Analysis of acidic oligosaccharides and glycopeptides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

2,5-Dihydroxybenzoic acid (DHB) is the most commonly used matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI/TOF) of oligosaccharides. Because acidic, sialylated oligosaccharides are detected at only the low picomole level with DHB, alternative matrices were screened to identify a matrix with a lower limit of detection. Negative-ion spectra of pure mono-, di-, and trisialylated oligosaccharides were acquired with either 6-aza-2-thiothymine or 2',4',6'-trihydroxyacetophenone (THAP) in the linear mode. Detection limits of less than 50 fmol with signal-to-noise ratios of better than 5:1 were achieved with both matrices. THAP was the preferred matrix because it provided a lower limit of detection and gave less prompt fragmentation. Incorporation of ammonium citrate into the matrix, along with vacuum drying of the sample, was required in order to obtain maximum sensitivity with THAP. No evidence of competition for ionization was found when a mixture of mono-, di-, and trisialylated oligosaccharides was analyzed with THAP. These findings suggest that MALDI/TOF analysis may provide a rapid means to identify changes in carbohydrate composition in glycoproteins. In addition, THAP offered improved sensitivity for detection of acidic glycopeptides over alpha-cyano-4-hydroxycinnamic acid.