Liquid-phase microextraction

Liquid-phase microextraction Click on the title to be taken to the full paper.

Abstract

The development of faster, simpler, inexpensive and more environmentally-friendly sample-preparation techniques is an important issue in chemical analysis.

Recent research trends involve miniaturization of the traditional liquid-liquid-extraction principle by greatly reducing the acceptor-to-donor ratio.

The current trend is towards simplification and miniaturization of sample preparation and decreasing the quantities of organic solvents used.

We discuss liquid-phasemicroextraction with the focus on extraction principles, historical development and performance.

Abbreviations

• AAS, Atomic absorption spectrometry; 
• BTEX, Benzene, toluene, ethylbenzene and xylenes; 
• DI, Direct immersion; 
• DLLME, Dispersive liquid-liquidmicroextraction; 
• DSDME, Directly-suspended dropletmicroextraction; 
• CE, Capillary electrophoresis; 
• CFME, Continuous-flow microextraction; 
• ECD, Electron-capture detector; 
• EME, Electrokinetic membrane extraction; 
• ETAAS, Electrothermal atomic absorption spectrometry; 
• ETV-ICP-OES/MS, Electrothermal vaporization-inductively coupled plasma optical emission spectrometry/mass spectrometry; 
• FID, Flame-ionization detector; 
• GC, Gas chromatography; 
• GF-AAS, Graphite-furnace atomic absorption spectrometry; 
• HPLC, High-performance liquid chromatography; 
• HFM, Hollow-fiber membrane; 
• HF-LPME, Hollow-fiber liquid-phasemicroextraction; 
• HS, Headspace; 
• IL, Ionicliquid; 
• LLE, Liquid-liquid extraction; 
• LLLME, Liquid-liquid-liquidmicroextraction; 
• LPME,Liquid-phasemicroextraction; 
• MASE, Microwave-assisted solvent extraction; 
• MS, Mass spectrometry;
• PAHs, Polycyclic aromatic hydrocarbons; 
• PCB, Polychlorinated biphenyl; 
• SDME, Single-dropmicroextraction; 
• SPE, Solid-phase extraction; 
• SPME, Solid-phasemicroextraction; 
• SLM, Supportedliquid membrane; 
• TD, Thermal desorption; 
• TILDLME, Temperature-controlled ionic liquid dispersiveliquid-phasemicroextraction; 
• THF, Tetrahydrofuran; 
• VOC, Volatile organic compounds; 
• VIS, Visible; 
• UV, Ultraviolet

Keywords

• Continuous-flow microextraction (CFME); 
• Directly-suspended droplet microextraction (DSDME);
• Dispersive liquid-liquidmicroextraction (DLLME); 
• Electrokinetic membrane extraction (EME); 
• Extraction;
• Hollow-fiber liquid-phasemicroextraction (HF-LPME); 
• Liquid-phasemicroextraction (LPME);
• Miniaturization; 
• Sample preparation; 
• Single-drop microextraction (SDME)

1. Introduction

In recent years, the development of fast, precise, accurate and sensitive methodologies has become an important issue. However, despite the advances in the development of highly efficient analytical instrumentation for the end-point determination of analytes in biological and environmental samples and pharmaceutical products, sample pre-treatment is usually necessary in order to extract, to isolate and to concentrate the analytes of interest from complex matrices because most of the analytical instruments cannot directly handle the matrix. A sample-preparation step is therefore commonly required.

Sample preparation can include clean-up procedures for very complex (dirty) samples. This step must also bring the analytes to a suitable concentration level. However, conventional sample-preparation techniques [i.e. liquid-liquid extraction (LLE) and solid-phase extraction (SPE)] have involved drawbacks (e.g., complicated, time-consuming procedures, large amounts of sample and organic solvents and difficulty in automation). Using harmful chemicals and large amounts of solvents causes environmental pollution, health hazards to laboratory personnel and extra operational costs for waste treatment. Ideally, sample-preparation techniques should be fast, easy to use, inexpensive and compatible with a range of analytical instruments, so the current trend is towards simplification and miniaturization of the sample-preparation steps and decrease in the quantities of organic solvents used.

In 1990, Arthur and Pawliszyn [1] introduced a new method termed solid-phase microextraction (SPME). A polymer-coated fiber, on which the investigated compound adsorbs, is placed in the sample or its headspace. SPME has several important advantages compared to the traditional sample-preparation techniques:

·         it is a rapid, simple, solvent free and sensitive method for the extraction of analytes;

·         it is a simple, effective adsorption/desorption technique;

·         it is compatible with analyte separation and detection by high-performance liquid chromatography with ultraviolet detection (HPLC-UV);

·         it provides linear results for a wide range of concentrations of analytes;

·         it has a small size, which is convenient for designing portable devices for field sampling; and,

·         it gives highly consistent, quantifiable results from very low concentrations of analytes.

Although the use of SPME fibers is increasingly popular, they have significant drawbacks, e.g.:

(i)                  their relatively low recommended operating temperature (generally in the range 240–280°C);

(ii)                their instability and swelling in organic solvents (greatly restricting their use with HPLC);

(iii)               fiber breakage;

(iv)              stripping of coatings; and,

(v)                the bending of needles and their expense [2].

In order to overcome these problems, simple, inexpensive liquid-phase microextraction (LPME) was introduced recently. LPME is a solvent-minimized sample-pretreatment procedure of LLE, in which only several μL of solvent are required to concentrate analytes from various samples rather than hundreds of mL needed in traditional LLE. It is compatible with capillary gas chromatography (GC), capillary electrophoresis (CE) and HPLC.

In LPME, extraction normally takes place into a small amount of a water-immiscible solvent (acceptorphase) from an aqueous sample containing analytes (donor phase). It can be divided into three main categories:

(1) single-drop microextraction (SDME)

(2) dispersive liquid–liquid microextraction (DLLME)

(3) hollow-fiber microextraction (HF-LPME)

We devote this article to discussion of microextraction techniques and performance and conclude with advantages and drawbacks.

References used in this Introduction:

1. C.L. Arthur, J. Pawliszyn, Anal. Chem., 63 (1990), p. 2145

2. A. Kumar, Gaurav, A.K. Malik, D.K. Tewary, B. Singh, Anal. Chim. Acta, 610 (2008), p. 1 - Article |  PDF (190 K) | View Record in Scopus | Full Text via CrossRef