Since most of the elemental detection methods are based on atomic spectrometry or elemental mass spectrometry, they do not provide any species related information. Using these techniques for speciation analysis, species separation has to performed before by a selective separation technique. For simple binary speciation or preliminary fractionation, liquid-liquid extraction can serve such purpose.
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.
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Related EVISA Resources: Brief summaries
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 Further chapters on techniques and methodology for 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
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
last time modified: September 23, 2019