Salt-assisted dispersive liquid–liquid microextraction coupled with programmed temperature vaporization gas chromatography–mass spectrometry for the determination of haloacetonitriles in drinking water

Highlights

• A novel dispersant solvent-free MADLLME method was reported.

• The dispersion of extractant in aqueous sample was performed only by salt addition.

• Quantification of HANs was performed by PTV-GC–MS.

• Preservation and occurrence study for MIAN was performed for the first time.

• Six target HANs detected in different drinking waters.

Abstract

We report here a new analytical method for the simultaneous determination of seven haloacetonitriles (HANs) in drinking water by coupling salt-assisted dispersive liquid–liquid microextraction (SADLLME) with programmed temperature vaporizer-gas chromatography–mass spectrometry (PTV-GC–MS). The newly developed method involves the dispersion of the extractant in aqueous sample by addition of a few grams of salt and no dispersion liquid was required as compared to the traditional DLLME methods. The extractant (CH₂Cl₂, 50 μL) and the salt (Na₂SO₄, 2.4 g) were successively added to water (8 mL) in a conical centrifuge tube that was shaken for 1 min and centrifuged (3500 rpm, 3 min). The aliquot of sedimented phase (4 μL) was then directly injected into the PTV-GC–MS system. The limits of detection and quantification for the HANs were 0.4–13.2 ng L⁻¹ and 1.2–43.9 ng L⁻¹, respectively. The calibration curves showed good linearity (r² ≥ 0.9904) over 3 orders of magnitude. The repeatability of the method was investigated by evaluating the intra- and inter-day precisions. The relative standard deviations (RSDs) obtained were lower than 10.2% and 7.8% at low and high concentration levels. The relative recoveries ranged from 79.3% to 105.1%. The developed methodology was applied for the analysis of seven HANs in several drinking water samples in coastal and inland cities of China. It was demonstrated to be a simple, sensible, reproducible and environment friendly method for the determination of trace HANs in drinking water samples.

Introduction

Haloacetonitriles (HANs) are a group of emerging halogenated nitrogenous disinfection by-products (N-DBPs) that can be formed as by-products from the reactions between chlorine, chloramine or bromine disinfectants and organic nitrogen present in source water [1], [2], [3]. Although the total HANs levels in finished drinking water are about 10% of the trihalomethanes (THMs) concentrations [4], [5], [6], the cytotoxicity and genotoxicity of some HANs are significantly higher than those of the regulated THMs or haloacetic acids (HAAs) [7], [8], [9]. Due to their potential health effects, the World Health Organization has suggested guideline values of 20 μg L⁻¹ for dichloroacetonitrile (DCAN) and 70 μg L⁻¹ for dibromoacetonitrile (DBAN). Moreover, HANs have been included in the US Environmental Protection Agency Information Collection Rules and they may be considered in future US EPA regulations. Therefore, the increasing public concern prompted us to develop a simple and reliable method for the determination of HANs in drinking water.

Gas chromatography (GC) coupled with electron capture detection (ECD), or in combination with mass spectrometry (MS) is generally used for the determination of HANs. The sample preparation step usually involves a liquid–liquid extraction (LLE) method which is based on the US EPA method 551.1 [10], [11]. However, this methodology has serious drawbacks such as long sample preparation time, relatively large amounts of toxic organic solvents and a low sample-to-solvent ratio. In addition, the headspace solid-phase microextraction (HS-SPME) for HANs in drinking water samples was developed in two studies [12], [13]. However, the inherent limitations of SPME such as sample carry-over, poor inter-fiber reproducibility, high cost and limited lifetime of the fiber cannot be ignored.

Since 2006, a novel liquid phase microextraction (LPME) technique, termed dispersive liquid–liquid microextraction (DLLME), has attracted much attention due to the merits of small amount of solvent, short extraction time, ease of operation, as well as high enrichment factors for analytes [14]. This technique is generally based on the addition of an immiscible and higher density solvent to the aqueous sample. In order to concentrate analytes in the extractant phase it is also essential to add a dispersant solvent which increases the interface between the two immiscible solvents. The main drawback of this process is that the third component (dispersant solvent) usually increases the solvent consumption (normally 0.5–2 mL), and decreases the partitioning of analytes into the extractant solvent, especially when the analytes are more polar. HANs are a group of weak polar compounds, and therefore the application of traditional DLLME to the analysis of HANs was inevitably limited.

In the present work, a novel dispersant solvent-free DLLME technique, termed as salt-assisted dispersive liquid–liquid microextraction (SADLLME), for HANs in water was developed. The dispersion of the extractant phase into aqueous phase was achieved by salt addition followed by manual shaking. The salt addition not only decreases the interfacial tension between aqueous and extractant phases, accordingly achieving sufficient dispersion of extract microdroplets into aqueous phase under the assistance of external stirring, but also can significantly improve the extraction efficiency of HANs with weak polarity due to the salting-out effect.

The developed SADLLME method was used for the extraction of trace-level HANs in drinking water, including monochloroacetonitrile (MCAN), monobromoacetonitrile (MBAN), monoiodoacetonitrile (MIAN), DCAN, DBAN, bromochloroacetonitrile (BCAN), and trichloroacetonitrile (TCAN). To our best knowledge, it is the first time that MIAN in drinking water as target compound has been investigated. It has been reported that MIAN is more cytotoxic and genotoxic than chloro/bromo acetonitrile [9]. Furthermore, the quantification of HANs was performed by GC–MS equipped with a programmed temperature vaporizer (PTV) inlet, which can remarkably reduce the background noise and column burden by selective transfer of target analytes onto column [15], [16]. In this study, the parameters affecting the developed SADLLME-PTV-GC–MS method for HANs analysis in drinking water were carefully optimized, and its performance was assessed.