Elsevier

Analytica Chimica Acta

Volume 743, 19 September 2012, Pages 101-110
Analytica Chimica Acta

Assessment of benzophenone-4 reactivity with free chlorine by liquid chromatography quadrupole time-of-flight mass spectrometry

https://doi.org/10.1016/j.aca.2012.07.016Get rights and content

Abstract

The stability of the UV filter benzophenone-4 (BP-4) in free chlorine-containing water was investigated, for the first time, by liquid chromatography quadrupole time-of-flight mass spectrometry (LC–QqTOF-MS). High mass accuracy and resolution capabilities of this hybrid mass spectrometer were used for the reliable assignation of empirical formulae and chemical structures of BP-4 derivatives. Time-course profiles of the parent compound and its by-products were simultaneously recorded by direct injection of sample aliquots, after quenching the excess of chlorine, in the LC–QqTOF-MS system. At neutral pHs, in excess of chlorine, BP-4 showed a limited stability fitting a pseudo-first-order degradation kinetics. A noticeable reduction in the half-lives of BP-4 was observed when increasing the sample pH between 6 and 8 units and also in presence of bromide traces. The reaction pathway of this UV filter involved a first electrophilic substitution of hydrogen per chlorine (or bromide) in the phenolic ring, followed by oxidation of the carbonyl moiety to an ester group, which induced a further electrophilic substitution in the same aromatic ring. Above reactions were also noticed when mixing a BP-4 containing personal care product with chlorinated tap water and in chlorinated swimming pool and sewage water, previously spiked with a BP-4 standard.

Highlights

► LC–QTOF-MS was used to investigate the reactivity of BP-4 with chlorine. ► In excess of free chlorine, BP-4 degrades following a pseudo-first order rate. ► Water pH and bromide traces affect the half-life of BP-4. ► BP-4 by-products arise from oxidation and halogenation processes.

Introduction

Worldwide usage, endocrine disruption activity and incomplete removal in sewage plants are concerning environmental aspects of the so-called UV filters [1]. The toxicity, fate and behavior of these emerging pollutants in the aquatic media greatly vary depending on their chemical class, functionalities and octanol–water partition coefficients [2], [3]. Benzophenone-type UV filters display medium to high polarities and moderate estrogenic activity. The most often used species, and the only benzophenones approved by the European Legislation to be used in sunscreens, are benzophenone-3 (2-hydroxy-4-methoxybenzophenone, BP-3) and benzophenone-4 (2-hydroxy-4-methoxybenzophenone-5-sulfonic acid, BP-4) [4]. The first species has been reported in surface and sewage water as well as in sludge samples [5], [6], [7]. On the other hand, due to its higher polarity, BP-4 was mainly found in the water phase [8], with similar levels in the inlet and outlet streams of activated sludge sewage plants [9], [10]. This behavior suggests stability versus conventional bacterial biodegradation processes and high mobility of this emerging pollutant in the aquatic environment, with the risk of interfering the sexual hormone system of fish and other aquatic organisms [11].

In addition to sewage biodegradation and sludge adsorption of lipophilic species, another important factor determining the occurrence and the fate of emerging pollutants in the aquatic media is their stability/reactivity in contact with chlorine containing water. This point is of crucial importance in the case of UV filters since most personal care products incorporating these compounds (e.g. sunscreens, shampoos, hair dyes, soaps) are in contact with chlorinated waters used in recreational activities, and also in our homes, e.g. during showering and hair washing. Chlorine can be also employed to improve the efficiency of sewage plants; thus, compounds surviving to primary and biological treatments (case of BP-4) might get in contact with this oxidant when used in tertiary treatments. In contact with chlorine, parent pollutants might evolve into new species with a different persistence and toxicity than their precursors. In this vein, under model conditions, it has been demonstrated that several pharmaceuticals and personal care products show favorable chlorination rates rendering halogenated by-products [12], [13], [14] which, in some cases, have been further identified in treated wastewater and even in surface and tap water [15], [16].

As regards UV filters halogenated by-products, Sakkas and co-workers [17] reported, for first time, the existence of chlorinated forms of 2-ethylhexyl (4-dimethylamino) benzoate in swimming-pool water samples. In a further study, our research group has also demonstrated that BP-1 (2,4-dihydroxybenzophenone) and BP-3 show a reduced stability in chlorinated water, identifying several electrophilic substitution by-products as well as other species resulting from cleavage of the benzophenone structure [18]. However, to the best of our knowledge, the stability of BP-4 in contact with chlorinated water has not been investigated yet.

Kinetics of analytes degradation and assessment of by-products formation in chlorinated water have been traditionally investigated by gas chromatography with mass spectrometry (GC–MS), based on ion-trap type mass analyzers [12], [13], [18]. This approach requires a previous extraction step to transfer the parent species, and its potential by-products, from the water phase to an organic solvent compatible with the GC technique. As a consequence, only those species (analytes and by-products) with suitable characteristics for extraction and GC analysis can be monitored. The availability of new generation liquid chromatography (LC) time-of-flight (TOF) mass spectrometry (MS) systems, displaying accurate mass capabilities together with an enhanced sensitivity and wide linear response ranges, greatly simplifies the study of polar species reactivity in water, since sample aliquots can be directly injected in the LC–MS system, just after quenching the chlorination reaction [19]. Furthermore, the use of hybrid mass analyzers (quadrupole (Q)-TOF) improves the reliability of by-products identification on the basis of their accurate ion product scan MS–MS spectra [16], [20], [21].

The aim of this research was to evaluate the stability of BP-4 in chlorinated water samples, assessing the influence of different experimental variables on its degradation rate and addressing the occurrence of above reactions when BP-4 containing personal care products get in contact with tap water. Furthermore, additional series of degradation assays were performed with chlorinated swimming pool water and sewage. The concentration of the parent compound throughout above experiments was followed by direct injection of sample aliquots, collected at different reaction times, in a LC–QqTOF-MS system, which was also used to propose the empirical formulae of transformation by-products. Their chemical structures were elucidated on the basis of their accurate MS–MS spectra and the expected reactivity of the different functional groups existing in the structure of BP-4. The degradation pathway proposed for BP-4 is also compared with that of BP-3, in order to better understand the effect of substituents attached to the phenolic ring of both UV filters in their stabilities.

Section snippets

Standards, solvents, reagents and samples

BP-4 and BP-3 were acquired from Aldrich (Milwaukee, WI, USA). Individual stock solutions of these compounds (ca. 1000 μg mL−1) were prepared in methanol and stored at −20 °C, diluted standards, including calibration standards (from 0.001 to 2 μg mL−1), were made in ultrapure water and stored at 4 °C for a maximum of 1 week. Ammonium acetate, used as mobile phase additive in LC separations, and methanol were purchased from Merck (Darmstadt, Germany). Ultrapure water was obtained from a Milli-Q system

Performance of BP-4 determination

The performance of the LC–QqTOF-MS system for BP-4 determination was assessed with standards in ultrapure water. Highly selective MS chromatograms were extracted using a mass window of 10 ppm around the [M−H] precursor ion (m/z 307.0282) of BP-4. Considering an injection volume of 75 μL, the system provided linear responses in the range between 0.001 and 2 μg mL−1, with a determination coefficient (R2) above 0.999, and a limit of quantification (LOQ), defined as the concentration of BP-4 rendering

Conclusions

In aqueous media, BP-4 reacts with free chlorine following a pseudo-first order kinetics in excess of oxidant. The rate of the above reaction is enhanced by the increase of sample pH (from 6 to 8 units) and the presence of bromide salts. By-products generated from BP-4 are the result of a first electrophilic substitution of hydrogen per chlorine (or bromine), likely in position number 3 of the phenolic ring, followed by oxidation of the carbonyl moiety to an ester group and a further

Acknowledgments

Financial support from the Spanish Government, Xunta de Galicia and E.U. FEDER funds (projects CTQ2009-08377 and 10MDS700006PR) is acknowledged. N.N. and R.R. thank their FPU and Ramón y Cajal contracts to the Spanish Ministry of Education and Science and Innovation, respectively.

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      As far as the degradation of UV filters is concerned, there are two important sources to emphasize: disinfection-induced degradation; and photo-induced degradation. A few studies exist on the degradation of these compounds (Negreira et al., 2012, 2008; Nakajima et al., 2009; Sakkas et al., 2003; Duirk et al., 2013; Grbović et al., 2013), but these are either focused on the same small number of filters, namely those relatively considered more popular in some commercial sunscreen formulations, or they regard solely one degradation scenario, in particular the disinfection-induced degradation, which is fundamentally achieved worldwide by the use of chlorine or chlorine-related water disinfecting agents. One of the problems highlighted by the analysis of these publications, besides the issue with addressing only one degradation source, is the fact that authors seldom focus on other common UV filters, out of a comprehensive list of more than 40 compounds currently regulated and used by pharmaceutical and cosmetic companies in their formulations (Shaath, 2010; Chisvert and Salvador, 2018).

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