Enhancement of the quality of MALDI mass spectra of highly acidic Oligosaccharides by using a nafion-coated probe

Spectra of highly acidic oligosaccharides obtained by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) can be greatly enhanc ed in quality by coating the MALDI probe surface with a film consisting of a perfluorosulfonated ionomer (Nafion) prior to the addition of the sample- matrix mixture. For comparison, three mixtures containing highly acidic oli go- and polysaccharides derived from partial acidic hydrolysis of alginate, pectin, or carboxymethyl cellulose (CMC) were analyzed by employing probes with an uncoated gold surface or a surface coated with a Nafion or nitroce llulose film. The negative ion-mode MALDI spectra of the oligouronates (oli gomers containing mannuronic/guluronic and galacturonic acid residues) obta ined using uncoated or nitrocellulose-coated probes consisted of a series o f broad, multiple peaks. These multiple peaks were assigned to the molecula r ions of the nondissociated [M - H](-) and partially sodiated [M-nNa - H]( -), where n = 1, 2, or 3, oligomers. In contrast, the corresponding spectra obtained with Nafion-coated probes contained only a single series of sharp peaks originating from the molecular ions ([M - H](-)) of nondissociated o ligomers exhibiting chain lengths of as many as similar to 15 uronic acid r esidues. The Nation coating was apparently capable of removing the sodium c ounterions remaining in the deposit of the sample-matrix mixture on the pro be, thereby greatly enhancing the signal-to-noise ratios of the peaks in th e spectra. In a similar manner, higher quality spectra could also be obtain ed by using Nafion-coated probes for analysis of the oligouronates and CMC oligomers by positive ion-mode MALDI-MS.