Investigations of electrospray sample deposition for polymer MALDI mass spectrometry

Quantitative MALDI-TOF Mass Spectrometry of Star-Shaped Polylactides Based on Chromatographic Hyphenation

The end groups of three- and four-arm star-shaped polylactides (PLA) with trimethylolpropane and pentaerythritol core structures were functionalized with acetic acid. Reaction products with different degrees of functionalization were analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Additional gradient elution liquid adsorption chromatography (GELAC) measurements were performed to determine the degree of functionalization. This technique enabled clear separation and sufficient quantification of the formed species. These chromatographic data could be used inversely to quantify mass spectrometric results, which are usually biased by the unknown ionization probabilities of different polymer end group structures. Our results showed that, in this particular case, the peak intensity in the MALDI-TOF mass spectra can be used to semiquantitatively determine the degree of functionalization in incompletely functionalized multiarm PLA.

Using solid-state nuclear magnetic resonance to rationalize best efficiency of 2,6-dihydroxybenzoic acid over other 2,X-dihydroxybenzoic acid isomers in solvent-free matrix-assisted laser desorption/ionization of poly(ethylene glycol)

RATIONALE

Among isomers of dihydroxybenzoic acid (DHB), 2,5-DHB is often the most efficient matrix in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for a great variety of compounds. Yet, when performing solvent-free MALDI, 2,6-DHB yields better results for poly(ethylene glycol [PEG]). This intriguing feature is explored here using solid-state nuclear magnetic resonance (NMR).

METHODS

Ternary mixtures were prepared by grinding 2,X-DHB (X = 3-6), poly(ethylene glycol) (M_n = 2000 g mol^-1 ) and lithium fluoride (LiF) in a matrix/analyte/salt molar ratio of 50/1/10 for 16 min under a controlled atmosphere. After mixing, a few grains were applied to the MALDI target for MS analysis, whereas the major part of the ground sample was transferred into rotors to perform ¹³ C, ⁷ Li, and ¹⁹ F NMR experiments.

RESULTS

Lithiated PEG chains are mainly formed with 2,6-DHB in solvent-free MALDI, but their abundance increases with 2,3-DHB and 2,4-DHB when water uptake is favored by a humid atmosphere. Solid-state NMR shows that grinding 2,6-DHB-based samples in atmospheric conditions leads to a solid phase in which the matrix, PEG, and salt molecules exhibit a high mobility compared with systems involving other 2,X-DHB isomers. This mobile environment would favor (as a solvent) LiF dissociation and best promote PEG cationization.

CONCLUSIONS

Complementary data in ¹³ C, ⁷ Li, and ¹⁹ F NMR spectra are consistent with the formation of a solid phase of high mobility composed of 2,6-DHB, PEG, and the two salt components that ultimately favor the production of lithiated PEG chains.

Recent developments in pre-treatment and analytical techniques for synthetic polymers by MALDI-TOF mass spectrometry

A great deal of effort has been expended to develop accurate means of determining the properties of synthetic polymers using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). Many studies have focused on the importance of sample pre-treatment to obtain accurate analysis results. This review discusses the history of synthetic polymer characterization and highlights several applications of MALDI-TOF MS that recognize the importance of pre-treatment technologies. The subject area is of significance in the field of analytical chemistry, especially for users of the MALDI technique. Since the 2000s, many such technologies have been developed that feature improved methods and conditions, including solvent-free systems. In addition, the recent diversification of matrix types and the development of carbon-based matrix materials are described herein together with the current status and future directions of MALDI-TOF MS hardware and software development. We provide a summary of processes used for obtaining the best analytical results with synthetic polymeric materials using MALDI-TOF MS.

The cationization of synthetic polymers in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Investigations of the salt-to-analyte ratio

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique that when used to analyze synthetic polymer analytes often requires the addition of a metal cationization agent (herein termed the "salt"). The choice of both the matrix and the cationization agent needs to be taken into account when considering the polymer under study; different polymers have shown different affinities toward different cationization agents, and their selectivity can change as the matrix changes. Salt-to-analyte ratio (S/A) plots are used in this work to investigate the effect of the quantity of cationization agent employed in the analysis of a poly (methylmethacrylate) (PMMA) analyte with different MALDI matrices. The point at which analyte signal stops increasing with the added cationization agent is termed the "cation saturation point," and it was found to occur around a S/A of 1. When the analyte signal after this point remains constant, it is termed an "ideal case." The "non-ideal case" occurs when the analyte signal decreases after the cation saturation point. The amount of matrix present (measured as the matrix-to-analyte molar ratio, M/A) and the use of different counterions for the salt are also found to affect the intensity of the analyte signal. In non-ideal cases, changes in the counterion or an increase in the M/A are found to increase the analyte signal, often converting an initially observed non-ideal case into an ideal case. Several experiments attempting to uncover the reason for observation of the non-ideal S/A behavior are also described.

Improvement of m/z accuracy for linear matrix-assisted laser desorption/ionization time-of-flight mass spectrometer

RATIONALE

Linear matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is widely used in analytical and biomedical applications. The use of delayed extraction increases the resolution, but the roughness of the matrix crystals and the misalignment of the target plate in the order of a few micrometers cause a substantial spread in the ion TOF values and a decrease in mass accuracy.

METHODS

The method of mass spectra correction based on the correlation of matrix fragment peaks in MALDI mass spectra was used. Experiments were performed using the MALDI-TOF instrument CMI-1600. SIMION 8.1 and MATLAB were used for ion motion simulations. Data analysis was done using the home-built custom-developed software and MATLAB.

RESULTS

It was shown that the peak position drift in the MALDI-TOF mass spectra depends linearly on the TOF in a wide mass range. While using the linear correction of the TOF scale, an increase in m/z accuracy of more than 10 times was achieved. The mass accuracy was limited by the resolution of the fast Analog-to-Digital Converter (ADC) used.

CONCLUSIONS

It is expected that the proposed method will significantly increase the dynamic range, since it becomes possible to sum up corrected individual mass spectra without a significant loss of resolution. Timescale adjusting can be used for both linear TOF instruments and reflector systems of various configurations.

Assessment of Reproducibility of Laser Electrospray Mass Spectrometry using Electrospray Deposition of Analyte

A nonresonant, femtosecond (fs) laser is employed to desorb samples of Victoria blue deposited on stainless steel or indium tin oxide (ITO) slides using either electrospray deposition (ESD) or dried droplet deposition. The use of ESD resulted in uniform films of Victoria blue whereas the dried droplet method resulted in the formation of a ring pattern of the dye. Laser electrospray mass spectrometry (LEMS) measurements of the ESD-prepared films on either substrate were similar and revealed lower average relative standard deviations for measurements within-film (20.9%) and between-films (8.7%) in comparison to dried droplet (75.5% and 40.2%, respectively). The mass spectral response for ESD samples on both substrates was linear (R² > 0.99), enabling quantitative measurements over the selected range of 7.0 × 10^-11 to 2.8 × 10^-9 mol, as opposed to the dried droplet samples where quantitation was not possible (R² = 0.56). The limit of detection was measured to be 210 fmol. Graphical Abstract ᅟ.

Independent assessment of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) sample preparation quality: A novel statistical approach for quality scoring

Preparation of samples according to an optimized method is crucial for accurate determination of polymer sample characteristics by Matrix-Assisted Laser Desorption Ionization (MALDI) analysis. Sample preparation conditions such as matrix choice, cationization agent, deposition technique or even the deposition volume should be chosen to suit the sample of interest. Many sample preparation protocols have been developed and employed, yet finding the optimal sample preparation protocol remains a challenge. Because an objective comparison between the results of diverse protocols is not possible, "gut-feeling" or "good enough" is often decisive in the search for an optimum. This implies that sub-optimal protocols are used, leading to a loss of mass spectral information quality. To address this problem a novel analytical strategy based on MALDI imaging and statistical data processing was developed in which eight parameters were formulated to objectively quantify the quality of sample deposition and optimal MALDI matrix composition and finally sum up to an overall quality score of the sample deposition. These parameters can be established in a fully automated way using commercially available mass spectrometry imaging instruments without any hardware adjustments. With the newly developed analytical strategy the highest quality MALDI spots were selected, resulting in more reproducible and more valuable spectra for PEG in a variety of matrices. Moreover, our method enables an objective comparison of sample preparation protocols for any analyte and opens up new fields of investigation by presenting MALDI performance data in a clear and concise way.

Advances in coupling microfluidic chips to mass spectrometry

Microfluidic technology has shown advantages of low sample consumption, reduced analysis time, high throughput, and potential for integration and automation. Coupling microfluidic chips to mass spectrometry (Chip-MS) can greatly improve the overall analytical performance of MS-based approaches and expand their potential applications. In this article, we review the advances of Chip-MS in the past decade, covering innovations in microchip fabrication, microchips coupled to electrospray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-MS. Development of integrated microfluidic systems for automated MS analysis will be further documented, as well as recent applications of Chip-MS in proteomics, metabolomics, cell analysis, and clinical diagnosis.

Matrix normalized MALDI-TOF quantification of a fluorotelomer-based acrylate polymer

The degradation of fluorotelomer-based acrylate polymers (FTACPs) has been hypothesized to serve as a source of the environmental contaminants, perfluoroalkyl carboxylates (PFCAs). Studies have relied on indirect measurement of presumed degradation products to evaluate the environmental fate of FTACPs; however, this approach leaves a degree of uncertainty. The present study describes the development of a quantitative matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry method as the first direct analysis method for FTACPs. The model FTACP used in this study was poly(8:2 FTAC-co-HDA), a copolymer of 8:2 fluorotelomer acrylate (8:2 FTAC) and hexadecyl acrylate (HDA). Instead of relying on an internal standard polymer, the intensities of 40 poly(8:2 FTAC-co-HDA) signals (911-4612 Da) were normalized to the signal intensity of a matrix-sodium cluster (659 Da). We termed this value the normalized polymer response (P(N)). By using the same dithranol solution for the sample preparation of poly(8:2 FTAC-co-HDA) standards, calibration curves with coefficient of determinations (R(2)) typically >0.98 were produced. When poly(8:2 FTAC-co-HDA) samples were prepared with the same dithranol solution as the poly(8:2 FTAC-co-HDA) standards, quantification to within 25% of the theoretical concentration was achieved. This approach minimized the sample-to-sample variability that typically plagues MALDI-TOF, and is the first method developed to directly quantify FTACPs.

Analysis of monoclonal antibody by a novel CE-UV/MALDI-MS interface

mAbs are highly complex proteins that present a wide range of microheterogeneity that requires multiple analytical methods for full structure assessment and quality control. As a consequence, the characterization of mAbs on different levels is particularly product- and time-consuming. CE-MS couplings, especially to MALDI, appear really attractive methods for the characterization of biological samples. In this work, we report the last instrumental development and performance of the first totally automated off-line CE-UV/MALDI-MS/MS. This interface is based on the removal of the original UV cell of the CE apparatus, modification of the spotting device geometry, and creation of an integrated delivery matrix system. The performance of the method was evaluated with separation of five intact proteins and a tryptic digest mixture of nine proteins. Intact protein application shows the acquisition of electropherograms with high resolution and high repeatability. In the peptide mapping approach, a total number of 154 unique identified peptides were characterized using MS/MS spectra corresponding to average sequence coverage of 64.1%. Comparison with NanoLC/MALDI-MS/MS showed complementarity at the peptide level with an increase of 42% when using CE/MALDI-MS coupling. Finally, this work represents the first analysis of intact mAb charge variants by CZE using an MS detection. Moreover, using a peptide mapping approach CE-UV/MALDI-MS/MS fragmentation allowed 100% sequence coverage of the light chain and 92% of the heavy chain, and the separation of four major glycosylated peptides and their structural characterization.

Quantification of saccharides in honey samples through surface-assisted laser desorption/ionization mass spectrometry using HgTe nanostructures

Quantification of monosaccharides and disaccharides in five honey samples through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using HgTe nanostructures as the matrix and sucralose as an internal standard has been demonstrated. Under optimal conditions (1× HgTe nanostructure, 0.2 mM ammonium citrate at pH 9.0), the SALDI-MS approach allows detection of fructose and maltose at the concentrations down to 15 and 10 μM, respectively. Without conducting tedious sample pretreatment and separation, the SALDI-MS approach allows determination of the contents of monosaccharides and disaccharides in honey samples within 30 min, with reproducibility (relative standard deviation <15%). Unlike only sodium adducts of standard saccharides detected, sodium adducts and potassium adducts with differential amounts have been found among various samples, showing different amounts of sodium and potassium ions in the honey samples. The SALDI-MS data reveal that the contents of monosaccharides and disaccharides in various honey samples are dependent on their nectar sources. In addition to the abundant amounts of monosaccharides and disaccharides, oligosaccharides in m/z range of 650 - 2700 are only detected in pomelo honey. Having advantages of simplicity, rapidity, and reproducibility, this SALDI-MS holds great potential for the analysis of honey samples.

Evaluating Atmospheric pressure Solids Analysis Probe (ASAP) mass spectrometry for the analysis of low molecular weight synthetic polymers

Atmospheric pressure Solids Analysis Probe (ASAP) mass spectrometry has facilitated the ionisation of oligomers from low molecular weight synthetic polymers, poly(ethylene glycol) (PEG: M(n) = 1430) and poly(styrene) (PS: M(n) = 1770), directly from solids, providing a fast and efficient method of identification. Ion source conditions were evaluated and it was found that the key instrument parameter was the ion source desolvation temperature which, when set to 600 °C was sufficient to vapourise the heavier oligomers for ionisation. PS, a non-polar polymer that is very challenging to analyse by MALDI or ESI without the aid of metal salts to promote cationisation, was ionised promptly by ASAP resulting in the production of radical cations. A small degree of in-source dissociation could be eliminated by control of the instrument ion source voltages. The fragmentation observed through in-source dissociation could be duplicated in a controlled manner through Collision-Induced Dissociation (CID) of the radical cations. PEG, which preferentially ionises through adduction with alkali metal cations in MALDI and ESI, was observed as a protonated molecular ion by ASAP. In-source dissociation could not be eliminated entirely and the fragmentation observed resulted from cleavage of the C-C and C-O backbone bonds, as opposed to only C-O bond cleavage observed from tandem mass spectrometry.

Sample preconcentration in open microchannels combined with MALDI-MS

In this work, a method for preconcentrating samples in 1 cm long, 50-150 μm wide open microchannels is presented. Platinum electrodes were positioned at the channel ends, voltage was applied, and charged analyte was preconcentrated at the oppositely charged side during continuous supply of sample. The preconcentration was initially studied in a closed system, where an influence on the analyte position from a pH gradient, generated by water electrolysis, was observed. In the open channel, the analyte distribution after preconcentration was evaluated using MALDI-MS with the channel as MALDI target. MALDI matrix was applied with an airbrush or by electrospray matrix deposition and by using the latter technique higher degrees of crystallization in the channels were obtained. After preconcentrating a 1 nM cytochrome c solution for 5 min, corresponding to a supplied amount of 1.25 fmol, a signal on the cathodic channel end could be detected. When a solution of cytochrome c trypsin digest was supplied, the peptides were preconcentrated at different positions along the channel depending on their charge.

Advancing matrix-assisted laser desorption/ionization-mass spectrometric imaging for capillary electrophoresis analysis of peptides

In this work, the utilization of matrix-assisted laser desorption/ionization-mass spectrometric imaging (MALDI-MSI) for capillary electrophoresis (CE) analysis of peptides based on a simple and robust off-line interface has been investigated. The interface involves sliding the CE capillary distal end within a machined groove on a MALDI sample plate, which is precoated with a thin layer of matrix for continuous sample deposition. MALDI-MSI by time of flight (TOF)/TOF along the CE track enables high-resolution and high-sensitivity detection of peptides, allowing the reconstruction of a CE electropherogram while providing accurate mass measurements and structural identification of molecules. Neuropeptide standards and their H/D isotopic formaldehyde-labeled derivatives were analyzed using this new platform. Normalized intensity ratios of individual ions extracted from the CE trace were compared to MALDI-MS direct analysis and the theoretical ratios. The CE-MALDI-MSI results show potential for sensitive and quantitative analysis of peptide mixtures spanning a wide dynamic range.

Online deposition of nano-aerosol for matrix-assisted laser desorption/ionization mass spectrometry

An online nano-aerosol sample deposition method for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is described in which matrix and analyte particles between 50 and 500 nm are aerodynamically focused onto a tight spot, ca. 200 microm in diameter, on the target plate under vacuum. MALDI analysis of the target is performed without additional sample preparation. The method is evaluated with insulin as the analyte and alpha-cyano-4-hydroxycinnamic acid (CHCA) as the matrix. Two preparation modes are compared with conventional dried-droplet deposition: mixture deposition where a single layer is deposited consisting of particles that contain both matrix and analyte, and layered deposition where an underlayer of matrix particles and an overlayer of analyte particles are deposited separately. Desalting is performed by adding ammonium sulfate to the solution used to generate the matrix aerosol. With mixture deposition, the optimum matrix-to-analyte mole ratio is about 500:1 compared with 5000:1 for the conventional dried-droplet method. With layered deposition, the thicknesses of the matrix and analyte layers are more important determinants of the analyte signal intensity than the matrix-to-analyte mole ratio. Analyte signal intensities are independent of matrix layer thickness above 200 nm, and the optimum analyte signal is obtained with an analyte layer thickness of about 100 nm.

Optimization of a modified aerospray deposition device for the preparation of samples for quantitative analysis by MALDI-TOFMS

A modified aerospray apparatus was used to prepare a thin layer sample of matrix and analyte for quantitative analysis by MALDI-TOFMS. The apparatus consists of a set of coaxial tubing; the liquid sample is forced by a syringe pump through the inner capillary and it is nebulized by a flow of gas through the outer capillary. The small droplets of sample exiting the device are deposited onto a rotating plate, which serves as the sample surface for a time-of-flight mass spectrometer. An optimization was carried out after initial experiments with the device resulted in poorer than expected reproducibility of analyte signal. A two-level plus center point factorial experiment was performed investigating several factors, including the inner capillary internal diameter, gas pressure, liquid flow, spray distance, and time. After optimization the within-sample reproducibility of the analyte signal improved 3-fold, while the sample-to-sample reproducibility improved 4.5-fold.

Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

Solid-state Nuclear magnetic resonance (NMR) was used here to explore structural characteristics of samples to be subjected to matrix-assisted laser desorption/ionization (MALDI) and prepared without the use of any solvent. The analytical systems scrutinized in NMR were mixtures of a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and caesium fluoride (CsF), used as the cationization agent in synthetic polymer MALDI mass analysis, at different molar ratios (1:1, 5:1, and 10:1). Complementary information could be obtained from 13C, 133Cs, and 19F NMR spectra. Grinding the matrix together with the salt in the solid state was shown to induce a strong modification in the molecular organization within the MALDI sample. The evidenced mechano-induced reactions allow strong interactions between the matrix and the cation, up to the formation of a salt, and only occur in the presence of some water molecules. Addition of a poly(ethylene oxide) polymer as the analyte did not further modify the observed molecular organizations. Although relative matrix and salt concentrations in the scrutinized samples were unusual for MALDI analysis, mass spectra of good quality could be obtained and revealed that cation attachment on polymers during the MALDI process is not a matrix-independent event since a lower ionization efficiency was obtained from highly organized solid samples, mostly consisting of 2,5-DHB caesium salt species.

A general method for quantitative measurement of molecular mass distribution by mass spectrometry

A method is presented to test whether the conversion of the mass spectrum of a polydisperse analyte to its molecular mass distribution is quantitative. Mixtures of samples with different average molecular masses, coupled with a Taylor's expansion mathematical formalism, were used to ascertain the reliability of molecular mass distributions derived from mass spectra. Additionally, the method describes how the molecular mass distributions may be corrected if the degree of mass bias is within certain defined limits. This method was demonstrated on polydisperse samples of C(60) fullerenes functionalized with ethylpyrrolidine groups measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; however, it is applicable to any polydisperse analyte and mass spectrometric method as long as spectrum resolution allows individual oligomers to be identified. Mass spectra of the derivatized fullerenes taken in positive ion mode were shown to give an accurate measurement of the molecular mass distribution while those taken in negative ion mode were not. Differences in the mechanisms for ion formation are used to explain the discrepancy. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States of America.

Determining the time needed for the vortex method for preparing solvent-free MALDI samples of low molecular mass polymers

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data is used to determine the polymer average molecular weights, repeat units, and end groups. The development of the vortex method of solvent-free sample preparation showed that remarkably short mixing times could prepare samples that yielded high quality MALDI mass spectra. In this paper, we use microscopy images and MALDI mass spectra to evaluate the mixing time required by the vortex method to produce mass spectra for low molecular mass polymer samples. Our results show that mixing times of as little as 10 s can generate homogeneous thin films that produce high quality mass spectra with S/N approximately 100. In addition, ultrashort mixing times of only 2 s still produce samples with mostly smooth morphology and mass spectra with S/N approximately 10.

Quantitative analysis of antiretroviral drugs in lysates of peripheral blood mononuclear cells using MALDI-triple quadrupole mass spectrometry

We report here on the use of a prototype matrix-assisted laser desorption/ionization (MALDI)-triple quadrupole mass spectrometer for quantitative analysis of six antiretroviral drugs in lysates of peripheral blood mononuclear cells (PBMC). Of the five investigated MALDI matrixes, 2,5-dihydroxybenzoic acid (DHB) and the novel 7-hydroxy-4-(trifluoromethyl)coumarin (HFMC) showed the broadest application ranges for the antiretroviral drugs. For DHB, the mean relative errors ranged from 8.3 (ritonavir) to 4.3% (saquinavir). The mean precisions (CV) ranged from 17.3 (nevirapine) to 10.8% (saquinavir). The obtained lower limits of quantitation (LLOQ) readily allow clinical applications using just 1 million PBMC from HIV-infected patients under therapy. The new matrix HFMC was used for quantitative analysis of the HIV protease inhibitor indinavir using a stainless steel target plate as well as a target plate with a novel, strongly hydrophobic fluoropolymer coating. Using the coated target plate, the mean relative error improved from 10.1 to 4.6%, the mean precision from 33.9 to 9.9% CV, and the LLOQ from 16 to 1 fmol. In addition, the measurement time for one spot went down from 6 to only 2.5 s.

Imaging the morphology of solvent-free prepared MALDI samples

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. The development of solvent-free sample preparation methods has enabled MALDI to analyze insoluble materials and, interestingly, can provide higher-quality mass spectral data. Although the utility of solvent-free sample preparation for MALDI has been demonstrated, the reasons for its success are only now being discovered. In this study, we use microscopy tools to image samples prepared using solvent-free methods to examine the morphology of these samples. The samples are prepared using a simple vortex method. Our results show that the average particle size of typical MALDI matrices is reduced from their original tens to hundreds of micrometers to hundreds of nanometers. This size reduction of the matrix occurs in one minute using the vortex method. We also observe remarkably smooth and homogeneous sample morphologies for the laser to interrogate, especially considering the relatively crude methods used to prepare our samples.

Development of a dual-spray electrospray deposition system for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A new method of matrix-assisted laser desorption/ionization (MALDI) sample preparation using a dual-spray electrospray deposition system is demonstrated and employed for the investigation of gas-phase cationization reactions in the MALDI plume. The dual-spray electrospray system is found to increase the homogeneity of the sample similarly to that of a conventional single-spray electrospray system. The dual-spray electrospray system allows for intimate mixing of separately prepared sample components and results in improved quantitative results. The development of this device also leads to the possibility of mixing sample components prepared in different solvents without the need to be concerned with solvent miscibility.

Imaging mass spectrometry

Imaging mass spectrometry combines the chemical specificity and parallel detection of mass spectrometry with microscopic imaging capabilities. The ability to simultaneously obtain images from all analytes detected, from atomic to macromolecular ions, allows the analyst to probe the chemical organization of a sample and to correlate this with physical features. The sensitivity of the ionization step, sample preparation, the spatial resolution, and the speed of the technique are all important parameters that affect the type of information obtained. Recently, significant progress has been made in each of these steps for both secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization (MALDI) imaging of biological samples. Examples demonstrating localization of proteins in tumors, a reduction of lamellar phospholipids in the region binding two single celled organisms, and sub-cellular distributions of several biomolecules have all contributed to an increasing upsurge in interest in imaging mass spectrometry. Here we review many of the instrumental developments and methodological approaches responsible for this increased interest, compare and contrast the information provided by SIMS and MALDI imaging, and discuss future possibilities.

Archiving and absolute quantitation of solutes separated by single charged droplet coulomb explosion

The electrospray (ES) ion source relies on the transfer of low-volatility solutes to the gas phase as an outcome of coulomb explosion events of charged droplets generated by electrical atomization. Introduced here are two methods for archiving compounds separated by coulomb explosion of single droplets having net charge that had been levitated in an electrodynamic balance. We categorized compounds separated by the explosion as either material ejected, including progeny droplets, or the material retained in the main residue. The potential for this methodology is illustrated by (i) qualitative characterization of solute states, aqueous versus precipitated in the separated material, and (ii) absolute quantitation of solutes separated by such an event. For a droplet containing 5 x 10(7) 20-nm-diameter fluospheres, its first encountered coulomb explosion event resulted in the ejection of 1.70% of them. The capability to acquire such detailed information regarding the individual steps in the process of transferring low-volatility solutes to the gas phase in an ES ion source is essential to develop strategies for absolute quantitation in applications of ES mass spectrometry.

Surfactant-mediated matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of small molecules

A variety of surfactants have been tested as matrix-ion suppressors for the analysis of small molecules by matrix-assisted laser desorption/ionization time-of flight mass spectrometry. Their addition to the common matrix alpha-cyano-4-hydroxycinnamic acid (CHCA) greatly reduces the presence of matrix-related ions when added at the appropriate mole ratio of CHCA/surfactant, while still allowing the analyte signal to be observed. A range of cationic quaternary ammonium surfactants, as well as a neutral and anionic surfactant, was tested for the analysis of phenolics, phenolic acids, peptides and caffeine. It was found that the cationic surfactants, particularly cetyltrimethylammonium bromide (CTAB), were suitable for the analysis of acidic analytes. The anionic surfactant, sodium dodecyl sulfate, showed promise for peptide analysis. For trialanine, the detection limit was observed to be in the 100 femtomole range. The final matrix/surfactant mole ratio was a critical parameter for matrix ion suppression and resulting intensity of analyte signal. It was also found that the mass resolution of analytes was improved by 25-75%. Depth profiling of sample spots, by varying the number of laser shots, revealed that the surfactants tend to migrate toward the top of the droplet during crystallization, and that it is likely that the analyte is also enriched in this surface region. Here, higher analyte/surfactant concentration would reduce matrix-matrix interactions (known to be a source of matrix-derived ions).

Analysis of Insect Cuticular Hydrocarbons Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Insect cuticular hydrocarbons (CHCs) were probed by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry with a lithium 2,5-dihydroxybenzoate matrix. CHC profiles were obtained for 12 species of diverse insect taxa (termites, ants, a cockroach, and a flesh fly). MALDI spectra revealed the presence of high molecular weight CHCs on the insect cuticle. Hydrocarbons with more than 70 carbon atoms, both saturated and unsaturated, were detected. When compared with gas chromatography/mass spectrometry (GC/MS), MALDI-TOF covered a wider range of CHCs and enabled CHCs of considerably higher molecular weight to be detected. Good congruity between GC/MS and MALDI-TOF was observed in the overlapping region of molecular weights. Moreover, a number of previously undiscovered hydrocarbons were detected in the high mass range beyond the analytical capabilities of current GC/MS instruments. MALDI was shown to hold potential to become an alternative analytical method for insect CHC analyses. The ability of MALDI to discriminate among species varying in the degree of their relatedness was found to be similar to GC/MS. However, neither MALDI-MS nor GC/MS data were able to describe the phylogenetic relationships.

Significant parameters in the optimization of MALDI-TOF-MS for synthetic polymers

One of the most significant issues in any analytical practice is optimization. Optimization and calibration are key factors in quantitation. In matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), instrument optimization is a limitation restricting quantitation. An understanding of the parameters that are most influential and the effects of these parameters on the mass spectrum is required for optimization. This understanding is especially important when characterizing synthetic polymers by MALDI-TOF-MS, due to the breadth of the polymer molecular mass distribution (MMD). Two considerations are important in quantitation, additivity of signal and signal-to-noise (S/N). In this study, the effects of several instrument parameters were studied using an orthogonal experimental design to understand effects on the signal-to-noise (S/N) of a polystyrene distribution. The instrument parameters examined included detector voltage, laser energy, delay time, extraction voltage, and lens voltage. Other parameters considered were polymer concentration and matrix. The results showed detector voltage and delay time were the most influential of the instrument parameters for polystyrene using all trans-retinoic acid (RA) as the matrix. These parameters, as well as laser energy, were most influential for the polystyrene with dithranol as the matrix.

Improved cell typing by charge-state deconvolution of matrix-assisted laser desorption/ionization mass spectra

Robust, specific, and rapid identification of toxic strains of bacteria and viruses, to guide the mitigation of their adverse health effects and optimum implementation of other response actions, remains a major analytical challenge. This need has driven the development of methods for classification of microorganisms using mass spectrometry, particularly matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), that allows high-throughput analyses with minimum sample preparation. We describe a novel approach to cell typing based on pattern recognition of MALDI mass spectra, which involves charge-state deconvolution in conjunction with a new correlation analysis procedure. The method is applicable to both prokaryotic and eukaryotic cells. Charge-state deconvolution improves the quantitative reproducibility of spectra because multiply charged ions resulting from the same biomarker attaching a different number of protons are recognized and their abundances are combined. This allows a clearer distinction of bacterial strains or of cancerous and normal liver cells. Improved class distinction provided by charge-state deconvolution was demonstrated by cluster spacing on canonical variate score charts and by correlation analyses. Deconvolution may enhance detection of early disease state or therapy progress markers in various tissues analyzed by MALDI-MS.

Ion formation mechanisms in UV-MALDI

Matrix Assisted Laser Desorption/Ionization (MALDI) is a very widely used analytical method, but has been developed in a highly empirical manner. Deeper understanding of ionization mechanisms could help to design better methods and improve interpretation of mass spectra. This review summarizes current mechanistic thinking, with emphasis on the most common MALDI variant using ultraviolet laser excitation. A two-step framework is gaining acceptance as a useful model for many MALDI experiments. The steps are primary ionization during or shortly after the laser pulse, followed by secondary reactions in the expanding plume of desorbed material. Primary ionization in UV-MALDI remains somewhat controversial, the two main approaches are the cluster and pooling/photoionization models. Secondary events are less contentious, ion-molecule reaction thermodynamics and kinetics are often invoked, but details differ. To the extent that local thermal equilibrium is approached in the plume, the mass spectra may be straightforwardly interpreted in terms of charge transfer thermodynamics.

Electrospray interfacing of polymer microfluidics to MALDI-MS

The off-line coupling of polymer microfluidics to MALDI-MS is presented using electrospray deposition. Using polycarbonate microfluidic chips with integrated hydrophobic membrane electrospray tips, peptides and proteins are deposited onto a stainless steel target followed by MALDI-MS analysis. Microchip electrospray deposition is found to yield excellent spatial control and homogeneity of deposited peptide spots, and significantly improved MALDI-MS spectral reproducibility compared to traditional target preparation methods. A detection limit of 3.5 fmol is demonstrated for angiotensin. Furthermore, multiple electrospray tips on a single chip provide the ability to simultaneously elute parallel sample streams onto a MALDI target for high-throughput multiplexed analysis. Using a three-element electrospray tip array with 150 microm spacing, the simultaneous deposition of bradykinin, fibrinopeptide, and angiotensin is achieved with no cross talk between deposited samples. In addition, in-line proteolytic digestion of intact proteins is successfully achieved during the electrospray process by binding trypsin within the electrospray membrane, eliminating the need for on-probe digestion prior to MALDI-MS. The technology offers promise for a range of microfluidic platforms designed for high-throughput multiplexed proteomic analyses in which simultaneous on-chip separations require an effective interface to MS.

Study of Chemistry in Droplets with Net Charge before and after Coulomb Explosion: Ion-Induced Nucleation in Solution and Implications for Ion Production in an Electrospray

Droplets with net charge are essential intermediaries in the production of gaseous ions in the electrospray (ES) ion source. There could be a wealth of knowledge regarding the chemistry that occurs in such droplets as a result of their violation of electroneutrality. Such information could lead to improved understanding of the ion generation process in an ES along with factors that affect it. The experiments performed involved the levitation of individually charged droplets that were, and were not, allowed to undergo Coulomb explosion while they remained levitated in an electrodynamic balance (EDB). Through examination of precipitates formed within the levitated droplets, it was observed that onset of NaCl precipitation was promoted in droplets (glycerol:water 9:1 v/v) that had mass-to-net-charge (m/z) ratio <-4.8 x 10(9) amu/e. This threshold m/z value is exceeded in essentially all droplets generated in an ES because it is above the calculated threshold for Coulomb explosion. This finding suggests that cluster formation in droplets having net charge could occur at solute concentrations lower than would be anticipated on the basis of homogeneous nucleation. The effect of large entities such as precipitates existing in the droplet on the dynamics of droplet Coulomb explosion was also examined. Using droplets whose initial size and magnitude of net charge were equivalent within experimental error but having different concentration of solutes, we showed that the m/z of their main residues following Coulomb explosion were different. Micrometer-size droplets containing 20 nm fluorescent beads that underwent Coulomb explosion resulted in main residues that had higher m/z for higher initial bead concentration in the starting solution (320 nM) when compared to main residues resultant from starting solutions having lower initial bead concentration (21 nM).

An operator-independent approach to mass spectral peak identification and integration

A mathematical algorithm is presented that locates and calculates the area beneath peaks from real data using only reproducible mathematical operations and no user-selected parameters. It makes no assumptions about peak shape and requires no smoothing or preprocessing of the data. In fact, it is shown that for matrix-assisted laser desorption time-of-flight mass spectra noise exists at all frequency ranges making the smoothing of data without distortion of peak areas impossible. The algorithm is based on a time-series segmentation routine that reduces the data set to groups of three strategic points where each group defines the beginning, center, and ending of each peak located. The peak areas are found from the strategic points using a commonplace polygonal area calculation routine. Peaks with statistically insignificant height or area are then discarded. The performance of the algorithm is demonstrated on a polystyrene mass spectrum with varying degrees of noise added either mathematically or experimentally. An on-line implementation of the method, termed MassSpectator, for public use can be found at www.nist.gov/maldi.

Subcellular imaging mass spectrometry of brain tissue

Imaging mass spectrometry provides both chemical information and the spatial distribution of each analyte detected. Here it is demonstrated how imaging mass spectrometry of tissue at subcellular resolution can be achieved by combining the high spatial resolution of secondary ion mass spectrometry (SIMS) with the sample preparation protocols of matrix-assisted laser desorption/ionization (MALDI). Despite mechanistic differences and sampling 10(5) times less material, matrix-enhanced (ME)-SIMS of tissue samples yields similar results to MALDI (up to m/z 2500), in agreement with previous studies on standard compounds. In this regard ME-SIMS represents an attractive alternative to polyatomic primary ions for increasing the molecular ion yield. ME-SIMS of whole organs and thin sections of the cerebral ganglia of Lymnaea stagnalis demonstrate the advantages of ME-SIMS for chemical imaging mass spectrometry. Subcellular distributions of cellular analytes are clearly obtained, and the matrix provides an in situ height map of the tissue, allowing the user to identify rapidly regions prone to topographical artifacts and to deconvolute topographical losses in mass resolution and signal-to-noise ratio.

Improved matrix-assisted laser desorption/ionisation sample preparation of a partially depolymerised cellulose derivative by continuous spray deposition and interfacing with size-exclusion chromatography

Continuous spray deposition (CSD) of aqueous solutions of partially depolymerised methyl cellulose was found to improve matrix-assisted laser desorption/ionisation (MALDI) sample preparation. One feature was that the sensitivity in MALDI time-of-flight mass spectrometry increased up to an order of magnitude compared with the standard sample preparation method. Another feature was that CSD provided targets for MALDI with homogeneously distributed analyte. This resulted in a more even signal intensity and a higher reproducibility than in the standard method. High-mass discrimination was more pronounced in CSD than in the standard method. Size-exclusion chromatography with aqueous eluent was coupled online to CSD onto matrix-precoated foils. The suitability for determination of the molar mass distribution of methyl cellulose was investigated.