Determination of organic compounds in water using dispersive liquid–liquid microextraction

https://doi.org/10.1016/j.chroma.2006.03.007Get rights and content

Abstract

A new microextraction technique termed dispersive liquid–liquid microextraction (DLLME) was developed. DLLME is a very simple and rapid method for extraction and preconcentration of organic compounds from water samples. In this method, the appropriate mixture of extraction solvent (8.0 μL C2Cl4) and disperser solvent (1.00 mL acetone) are injected into the aqueous sample (5.00 mL) by syringe, rapidly. Therefore, cloudy solution is formed. In fact, it is consisted of fine particles of extraction solvent which is dispersed entirely into aqueous phase. After centrifuging, the fine particles of extraction solvent are sedimented in the bottom of the conical test tube (5.0 ± 0.2 μL). The performance of DLLME is illustrated with the determination of polycyclic aromatic hydrocarbons (PAHs) in water samples by using gas chromatography-flame ionization detection (GC-FID). Some important parameters, such as kind of extraction and disperser solvent and volume of them, and extraction time were investigated. Under the optimum conditions the enrichment factor ranged from 603 to 1113 and the recovery ranged from 60.3 to 111.3%. The linear range was 0.02–200 μg/L (four orders of magnitude) and limit of detection was 0.007–0.030 μg/L for most of analytes. The relative standard deviations (RSDs) for 2 μg/L of PAHs in water by using internal standard were in the range 1.4–10.2% (n = 5). The recoveries of PAHs from surface water at spiking level of 5.0 μg/L were 82.0–111.0%. The ability of DLLME technique in the extraction of other organic compounds such as organochlorine pesticides, organophosphorus pesticides and substituted benzene compounds (benzene, toluene, ethyl benzene, and xylenes) from water samples were studied. The advantages of DLLME method are simplicity of operation, rapidity, low cost, high recovery, and enrichment factor.

Introduction

The sample preparation step in an analytical process typically consists of an extraction procedure that results in the isolation and enrichment of components of interest from a sample matrix. Extraction can vary in degree of selectivity, speed and convenience and depends not only on the approach and conditions used but also on the geometric configurations of the extraction phase [1]. Liquid–liquid extraction (LLE) is among the oldest of the preconcentration and matrix isolation techniques in analytical chemistry [2]. However, LLE is time-consuming and requires large amounts of organic solvent. Solid-phase extraction (SPE) uses much less solvent than LLE, but can be relatively expensive. Supercritical fluid extraction (SFE) can also be relatively expensive [3].

Recent research activities are oriented towards the development of efficient, economical, and miniaturized sample preparation methods. As a result, solid-phase microextraction (SPME) [4], [5], [6] and solvent microextraction (SME) [7], [8] have been developed, among the others. Compared with LLE, SPME is a solvent-free process developed by Arthur and Pawliszyn [9] that includes simultaneous extraction and preconcentration of analytes from aqueous samples or the headspace of the samples. However, SPME is also expensive, its fiber is fragile and has limited lifetime and sample carry-over can be a problem [10]. Jeannot and Cantwell developed a liquid–liquid microextraction (LLME) system by which extraction was achieved into a single drop [11]. He and Lee reported liquid-phase microextraction (LPME) in 1997 [12], [13]. Single drop microextraction (SDME) was developed as a solvent-minimized sample pretreatment procedure which is inexpensive, and since very little solvent is used, there is minimal exposure to toxic organic solvents [14], [15]. However, the disadvantages of these methods are as follows: fast stirring would tend to break up the organic drop; air bubble formation [3]; extraction is time-consuming and equilibrium could not be attained after a long time in most cases [15]. Furthermore, as a result of the demand for ultra-trace analysis, the need for powerful methods has increased in particular for environmental analysis. Therefore, simple, rapid, clean and efficient techniques that can be performed easily are required.

Cloud-point extraction (CPE) uses surfactants for extraction of materials. Surfactants for extraction have been known to human beings for long for their capability to enhance the solubility of hydrophobic material [16], [17]. The advantage of CPE is the preferable use of water as the solvent in the micellar solution, which is benign to the environment, as compared with the organic solvents still used in other preconcentration procedures. Despite many benefits of using cloud-point extraction, the choices of the surfactants often bring the nuisance to the analysis of analytes using some instruments analyses such as GC and HPLC [18], [19]. In addition, the use of anionic surfactants as effective extractant in the cloud-point extraction separation often requires of salts and adjustments of pH [20], [21]. Pressure and temperature effect in CPE. Hence, it is very important to optimize them in order to obtain the good recovery [22].

Homogeneous liquid–liquid extraction (HLLE) utilizes the phase separation phenomenon from a homogeneous solution, and the target solutes are extracted into a separated phase. In homogeneous liquid–liquid extraction, the initial condition (before phase separation) is homogeneous solution; namely, there is no interface between the water phase and the water-miscible organic solvent phase. In other words, the surface area of the interface is infinitely large initially. Accordingly, no vigorous mechanical shaking is necessary. The procedure is simple and requires only the addition of a reagent. The ternary component solvent system and the perfluorinated surfactant system are the two usual modes of homogeneous liquid–liquid extraction [23], [24], [25], [26].

However, HLLE has some problem; for instance, sometimes it is not compatible with some instrumental analysis and also it requires the addition of reagent such as acid, base, salt, etc. As a result of that, probably some interested compounds are destroyed; moreover, the addition of reagent causes to release of heat during extraction.

The authors demonstrated a novel microextraction technique as a high performance and powerful preconcentration method which is dispersive liquid–liquid microextraction (DLLME). It is based on ternary component solvent system such as HLLE and CPE. In this method, the appropriate mixture of extraction solvent and disperser solvent is injected into aqueous sample by syringe, rapidly. Thereby, cloudy solution is formed. The advantages of DLLME method are simplicity of operation, rapidity, low cost, high recovery, and enrichment factor. The performance of DLLME is illustrated with the determination of polycyclic aromatic hydrocarbons (PAHs) in water samples by using gas chromatography-flame ionization detection (GC-FID). The effects of various experimental parameters on the extraction of PAHs from water samples were investigated. Also, the ability of DLLME technique in the extraction of other organic compounds such as organochlorine pesticides (OCPs), organophosphorus pesticides (OPPs) and benzene, toluene, ethyl benzene and xylenes (BTEX) from water samples was studied.

Section snippets

Reagents and standards

All PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, flouranthene, pyrene, benzofluorene, benzo[a]anthracene, chrysene, benzo[e]acephenanthylene, benzo[e]pyrene, benzo[a]pyrene, perylene, and benzo[ghi]perylene) used were purchased from Aldrich (USA). Tetrachloroethylene (for spectroscopy), carbon tetrachloride (GR), and carbon disulfide (GR) were obtained from Merck (Germany). These solvents were distillated at least four times and were used as extraction

Result and discussion

There are different factors that affect the extraction process. Some of them are selection of suitable extraction solvent, selection of suitable disperser solvent, volume of extraction solvent, volume of disperser solvent, and extraction time. It is very important to optimize them in order to obtain the good recovery strategy forms. We selected eight compounds as representative of the PAHs, and showed their behavior under these extraction conditions.

Conclusion

In the present study, a new mode of microextraction technique was described as a dispersive liquid–liquid microextraction (DLLME) which has been developed. DLLME provides high recovery and enrichment factor within a very short time (a few seconds). PAHs were employed as model compounds to assess the extraction procedure and were determined by GC-FID. The performance of this procedure in the extraction of PAHs from surface, river, and well waters was excellent. Also, the ability of DLLME

Acknowledgements

Financial support from Iran University of Science and Technology is gratefully acknowledged. The authors thank Dr. Professor M. Ashraf-Khorasani and Dr. Professor M. Jalali-Heravi.

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