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

"Magic" Ionization Mass Spectrometry

The systematic study of the temperature and pressure dependence of matrix-assisted ionization (MAI) led us to the discovery of the seemingly impossible, initially explained by some reviewers as either sleight of hand or the misinterpretation by an overzealous young scientist of results reported many years before and having little utility. The “magic” that we were attempting to report was that with matrix assistance, molecules, at least as large as bovine serum albumin (66 kDa), are lifted into the gas phase as multiply charged ions simply by exposure of the matrix:analyte sample to the vacuum of a mass spectrometer. Applied heat, a laser, or voltages are not necessary to achieve charge states and ion abundances only previously observed with electrospray ionization (ESI). The fundamentals of how solid phase volatile or nonvolatile compounds are converted to gas-phase ions without added energy currently involves speculation providing a great opportunity to rethink mechanistic understanding of ionization processes used in mass spectrometry. Improved understanding of the mechanism(s) of these processes and their connection to ESI and matrix-assisted laser desorption/ionization may provide opportunities to further develop new ionization strategies for traditional and yet unforeseen applications of mass spectrometry. This Critical Insights article covers developments leading to the discovery of a seemingly magic ionization process that is simple to use, fast, sensitive, robust, and can be directly applied to surface characterization using portable or high performance mass spectrometers.

Matrix assisted ionization in vacuum, a sensitive and widely applicable ionization method for mass spectrometry

An astonishingly simple new method to produce gas-phase ions of small molecules as well as proteins from the solid state under cold vacuum conditions is described. This matrix assisted ionization vacuum (MAIV) mass spectrometry (MS) method produces multiply charged ions similar to those that typify electrospray ionization (ESI) and uses sample preparation methods that are nearly identical to matrix-assisted laser desorption/ionization (MALDI). Unlike these established methods, MAIV does not require a laser or voltage for ionization, and unlike the recently introduced matrix assisted ionization inlet method, does not require added heat. MAIV-MS requires only introduction of a crystalline mixture of the analyte incorporated with a suitable small molecule matrix compound such as 3-nitrobenzonitrile directly to the vacuum of the mass spectrometer. Vacuum intermediate pressure MALDI sources and modified ESI sources successfully produce ions for analysis by MS with this method. As in ESI-MS, ion formation is continuous and, without a laser, little chemical background is observed. MAIV, operating from a surface offers the possibility of significantly improved sensitivity relative to atmospheric pressure ionization because ions are produced in the vacuum region of the mass spectrometer eliminating losses associated with ion transfer from atmospheric pressure to vacuum. Mechanistic aspects and potential applications for this new ionization method are discussed.

A mechanism for ionization of nonvolatile compounds in mass spectrometry: considerations from MALDI and inlet ionization

Mechanistic arguments relative to matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) address observations that predominately singly charged ions are detected. However, recently a matrix assisted laser ablation method, laserspray ionization (LSI), was introduced that can use the same sample preparation and laser as MALDI, but produce highly charged ions from proteins. In MALDI, ions are generated from neutral molecules by the photon energy provided to a matrix, while in LSI ions are produced inside a heated inlet tube linking atmospheric pressure and the first vacuum region of the mass spectrometer. Some LSI matrices also produce highly charged ions with MALDI ion sources operated at intermediate pressure or high vacuum. The operational similarity of LSI to MALDI, and the large difference in charge states observed by these methods, provides information of fundamental importance to proposed ionization mechanisms for LSI and MALDI. Here, we present data suggesting that the prompt and delayed ionization reported for vacuum MALDI are both fast processes relative to producing highly charged ions by LSI. The energy supplied to produce these charged clusters/droplets as well as their size and time available for desolvation are determining factors in the charge states of the ions observed. Further, charged droplets/clusters may be a common link for ionization of nonvolatile compounds by a variety of MS ionization methods, including MALDI and LSI.

Ion spectrometric detection technologies for ultra-traces of explosives: a review

In recent years, explosive materials have been widely employed for various military applications and civilian conflicts; their use for hostile purposes has increased considerably. The detection of different kind of explosive agents has become crucially important for protection of human lives, infrastructures, and properties. Moreover, both the environmental aspects such as the risk of soil and water contamination and health risks related to the release of explosive particles need to be taken into account. For these reasons, there is a growing need to develop analyzing methods which are faster and more sensitive for detecting explosives. The detection techniques of the explosive materials should ideally serve fast real-time analysis in high accuracy and resolution from a minimal quantity of explosive without involving complicated sample preparation. The performance of the in-field analysis of extremely hazardous material has to be user-friendly and safe for operators. The two closely related ion spectrometric methods used in explosive analyses include mass spectrometry (MS) and ion mobility spectrometry (IMS). The four requirements-speed, selectivity, sensitivity, and sampling-are fulfilled with both of these methods.

Effect of water treatment on analyte and matrix ion yields in matrix-assisted time-of-flight secondary ion mass spectrometry: the case of insulin in and on hydroxycinnamic acid

A systematic study was performed to identify the origin of surprisingly high analyte-to-matrix yield ratios recently observed in time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of oligo- and polypeptides mixed in matrices of alpha-cyano-4-hydroxycinnamic acid (4HCCA). Several sets of samples of porcine insulin in 4HCCA (1:3100 molar) were prepared from liquid solutions by a nebuliser technique, with more than one order of magnitude variation in sprayed material (substrate silicon). Following different periods of storage in air and/or vacuum as well as exposure to high-purity water, TOF-SIMS analysis was performed under oblique impact of 22 keV SF5+. Treatment with water involved either deposition of a droplet covering the whole sample for times between 1 and 20 min or spraying with water in droplet equivalent quantities. The analyte and matrix molecules were detected as protonated molecules (insulin also in doubly protonated form). Even the as-prepared samples usually showed insulin-to-4HCCA yield ratios exceeding the molar ratio of the mixed material. Upon ageing in vacuum the matrix ion yields remained constant but the analyte yields decreased, partly due to break-up of intrachain disulfide bonds. Water treatment resulted in a pronounced decrease in the 4HCCA yield, typically by a factor of five, in parallel with an increase of the insulin yield, by up to a factor of four. Evidence is provided that these changes occur concurrently with a partial dissolution of 4HCCA at the sample surface. The enhanced insulin yield was not correlated with the Na+ yield. The typically 20-fold increase in the insulin-to-4HCCA yield ratio, generated by water exposure of the samples, provides the explanation for the high yield ratios observed previously with water-treated samples. Spraying with water or repeated exposure to water droplets caused a pronounced degradation of the insulin parent yields in combination with an increasing appearance of signals due to the B- and A-chains of insulin. To clarify the issue of surface segregation, a few samples were prepared by spraying acetone-diluted solutions of insulin on previously deposited layers of 4HCCA. Whereas the insulin yields from as-prepared samples were rather low, the yields observed after water treatment were comparable with those observed with samples of insulin in 4HCCA. The results suggest that a large amount of insulin is present at the surface of samples prepared from liquid mixtures of insulin in 4HCCA. With both methods of sample preparation, however, high secondary ion yields of insulin were only obtained after exposure of the samples to water. The chemical changes responsible for this beneficial effect still need to be identified.

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.