Ionisation techniques liquid chromatography-mass spectrometry

Liquid chromatography-mass spectrometry (LC-MS) combines the separating power of LC with the uniquely powerful detection capabilities of MS. With this technique, it is now possible to separate, identify and quantify components in a mixture. As with GC-MS, there is a choice of mass spectrometric ionisation techniques and analysers available. LC-MS can also be taken a step further as LC-MS" (otherwise known as LC-tandem MS) where ions can be subjected to fragmentation so as to gain information on their structures. 

Leinonen, A., Kuuranne, T., Kostiainen, R. (2002) Liquid chromatography/mass spectrometry in anabolic steroid analysis—optimization and comparison of three ionization techniques electrospray ionisation, atmospheric pressure chemical ionization and atmospheric pressure photoionization. /. Mass Spectrom., 37, 693-698. 

Whilst these methods are informative for the characterisation of synthetic mixtures, the information gained and the nature of these techniques precludes their use in routine quantitative analysis of environmental samples, which requires methods amenable to the direct introduction of aqueous samples and in particular selective and sensitive detection. Conventionally, online separation techniques coupled to mass spectrometric detection are used for this, namely gas (GC) and liquid chromatography (LC). As a technique for agrochemical and environmental analyses, high performance liquid chromatography (HPLC) coupled to atmospheric pressure ionisation-mass spectrometry (API-MS) is extremely attractive, with the ability to analyse relatively polar compounds and provide detection to very low levels. 

White, S., Catterick, T., Fairman, B. and Webb, K. (1998) Speciation of organo-tin compounds using liquid chromatography atmospheric pressure ionisation mass spectrometry and liquid chromatography inductively coupled plasma mass spectrometry as complementary techniques./. Chromatogr. A, 794, 211-218. 

Rather limited use has been made of mass spectrometry in the study of organotin compounds,23-24 though MS linked to gas-liquid chromatography is now being used for the identification of organotin compounds, particularly in environmental studies. Most of the early work involved electron ionisation (El), but in recent years, other techniques such as chemical ionisation (Cl),25 fast atom bombardment (FAB),26, 27 field desorption,28 surface ionisation,29 and, particularly, electrospray (ES),30 31 have been used. 

Automated in-tube solid-phase microextraction (SPME) has recently been coupled with liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS), e. g. for the determination of drugs in urine [60, 62]. In-tube SPME is an extraction technique in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. The analyte is then desorbed with methanol and transferred to an analytical HPLC-column. 

Recent work includes extraction studies of aqueous extractants from acrylate coatings and adhesives using liquid chromatography LC)-mass spectrometry MS)/LC-MS-MS detection methods [21], extractant studies on elastomeric materials using MS techniques [22], and studies on rubbers using LC-atmospheric pressure chemical ionisation MS [23]. 

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