Interaction between explosive and analyte layers in explosive matrix-assisted plasma desorption mass spectrometry

An HMX/insulin two-layer system was chosen as a model for further investiga tion of the matrix properties of explosive materials for protein analytes i n plasma desorption mass spectrometry, The dependencies of the molecular io n yield and average charge state as a function of the analyte thickness wer e studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assis ted laser desorption/ionization and closer to the value obtained in electro spray ionization under standard acidic conditions. Observed charge state di stributions are significantly narrower than the corresponding Poisson distr ibutions, which suggests that the protonation of insulin is limited in plas ma desorption by the number of basic sites in the molecule, similar to elec trospray ionization. Both the curve displaying total molecular ion yield an d the one showing the total charge (proton) yield as a function of the insu lin thickness have maxima at a thickness different from an insulin monolaye r, These observations diminish the significance of a matrix/ analyte interf ace mechanism for the explosive matrix assistance. Instead, a mechanism rel ated to the chemical energy release during conversion of the explosive afte r the ion impact is proposed. As additional mechanisms, enhanced protonatio n of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular io ns, Copyright (C) 1999 John Wiley & Sons, Ltd.