Lipids ionisation methods

ESI enables the production of molecular ions directly from samples in solution. It can be used for small as well as large biopolymers up to about 200,000 Da including peptides, proteins, carbohydrates, DNA fragments and lipids. Unlike MALDI, ESI is a continuous ionisation method and suitable for coupling with liquid separation methods like HPLC (chapter 2) or CE (chapter 3.3). 

Ylinen et al. [53] developed an ion-pair extraction procedure employing tetrabutylamonium (TBA) counter ions for determination of PFOA in plasma and urine in combination with gas chromatography (GC) and flame ionisation detection (FID). Later on, Hansen et al. [35] improved the sensitivity of the ion-pair extraction approach using methyl tertiary butyl ether (MTBE) and by the inclusion of a filtration step to remove solids from the extract making it amenable to liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) determination. Ion-pair extraction procedure has been the basis of several procedures for biota [49,54-58] and food samples [50,59,60]. However, this method has shown to have some limitations, such as (1) co-extraction of lipids and other matrix constituents and the absence of a clean-up step to overcome the effects of matrix compounds and (2) the wide variety of recoveries observed, typically ranging. 

Ashraf, J., Butterfield, D. A., Jamefelt, J., and Laine, R. A. (1980). Enhancement of the Yu and Ledeen gas—liquid chromatographic, method for sialic acid estimation Use of methane chemical ionisation mass fragmentography. J. Lipid Res. 21, 1137-1141. 

In the case of acidic glycolipids the relative proton affinity of chemicals can shift the balance for negative ionisation in favor of co-eluted compounds. Pre-analytical separation under acidic conditions serves also to reduce as much as possible the dispersion in the MS spectrum of the metabolite into multiple m/ z representing the various adducts of counterions Na, K, NH4, organic amines, ... which improves sensitivity of the test. Sulfatides are lost during the partition between the hexane and the methanol/water phase. The analysis of sulfatides involves the isolation of the glycosphingolipid fraction and the subsequent separation of sulfatides from neutral lipids by chromatography on DEAE-sephadex or DEAE-cellulose column (the variety of methods are referenced in the website CyberLipid (http //www.cyberlipid.org/). 

Novel methods for the direct mass spectral analysis of breath samples have been reported. Most of these detect volatile components but one has used extractive electrospray ionisation MS to directly detect non-volatile lipids, peptides and carbohydrates as well as volatile compounds in the breath of subjects. ... 

Kallio, H. and Currie, G. (1993a) Analysis of low erucic acid turnip rapeseed oil (Brassica campesteris) by negative ion chemical ionisation tandem mass spectrometry. A method giving information on the fatty acid composition in positions sn-2 and sn- /3 of triacylglycerols. Lipids, 28, 207-15. 

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