The signal-to-noise ratio as the measure for the quantification of lysophospholipids by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry

The subject of this report is the determination of lysophospholipids; lysop hosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylserine an d lysophosphatidic acid, by matrix-assisted laser desorption/ionisation tim e-of-flight mass spectrometry (MALDI-TOF MS). The mean signal-to-noise rati o (S/N) was used for the first time as a measure of lysophospholipid concen tration. Two different sample preparation procedures were applied, the 'sta ndard' procedure and the 'premix' in order to check to what extent these me thods influence the results of the lysophospholipid quantification. Results can be summarised as follows: (a) All classes of lysophospholipids can be easily and sensitively analysed by MALDI-TOF MS. The smallest detectable am ount of lysophospholipids was 0.09 pmol on the sample plate. That is about two orders of magnitude lower than the amount detectable by standard chroma tographic methods. (b) The mean S/N of all peaks detected in the positive i on mass spectra can be used as a measure of the lysophospholipid concentrat ion. Whereas the S/N for neutral lysophospholipids correlated with the appl ied concentrations only when the samples were analysed as 'premix', the sam ple preparation and application procedure did not influence the quantificat ion of acidic lysophospholipids. The standard deviations were not higher th an 10% of the mean value. (c) All spectra were additionally analysed in the presence of CsCl. The addition of caesium ions makes the peak identificati on unambiguous in phospholipid mixtures, but the Cs adducts of lysophosphol ipids do not properly reflect their concentration and, therefore, they were not useful for quantification. (d) The applicability of the method was dem onstrated on the organic extract of human neutrophils.