Sample-first preparation: A method for surface-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of cyclic oligosaccharides

A new sample preparation method for the analysis of cyclic oligosaccharides in surface-assisted laser desorption/ ionization mass spectrometry (SALDI-MS) is presented. We call this new technique sample first method, in which a sample is deposited first and then bare gold nanoparticles (AuNPs), which serve as the SALDI matrixes, are added to the top of the sample layer. The use of the sample first method offers significant advantages for improving shot-to-shot reproducibility, enhancing the ionization efficiency of the analyte, and reducing sample preparation time as compared to the dried-droplet method, wherein samples and bare AuNPs are mixed and dried together. The relative standard deviation (RSD) values of the signal intensity as calculated from 65 sample spots was 25% when the sample first methods were applied to the analysis of beta-cyclodextrin. The results were more homogeneous as compared to the outcome using dried-droplet preparation of AuNPs (RSD = 66%) and 2,5-dihydroxybenzoic acid (RSD = 209%). We also found out that the optimal concentration of AuNP for ionization efficiency is 7.4 nM (4.52 x 1012 particles/mL) while the lowest detectable concentration of cyclic oligosaccharides through this approach is 0.25 mu M. Except for the cyclic oligosaccharide, the proposed method was also applied to the analyses of other biological samples, including neutral carbohydrate and steroid, aminothiols, and peptides as well as proteins.