The solvent shells of cluster ions produced by direct electric field extraction from glycerol/water mixtures

Electric field extraction of gaseous negative ions directly from water/glycerol solutions by use of a track membrane technique was investigated. The distributions of numbers of solvent molecules in the extracted cluster ions for different compounds were obtained. It is shown that the extraction mechanism is a direct field-stimulated evaporation of cluster ions from liquid, with a subsequent loss of several solvent molecules in the vacuum. For relatively simple ions a good correspondence of results was obtained with a continuous medium model. It was found that the number of solvent molecules in a cluster shell, for more complicated ions such as amino acids, is significantly greater than that for halide ions or ions of simple organic acids. An increase in the number of solvent molecules in the case of amino acid negative ions is rationalized in terms of the existence of several charged groups, each of which gives an additional contribution to the cluster shell.

Low-mass ions observed in plasma desorption mass spectrometry of high explosives

The low-mass ions observed in both positive and negative plasma desorption mass spectrometry (PDMS) of the high explosives HMX, RDX, CL-20, NC, PETN and TNT are reported. Possible identities of the most abundant ions are suggested and their presence or absence in the different spectra is related to the properties of the explosives as matrices in PDMS. The detection of abundant NO+ and NO2- ions for HMX, RDX and CL-20, which are efficient matrices, indicates that explosive decomposition takes place in PDMS of these three substances and that a contribution from the corresponding chemical energy release is possible. The observation of abundant C2H4N+ and CH2N+ ions, which have high protonation properties, might also explain the higher protein charge states observed with these matrices. Also, the observation of NO2-, possibly formed by electron scavenging which increases the survival probability of positively charged protein molecular ions, completes the pattern. TNT does not give any of these ions and it is thereby possible to explain why it does not work as a PDMS matrix. For NC and PETN, decomposition does not seem to be as pronounced as for HMX, RDX and CL-20, and also no particularly abundant ions with high protonation properties are observed. The fact that NC works well as a matrix might be related to other properties of this compound, such as its high adsorption ability.

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 investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were 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-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation 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 ions. Copyright 1999 John Wiley & Sons, Ltd.

Insight into absorption of radiation/energy transfer in infrared matrix-assisted laser desorption/ionization: the roles of matrices, water and metal substrates

Although the ionization/desorption mechanisms in matrix-assisted laser desorption/ionization (MALDI) remain poorly understood, there is a clear difference between the energy absorption processes in the ultraviolet (UV) and infrared (IR) modes of operation. UV-MALDI demands an on-resonance electronic transition in the matrix compound, whereas results presented here support earlier work showing that a corresponding resonant vibrational transition is not a requirement for IR-MALDI. In fact, data from the present study suggest that significant absorption of radiant energy by a potential matrix impairs its performance, although this observation is at variance with some other reports. For example, sinapinic acid, with an IR absorption maximum close to the 2.94 micrometer wavelength of the Er-YAG laser, has been little used as an IR-MALDI matrix. By contrast, succinic acid, with much lower IR absorption and no history of use in UV-MALDI as it has no UV absorption at the wavelength of common UV lasers, has become widely recognized as a good general purpose matrix for IR-MALDI. Despite reports by others that glycerol is an effective matrix for IR-MALDI, we find that glycerol, which also absorbs strongly at 2.94 micrometer, is useful only if applied as a very thin film. Thus the cumulative evidence for the role of the matrix in IR-MALDI appears confusing and often contradictory. Water has been postulated to be a major contributor to the absorption of energy in IR-MALDI. Consistent with this, we find that samples dried from D(2)O, which does not absorb at 2.94 micrometer, give spectra of inferior quality compared with the same samples from H(2)O. Similarly, samples dried under vacuum, that probably contain less water than those dried in the open laboratory, give weaker and more erratic spectra. Another potential participant in energy absorption and energy transfer is the surface of the metal support, an alternative mechanism for IR-MALDI, for which some evidence is presented here.

Enhancement of the molecular ion yield in plasma desorption mass spectrometry using explosive matrices

The working hypothesis of this study was that the chemical energy of matrix material may be released, although only on a microscale, under MeV ion bombardment and may assist ejection of large intact bioorganic molecules. To test the hypothesis, the performance of several common explosives, as matrices in plasma desorption mass spectrometry, was compared to the standard matrix, nitrocellulose (NC), which is also a high explosive. Two explosives, RDX and HMX, were found to be new, effective matrices for peptides and proteins. While the performance of RDX was comparable with that of nitrocellulose, HMX gave a superior molecular ion yield and a higher average charge state of desorbed molecular ions compared with NC. Noth RDX and HMX have a similar chemical composition and structure, although the latter is a more powerful explosive. The measured total ion yield allows the conclusion that the increase in the amount of ejected material, due to the chemical energy release in high explosives under MeV ion bombardment, is limited, perhaps to a factor of 2 to 3. The fact that not all tested explosives gave molecular ions from peptides and proteins suggests that other factors, such as gas-phase chemistry, may play a significant role in molecular ion formation.