Generation of Highly Charged Peptide and Protein Ions by Atmospheric Pressure Matrix-Assisted Infrared Laser Desorption/Ionization Ion Trap Mass Spectrometry

Direct sub-atmospheric pressure ionization mass spectrometry: Evaporation/sublimation-driven ionization is amazing, fundamentally, and practically

This paper covers direct sub-atmospheric pressure ionization mass spectrometry (MS). The discovery, applications, and mechanistic aspects of novel ionization processes for use in MS that are not based on the high-energy input from voltage, laser, and/or high temperature but on sublimation/evaporation within a region linking a higher to lower pressure and modulated by heat and collisions, are discussed, including how this new reality has guided a series of discoveries, instrument developments, and commercialization. A research focus, inter alia, is on how best to understand, improve, and use these novel ionization processes, which convert volatile and nonvolatile compounds from solids (sublimation) or liquids (evaporation) into gas-phase ions for analysis by MS providing reproducible, accurate, sensitive, and prompt results. Our perception on how these unprecedented versus traditional ionization processes/methods relate to each other, how they can be made to coexist on the same mass spectrometer, and an outlook on new and expanded applications (e.g., clinical, portable, fast, safe, and autonomous) is presented, and is based on ST's Opening lecture presentation at the Nordic Mass spectrometry Conference, Geilo, Norway, January 2023. Focus will be on matrix-assisted ionization (MAI) and solvent-assisted ionization (SAI) MS covering the period from 2010 to 2023; a potential paradigm shift in the making.

Surface-sampling mass spectrometry to study proteins and protein complexes

This review aims to summarise the current capabilities of surface mass spectrometry (MS) approaches that offer intact protein analysis, and that of non-covalent complexes. Protein analysis is largely achieved via matrix-assisted laser desorption/ionisation (MALDI), which is in itself a surface analysis approach or solvent-based electrospray ionisation (ESI). Several surface sampling approaches have been developed based on ESI, and those that have been used for intact protein analysis will be discussed below. The extent of protein coverage, top-down elucidation, and probing of protein structure for native proteins and non-covalent complexes will be discussed for each approach. Strategies for improving protein analysis, ranging from sample preparation, and sampling methods to instrument modifications and the inclusion of ion mobility separation in the workflow will also be discussed. The relative benefits and drawbacks of each approach will be summarised, providing an overview of current capabilities.

A nanoparticle co-matrix for multiple charging in matrix-assisted laser desorption ionization imaging of tissue

RATIONALE

A two-component matrix of 2-nitrophloroglucinol (2-NPG) and silica nanoparticles was used for matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging of high-charge-state biomolecules in tissue. Potential advantages include increased effective mass range and efficiency of fragmentation.

METHODS

A mixture of 2-NPG matrix and silica nanoparticles was applied to cyrosectioned 10 μm thick mouse brain tissue. The mixture was pipetted onto the tissue for profiling and sprayed for tissue imaging. MALDI images were obtained under high vacuum in a commercial time-of-flight mass spectrometer.

RESULTS

The combined 2-NPG and nanoparticle matrix produced highly charged ions from tissue with high-vacuum MALDI. Nanoparticles of 20, 70, 400, and 1000 nm in diameter were tested, the 20 nm particles producing the highest charge states. Images of mouse brain tissue obtained from highly charged ions show similar spatial localization.

CONCLUSIONS

The combined 2-NPG and nanoparticle matrix produces highly charged ions from tissue through a mechanism that may rely on the high surface area of the particles which can dry the tissue, and their ability to bind analyte molecules thereby assisting in crystal formation and production of multiply charged ions on laser irradiation.

Detection and fragmentation of doubly charged peptide ions in MALDI-Q-TOF-MS by ion mobility spectrometry for improved protein identification

Today, bottom-up protein identification in MALDI-MS is based on employing singly charged peptide ions, which are predominantly formed in the ionization process. However, peptide mass fingerprinting (PMF) with subsequent tandem MS confirmation using these peptide ions is often hampered due to the lower quality of fragment ion mass spectra caused by the higher collision energy necessary for fragmenting singly protonated peptides. Accordingly, peptide ions of higher charge states would be of high interest for analytical purposes, but they are usually not detected in MALDI-MS experiments as they overlap with singly charged matrix clusters and peptide ions. However, when utilizing ion mobility spectrometry (IMS), doubly charged peptide ions can be actively used by separating them from the singly protonated peptides, visualized, and selectively targeted for tandem MS experiments. The generated peptide fragment ion spectra can be used for a more confident protein identification using PMF with tandem MS confirmation, as most doubly protonated peptide ions yield fragment ion mass spectra of higher quality compared to tandem mass spectra of the corresponding singly protonated precursor ions. Mascot protein scores can be increased by approximately 50% when using tandem mass spectra of doubly charged peptide ions, with ion scores up to six times higher compared with ion scores of tandem mass spectra from singly charged precursors.

Enhanced Ion Transmission Efficiency up to m/ z 24 000 for MALDI Protein Imaging Mass Spectrometry

The molecular identification of species of interest is an important part of an imaging mass spectrometry (IMS) experiment. The high resolution accurate mass capabilities of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) have recently been shown to facilitate the identification of proteins in matrix-assisted laser desorption/ionization (MALDI) IMS. However, these experiments are typically limited to proteins giving rise to ions of relatively low m/ z due to difficulties transmitting and measuring large molecular weight ions of low charge states. Herein we have modified the source gas manifold of a commercial MALDI FT-ICR MS to regulate the gas flow and pressure to maximize the transmission of large m/ z protein ions through the ion funnel region of the instrument. By minimizing the contribution of off-axis gas disruption to ion focusing and maximizing the effective potential wall confining the ions through pressure optimization, the signal-to-noise ratios (S/N) of most protein species were improved by roughly 1 order of magnitude compared to normal source conditions. These modifications enabled the detection of protein standards up to m/ z 24 000 and the detection of proteins from tissue up to m/ z 22 000 with good S/N, roughly doubling the mass range for which high quality protein ion images from rat brain and kidney tissue could be produced. Due to the long time-domain transients (>4 s) required to isotopically resolve high m/ z proteins, we have used these data as part of an FT-ICR IMS-microscopy data-driven image fusion workflow to produce estimated protein images with both high mass and high spatial resolutions.

Remote Atmospheric Pressure Infrared Matrix-Assisted Laser Desorption-Ionization Mass Spectrometry (Remote IR-MALDI MS) of Proteins

Remote Infrared Matrix-Assisted Laser Desorption/Ionization (Remote IR-MALDI) system using tissue endogenous water as matrix was shown to enable in vivo real-time mass spectrometry analysis with minimal invasiveness. Initially the system was used to detect metabolites and lipids. Here, we demonstrate its capability to detect and analyze peptides and proteins. Very interestingly, the corresponding mass spectra show ESI-like charge state distribution, opening many applications for structural elucidation to be performed in real-time by Top-Down strategy. The charge states show no dependence toward laser wavelength or length of the transfer tube. Indeed, remote analysis can be performed 5 m away from the mass spectrometer without modification of spectra. On the contrary, addition of glycerol to water shift the charge state distributions toward even higher charge states. The desorption/ionization process is very soft, allowing to maintain protein conformation as in ESI. Observation of proteins and similar spectral features on tissue, when protein standards are deposited on raw tissue pieces, could potentially open the way to their direct analysis from biological samples. This also brings interesting features that could contribute to the understanding of IR MALDI ionization mechanism.

Soft Picosecond Infrared Laser Extraction of Highly Charged Proteins and Peptides from Bulk Liquid Water for Mass Spectrometry

We report the soft laser extraction and production of highly charged peptide and protein ions for mass spectrometry directly from bulk liquid water at atmospheric pressure and room temperature, using picosecond infrared laser ablation. Stable ion signal from singly charged small molecules, as well as highly charged biomolecular ions, from aqueous solutions at low laser pulse fluence (∼0.3 J cm^-2) is demonstrated. Sampling via single picosecond laser pulses is shown to extract less than 27 pL of volume from the sample, producing highly charged peptide and protein ions for mass spectrometry detection. The ablation and ion generation is demonstrated to be soft in nature, producing natively folded proteins ions under sample conditions described for native mass spectrometry. The method provides laser-based sampling flexibility, precision and control with highly charged ion production directly from water at low and near neutral pH. This approach does not require an additional ionization device or high voltage applied directly to the sample.

The composition of liquid atmospheric pressure matrix-assisted laser desorption/ionization matrices and its effect on ionization in mass spectrometry

New liquid atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) matrices that produce predominantly multiply charged ions have been developed and evaluated with respect to their performance for peptide and protein analysis by mass spectrometry (MS). Both the chromophore and the viscous support liquid in these matrices were optimized for highest MS signal intensity, S/N values and maximum charge state. The best performance in both protein and peptide analysis was achieved employing light diols as matrix support liquids (e.g. ethylene glycol and propylene glycol). Investigating the influence of the chromophore, it was found that 2,5-dihydroxybenzoic acid resulted in a higher analyte ion signal intensity for the analysis of small peptides; however, larger molecules (>17 kDa) were undetectable. For larger molecules, a sample preparation based on α-cyano-4-hydroxycinnammic acid as the chromophore was developed and multiply protonated analytes with charge states of more than 50 were detected. Thus, for the first time it was possible to detect with MALDI MS proteins as large as ∼80 kDa with a high number of charge states, i.e. m/z values below 2000. Systematic investigations of various matrix support liquids have revealed a linear dependency between laser threshold energy and surface tension of the liquid MALDI sample.

Continuous flow reduced-pressure infrared laser desorption/ionization mass spectrometry

RATIONALE

Continuous flow ionization methods using infrared (IR) lasers have several favorable characteristics, including ionization without any additional matrices and tolerance to contaminants such as detergents and buffer salts. However, poor sensitivity due to low ion-transfer efficiency from the sample plate to the inlet capillary of the mass spectrometer under atmospheric pressure remains a serious problem.

METHODS

We developed a new continuous flow IR laser desorption/ionization (IR-LDI) method using a frit plate and wavelength-tunable mid-IR laser with an optical parametric oscillator. Continuous flow samples were directly injected into the ion source without any additional matrices. The ion source was covered with a decompression chamber, and could vary the pressure of the ion source from 21 to 101 kPa.

RESULTS

Reduction of the pressure of the IR-LDI source from 101 to 71 kPa increased the signal intensity for the [M + H]⁺ ion of angiotensin II by 1.8-fold. On the other hand, the ion signal intensity was reduced at pressures lower than 71 kPa. It became clear that reducing pressure was more effective when ionization occurred with lower laser pulse energy and lower ion source temperature. In addition, signal intensities for the [M + 2H]²⁺ and [M + 3H]³⁺ ions of insulin were also increased, by 1.4-fold and 1.1-fold, respectively, upon reduction of the pressure to 91 and 81 kPa.

CONCLUSIONS

Although many studies have described IR-LDI using a differential pumping mass spectrometer, the optimal pressure of the ion source has never been investigated. We found that a slight reduction in pressure enhances sensitivity. This knowledge may be applicable to a number of ambient ionization methods using IR lasers.

Investigation and optimization of parameters affecting the multiply charged ion yield in AP-MALDI MS

Liquid matrix-assisted laser desorption/ionization (MALDI) allows the generation of predominantly multiply charged ions in atmospheric pressure (AP) MALDI ion sources for mass spectrometry (MS) analysis. The charge state distribution of the generated ions and the efficiency of the ion source in generating such ions crucially depend on the desolvation regime of the MALDI plume after desorption in the AP-to-vacuum inlet. Both high temperature and a flow regime with increased residence time of the desorbed plume in the desolvation region promote the generation of multiply charged ions. Without such measures the application of an electric ion extraction field significantly increases the ion signal intensity of singly charged species while the detection of multiply charged species is less dependent on the extraction field. In general, optimization of high temperature application facilitates the predominant formation and detection of multiply charged compared to singly charged ion species. In this study an experimental set-up and optimization strategy is described for liquid AP-MALDI MS which improves the ionization efficiency of selected ion species up to 14 times. In combination with ion mobility separation, the method allows the detection of multiply charged peptide and protein ions for analyte solution concentrations as low as 2fmol/μL (0.5μL, i.e. 1fmol, deposited on the target) with very low sample consumption in the low nL-range.

Continuous flow atmospheric pressure laser desorption/ionization using a 6-7-µm-band mid-infrared tunable laser for biomolecular mass spectrometry

A continuous flow atmospheric pressure laser desorption/ionization technique using a porous stainless steel probe and a 6-7-µm-band mid-infrared tunable laser was developed. This ion source is capable of direct ionization from a continuous flow with a high temporal stability. The 6-7-µm wavelength region corresponds to the characteristic absorption bands of various molecular vibration modes, including O-H, C=O, CH3 and C-N bonds. Consequently, many organic compounds and solvents, including water, have characteristic absorption peaks in this region. This ion source requires no additional matrix, and utilizes water or acetonitrile as the solvent matrix at several absorption peak wavelengths (6.05 and 7.27 µm, respectively). The distribution of multiply-charged peptide ions is extremely sensitive to the temperature of the heated capillary, which is the inlet of the mass spectrometer. This ionization technique has potential for the interface of liquid chromatography/mass spectrometry (LC/MS).

Recent methodological advances in MALDI mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is widely used for characterization of large, thermally labile biomolecules. Advantages of this analytical technique are high sensitivity, robustness, high-throughput capacity, and applicability to a wide range of compound classes. For some years, MALDI-MS has also been increasingly used for mass spectrometric imaging as well as in other areas of clinical research. Recently, several new concepts have been presented that have the potential to further advance the performance characteristics of MALDI. Among these innovations are novel matrices with low proton affinities for particularly efficient protonation of analyte molecules, use of wavelength-tunable lasers to achieve optimum excitation conditions, and use of liquid matrices for improved quantification. Instrumental modifications have also made possible MALDI-MS imaging with cellular resolution as well as an efficient generation of multiply charged MALDI ions by use of heated vacuum interfaces. This article reviews these recent innovations and gives the author's personal outlook of possible future developments.

Ambient femtosecond laser vaporization and nanosecond laser desorption electrospray ionization mass spectrometry

Recent investigations of ambient laser-based transfer of molecules into the gas phase for subsequent mass spectral analysis have undergone a renaissance resulting from the separation of vaporization and ionization events. Here, we seek to provide a snapshot of recent femtosecond (fs) duration laser vaporization and nanosecond (ns) duration laser desorption electrospray ionization mass spectrometry experiments. The former employs pulse durations of <100 fs to enable matrix-free laser vaporization with little or no fragmentation. When coupled to electrospray ionization, femtosecond laser vaporization provides a universal, rapid mass spectral analysis method requiring no sample workup. Remarkably, laser pulses with intensities exceeding 10(13) W cm(-2) desorb intact macromolecules, such as proteins, and even preserve the condensed phase of folded or unfolded protein structures according to the mass spectral charge state distribution, as demonstrated for cytochrome c and lysozyme. Because of the ability to vaporize and ionize multiple components from complex mixtures for subsequent analysis, near perfect classification of explosive formulations, plant tissue phenotypes, and even the identity of the manufacturer of smokeless powders can be determined by multivariate statistics. We also review the more mature field of nanosecond laser desorption for ambient mass spectrometry, covering the wide range of systems analyzed, the need for resonant absorption, and the spatial imaging of complex systems like tissue samples.

Direct analysis of samples under ambient condition by high-voltage-assisted laser desorption ionization mass spectrometry in both positive and negative ion mode

RATIONALE

With the rapid development of ambient mass spectrometry, the hybrid laser-based ambient ionization methods which can generate multiply charged ions of large biomolecules and also characterize small molecules with good signal-to-noise in both positive and negative ion modes are of particular interest.

METHODS

An ambient ionization method termed high-voltage-assisted laser desorption ionization (HALDI) is developed, in which a 1064 nm laser is used to desorb various liquid samples from the sample target biased at a high potential without the need for an organic matrix. The pre-charged liquid samples are desorbed by the laser to form small charged droplets which may undergo an electrospray-like ionization process to produce multiply charged ions of large biomolecules.

RESULTS

Various samples including proteins, oligonucleotides (ODNs), drugs, whole milk and chicken eggs have been analyzed by HALDI-MS in both positive and negative ion mode with little or no sample preparation. In addition, HALDI can generate intense signals with better signal-to-noise in negative ion mode than laser desorption spay post-ionization (LDSPI) from the same samples, such as ODNs and some carboxylic-group-containing small drug molecules.

CONCLUSIONS

HALDI-MS can directly analyze a variety of liquid samples including proteins, ODNs, pharmaceuticals and biological fluids in both positive and negative ion mode without the use of an organic matrix. This technique may be further developed into a useful tool for rapid analysis in many different fields such as pharmaceutical, food, and biological sciences.

Analysis of noncovalent chitinase-chito-oligosaccharide complexes by infrared-matrix assisted laser desorption ionization and nanoelectrospray ionization mass spectrometry

Transferring noncovalently bound complexes from the condensed phase into the gas phase represents a challenging task due to weak intermolecular bonds that have to be maintained during the phase transition. Currently, electrospray ionization (ESI) is the standard mass spectrometric (MS) technique to analyze noncovalent complexes. Although infrared matrix-assisted laser desorption ionization (IR-MALDI)-MS also provides particular soft desorption/ionization conditions, this method has so far hardly been applied for the analysis of noncovalent complexes. In this study, we employed IR-MALDI orthogonal time-of-flight (o-TOF)-MS in combination with the liquid matrix glycerol to characterize the specific complex formation of chito-oligosaccharide (CHOS) ligands with two variants of Chitinase A (ChiA) from Serratia marcescens, the inactive E315Q mutant and the active W167A mutant, respectively. The IR-MALDI-o-TOF-MS results were compared to those obtained using nano-ESI-quadrupole (q)-TOF-MS and ultraviolet (UV)-MALDI-o-TOF-MS. Using IR-MALDI-o-TOF-MS, specific noncovalent complexes between ChiA and CHOS were detected with distributions between enzymes with bound oligosaccharides vs free enzymes that were essentially identical to those obtained by nano-ESI-q-TOF-MS. Chitinase-CHOS complexes were not detected when UV-MALDI was employed for desorption/ionization. The results show that IR-MALDI-MS can be a valuable tool for fast and simple screening of noncovalent enzyme-ligand interactions.

Laserspray ionization using an atmospheric solids analysis probe for sample introduction

A newly introduced high sensitivity laserspray (LSI) mass spectrometry (MS) method that uses laser ablation of a matrix/analyte mixture at atmospheric pressure (AP) to obtain multiply charged ions from nonvolatile as well as high-mass compounds is now implemented using a simple probe device. The probe used in the LSI approach was originally designed for sample introduction into an AP ionization source using the atmospheric solids analysis probe (ASAP) method. Multiply charged mass spectra of peptides and proteins in 2,5-dihydroxybenzoic acid matrix were readily obtained on two mass spectrometers from different manufacturers with sample introduction using melting point tubes. Here we demonstrate rapid analysis by placing four peptide and protein samples on a single melting point tube. Mass spectra were obtained at high-resolution and using ion mobility spectrometry/MS.

Covalent attachment and dissociative loss of sinapinic acid to/from cysteine-containing proteins from bacterial cell lysates analyzed by MALDI-TOF-TOF mass spectrometry

We report covalent attachment via a thiol ester linkage of 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid or SA) to cysteine-containing protein biomarkers from bacterial cell lysates of E. coli analyzed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry when using SA as the matrix. Evidence to support this conclusion is the appearance of additional peaks in the MS spectra when using SA, which are absent when using alpha-cyano-4-hydroxycinnamic acid (HCCA). The additional peaks appear at a mass-to-charge (m/z) approximately 208 greater to the m/z of a more abundant protein ion peak. Protein biomarkers were identified by tandem mass spectrometry (MS/MS) using a MALDI time-of-flight/time-of-flight (TOF-TOF) mass spectrometer and top-down proteomics. Three protein biomarkers, HdeA, HdeB, and homeobox or YbgS (each containing two cysteine residues) were identified as having reactivity to SA. Non-cysteine-containing protein biomarkers showed no evidence of reactivity to SA. MS ions and MS/MS fragment ions were consistent with covalent attachment of SA via a thiol ester linkage to the side-chain of cysteine residues. MS/MS of a protein biomarker ion with a covalently attached SA revealed fragment ion peaks suggesting dissociative loss SA. We propose dissociative loss of SA is facilitated by a pentacyclic transition-state followed by proton abstraction of the beta-hydrogen of the bound SA by a sulfur lone pair followed by dissociative loss of 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-ynal. The apparent reactivity of SA to cysteine/disulfide-containing proteins may complicate identification of such proteins, however the apparent differential reactivity of SA and HCCA toward cysteine/disulfide-containing proteins may be exploited for identification of unknown cysteine-containing proteins.

Atmospheric pressure infrared (10.6 microm) laser desorption electrospray ionization (IR-LDESI) coupled to a LTQ Fourier transform ion cyclotron resonance mass spectrometer

We present the implementation of a CO(2) laser emitting infrared laser irradiation at 10.6 microm onto the versatile atmospheric pressure ionization platform. Infrared laser desorption electrospray ionization (IR-LDESI) is demonstrated from liquid-state samples at atmospheric pressure with and without ESI postionization. Multiply charged proteins ranging in molecular mass from 8.6 to 17 kDa were detected from liquid-state samples without the addition of matrix.

Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry

The number and type of atmospheric pressure techniques suitable for sampling analytes from surfaces, forming ions from these analytes, and subsequently transporting these ions into vacuum for interrogation by MS have rapidly expanded over the last several years. Moreover, the literature in this area is complicated by an explosion in acronyms for these techniques, many of which provide no information relating to the chemical or physical processes involved. In this tutorial article, we sort this vast array of techniques into relatively few categories on the basis of the approaches used for surface sampling and ionization. For each technique, we explain, as best known, many of the underlying principles of operation, describe representative applications, and in some cases, discuss needed research or advancements and attempt to forecast their future analytical utility.

Comparison of the internal energy deposition of Venturi-assisted electrospray ionization and a Venturi-assisted array of micromachined ultrasonic electrosprays (AMUSE)

The internal energy deposition of a Venturi-assisted array of micromachined ultrasonic electrosprays (AMUSE), with and without the application of a DC charging potential, is compared with equivalent experiments for Venturi-assisted electrospray ionization (ESI) using the "survival yield" method on a series of para-substituted benzylpyridinium salts. Under conditions previously shown to provide maximum ion yields for standard compounds, the observed mean internal energies were nearly identical (1.93-2.01 eV). Operation of AMUSE without nitrogen flow to sustain the air amplifier focusing effect generated energetically colder ions with mean internal energies that were up to 39% lower than those for ESI. A balance between improved ion transfer, adequate desolvation, and favorable ion energetics was achieved by selection of optimum operational ranges for the parameters that most strongly influence the ion population: the air amplifier gas flow rate and API capillary temperature. Examination of the energy landscapes obtained for combinations of these parameters showed that a low internal energy region (<or=1.0 eV) was present at nitrogen flow rates between 2 and 4 L min(-1) and capillary temperatures up to 250 degrees C using ESI (9% of all parameter combinations tested). Using AMUSE, this region was present at nitrogen flow rates up to 2.5 L min(-1) and all capillary temperatures (13% of combinations tested). The signal-to-noise (S/N) ratio of the intact p-methylbenzylpyridinium ion obtained from a 5 microM mixture of thermometer compounds using AMUSE at the extremes of the studied temperature range was at least fivefold higher than that of ESI, demonstrating the potential of AMUSE ionization as a soft method for the characterization of labile species by mass spectrometry.

Development and characterization of an ionization technique for analysis of biological macromolecules: liquid matrix-assisted laser desorption electrospray ionization

We have developed an atmospheric pressure ionization technique called liquid matrix-assisted laser desorption electrospray ionization (liq-MALDESI) for the generation of multiply charged ions by laser desorption from liquid samples deposited onto a stainless steel sample target biased at a high potential. This variant of our previously reported MALDESI source does not utilize an ESI emitter to postionize neutrals. Conversely, we report desorption and ionization from a macroscopic charged droplet. We demonstrate high mass resolving power single-acquisition FT-ICR-MS analysis of peptides and proteins ranging from 1 to 8.6 kDa at atmospheric pressure. The liquid sample acts as a macroscopic charged droplet similar to those generated by electrospray ionization, whereby laser irradiation desorbs analyte from organic matrix containing charged droplets generating multiply charged ions. We have observed a singly charged radical cation of an electrochemically active species indicating oxidation occurs for analytes and therefore water; the latter would play a key role in the mechanism of ionization. Moreover, we demonstrate an increase in ion abundance and a concurrent decrease in surface tension with an increase in the applied potential.

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.