Determination of 4-nonylphenol and 4-tert.-octylphenol in water samples by stir bar sorptive extraction and thermal desorption–gas chromatography–mass spectrometry

doi:10.1016/j.aca.2003.10.064

Analytica Chimica Acta

Volume 505, Issue 2, 10 March 2004, Pages 217-222

https://doi.org/10.1016/j.aca.2003.10.064 Get rights and content

Abstract

A simple and highly sensitive method called thermal desorption (TD)–gas chromatography–mass spectrometry (GC–MS), which is used for the determination of trace amounts of 4-nonylphenol (NP) and 4-tert.-octylphenol (OP) in water samples, is described. NP and OP in samples are extracted from water samples and concentrated by the stir bar sorptive extraction (SBSE) technique. A stir bar coated with polydimethylsiloxane (PDMS) is added to a 2.0 ml water sample and stirring is carried out for 60 min at room temperature (25 °C) in a headspace vial. Then the extract is high sensitively analyzed by TD–GC–MS without any derivatization step. The optimum SBSE conditions are realized at an extraction time of 60 min. The detection limits are 0.02 ng ml⁻¹ for NP and 0.002 ng ml⁻¹ for OP. The method shows good linearity over the concentration range of 0.1–10 ng ml⁻¹ for NP and 0.01–10 ng ml⁻¹ for OP, and the correlation coefficients are higher than 0.999. The average recoveries of NP and OP are higher than 97% (R.S.D.: 3.6–6.2%) with correction using the added surrogate standards, 4-(1-methyl) octylphenol-d₅ and deuterium 4-tert.-octylphenol. This simple, accurate, sensitive and selective analytical method may be used in the determination of trace amounts of NP and OP in tap and river water samples.

Introduction

4-Nonylphenol (NP) and 4-tert.-octylphenol (OP) are the degradation products of such non-ionic surfactants as alkylphenolpolyethoxylates (APEOs) that exist mainly as intermediates in the manufacturing industry. Alkylphenols, NP and OP, have been detected in river water, sewage sludge and fish tissue [1], [2], [3], [4]. In addition, the estrogenic activity of NP and OP has been extensively evaluated by a variety of assays [5], [6], [7] because of the lack of a simple and accurate analytical method for quantifying NP and OP in environmental samples.

Recently, many analytical methods for the determination of NP and OP in various samples have been reported. Reliable methods for NP analysis include liquid chromatography (LC) with coulometric array detection [8], [9], and LC with on-line solid-phase extraction (SPE) [10], [11]. There are many reports on the use of LC with mass spectrometry (LC–MS) for the determination of NP in environmental samples [12], [13], [14], [15]. However, the LC methods have low resolution and the matrix is frequently affected. On the other hand, gas chromatography–mass spectrometry (GC–MS) was initially used for the determination of phenol compounds including NP and OP, even though derivatization was required [16], [17], [18], [19], [20], [21]. The derivatization leads to shaper peaks and hence to a better separation and sensitivity of the phenols. However, the derivatization required by GC–MS presents a difficult problem because high backgrounds are often encountered. In other words, the derivatization faces the risk of contamination and hence an overestimation of NP and OP concentrations. Therefore, the development of a derivatization-free and highly sensitive GC–MS method is required. Recently, solid-phase microextraction (SPME)–GC–MS without derivatization have been successfully applied to NP determination with quantification limits below 0.6 ng ml⁻¹[22]. However, the sensitivity is still considered to be limited. Because SPME with polydimethylsiloxane (PDMS) is by nature an equilibrium technique based on the partitioning of the solute between the stationary phase and the aqueous sample, enrichment factors are dependent on the distribution coefficients of the analyte between the different phases. As a consequence, the limited enrichment on the SPME fiber is mainly due to the amount of PDMS phase (typically 0.5 μl or less). Increasing the amount of PDMS relative to the aqueous matrix would dramatically increase the enrichment of the analyte. Recently, a new sorptive extraction technique that uses a stir bar coated with 50–300 μl of PDMS was developed [23]. The technique is known as stir bar sorptive extraction (SBSE) and its main advantage is its wide application range that can includes volatile aromatics, halogenated solvents, polyaromatic hydrocarbons, polychlorinated biphenyls (PCBs), pesticides, preservatives, odor compounds and organotin compounds [23], [24], [25], [26], [27], [28].

The aim of this study was to determine trace amounts of NP and OP in water samples by SBSE and TD–GC–MS without derivatization step. This method was applied to tap and river water samples.

Section snippets

Materials and reagents

4-Nonylphenol (mixture type) (NP) and 4-tert.-octylphenol (OP) of environmental analytical grade were purchased from Kanto Chemical, Inc., Tokyo, Japan. 4-(1-Methyl)octylphenol-d₅ (m-OP-d₅) and deuterium 4-tert.-octylphenol (the mixture by which the hydrogen of OP was replaced by 1-12 deuterium) (OP-d) were purchased from Hayashi, Osaka, Japan. The chemical structures are shown in Fig. 1. Methanol of pesticide grade was purchased from Wako Pure Chemical Inc., Osaka, Japan. The water

Theoretical recoveries

Table 1 shows the log K_o/w and the calculated theoretical recoveries for the compounds under consideration in this work. The K_o/w values are calculated from the K_o/w Win program, which is available from Syracuse Research Corp. (SRC, USA). Theoretical recoveries are calculated by the following equations: $Theoretical recovery =K_{o/w} /β/(1+K_{o/w} /β)$ where β=V_w/V_PDMS, V_PDMS the volume of PDMS and V_w the volume of water. Theoretical recoveries are calculated on the basis of a 2 ml sample volume and stir bar

Conclusions

The determination of trace amounts of NP and OP in water samples using the SBSE technique followed by TD–GC–MS without derivatization was described. The proposed method has many practical advantages such as a small sample volume (2 ml) and simplicity of the extraction; it is also solvent-free and high sensitivity. The detection limits were 0.02 ng ml⁻¹ for NP and 0.002 ng ml⁻¹ for OP. In addition, the present method showed good linearity and correlation coefficients using surrogate standards. Also,

Acknowledgements

This study was supported by Health Sciences Research grants and the Grant-in-Aid for Cancer research (15–22) from the Ministry of Health, Labour and Welfare of Japan and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology.

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Determination of 4-nonylphenol and 4-tert.-octylphenol in water samples by stir bar sorptive extraction and thermal desorption–gas chromatography–mass spectrometry

Abstract

Introduction

Section snippets

Materials and reagents

Theoretical recoveries

Conclusions

Acknowledgements

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