ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2013, том 68, № 8, с. 741-749
ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
DEVELOPMENT OF DRY DERIVATIZATION AND HEAD SPACE SOLID-PHASE MICROEXTRACTION TECHNIQUE FOR THE GC-ECD DETERMINATION OF HALOACETIC ACIDS IN TAP WATER © 2013 г. B. Hammami, M. R. Driss
Faculté des sciences de Bizerte, Unité de recherche Chimie Analytique Appliquée Bizerte, Zarzouna, 7021 Tunisia Received 25.11.2010; in final form 17.01.2012
A simple, fast and efficient liquid -liquid extraction (LLE) technique using headspace solid-phase microextraction (HS-SPME), in conjunction with gas chromatography—electron capture detection (GC-ECD) has been developed for the determination of haloacetic acids (HAAs) in tap water. The analytical procedure involves LLE, evaporation to dryness of extraction solvent, derivatization of HAAs into their methyl esters with acidic methanol, HS-SPME using 100-^m polydimethylsiloxane (PDMS) fiber, and GC-ECD determination. The derivatization process was optimized in dry conditions to achieve maximum sensitivity using the following conditions: esterification for 10 min at 55°C in 50 p.L methanol, 30 p.L sulphuric acid and 0.1 g anhydrous sodium sulphate. The HS-SPME conditions were also optimized and good sensitivity was obtained at a sampling temperature of 25°C, an absorption time of 10 min and a desorption time of 2 min. The linear calibration curves were observed for the concentration ranging from 0.1 to 200 p.g/L with the correlation coefficients (R2) greater than 0.993 and the relative standard deviation (RSD) less than 12%. The method detection limits of all analytes ranging from 0.02 to 0.7 p.g/L were obtained. The proposed method is compared directly to standard EPA method 552.2 in drinking water, and significant advantage in terms of selectivity was observed. Finally the optimized procedure was applied to the analysis of HAAs in Bizerte drinking water. The studied HAA were detected in all the water samples and the concentration of total HAA5 ranged from 17.8 to 70.3 ^g/L.
Keywords: haloacetic acids, solid-phase microextraction, derivatization, drinking water, disinfection byproduct.
DOI: 10.7868/S0044450213080057
Chlorine used to disinfect tap water reacts with natural organic matter in surface water, resulting in the formation of a complex mixture of disinfection byproducts (DBPs) including trihalomethanes (THMs) and haloacetic acids (HAAs). Trihalomethanes (THMs) are the major volatile DBPs, while haloacetic acids (HAAs) make up the main non-volatile components. There are a total of nine HAAs congeners (HAA9) containing chlorine and bromide: monochlo-ro-, dichloro-, and trichloro acetic acids (MCAA, DCAA, TCAA); monobromo-, dibromo- and tribromo acetic acids (MBAA, DBAA, TBAA) and the mixed species bromochloro-, bromodichloro- and dibromo-chloro acetic acids (BCAA, BDCAA, DBCAA).
In recent years, the adverse effects of HAAs on human health and the environment have been increasingly recognized. These compounds are toxic to humans and plants [1—3]. The US Environmental Protection Agency (EPA) has established maximum contaminants levels (MCL) for the sum of five HAAs (HAA5: MCAA, DCAA, TCAA, MBAA and DBAA) (60 |g/L) [4]. The World health Organization (WHO)
has guideline value for DCAA (50 ^g/L) and TCAA (200 ^g/L) [5]. Consequently, efforts must be made to develop fast and accurate analytical methods of monitoring concentration, behaviour and distribution of HAAs in water. Some applications have been using HAAs analysis by high-performance liquid chromatography (HPLC) [6, 7], ion chromatography [8], and capillary electrophoresis (CE) [9, 10]. These methods are not suitable for drinking water samples because of their higher detection limit compared to gas chromatographic methods and insufficient selectively. Actually, most of the analytical methods reported so far involve gas chromatography (GC) either with electron captures detection (ECD) [3, 11—14] or coupled with mass spectrometry (GC-MS) [1, 12, 13, 15, 16].
Generally, previous to the analysis by GC, the HAAs were first preconcentrated by liquid - liquid or solid-phase extraction, then converted to their methyl or ethyl esters to make HAAs volatile enough to pass through a GC column using varied reagent such as di-azomethane [17, 18], acidic methanol [16, 19, 20],
acidic ethanol [21], BF3-methanol [22] and aniline [23]. Alternative derivatization methods have been developed in aqueous medium by using an ion-pairing agent [21, 24]. Occasionally, the derivatization steps could also be performed in dry conditions after evaporation of the sample matrix [19]. Simultaneously or after the derivatization, the methyl haloacetates are separated by liquid - liquid extraction (LLE) [25, 26], liquid - phase microextraction (LPME) [27—29], hollow fiber membrane liquid-phase microextraction (HF-LPME) [16] or headspace solid-phase microextraction (HS-SPME) [11, 21, 30].
In this study we have proposed a HS-GC-ECD method for HAAs determination in drinking water. The derivatization step is conducted in dry conditions after evaporation of the extract, following by HS-SPME and GC-ECD determination. The proposed conditions have improved sensitivity and reduced the treatment time compared to most of the methods described in the literature which require treatment time between 40 and 120 min. On the other hand, the proposed method was validated against the EPA method 552.2 by analyzing real water samples. Finally, the proposed method was applied for the determination of HAAs in Bizerte drinking water. To our knowledge, this is the first published study that has measured HAA levels in Bizerte drinking water.
EXPERIMENTAL
Instrumentation. All analyses were carried out on an Agilent 6890 gas chromatograph system equipped with a 63Ni electron capture detector (GC-ECD) operated by HP Chemstation software. The analytical column purchased from Supelco (Bellefonte, PA, USA) was SPB-1701, coated with 14% cyanopropyl-phenyl, 86% dimethylpolysiloxane (30 m x 0.32 mm i.d., 0.25 ^m film thickness). Helium was used as carrier gas with a flow rate 1.5 mL/min. The injector was operated in the splitless mode. The injector temperature was 250°C. The oven temperature was programmed as 40°C for 2 min, increased to 120 at 5°C/min and then increased to 250 at 25°C/min for 10 min. The detector temperature was held at 300°C. Nitrogen was utilized as a make-up gas at 60 mL/min.
Chemicals and reagents. The studied haloacetic acids (MCAA, DCAA, TCAA, MBAA, DBAA, TBAA, and BCAA) were obtained as individual products from Fluka (Buchs, Switzerland) of purity higher than 98%. The 2,3-dibromopropanoic acid and 1,2-dibromopro-pane, used as surrogate standard and internal standard, respectively, for the EPA Method 552 were purchased from Supelco. The solvent methanol of residue analysis grade and sulphuric acid for analysis were supplied by Fisher Scientific (Loughborough, UK), whereas methyl tert-butyl ether (MTBE) of residue analysis grade and anhydrous sodium sulphate were purchased from Fluka. Laboratory grade water was
from Milli-Q water purification system (Millipore, Bedford, MA). 100-^m film thickness polydimethyl-siloxane (PDMS) fibre housed in a manual holder was supplied by Supelco.
The HAAs individual stock standard solutions of 1000 mg/L were prepared by weight in Milli-Q water. Standard mixtures were prepared weekly and intermediate standard solution of 10—100 mg/L in water was prepared before analysis. All solutions were stored in the dark at 4°C and warmed to ambient temperature before use. For methylation and HS-SPME optimization studies, individual stock standard solutions for each HAA and standard mixtures of 1000 mg/L were prepared by weight in methanol. Calibration solutions were prepared by spiking of the intermediate standard solution into appropriate volumes of Milli-Q water.
Sample preparation 1: EPA method 552.2. EPA
Method 552.2 [26] was used to process samples for our experiments. Sample preparation procedures are briefly summarized below. The surrogate is spiked into a 40 mL tap water sample in an extraction vial. The pH is adjusted to <0.5 with sulphuric acid, followed by addition of sodium sulphate and copper sulphate. The extraction solvent (4 mL MTBE) is added, and the capped vial is shaken for several minutes. Three milliliters of the extraction solvent are placed into a 15-mL conical test tube, 1 mL of 10% sulphuric acid in methanol (v/v) is added, and the capped tube is heated at 50°C for 2 h. The cooled mixture is neutralized with four milliliters of saturated sodium bicarbonate solution. After neutralization, the aqueous layer is discarded. The internal standard is added to 1 mL aliquots of the neutralized extract and the aliquots are sealed in amber vials for GC analysis.
Sample preparation 2: Proposed method. Tap Water (40 mL) was placed in a 100-mL vial and the following reagents were added: 2 mL of concentrated sulphuric acid (to give pH < 0.5), 15 g of anhydrous sodium sulphate and 3 mL of MTBE. The flask was sealed, vor-texed, mixed for 5 min and allowed to stand for 2 min. Then, 2 mL of the extract were transferred into a 3-mL screw-capped septum vial and evaporated to dryness under a gentle stream of nitrogen (99.99% pure) during 4 min; then 0.1 g of anhydrous sodium sulphate, 30 ^L of concentrated sulphuric acid and 50 ^L of methanol were added to the vial, which was then sealed with the septum. The solution was mixed with magnetic stirrer at 55°C for 10 min to derivatize the HAAs. After cooling, the vial was placed in a thermostatic bath at 25°C, and the 100-^m PDMS fibre was exposed to the headspace during 10 min to extract the haloacetic methyl esters. Finally, the corresponding haloacetates were desorbed after 2 min in the injector port of the gas chromatograph at 250°C.
Sample collection. All the samples were collected in 200 mL amber glass bottles with PTFE-faced septa and propylene screw caps. Ammonium chloride (NH4Cl
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