Research progress in material preparation and application of magnetism-enhanced in-tube solid-phase microextraction

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Abstract
Selecting a suitable sample preparation method is a significant step prior to chromatographic separation and detection. Directly analyzing samples instrumentally is difficult owing to the complexity of the sample matrix and the trace concentration of analytes. Most sample preparation methods have disadvantages， including complicated operating procedures， the use of large amounts of organic solvent， and ease of analyte loss during multistep processes； consequently， they do not meet the high analytical sample detection requirements of modern industry. The development of simple， environmentally friendly， efficient， and rapid preparation methods is a continuing frontier research area in the analytical chemistry field. Among the many available sample preparation techniques， in-tube solid-phase microextraction （IT-SPME） is receiving extensive attention. IT-SPME enriches the target analytes by extracting them to the inner surface of the capillary tube， and has been applied to extract various analytes in the environmental and food fields. IT-SPME is advantageous because it consumes low amounts of organic solvent and capillaries are mechanical stable； consequently， IT-SPME is a promising sample preparation technique. Magnetic field has been introduced to the IT-SPME system to further improve extraction efficiency and selectivity， leading to the development of magnetism-enhanced in-tube solid-phase microextraction （ME-IT-SPME） as a new technology. ME-IT-SPME uses magnetic field to separate and enrich targets， with different magnetic-field strengths applied to the extraction column during adsorption and elution. Diamagnetic substances in a paramagnetic medium tend to concentrate in regions where the magnetic field is weak when an external magnetic field is applied. Target analytes are detected chromatographically following elution. Conditions are optimized and an analytical method is established and used to detect targets in actual samples， leading to improved extraction sensitivity and precision compared to those obtained using IT-SPME， including shorter analysis time and superior extraction efficiency. This paper reviews the applications of ME-IT-SPME technology in combination with various analytical instruments since its inception in 2012， and analyzes its analysis and detection advantages. Based on hydrophobic interactions， hydrogen bonding， π-π and polarity interactions， coordination， and other extraction mechanisms with analytes， ME-IT-SPME uses innovative functional extraction materials， including nanomaterials， monolithic materials， and magnetic hybrid materials， all of which have high surface areas and numerous adsorption sites. Capillary microextraction columns are prepared using open-tubular capillary， particle-filling capillary， or monolithic capillaries. Diverse analytes are detected when ME-IT-SPME is combined with chromatograph， including organic pesticide residues， heavy-metal ions， herbicides， preservatives， and drug molecules. ME-IT-SPME technology is widely used in the environmental-analysis， food-analysis， and biomedical fields. Future， ME-IT-SPME technological developments should include： （1） focus on the reusability and stability of the magnetic extraction material； （2） discovering new extraction materials that are highly enriching and selective in order to analyze a greater variety of targets； （3） further innovating ME-IT-SPME technology by combining it with other more-sensitive analytical methods and considering its use in other fields； （4） connecting different capillaries to simultaneously enrich a variety of analytes； （5） exploring how the higher magnetic field influences extraction efficiency by designing new magnetic-field-regulating devices with small thermal interference； （6） combining the technology with advanced portable analytical instruments to realize real-time target analysis in the field； （7） exploiting immuno-affinitive extraction tubes that can be used to highly efficiently and selectively extract biological macromolecular drugs.

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