Elsevier

Talanta

Volume 144, 1 November 2015, Pages 836-843
Talanta

Simultaneous determination of 9-ethylphenanthrene, pyrene and 1-hydroxypyrene in an aqueous solution by synchronous fluorimetry using the double scans method and hydroxyl-propyl beta-cyclodextrin as a sensitizer

https://doi.org/10.1016/j.talanta.2015.05.067Get rights and content

Highlights

  • Simultaneous measurement of three analytes was achieved using synchronous fluorimetry.

  • 1-OH-Pyr-HPCD formed more stable inclusion complex because of hydrogen bonding.

  • Sensitizing effect of HPCD was different among three analytes.

Abstract

A novel method for the simultaneous determination of 9-ethylphenanthrene (9-EP), pyrene (Pyr) and 1-hydroxypyrene (1-OH-Pyr) in an aqueous solution using hydroxyl-propyl β-cyclodextrin (HPCD) as a sensitizer has been established. The overlap of the conventional fluorescence spectra of these molecules is resolved using synchronous fluorescence spectrometry with the double scans method. The simultaneous quantitative determination of three compounds was carried out with Δλ=36 nm and Δλ=55 nm. The signals detected at these three wavelengths (i.e., 298 nm, 337 nm and 351 nm) vary linearly when the concentrations of 9-EP, Pyr and 1-OH-Pyr were in the range of 5.00×108–1.60×10–6 mol L–1, 2.00×10–8–1.80×10–6 mol L–1, and 2.00×10–8–1.20×105 mol L–1, respectively. The limits of detection (LOD) for 9-EP, Pyr and 1-OH-Pyr were 3.97×10–9 mol L–1, 5.25×109 mol L1, 4.20×10–9 mol L–1, respectively, with relative standard deviations (R.S.D.) of 1.62%, 2.45% and 1.73% (n=9), respectively. The inclusion behaviors between HPCD and the guest molecules were observed by synchronous fluorimetry and the association constants for the 1:1 complexes with HPCD were determined. The binding and complexation energies for different orientations are discussed. The proposed method was successfully applied to the analysis of 9-EP, Pyr and 1-OH-Pyr in tap and lake water with good recoveries in the range of 92.9–110.0%.

Introduction

Water quality is currently an important issue in several regions of the world. Among the several contaminants that are recognized as threats to aquatic systems, much concern has been focused on polycyclic aromatic hydrocarbons (PAHs). PAHs have been recognized as mutagenic, carcinogenic and teratogenic. In addition, PAHs have been classified as highly hazardous by the United States Environmental Protection Agency (US EPA) and the European Union (EU) due to their toxicity [1]. Therefore, there is increasing concern regarding the deleterious effects of these compounds in estuarine and coastal ecosystems because unlike other hazardous organic chemicals that have been banned or regulated in discharges, PAHs continue to be released into the environment due to several natural phenomena in addition to anthropogenic activities, such as burning of fossil fuels, oil and gas extraction, transformation and transport [2]. In the aquatic environment, although concentrations of PAHs are relatively low compared to other pollutants, its toxicity towards aquatic organisms cannot be ignored. Accumulation of PAHs in aquatic organisms can effectively alter their activities, such as development, reproduction, and behaviors. Potential threats to the ecosystem and human health can occur via bioconcentration and biomagnification. Currently, studies on the environmental behaviors and toxicity of PAHs are primarily focused on the parent PAHs, which are listed by the US EPA as priority pollutants. However, alkyl PAHs, which are important components of the entire PAHs family, have received less attention [3]. The reported concentrations of alkyl PAHs were quite comparable to those of their parent forms in air [4], water [5] and sediment samples [6]. In crude oil, the parent PAHs constitute 1–3% of the total PAH content, and alkyl PAHs typically comprise more than 90% of the total PAH content [7]. The results of previous studies indicated that unmeasured alkyl PAHs can result in a substantial underestimation of the PAH risk [8] because the parent PAHs and alkyl PAHs co-exist in real environments and may be further metabolized. To effectively and scientifically evaluate the toxicological effects of PAHs including alkyl PAHs in the aquatic environment, an analytical method for the simultaneous determination of the parent PAHs, alkyl PAHs and their metabolites is required.

Analytical methods for the determination of the parent PAHs have improved during the past decade [9], [10], [11]. However, studies on the measurement of alkyl PAHs and PAH metabolites are still rather limited, especially for alkyl PAHs. Several studies proposed methods, such as gas chromatography coupled to mass spectrometry (GC–MS) [3], [6], [12] and liquid chromatography coupled to mass spectrometry (LC–MS) [13]. In general, these methods were expensive, time-consuming, and required complicated pretreatment procedures, which are disadvantageous for routine environmental monitoring. Alkyl PAHs and PAH metabolites obtained by alkylation and metabolization processes that altered their original structures resulted in aromatic structures with delocalized π-electrons that exhibit relatively high fluorescence quantum yields. Therefore, these compounds could be determined using fluorescence spectrometry. However, chemical compounds with similar structures typically suffered from overlapping spectra, which posed a challenge for multi-component analysis. Synchronous fluorescence spectrometry is a simple modification of the conventional fluorescence technique to afford higher selectivity due to the narrowing of the spectral bands and simplification of the spectra [14]. After its introduced by Lloyd [15], this technique has been exploited for multi-components measurements [16], [17], [18]. Because PAHs have a low solubility limit, the application of the fluorescence method was hindered by only the dissolved fraction being measured. These disadvantages can be overcome by the introduction of cyclodextrins (CDs), which form inclusion complexes with poorly soluble organic compounds to improve their solubility and dissolution rate [19]. Among the CDs, beta-CD (β-CD) and hydroxyl-propyl beta-CD (HPCD) are the primary options because they have suitable cavity sizes and relatively low costs [20]. HPCD deserves special attention due to its higher aqueous solubility, lower toxicity and more hydrophobic cavity compared to the parent compound, making it a good choice for incorporation of lipophilic molecules into the CD cavity [21]. Therefore, the introduction of CDs enabled us to improve analytical measurements, eliminate or reduce solubility issues with organic compounds, augment the sensitivity and precision, and increase the selectivity of the analytical method [22]. More importantly, studies on the bioavailability and toxicity of PAHs could address the restrictions on the solubility limit by introducing CDs [23].

To the best of our knowledge, few studies have focused on the simultaneous determination of parent PAHs, alkyl PAHs and PAH metabolites. The aim of the current study was to apply synchronous fluorescence spectrometry for the simultaneous determination of parent PAHs, alkyl PAHs and PAH metabolites by selecting 9-ethylphenanthrene (9-EP), pyrene (Pyr) and 1-hydroxy-pyrene (1-OH-Pyr) as model compounds. HPCD was selected to enhance the solubility and quantum yield of the target compounds and increase the sensitivity of the method. The inclusion behaviors between HPCD and the guest molecules as well as the mechanism of the enhancing effect were studied by fluorescence spectrometry and molecular modeling using Autodock. This novel method is expected to provide an approach for the continued study of the environmental behavior and toxicity of 9-EP, Pyr and 1-OH-Pyr in environmental science.

Section snippets

Materials

The stock solutions of analytes were prepared by adding the appropriate amount of solid 9-EP (Sigma Aldrich, purity>98%), Pyr (Acros Organics, purity>98%) and 1-OH-Pyr (J&K, purity>99%), which were separately dissolved in 100 mL of dichloromethane (Xilong Chemical Plant, purity>99%) to achieve a final concentration of 1.00×10–2 mol L–1. HPCD (J&K, purity>97%) was used without any further purification. Stock solution of HPCD (1.00×10–2 mol L–1) was prepared by dissolving HPCD in water. The working

Selection of synchronous fluorescence parameters

To the best of our knowledge, the simultaneous measurement of 9-EP, Pyr and 1-OH-Pyr in water has not been previously reported. Therefore, the feasibility of measuring three components must be investigated first. The results in Fig. 1 indicates that the fluorescence emission maximum of 9-EP (3.00×10–7 mol L–1), Pyr (5.00×10–7 mol L–1) and 1-OH-Pyr (6.00×10–7 mol L1) was located at 352 nm, 372 nm and 386 nm, respectively. The fluorescence spectra of 9-EP, Pyr and 1-OH-Pyr overlapped. Therefore, the

Conclusions

The results presented in this paper indicated that the established synchronous fluorescence method can be successfully used for the simultaneous determination of 9-EP, Pyr, and 1-OH-Pyr in aqueous solutions. Because the physicochemical parameters of 9-EP and Pyr and the m/z ratios of the three compounds were similar, good separation and detection are difficult to achieve using other methods, such as GC–MS and LC–MS. The comparison of the main characteristics of the established method with other

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China (21177102), Research Fund for the Doctoral Program of Higher Education of China (20130121130005), Open Fund of Key Laboratory of Marine Spill Oil Identification and Damage Assessment Technology (MOIDAT) (201405), and Fundamental Research Funds for the Central Universities (2013121052).

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