CsCl as an auxiliary reagent for the analysis of phosphatidylcholine mixtures by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS).

Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) is mainly used for protein and peptide analysis. However, there is growing evidence that also phospholipids like phosphatidylcholines (PC) can be easily analyzed by MALDI-TOF MS. In MALDI-TOF methodology, the sample is cationized by the addition of inorganic ions. This process is strongly dependent on the corresponding ion concentration. In biological samples various cations are present (mainly H+, Na+ and K+) and, therefore, a mixture of different adducts is formed. Since phospholipids exhibit a wide distribution of different fatty acid residues a considerable peak overlap may occur. This is a major problem since the peak assignment in a mixture will be often unclear. In this paper we demonstrate that this problem can be easily overcome by mixing the analyte with caesium chloride (CsCl). This yields naturally non-occurring Cs+ adducts that are apparent due to the large shift of the molecular mass. The proposed method facilitates the clear assignment of most peaks. Besides that, we will show that CsCl can also be used for the determination of the relative fatty acid composition of a given PC sample. For this purpose naturally occurring mixtures of PCs as well as organic extracts of human lipoproteins-that are mainly composed of PC and sphingomyeline-are used.