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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 as well as 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 resolubilization 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-fiber 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 relativel 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 fiber to hold the extractant phase. Besides mechanical stabilization, the membrane is also a barrier against particles and large molecules and therfore 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.

Related Reviews

Inmaculada de la Calle, Francisco Pena-Pereira, Isela Lavilla, Carlos Bendicho, Liquid-phase microextraction combined with graphite furnace atomic absorption spectrometry: A review, Anal. Chim. Acta, 936 (2016) 12-39. doi: 10.1016/j.aca.2016.06.046

Karina Kocot, Katarzyna Pytlakowska, Beata Zawisza, Rafal Sitko, How to detect metal species preconcentrated by microextraction techniques?, Trends Anal. Chem., 82 (2016) 412–424. doi: 10.1016/j.trac.2016.07.003

Bin Hu, Man He, Beibei Chen, Linbo Xia, Liquid phase micro extraction for the analysis of trace elements and their speciation, Spectrochim. Acta Part B, 86 (2013) 14–30.  doi: 10.1016/j.sab.2013.05.025 

M.S. El-Shahawi, H.M. Al-Saidi, Dispersive liquid-liquid microextraction for chemical speciation and determination of ultra-trace concentrations of metal ions, Trends Anal. Chem., 44 (2013) 12-24. doi: 10.1016/j.trac.2012.10.011 

 Related EVISA Resources: Brief summaries

About Speciation

   Speciation as a discipline in Analytical Chemistry – Definitions   
   Why should elemental speciation be done ?
   Why is elemental speciation analysis not done routinely ?
   Speciation analysis as a tool to enhance the quality of life
   Speciation and Toxicity

Research fields related to elemental speciation

   Chemical speciation analysis for the life sciences
   Chemical speciation analysis for nutrition and food science
   Trace element speciation analysis for environmental sciences
   Speciation analysis for the study of metallodrugs and their biomolecular interactions

Speciation Analysis - Striving for Quality

   Problems to be solved in the field of speciation analysis
   Error sources in speciation analysis - Overview
   Sample preservation for speciation analysis - General recommendations
   Species transformation during speciation analysis
   Certified Reference Materials for Chemical Speciation Analysis
   Standard methods for elemental speciation analysis

Further chapters on techniques and methodology for speciation analysis:

Chapter 1: Tools for elemental speciation
Chapter 2: ICP-MS - A versatile detection system for speciation analysis
Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis
Chapter 4: GC-ICP-MS- A very sensitive hyphenated system for speciation analysis
Chapter 5: CE-ICP-MS for speciation analysis
Chapter 6: ESI-MS: The tool for the identification of species
Chapter 7: Speciation Analysis - Striving for Quality
Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Chapter 9: Gas chromatography for the separation of elemental species
Chapter 10: Plasma source detection techniques for gas chromatography
Chapter 11: Fractionation as a first step towards speciation analysis
Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis
Chapter 13: Gel electrophoresis combined with laser ablation inductively coupled plasma mass spectrometry for speciation analysis
Chapter 14: Non-chromatographic separation techniques for speciation analysis
Chapter 15: Liquid-liquid extraction in speciation analysis
Chapter 16: Solid-phase extraction in speciation analysis

last time modified: December 14, 2016

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