OPTIMIZATION OF SAMPLE DEPOSITION FOR PLASMA-DESORPTION MASS-SPECTROMETRY OF PEPTIDOGLYCAN MONOMERS

Three selected peptidoglycan monomers isolated from muramidase-digested Escherichia coli-sacculus were characterized by positive- and negative-ion Cf-252 plasma-desorption mass spectrometry (PDMS). Due to the high affinity of sugar moieties towards alkali cations multiple alkali attachment in PDMS can be a problem. Electrospray, nitrocellulose, nitrocellulose sandwich anthracene-9-carboxylic acid and 3-(3-pyridyl)acrylic acid deposition techniques were evaluated for the degree of (de)protonation and cationization of the molecular species by sodium and/or potassium. Electrosprayed and nitrocellulose-deposited samples mainly exhibited intense cationized molecular ion species whereas anthracene-9-carboxylic acid-deposited samples show a similar pattern but with much lower intensity. An extension of the nitrocellulose deposition technique, the so-called nitrocellulose sandwich sample preparation reduced the alkali attachment problem for these hydrophilic compounds. Application of the small molecule matrix 3-(3-pyridyl)acrylic acid yielded mass spectra with abundant [M + H]+ and [M - H]- ions in the positive- and in the negative-ion modes, respectively.