Liquid-liquid extraction in speciation analysis

Liquid-liquid extraction (LLE) is one of the most often used separation method based on the different solubility of the analytes in the different immiscible solvents applied. The technique has been applied for both biological and environmental materials. However, the operation of conventional LLE is time-consuming and complex; large volumes of solvent are needed; the direct injection of large amounts of organic solvents can affect the performance of the detection technique, as a result tedious evaporation and resolubilisation steps after extraction are required. Thus, the application of LLE in elemental speciation is limited.

Liquid-phase micro-extraction (LPME) was developed as an environmentally friendly alternative with significantly reduced use of organic solvents. Based on the applied extraction devices, LPME could be divided into single-drop microextraction (SDME), Hollow-fibre liquid-phase microextraction (HF-LPME), and dispersive liquid–liquid microextraction (DLLME). All these LPME modes have been applied for elemental speciation analysis of biological and environmental samples.

SDME is a simple, cheap, environmentally friendly extraction method providing a high enrichment factor (EF) based on the very small volume of the extractant phase used to collect the analytes from a relatively high sample volume. SDME can be performed in two different modes: headspace (HS)-SDME (the extractant drop is exposed to the headspace of the sample) which is suitable for the extraction of volatile elemental species, and direct-SDME (the extractant drop is immersed in sample solution). The most important factor in SDME is the extractant which should selected based on selectivity, extraction efficiency, incidence of drop loss, rate of drop dissolution, toxicity and the compatibility with the detection technique employed. The main shortcoming of SDME is the lack of stability of the micro-drop on the tip of the microsyringe. Especially at high stirring rates of the sample solution and long extraction times, the drop instability is  leading to poor reproducibility.

HF-LPME has been developed with the main goal to overcome the instability problem of SDME. HF-LPNME is using a hollow fibre to hold the extractant phase. Besides mechanical stabilization, the membrane is also a barrier against particles and large molecules and therefore can reduce matrix interferences. 

Dispersive liquid–liquid microextraction (DLLME) is another LPME method, that has been used for speciation analysis. In this method, a cloudy dispersion is formed when an appropriate mixture of extraction and dispersive solvents is injected into the aqueous sample. Hydrophobic solutes are enriched in the extraction solvent and can be separated by centrifugation. DLLME is simple to operate, fast, cheap with high recovery, high EF and very short extraction times (a few seconds). However, its main drawback is poor matrix tolerance, which greatly limits its use for complex sample matrices as present in biological and environmental samples.

B. Hu, F. Zheng, M. He, N. Zhang, Capillary microextraction (CME) and its application to trace elements and their speciation, Anal. Chim. Acta, 650/1 (2009) 23-32. DOI: 10.1016/j.aca.2009.04.002

M. de Almeida Bezerra, M.A. Zezzi Arruda, S.L. Costa Ferreira, Cloud Point Extraction as a Procedure of Separation and Pre-Concentration for Metal Determination Using Spectroianalytical Techniques: A Review, Appl. Spectrosc. Rev., 40/3 (2005) 269-299. DOI: 10.1080/05704920500230880.

Chapter 15: Liquid-liquid extraction in speciation analysis
Chapter 16: Solid-phase extraction in speciation analysis

Chapter 17: Mercury 'speciation analysis' by using thermal desorption analysis
Chapter 18: Multielement speciation analysis

Chapter 19: Chemical speciation modelling