ANALYSIS OF ACIDIC OLIGOSACCHARIDES AND GLYCOPEPTIDES BY MATRIX-ASSISTED LASER-DESORPTION IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY

2,5-Dihydrorrybenzoic acid (DHB) is the most commonly used matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectr ometry (MALDI/TOF) of oligosaccharides. Because acidic, sialylated oli gosaccharides are detected at only the low picomole level with DHB, al ternative matrices were screened to identify a matrix with a lower lim it of detection, Negative-ion spectra of pure mono-, di-, and trisialy lated oligosaccharides were acquired with either 6-aza-2-thiothymine o r 2',4',6'-trihydroxyacetophenone (THAP) in the linear mode. Detection limits of less than 50 fmol with signal-to-noise ratios of better tha n 5:1 were achieved with both matrices. THAP was the preferred matrix because it provided a lower limit of detection and gave less prompt fr agmentation. Incorporation of ammonium citrate into the matrix, along with vacuum drying of the sample, was required in order to obtain maxi mum sensitivity with THAP, No evidence of competition for ionization w as found when a mixture of mono-, di-, and trisialylated oligosacchari des was analyzed with THAP, These findings suggest that MALDI/TOF anal ysis may provide a rapid means to identify changes in carbohydrate com position in glycoproteins, In addition, THAP offered improved sensitiv ity for detection of acidic glycopeptides over alpha-cyano-4-hydroxyci nnamic acid.