3 recommended by Parker [19]. Additionally, fluorescence quenching was also used to determine the equilibrium time of PAH–DOM interactions. Analyses of phenanthrene and benzo[a]pyrene fluorescence in presence of

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1 mg L–1 of DOC were compared at different times during a 24 h period and it appeared that equilibrium was reached in less than 5 h for both PAHs without any agitation. In order to be sure all the experiments were performed at the equilibrium point, each sample was agitated manually for a few seconds and then left in the dark at 4 ◦ C overnight before fluorescence quenching and SPME analyses. 2.3.3. Dialysis experiment The third technique used to study the interactions between DOM and PAHs was dialysis coupled to PAH extraction by LLE and GC–MS analysis for quantification. The dialysis membranes were in cellulose ester (Spectra/Por® Biotech, wet in 0.1% sodium azide, Spectrum Europe, Netherlands) with molecular weight cut-offs of 500 and 1000 Da. The membranes were cut and washed before use. Several solvents (ultrapure water, ethanol 10% or hydrochloric acid 5%) were tested in order to get the best blank but neither ethanol nor hydrochloric acid improved the dialysis bag cleaning. In other words, the fluorescence intensity remained constant under both cleaning conditions. Consequently, the membranes were rinsed thoroughly and soaked overnight in ultrapure water and then rinsed thoroughly again. After that, they were knotted and filled either with 10 mL of the sample and submerged in a 500 mL bottle of ultrapure water, or with 10 mL of Milli-Q water and submerged in 500 mL of the sample. First, the sample was made with only ultrapure water spiked with PAHs at 1 ␮g L−1 in order to determine the time required to achieve equilibrium of PAH concentrations inside and outside the bag. To study interactions with DOM, the latter was added to PAHs in the dialysis bag. Once the equilibrium is reached, free PAHs are theoretically in the same concentration inside and outside the bag whereas DOM-bound PAHs exist only inside the bag. Thus, the concentration of bound PAHs is equal to the difference between the total concentration of PAHs in the bag and the concentration outside the bag. During all the dialysis experiments, the samples were agitated using a shaking table. 2.4. KDOC calculations In order to study the interactions of PAHs with DOM, the DOCwater partition coefficient was calculated using Eq. (1): KDOC =

[PAH]DOC [DOC][PAH]free

(1)

with [PAH]DOC the concentration of PAHs bound to DOM, [PAH]free the concentration of freely dissolved PAHs in water and [DOC] the concentration of DOC in water. SPME–GC–MS and dialysis-LLE–GC–MS give free and total PAH concentrations, so [PAH]DOC was deduced by the equation: [PAH]DOC = [PAH]total –[PAH]free . It follows from the Stern–Volmer equation that: [PAH]total = KDOC [DOC] + 1 [PAH]free

(2)

To be able to compare KDOC values with other studies, they were expressed in mL g−1 . With the technique of fluorescence quenching, PAH concentrations were not calculated. The intensities of PAHs in the absence and in the presence of DOM were used to calculate the partition ∗ coefficient KDOC by means of the Stern–Volmer Eq. (3): F0 ∗ = KDOC [DOC] + 1 F

(3)

with F0 the fluorescence intensity of PAHs without DOM and F in the presence of DOM.

Fig. 1. Sensitivity and reproducibility of different fibers. Extraction time = 30 min, [PAH] = 1 ␮g L−1 , n = 3. (a) PDMS 100 ␮m 24 g, (b) PDMS 100 ␮m 23 g, (c) metal alloy PDMS 100 ␮m 23 g, (d) PDMS-DVB 65 ␮m 24 g, (e) PDMS 7 ␮m 24 g.

3. Results and discussion 3.1. Optimization of SPME–GC–MS conditions First, optimizations of the SPME–GC–MS were performed with 4 PAHs (phenanthrene, fluoranthene, chrysene, benzo[a]pyrene). As far as the fiber is concerned, a non polar PDMS coating is recommended for the analysis of non polar compounds in a complex matrix. For the more polar compounds, PDMS-DVB can improve the fiber capacity of retention thanks to ␲–␲ interactions with aromatic DVB. The fiber thickness is also important since the thicker the coating, the more sensitive it is. Three kinds of fibers have therefore been tested: PDMS 100 ␮m, PDMS-DVB 65 ␮m and PDMS 7 ␮m and then PDMS 100 ␮m with a thicker gauge (23 g instead of 24 g) and one with a metal alloy core (instead of fused silica). The thicker gauge fiber and the metal alloy one were tested since they should reduce breakage of the fibers thanks, respectively, to the better solidity and flexibility of the needle. Moreover, it is advised that these types of fibers be used with a septum-less seal (Merlin Microseal® system), which prevents septum coring issues [20]. The areas of GC–MS peaks (obtained with the same extraction time) are reported in Fig. 1. All the fibers tested were neither new nor too old (between 25 and 70 extractions had already been performed with them). Indeed, the newest fibers (25- and 30-extractions) appeared to be the least reproducible for HMW PAHs; these compounds might require more conditioning cycles for the reproducibility to be maximum. The optimal age appeared to be about 50 extractions: they seemed less reproducible before and less sensitive after 130–150 extractions, or 200–250 for the PDMS 100 ␮m 23 g, probably because of progressive damage of the coating. The most sensitive fibers were the PDMS 100 ␮m 23 g and 24 g but the 23 g benefited from a longer durability. Contrary to expectations, the metal alloy fiber was far less sensitive than the other PDMS 100 ␮m ones. It seemed to be due to a bad desorption of compounds since the fiber blanks represented up to 20% in the area of the samples at 0.1 ␮g L−1 (instead of