Matrix-Free Formation of Gas-Phase Biomolecular Ions by Soft Cluster-Induced Desorption

Soft Deposition of Organic Molecules Based on Cluster-Induced Desorption for the Investigation of On-Surface and Surface-Mediated Reactions

Desorption/ionization induced by neutral clusters (DINeC) was employed for the soft transfer of organic and biomolecules, such as porphyrins and peptides, from a bulk sample onto any substrate of choice. Qualitative analysis of the deposition technique was performed by means of mass spectrometry, demonstrating that the deposited molecules remained intact due to the soft nature of the transfer process. Deposition rates were studied quantitatively using a quartz crystal microbalance; layers of intact biomolecules ranging from the submonolayer regime up to a few monolayers in thickness were realized. Mixed layers of molecules were deposited when two different sources of molecules were employed. The samples which were prepared based on this soft deposition method were used for the investigation of reactions of the deposited molecules with either coadsorbates on the surface or the surface itself. Examples include adduct formation of peptides with alkali metals on SiO2, the oxidation of peptides exposed to oxygen, as well as the metallization of porphyrins in interaction with the substrate.

Quantitative and Qualitative Analyses of Mass Spectra of OEL Materials by Artificial Neural Network and Interface Evaluation: Results from a VAMAS Interlaboratory Study

Quantitative analysis of binary mixtures of tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) and tris(8-hydroxyquinolinato)aluminum (Alq3) by using an artificial neural network (ANN) system to mass spectra was attempted based on the results of a VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study (TW2 A31) to evaluate matrix-effect correction and to investigate interface determination. Monolayers of binary mixtures having different Ir(ppy)3 ratios (0, 0.25, 0.50, 0.75, and 1.00), and the multilayers containing these mixtures and pure samples were measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS) with different primary ion beams, OrbiSIMS (SIMS with both Orbitrap and ToF mass spectrometers), laser desorption ionization (LDI), desorption/ionization induced by neutral clusters (DINeC), and X-ray photoelectron spectroscopy (XPS). The mass spectra were analyzed using a simple ANN with one hidden layer. The Ir(ppy)3 ratios of the unknown samples and the interfaces of the multilayers were predicted using the simple ANN system, even though the mass spectra of binary mixtures exhibited matrix effects. The Ir(ppy)3 ratios at the interfaces indicated by the simple ANN were consistent with the XPS results and the ToF-SIMS depth profiles. The simple ANN system not only provided quantitative information on unknown samples, but also indicated important mass peaks related to each molecule in the samples without a priori information. The important mass peaks indicated by the simple ANN depended on the ionization process. The simple ANN results of the spectra sets obtained by a softer ionization method, such as LDI and DINeC, suggested large ions such as trimers. From the first step of the investigation to build an ANN model for evaluating mixture samples influenced by matrix effects, it was indicated that the simple ANN method is useful for obtaining candidate mass peaks for identification and for assuming mixture conditions that are helpful for further analysis.

Bond-specific fragmentation of oligopeptides via electronic stopping of swift heavy ions in molecular films

Highly bond-specific fragmentation of oligopeptides induced by swift heavy ion (SHI) irradiation was investigated by means of mass spectrometry. In pronounced contrast to measurements of samples irradiated with keV ions, oligopeptides which were exposed to 946 MeV Au ions show a high abundance of specific fragments. The highly bond-specific nature of SHI-induced fragmentation is attributed to electronic stopping as the most relevant energy loss mechanism for SHI in the oligopeptide samples in combination with the subsequent coupling between the excited electronic and the atomic subsystem. Fragmentation induced by SHI is observed to be further influenced by the structure of the oligopeptides, suggesting that electronic excitation and/or the electronic-vibrational coupling depend on the details of the molecular structure.

Surface Properties of Ionic Liquids: A Mass Spectrometric View Based on Soft Cluster-Induced Desorption

Desorption/ionization induced by neutral clusters (DINeC) in combination with mass spectrometry (MS) was used for the investigation of the molecular composition of the surface of ionic liquids (IL). Based on the surface sensitivity of DINeC-MS, accumulation of either cations or anions was discriminated on the surface of bulk IL depending on the molecular structure of the IL components. In particular, cations with long alkyl chains aggregate on the surface, but this tendency is more reduced the larger the respective anion is; in the case of larger anions and smaller cations, it can be even reversed. For thin layers of IL, the ratio between cations and anions as detected in the mass spectra was found to be further influenced by the substrate surface.

Cluster-Induced Desorption/Ionization of Polystyrene: Desorption Mechanism and Effect of Polymer Chain Length on Desorption Probability

Soft cluster-induced desorption/ionization of polystyrene oligomers was investigated with respect to application in mass spectrometry. Clear peak progressions corresponding to intact polystyrene molecules were observed in the mass spectra, and no fragmentation was detected; efficient desorption was deduced from quartz crystal microbalance measurements. Molecular dynamics (MD) simulations of the process revealed that even in the case of the nonpolar polystyrene molecules cluster-induced desorption proceeds via dissolvation in the polar clusters. Experimentally, a significantly lower desorption efficiency was observed for polystyrene molecules with larger chain length. Taking into account MD simulations and further experiments with mixed samples consisting of long- and short-chain polystyrene oligomers, the reduced desorption efficiency for longer chain polystyrene molecules was attributed to a stronger entanglement of the larger polystyrene molecules.

Cluster-induced desorption/ionization mass spectrometry of highlighter ink: unambiguous identification of dyes and degradation processes based on fragmentation-free desorption

Highlighter inks were analyzed by means of soft Desorption/Ionization induced by Neutral SO₂ clusters (DINeC) in combination with mass spectrometry (MS). The dye molecules of the different inks were directly desorbed from dots of ink drawn on arbitrary substrates. Fragmentation free spectra were observed and the dyes used in the dye mixtures of the different highlighter inks were unambiguously identified. The soft nature of cluster-induced desorption was used to investigate the decomposition of the dye molecules induced by either heat or UV-light. The two processes lead to different decomposition products which are clearly distinguished in the DINeC spectra. The two different degradation processes can thus be discriminated using DINeC-MS.

Development of a focused high-energy macromolecular ion beam

In this work, we report the development of a focused macromolecular ion beam with kinetic energy of up to 110 keV. The system consists of a quadrupole ion trap (QIT), einzel lens and linear accelerator (LINAC). Based on the combination of matrix-assisted laser desorption ionization (MALDI) and quadrupole ion trapping (QIT), ions were desorbed from the surface and trapped with an ion trap to form biomolecular ion packets. Positive- and negative-pulsed voltages were applied on each end-cap electrode of the QIT to extract the ion packets and form an ion beam that was subsequently focused via an einzel lens and accelerated by stepwise pulsed voltages. The tabletop instrument was designed and successfully demonstrated via measurements of molecular ions of insulin, cytochrome c and bovine serum albumin (BSA) with mass-to-charge ratios (m/z) ranging from ∼5.8 to 66.5 k. This is the first report of both a focused and high-kinetic-energy protein ion beam. In addition, both secondary ions and electrons were observed from the surface by hypervelocity ion beam bombardment. This focused macromolecular ion beam has demonstrated its potential in the study of interactions between large molecular ions with other molecules either in the gas phase or upon a surface.

Large Molecular Cluster Formation from Liquid Materials and Its Application to ToF-SIMS

Since molecular cluster ion beams are expected to have various chemical effects, they are promising candidates for improving the secondary ion yield of Tof-SIMS. However, in order to clarify the effect and its mechanism, it is necessary to generate molecular cluster ion beams with various chemical properties and systematically examine it. In this study, we have established a method to stably form various molecular cluster ion beams from relatively small amounts of liquid materials for a long time by the bubbling method. Furthermore, we applied the cluster ion beams of water, methanol, methane, and benzene to the primary beam of SIMS and compared the molecular ion yields of aspartic acid. The effect of enhancing the yields of [M+H]+ ion of aspartic acid was found to be the largest for the water cluster and small for the methane and benzene clusters. These results indicate that the chemical effect contributes to the desorption/ionization process of organic molecules by the molecular cluster ion beam.

Direct Analysis of Ion-Induced Peptide Fragmentation in Secondary-Ion Mass Spectrometry

Primary-ion-induced fragmentation in organic molecules can strongly influence the results in secondary-ion mass spectrometry (SIMS) of organic and biomolecular samples. In order to characterize this ion-induced fragmentation, oligopeptide samples irradiated in SIMS experiments were investigated by means of desorption/ionization induced by neutral SO₂ clusters (DINeC). The latter is a nondestructive desorption method for mass spectrometry of biomolecules, which gives direct access to the fragments induced in the sample. Comparison of TOF-SIMS and DINeC mass spectra revealed qualitative differences between the fragments, which remain in the sample and the fragments sputtered during ion bombardment. The fragmentation strength and its spatial distribution were found to be quantitatively different for Bi₁⁺, Bi₃⁺, and Ar₁₀₀₀⁺ primary ions, leading to different distributions of the degree of fragmentation in the samples as directly measured by means of DINeC depth profiles.

Soft cluster-induced desorption/ionization mass spectrometry: How soft is soft?

Desorption/ionization induced by neutral clusters (DINeC) is used as an ultrasoft desorption/ionization method for the analysis of fragile biomolecules by means of mass spectrometry (MS). As a test molecule, the glycopeptide vancomycin was measured with DINeC-MS, and resulting mass spectra were compared to the results obtained with electrospray ionization (ESI), matrix assisted laser desorption ionization, and time-of-flight secondary ion MS. Of the desorption-based techniques, DINeC spectra show the lowest abundance of fragments comparable to ESI spectra. The soft desorption nature of DINeC was further demonstrated when applied to MS analysis of teicoplanin.

Combination of Thin-Layer Chromatography and Mass Spectrometry Using Cluster-Induced Desorption/Ionization

Desorption/ionization induced by neutral clusters (DINeC) was employed for mass spectrometry (MS) of oligopeptides and lipids after separation by means of thin-layer chromatography (TLC). Clear and fragmentation-free spectra were obtained from the TLC plates without any further sample treatment. Mass-resolved chromatograms were deduced when scanning the TLC plates with the cluster beam along the direction of solvent movement. Using vancomycin and noncovalently bound complexes, the soft nature of DINeC was demonstrated also when used in combination with TLC. As a test application, TLC and DINeC-MS were employed to separate and detect different phospholipids obtained from egg yolk.

Cluster-induced desorption investigated by means of molecular dynamics simulations-Microsolvation in clusters of polar and non-polar constituents

The desorption of surface-adsorbed molecules induced by cluster-surface collisions of neutral molecular clusters, in particular, SO₂ clusters, was investigated by means of molecular dynamics simulations. The desorption efficiency was found to be in general much higher for clusters of polar molecules when compared to nonpolar cluster constituents, for both nonpolar and polar adsorbates. In all cases, desorption is shown to proceed via dissolvation of the analyte in the cluster. In systems with nonpolar cluster constituents, the process is mainly driven by the increase in the entropy of the dissolved analyte in a larger cluster fragment. The latter process is enhanced by polar cluster constituents since the respective clusters show lower fragmentation at comparable kinetic energy and thus provide in average larger cluster fragments for the analytes to be dissolved in. In systems with clusters of polar constituents and polar adsorbates, the process is most efficient due to the additional energetic stabilization of the desorbed molecule in the solvation shell formed in the cluster fragment.

Electron microscopy of polyoxometalate ions on graphene by electrospray ion beam deposition

Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) has enabled atomically resolved imaging of molecules adsorbed on low-dimensional materials like carbon nanotubes, graphene oxide and few-layer-graphene. However, conventional methods for depositing molecules onto such supports lack selectivity and specificity. Here, we describe the chemically selective preparation and deposition of molecules-like polyoxometalate (POM) anions [PW₁₂O₄₀]^3- using electrospray ion-beam deposition (ES-IBD) along with high-resolution TEM imaging. This approach provides access to sub-monolayer coatings of intact molecules on freestanding graphene, which enables their atomically resolved ex situ characterization by low-voltage AC-HRTEM. The capability to tune the deposition parameters in either soft or reactive landing mode, combined with the well-defined high-vacuum deposition conditions, renders the ES-IBD based method advantageous over alternative methods such as drop-casting. Furthermore, it might be expanded towards depositing and imaging large and nonvolatile molecules with complex structures.

Chemical Analysis of Complex Surface-Adsorbed Molecules and Their Reactions by Means of Cluster-Induced Desorption/Ionization Mass Spectrometry

Desorption/ionization induced by neutral SO₂ clusters (DINeC) is used for mass spectrometry (MS) of surface-adsorbed molecules. The method is shown to be a surface-sensitive analysis tool capable of detecting molecular adsorbates in a wide range of molecular weights as well as their reactions on surfaces, which are otherwise difficult to access. Two different surface/adsorbate systems prepared by means of electrospray ion beam deposition (ES-IBD) were investigated: For the peptide angiotensin II on gold, intact molecules were desorbed from the surface when deposited by soft landing ES-IBD. By comparison to the well-controlled amount of substance deposited by ES-IBD, the sensitivity of DINeC-MS was shown to be on the order of 0.1% of a monolayer coverage, corresponding to femtomoles of analyte. Depending on deposition and sample conditions, the original state of charge of the molecules could be retrieved. Reaction of the adsorbed molecules both with surface atoms as well as with coadsorbed D₂O was monitored. Rhodamine 6G was also desorbed as an intact molecule when deposited with kinetic energies below 50 eV. For higher deposition energy, fragmentation of the dye molecules was observed by means of DINeC-MS.

Soft depth-profiling of mixed peptide/lipid samples by means of cluster induced desorption/ionization mass spectrometry-High depth resolution and low matrix effect

Mixed peptide/lipid samples were analyzed with respect to their chemical composition by means of desorption/ionization induced by neutral SO₂ clusters (DINeC) in combination with mass spectrometry (MS). Depth profiles of the mixed films indicated a segregation layer of lipid on top of all samples. The thickness of this layer as obtained by DINeC-MS was in the order of one nanometer what can be seen as an upper limit for the depth resolution of DINeC-MS. The relative amounts of the substance of peptide and lipid derived for the bulk material of mixed samples with different compositions were found to be close to the nominal values indicating a low matrix effect. Throughout the depth profiles, only intact molecular ions [M+H]⁺ as well as dimers of peptides and lipids were detectable, indicating the soft nature of DINeC even when used for depth profiling of biomolecular samples.

Real-Time Investigation of the H/D Exchange Kinetics of Porphyrins and Oligopeptides by Means of Neutral Cluster-Induced Desorption/Ionization Mass Spectrometry

The kinetics of the H/D exchange reaction in angiotensin II, hexaglycine (Gly₆), Co(II)tetra(3-carboxyphenyl)porphyrin, and tetra(4-carboxyphenyl)porphyrin were followed in real time by mass spectrometry employing desorption/ionization induced by neutral SO₂ clusters. The change of the isotope patterns with increasing degree of deuteration was recorded as a function of D₂O exposure and the underlying H/D exchange kinetics, i.e., the dependence of the different degrees of deuteration on time, were deduced. The results were modeled by means of Monte Carlo simulations taking into account different reaction constants for the H/D exchange reaction at different functional groups. In the case of the investigated porphyrins, the rate constants were directly assigned to the functional groups involved; in the case of the peptides, reaction at the explicit functional groups and the backbone chain of the molecules could be discriminated.

Time-of-flight secondary ion mass spectrometry using a new primary ion beam generated by vacuum electrospray of a protic ionic liquid, propylammonium nitrate

RATIONALE

Protic ionic liquids have the potential to be useful materials for primary ion beams in terms of protonation, since they have active protons. Selecting protic ionic liquids suitable for primary ion beams is of great importance to increase molecular secondary ion yields. Propylammonium nitrate ([C₃ H₇ NH₃ ][NO₃ ]) seems promising in view of its proton affinity. It is likely that [C₃ H₇ NH₃ ]⁺ cations can act as proton donors, and [NO₃ ]^- anions can work as proton acceptors.

METHODS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments have been performed to verify the usefulness of [C₃ H₇ NH₃ ][NO₃ ]. A primary propylammonium nitrate cluster ion beam was generated by vacuum electrospray, and then used to analyze amino acids (arginine, glutamic acid, aspartic acid), angiotensin II and polyethylene glycol. Positive and negative secondary ion mass spectra were obtained to study both protonation and deprotonation.

RESULTS

The propylammonium nitrate cluster ion beam successfully generated protonated molecules [M + H]⁺ of all the analytes in positive ion mode. The primary ion beam also generated deprotonated molecules [M - H]^- of glutamic acid, aspartic acid and angiotensin II in negative ion mode. Additionally, adduct ions related to [C₃ H₇ NH₃ ][NO₃ ] were detected in the case of arginine and polyethylene glycol.

CONCLUSIONS

The TOF-SIMS experiments confirmed that the propylammonium nitrate cluster ion beam was useful in generating molecular secondary ions, demonstrating that it is well suited for a primary ion beam in TOF-SIMS.

Influence of the cluster constituents' reactivity on the desorption/ionization process induced by neutral SO2 clusters

The influence of the chemical nature of the cluster constituents on the desorption/ionization process was investigated for desorption/ionization induced by neutral SO₂ clusters (DINeC). The polar clusters act as a transient matrix in which the desorbed analyte molecules are dissolved during the desorption process. For drop-cast samples, the desorption/ionization efficiency was found to be largely independent of the pH value of the initial solution the samples were prepared from; positive ions were almost always dominant and no multiply charged negative ions were observed. The results were traced back to the interaction of SO₂ with water present in the samples. Both H/D exchange experiments and surface charge measurements showed that SO₂ from the cluster beam interacts with water on and in the sample forming sulfurous acid. The latter then acts as an efficient proton supply leading to an enhanced ionization efficiency. The results demonstrate the possibility to control the ionization efficiency when using reactive cluster constituents in desorption-based ionization methods such as DINeC and cluster-based secondary ion mass spectrometry.

Two-Dimensional Folding of Polypeptides into Molecular Nanostructures at Surfaces

Herein we report the fabrication of molecular nanostructures on surfaces via two-dimensional (2D) folding of the nine amino acid peptide bradykinin. Soft-landing electrospray ion beam deposition in conjunction with high-resolution imaging by scanning tunneling microscopy is used to fabricate and investigate the molecular nanostructures. Subnanometer resolved images evidence the large conformational freedom of the molecules if thermal motion is inhibited and the formation of stable uniform dimers of only one specific conformation when diffusion can take place. Molecular dynamics modeling supported by density functional theory calculations give atomically precise insight into the induced-fit binding scheme when the folded dimer is formed. In the absence of solvent, we find a hierarchy of binding strength from polar to nonpolar, manifested in an inverted polar-nonpolar segregation which suppresses unspecific interactions at the rim of the nanostructure. The demonstrated 2D-folding scheme resembles many key properties of its native 3D counterpart and shows that functional, molecular nanostructures on surfaces fabricated by folding could be just as versatile and specific.

Mass spectrometry of oligopeptides in the presence of large amounts of alkali halides using desorption/ionization induced by neutral cluster impact

Oligopeptides in the presence of large amounts of salt were desorbed and ionized using desorption/ionization induced by neutral clusters (DINeC) for further analysis by means of mass spectrometry (MS). Using oligopeptides in alkali halide solutions as a model system, DINeC was shown to yield clear and fragmentation free mass spectra of the biomolecules even from environments with a large excess of salt. The results were traced back to a phase separation between salt and biomolecules during sample preparation. The ratio between alkali metal complexes [M+A](+) and bare biomolecules [M+H](+) was controlled using different preparation schemes. DINeC was applied to the products of a tryptic digest of bovine serum albumin in the presence of sodium chloride; the results of a mass fingerprint analysis did not show a major difference for the spectra with and without salt in the original solution. The metal-ion/peptide interaction was further investigated by means of tandem-MS.

Mass Spectra and Yields of Intact Charged Biomolecules Ejected by Massive Cluster Impact for Bioimaging in a Time-of-Flight Secondary Ion Microscope

Impacts of massive, highly charged glycerol clusters (≳10(6) Da, ≳ ± 100 charges) have been used to eject intact charged molecules of peptides, lipids, and small proteins from pure solid samples, enabling imaging using these ion species in a time-of-flight secondary ion microscope with few-micrometer spatial resolution. Here, we report mass spectra and useful ion yields (ratio of intact charged molecules detected to molecules sputtered) for several molecular species-two peptides, bradykinin and angiotensin II; two lipids, phosphatidylcholine and sphingomyelin; Irganox 1010 (a detergent); insulin; and rhodamine B-and show that useful ion yields are high enough to enable bioimaging of peptides and lipids in biological samples with few-micrometer resolution and acceptable signals. For example, several hundred molecular ion counts should be detectable from a 3 × 3 μm(2) area of a pure lipid bilayer given appropriate instrumentation or tens of counts from a minor constituent of such a layer.

Desorption/ionization induced by neutral cluster impact as a soft and efficient ionization source for ion trap mass spectrometry of biomolecules

RATIONALE

Desorption and ionization induced by neutral clusters (DINeC) using SO2 as cluster constituents was previously shown to produce clear and fragmentation-free spectra with low background from samples prepared with standard oligopeptides. Here we demonstrate a more general applicability of this method based on examples from different classes of (bio-)molecules. In order to make better use of the ions generated during the millisecond cluster-pulse, the DINeC source was combined with an ion trap mass spectrometer.

METHODS

Desorption and ionization was induced by neutral SO2 clusters with a mean size of 10(3) to 10(4) molecules seeded in a pulsed He beam. The desorbed ions were accumulated in an ion trap over the whole pulse duration prior to mass spectrometric analysis. Samples were prepared by simply drop casting the respective aqueous solution of biomolecules on Si/SiO2 substrates.

RESULTS

Clear and fragmentation-free spectra of oligopeptides were detected in single pulse operation mode. The very soft nature of the desorption process was demonstrated for phosphopeptides. DINeC spectra from bovine serum albumin samples after tryptic digest led to a clear identification of the original sequence using mass fingerprinting analysis. MS(n) capability was illustrated with two types of rhodamine dyes.

CONCLUSIONS

Desorption and ionization induced by neutral clusters can efficiently be combined with ion trap mass spectrometry since the pulse width and repetition rate of a typical pulsed cluster beam correspond well to the discontinuous accumulation time as well as the spectral rate of the ion trap. Clear mass spectra were obtained with such a setup for a variety of biosamples demonstrating the wider applicability of the DINeC process.

Atomic-scale observation of multiconformational binding and energy level alignment of ruthenium-based photosensitizers on TiO2 anatase

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dye's energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.

Quantification of the ionization probability during desorption/ionization of oligopeptides induced by neutral cluster impact

RATIONALE

Desorption-and-ionization induced by neutral cluster impact is a soft and matrix-free method, which leads to the formation of free ions of oligopeptides and smaller proteins without fragmentation. As a prerequisite for its successful application in bioanalytics, especially with respect to sensitivity, the ionization efficiency, i.e., the ion-to-neutral ratio of the desorbing molecules, was determined.

METHODS

Neutral SO2 clusters of 10(3) to 10(4) molecules in size were seeded in a pulsed He beam and used to desorb and ionize oligopeptides by means of cluster surface impact. The samples were prepared by drop casting a well-defined amount of substance on the substrate surface; the desorbing ions were identified by means of time-of-flight mass spectrometry. Furthermore, the ion current leaving the surface was determined for positive ions, which predominate in the investigated oligopeptides.

RESULTS

For angiotensin II, bradykinin (1-7), and adrenocorticotropic hormone (34-39), the number of ions desorbed from the respective samples was compared with the amount of substance applied on the substrate. Assuming that all biomolecules were desorbed during the experiment, the ion-to-neutral ratio or ionization efficiency η was determined. For the tested molecules, values of η between 0.5% and 3% were observed; the substrate material and the total amount of substance applied were shown to have a minor effect on the results.

CONCLUSIONS

The ion-to-neutral ratio in desorption/ionization of oligopeptides induced by neutral cluster impact was determined to be of the order of 10(-3) to 10(-2). The soft and matrix-free nature of the method in combination with this value of η might be interesting for applications in bioanalytics.

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.