Repeat MALDI MS imaging of a single tissue section using multiple matrices and tissue washes

Quantitative mass spectrometry imaging: therapeutics & biomolecules

Mass spectrometry imaging (MSI) has become increasingly utilized in the analysis of biological molecules. MSI grants the ability to spatially map thousands of molecules within one experimental run in a label-free manner. While MSI is considered by most to be a qualitative method, recent advancements in instrumentation, sample preparation, and development of standards has made quantitative MSI (qMSI) more common. In this feature article, we present a tailored review of recent advancements in qMSI of therapeutics and biomolecules such as lipids and peptides/proteins. We also provide detailed experimental considerations for conducting qMSI studies on biological samples, aiming to advance the methodology.

Applying Multimodal Mass Spectrometry to Image Tumors Undergoing Ferroptosis Following In Vivo Treatment with a Ferroptosis Inducer

Epithelial ovarian cancer (EOC) is the most common form of ovarian cancer. The poor prognosis generally associated with this disease has led to the search for improved therapies such as ferroptosis-inducing agents. Ferroptosis is a form of regulated cell death that is dependent on iron and is characterized by lipid peroxidation. Precise mapping of lipids and iron within tumors exposed to ferroptosis-inducing agents may provide insight into processes of ferroptosis in vivo and ultimately assist in the optimal deployment of ferroptosis inducers in cancer therapy. In this work, we present a method for combining matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) with secondary ion mass spectrometry (SIMS) to analyze changes in spatial lipidomics and metal composition, respectively, in ovarian tumors following exposure to a ferroptosis inducer. Tumors were obtained by injecting human ovarian cancer tumor-initiating cells into mice, followed by treatment with the ferroptosis inducer erastin. SIMS imaging detected iron accumulation in the tumor tissue, and sequential MALDI-MS imaging of the same tissue section displayed two chemically distinct regions of lipids. One region was associated with the iron-rich area detected with SIMS, and the other region encompassed the remainder of the tissue section. Bulk lipidomics confirmed the lipid assignments putatively assigned from the MALDI-MS data. Overall, we demonstrate the ability of multimodal MSI to identify the spatial locations of iron and lipids in the same tissue section and associate these regions with clinical pathology.

Peptide Mass Spectra from Micrometer-Thick Ice Films Produced with Femtosecond Pulses

We present a cryogenic mass spectrometry protocol with the capability to detect peptides in the attomole dilution range from ice films. Our approach employs femtosecond laser pulses and implements neither substrate modification nor proton donor agents in the aqueous solution, known to facilitate analyte detection in mass spectrometry. In a systematic study, we investigated the impact of temperature, substrate composition, and irradiation wavelength (513 and 1026 nm) on the bradykinin signal onset. Our findings show that substrate choice and irradiation wavelength have a minor impact on signal intensity once the preparation protocol is optimized. However, if the temperature is increased from -140 to 0 °C, which is accompanied by ice film thinning, a somehow complex picture of analyte desorption and ionization is recognizable, which has not been described in the literature yet. Under cryogenic conditions (-140 °C), obtaining a signal is only possible from isolated sweet spots across the film. If the thin ice film is between -100 and -70 °C of temperature, these sweet spots appear more frequently. Ice sublimation triggered by temperatures above -70 °C leads to an intense and robust signal onset that could be maintained for several hours. In addition to the above findings, we notice that a vibrant fragmentation pattern produced is strikingly similar with both wavelengths. Our findings suggest that while following an optimized protocol, femtosecond mass spectrometry has excellent potential to analyze small organic molecules and peptides with a mass range of up to 2.5 kDa in aqueous solution without any matrix, as employed in matrix-assisted laser desorption/ionization (MALDI) or any substrate surface modification, found in surface-assisted laser desorption/ionization (SALDI).

An optimized method for the detection and spatial distribution of aminoglycoside and vancomycin antibiotics in tissue sections by mass spectrometry imaging

Suboptimal antibiotic dosing has been identified as one of the key drivers in the development of multidrug-resistant (MDR) bacteria that have become a global health concern. Aminoglycosides and vancomycin are broad-spectrum antibiotics used to treat critically ill patients infected by a variety of MDR bacterial species. Resistance to these antibiotics is becoming more prevalent. In order to design proper antibiotic regimens that maximize efficacy and minimize the development of resistance, it is pivotal to obtain the in situ pharmacokinetic-pharmacodynamic profiles at the sites of infection. Mass spectrometry imaging (MSI) is the ideal technique to achieve this. Aminoglycosides, due to their structure, suffer from poor ionization efficiency. Additionally, ion suppression effects by endogenous molecules greatly inhibit the detection of aminoglycosides and vancomycin at therapeutic levels. In the current study, an optimized method was developed that enabled the detection of these antibiotics by MSI. Tissue spotting experiments demonstrated a 5-, 15-, 35-, and 54-fold increase in detection sensitivity in the washed samples for kanamycin, amikacin, streptomycin, and vancomycin, respectively. Tissue mimetic models were utilized to optimize the washing time and matrix additive concentration. These studies determined the improved limit of detection was 40 to 5 μg/g of tissue for vancomycin and streptomycin, and 40 to 10 μg/g of tissue for kanamycin and amikacin. The optimized protocol was applied to lung sections from mice dosed with therapeutic levels of kanamycin and vancomycin. The washing protocol enabled the first drug distribution investigations of aminoglycosides and vancomycin by MSI, paving the way for site-of-disease antibiotic penetration studies.

Pneumatically Sprayed Gold Nanoparticles for Mass Spectrometry Imaging of Neurotransmitters

Using citrate-capped gold nanoparticles (AuNPs) for laser desorption ionization mass spectrometry (LDI-MS) is an approach that has demonstrated broad applicability to ionization of different classes of molecules. Here, we show a simple AuNP-based approach for the ionization of neurotransmitters. Specifically, the detection of acetylcholine, dopamine, epinephrine, glutamine, 4-aminobutyric acid, norepinephrine, octopamine, and serotonin was achieved at physiologically relevant concentrations in serum and homogenized tissue. Additionally, pneumatic spraying of AuNPs onto tissue sections facilitated mass spectrometry imaging (MSI) of rabbit brain tissue sections, zebrafish embryos, and neuroblastoma cells for several neurotransmitters simultaneously using this quick and simple sample preparation. AuNP LDI-MS achieved mapping of neurotransmitters in fine structures of zebrafish embryos and neuroblastoma cells at a lateral spatial resolution of 5 μm. The use of AuNPs to ionize small aminergic neurotransmitters in situ provides a fast, high-spatial resolution method for simultaneous detection of a class of molecules that typically evade comprehensive detection with traditional matrixes.

Imaging drugs, metabolites and biomarkers in rodent lung: a DESI MS strategy for the evaluation of drug-induced lipidosis

Within drug development and pre-clinical trials, a common, significant and poorly understood event is the development of drug-induced lipidosis in tissues and cells. In this manuscript, we describe a mass spectrometry imaging strategy, involving repeated analysis of tissue sections by DESI MS, in positive and negative polarities, using MS and MS/MS modes. We present results of the detected distributions of the administered drug, drug metabolites, lipid molecules and a putative marker of lipidosis, di-docosahexaenoyl (22:6)-bis(monoacylglycerol) phosphate (di-22:6-BMP). A range of strategies have previously been reported for detection, isolation and identification of this compound, which is an isomer of di-docosahexaenoic (22:6 n-3) phosphatidylglycerol (di-22:6 PG), a commonly found lipid that acts as a surfactant in lung tissues. We show that MS imaging using MS/MS can be used to differentiate these compounds of identical mass, based upon the different distributions of abundant fragment ions. Registration of images of these fragments, and detected drugs and metabolites, is presented as a new method for studying drug-induced lipidosis in tissues. Graphical abstract.

Characterizing the Natural History of Acute Radiation Syndrome of the Gastrointestinal Tract: Combining High Mass and Spatial Resolution Using MALDI-FTICR-MSI

The acute radiation syndrome of the gastrointestinal tract has been histologically characterized, but the molecular and functional mechanisms that lead to these cellular alterations remain enigmatic. Mass spectrometry imaging is the only technique that enables the simultaneous detection and cellular or regional localization of hundreds of biomolecules in a single experiment. This current study utilized matrix-assisted laser desorption/ionization mass spectrometry imaging for the molecular characterization of the first natural history study of gastrointestinal acute radiation syndrome in the nonhuman primate. Jejunum samples were collected at days 4, 8, 11, 15, and 21 following 12-Gy partial-body irradiation with 2.5% bone marrow sparing. Mass spectrometry imaging investigations identified alterations in lipid species that further understanding of the functional alterations that occur over time in the different cellular regions of the jejunum following exposure to high doses of irradiation. Alterations in phosphatidylinositol species informed on dysfunctional epithelial cell differentiation and maturation. Differences in glycosphingolipids of the villi epithelium that would influence the absorptive capacity and functional structure of the brush border membrane were detected. Dichotomous alterations in cardiolipins indicated altered structural and functional integrity of mitochondria. Phosphatidylglycerol species, known regulators of toll-like receptors, were detected and localized to regions in the lamina propria that contained distinct immune cell populations. These results provide molecular insight that can inform on injury mechanism in a nonhuman primate model of the acute radiation syndrome of the gastrointestinal tract. Findings may contribute to the identification of therapeutic targets and the development of new medical countermeasures.

Novel Trimodal MALDI Imaging Mass Spectrometry (IMS3) at 10 μm Reveals Spatial Lipid and Peptide Correlates Implicated in Aβ Plaque Pathology in Alzheimer's Disease

Multimodal chemical imaging using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) can provide comprehensive molecular information in situ within the same tissue sections. This is of relevance for studying different brain pathologies such as Alzheimer's disease (AD), where recent data suggest a critical relevance of colocalizing Aβ peptides and neuronal lipids. We here developed a novel trimodal, high-resolution (10 μm) MALDI imaging MS (IMS) paradigm for negative and positive ion mode lipid analysis and subsequent protein ion imaging on the same tissue section. Matrix sublimation of 1,5-diaminonaphthalene (1,5-DAN) enabled dual polarity lipid MALDI IMS on the same pixel points at high spatial resolutions (10 μm) and with high spectral quality. This was followed by 10 μm resolution protein imaging on the same measurement area, which allowed correlation of lipid signals with protein distribution patterns within distinct cerebellar regions in mouse brain. The demonstrated trimodal imaging strategy (IMS3) was further shown to be an efficient approach for simultaneously probing Aβ plaque-associated lipids and Aβ peptides within the hippocampus of 18 month-old transgenic AD mice (tgArcSwe). Here, IMS3 revealed a strong colocalization of distinct lipid species including ceramides, phosphatidylinositols, sulfatides (Cer 18:0, PI 38:4, ST 24:0) and lysophosphatidylcholines (LPC 16:0, LPC 18:0) with plaque-associated Aβ isoforms (Aβ 1-37, Aβ 1-38, Aβ 1-40). This highlights the potential of IMS3 as an alternative, superior approach to consecutively performed immuno-based Aβ staining strategies. Furthermore, the IMS3 workflow allowed for multimodal in situ MS/MS analysis of both lipids and Aβ peptides. Altogether, the here presented IMS3 approach shows great potential for comprehensive, high-resolution molecular analysis of histological features at cellular length scales with high chemical specificity. It therefore represents a powerful approach for probing the complex molecular pathology of, e.g., neurodegenerative diseases that are characterized by neurotoxic protein aggregation.

Metabolite mapping by consecutive nanostructure and silver-assisted mass spectrometry imaging on tissue sections

RATIONALE

Nanostructure-based mass spectrometry imaging (MSI) is a promising technology for molecular imaging of small molecules, without the complex chemical background typically encountered in matrix-assisted molecular imaging approaches. Here, we have enhanced these surfaces with silver (Ag) to provide a second tier of MSI data from a single sample.

METHODS

MSI data was acquired through the application of laser desorption/ionization mass spectrometry to biological samples imprinted onto desorption/ionization on silicon (DIOS) substrates. Following initial analysis, ultra-thin Ag layers were overlaid onto the followed by MSI analysis (Ag-DIOS MSI). This approach was first demonstrated for fingermark small molecules including environmental contaminants and sebum components. Subsequently, this bimodal method was translated to lipids and metabolites in fore-stomach sections from a 6-bromoisatin chemopreventative murine mouse model.

RESULTS

DIOS MSI allowed mapping of common ions in fingermarks as well as 6-bromoisatin metabolites and lipids in murine fore-stomach. Furthermore, DIOS MSI was complemented by the Ag-DIOS MSI of Ag-adductable lipids such as wax esters in fingermarks and cholesterol in murine fore-stomach. Gastrointestinal acid condensation products of 6-bromoisatin, such as the 6,6'-dibromoindirubin mapped herein, are very challenging to isolate and characterize. By re-analyzing the same tissue imprints, this metabolite was readily detected by DIOS, placed in a tissue-specific spatial context, and subsequently overlaid with additional lipid distributions acquired using Ag-DIOS MSI.

CONCLUSIONS

The ability to place metabolite and lipid classes in a tissue-specific context makes this novel method suited to MSI analyses where the collection of additional information from the same sample maximises resource use, and also maximises the number of annotated small molecules, in particular for metabolites that are typically undetectable with traditional platforms. Copyright © 2017 John Wiley & Sons, Ltd.

A Nanostructured Matrices Assessment to Study Drug Distribution in Solid Tumor Tissues by Mass Spectrometry Imaging

The imaging of drugs inside tissues is pivotal in oncology to assess whether a drug reaches all cells in an adequate enough concentration to eradicate the tumor. Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Imaging (MALDI-MSI) is one of the most promising imaging techniques that enables the simultaneous visualization of multiple compounds inside tissues. The choice of a suitable matrix constitutes a critical aspect during the development of a MALDI-MSI protocol since the matrix ionization efficiency changes depending on the analyte structure and its physico-chemical properties. The objective of this study is the improvement of the MALDI-MSI technique in the field of pharmacology; developing specifically designed nanostructured surfaces that allow the imaging of different drugs with high sensitivity and reproducibility. Among several nanomaterials, we tested the behavior of gold and titanium nanoparticles, and halloysites and carbon nanotubes as possible matrices. All nanomaterials were firstly screened by co-spotting them with drugs on a MALDI plate, evaluating the drug signal intensity and the signal-to-noise ratio. The best performing matrices were tested on control tumor slices, and were spotted with drugs to check the ion suppression effect of the biological matrix. Finally; the best nanomaterials were employed in a preliminary drug distribution study inside tumors from treated mice.

Integrated mass spectrometry imaging and omics workflows on the same tissue section using grid-aided, parafilm-assisted microdissection

BACKGROUND

In spite of the number of applications describing the use of MALDI MSI, one of its major drawbacks is the limited capability of identifying multiple compound classes directly on the same tissue section.

METHODS

We demonstrate the use of grid-aided, parafilm-assisted microdissection to perform MALDI MS imaging and shotgun proteomics and metabolomics in a combined workflow and using only a single tissue section. The grid is generated by microspotting acid dye 25 using a piezoelectric microspotter, and this grid was used as a guide to locate regions of interest and as an aid during manual microdissection. Subjecting the dissected pieces to the modified Folch method allows to separate the metabolites from proteins. The proteins can then be subjected to digestion under controlled conditions to improve protein identification yields.

RESULTS

The proof of concept experiment on rat brain generated 162 and 140 metabolite assignments from three ROIs (cerebellum, hippocampus and midbrain/hypothalamus) in positive and negative modes, respectively, and 890, 1303 and 1059 unique proteins. Integrated metabolite and protein overrepresentation analysis identified pathways associated with the biological functions of each ROI, most of which were not identified when looking at the protein and metabolite lists individually.

CONCLUSIONS

This combined MALDI MS imaging and multi-omics approach further extends the amount of information that can be generated from single tissue sections.

GENERAL SIGNIFICANCE

To the best of our knowledge, this is the first report combining both imaging and multi-omics analyses in the same workflow and on the same tissue section.

Molecular imaging of brain localization of liposomes in mice using MALDI mass spectrometry

Phospholipids have excellent biocompatibility and are therefore often used as main components of liposomal drug carriers. In traditional bioanalytics, the in-vivo distribution of liposomal drug carriers is assessed using radiolabeled liposomal constituents. This study presents matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) as an alternative, label-free method for ex-vivo molecular imaging of liposomal drug carriers in mouse tissue. To this end, indocyanine green as cargo and two liposomal markers, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated with monodisperse polyethylene glycol (PEG36-DSPE) were incorporated into liposomal carriers and administered to mice. We used MALDI MSI of the two lipid markers in both positive and negative ion mode for visualization of liposome integrity and distribution in mouse organs. Additional MSI of hemoglobin in the same tissue slice and pixel-by-pixel computational analysis of co-occurrence of lipid markers and hemoglobin served as indicator of liposome localization either in parenchyma or in blood vessels. Our proof-of-concept study suggests that liposomal components and indocyanine green distributed into all investigated organs.

MALDI Imaging of Liquid Extraction Surface Analysis Sampled Tissue

Combined mass spectrometry imaging methods in which two different techniques are executed on the same sample have recently been reported for a number of sample types. Such an approach can be used to examine the sampling effects of the first technique with a second, higher resolution method and also combines the advantages of each technique for a more complete analysis. In this work matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) was used to study the effects of liquid extraction surface analysis (LESA) sampling on mouse brain tissue. Complementary multivariate analysis techniques including principal component analysis, non-negative matrix factorization, and t-distributed stochastic neighbor embedding were applied to MALDI MS images acquired from tissue which had been sampled by LESA to gain a better understanding of localized tissue washing in LESA sampling. It was found that MALDI MS images could be used to visualize regions sampled by LESA. The variability in sampling area, spatial precision, and delocalization of analytes in tissue induced by LESA were assessed using both single-ion images and images provided by multivariate analysis.

Multimodal Mass Spectrometry Imaging of N-Glycans and Proteins from the Same Tissue Section

On-tissue digestion matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) can be used to record spatially correlated molecular information from formalin-fixed, paraffin-embedded (FFPE) tissue sections. In this work, we present the in situ multimodal analysis of N-linked glycans and proteins from the same FFPE tissue section. The robustness and applicability of the method are demonstrated for several tumors, including epithelial and mesenchymal tumor types. Major analytical aspects, such as lateral diffusion of the analyte molecules and differences in measurement sensitivity due to the additional sample preparation methods, have been investigated for both N-glycans and proteolytic peptides. By combining the MSI approach with extract analysis, we were also able to assess which mass spectral peaks generated by MALDI-MSI could be assigned to unique N-glycan and peptide identities.

Multigrid MALDI mass spectrometry imaging (mMALDI MSI)

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is an important technique for the spatially resolved molecular analysis of tissue sections. The selection of matrices influences the resulting mass spectra to a high degree. For extensive and simultaneous analysis, the application of different matrices to one tissue sample is desirable. To date, only a single matrix could be applied to a tissue section per experiment. However, repetitive removal of the matrix makes this approach time-consuming and damaging to tissue samples. To overcome these drawbacks, we developed a multigrid MALDI MSI technique (mMALDI MSI) that relies on automated inkjet printing to place differing matrices onto predefined dot grids. We used a cooled printhead to prevent cavitation of low viscosity solvents in the printhead nozzle. Improved spatial resolution of the dot grids was achieved by using a triple-pulse procedure that reduced droplet volume. The matrices can either be applied directly to the thaw-mounted tissue sample or by precoating the slide followed by mounting of the tissue sample. During the MALDI imaging process, we were able to precisely target different matrix point grids with the laser to simultaneously produce distinct mass spectra. Unlike the standard method, the prespotting approach optimizes the spectra quality, avoids analyte delocalization, and enables subsequent hematoxylin and eosin (H&E) staining. Graphical Abstract Scheme of the pre-spotted multigrid MALDI MSI workflow.

Automated Protein Localization of Blood Brain Barrier Vasculature in Brightfield IHC Images

In this paper, we present an objective method for localization of proteins in blood brain barrier (BBB) vasculature using standard immunohistochemistry (IHC) techniques and bright-field microscopy. Images from the hippocampal region at the BBB are acquired using bright-field microscopy and subjected to our segmentation pipeline which is designed to automatically identify and segment microvessels containing the protein glucose transporter 1 (GLUT1). Gabor filtering and k-means clustering are employed to isolate potential vascular structures within cryosectioned slabs of the hippocampus, which are subsequently subjected to feature extraction followed by classification via decision forest. The false positive rate (FPR) of microvessel classification is characterized using synthetic and non-synthetic IHC image data for image entropies ranging between 3 and 8 bits. The average FPR for synthetic and non-synthetic IHC image data was found to be 5.48% and 5.04%, respectively.

A MALDI-MSI Approach to the Characterization of Radiation-Induced Lung Injury and Medical Countermeasure Development

Radiation-induced lung injury is highly complex and characterized by multiple pathologies, which occur over time and sporadically throughout the lung. This complexity makes biomarker investigations and medical countermeasure screenings challenging. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) has the ability to resolve differences spatially in molecular profiles within the lung following radiation exposure and can aid in biomarker identification and pharmaceutical efficacy investigations. MALDI-MSI was applied to the investigation of a whole-thorax lung irradiation model in non-human primates (NHP) for lipidomic analysis and medical countermeasure distribution.

Adsorption of uranium composites onto saltrock oxides - experimental and theoretical study

The study encompassed experimental mass spectrometric and theoretical quantum chemical studies on adsorption of uranium species in different oxidation states of the metal ion, and oxides of UxOy(n+) type, where x = 1 or 3, y = 2 or 8, and n = 0, 1 or 2 onto nanosize-particles of saltrock oxides MO (M = Mg(II), Ca(II), Ni(II), Co(II), Sr(II) or Ba(II)), M2Oy (M = Au(III) or Ag(I), y = 3 or 1) silicates 3Al2O3.2SiO2, natural kaolinite (Al2O2·2SiO2·2H2O), illite (K0.78Ca0.02Na0.02(Mg0.34Al1.69Fe(III)0.02)[Si3.35Al0.65]O10(OH)2·nH2O), CaSiO3, 3MgO·4SiO2,H2O, and M(1)M(2)(SiO4)X2 (M(1) = M(2) = Al or M(1) = K, M(2) = Al, X = F or Cl), respectively. The UV-MALDI-Orbitrap mass spectrometry was utilized in solid-state and semi-liquid colloidal state, involving the laser ablation at λex = 337.2 nm. The theoretical modeling and experimental design was based on chemical-, physico-chemical, physical and biological processes involving uranium species under environmental conditions. Therefore, the results reported are crucial for quality control and monitoring programs for assessment of radionuclide migration. They impact significantly the methodology for evaluation of human health risk from radioactive contamination. The study has importance for understanding the coordination and red-ox chemistry of uranium compounds as well. Due to the double nature of uranium between rare element and superconductivity like materials as well as variety of oxidation states ∈ (+1)-(+6), the there remain challenging areas for theoretical and experimental research, which are of significant importance for management of nuclear fuel cycles and waste storage.

High-resolution droplet-based fractionation of nano-LC separations onto microarrays for MALDI-MS analysis

We present a robust droplet-based device, which enables the fractionation of ultralow flow rate nanoflow liquid chromatography (nano-LC) eluate streams at high frequencies and high peak resolution. This is achieved by directly interfacing the separation column to a micro T-junction, where the eluate stream is compartmentalized into picoliter droplets. This immediate compartmentalization prevents peak dispersion during eluate transport and conserves the chromatographic performance. Subsequently, nanoliter eluate fractions are collected at a rate of one fraction per second on a high-density microarray to retain the separation with high temporal resolution. Chromatographic separations of up to 45 min runtime can thus be archived on a single microarray possessing 2700 sample spots. The performance of this device is demonstrated by fractionating the separation of a tryptic digest of a known protein mixture onto the microarray chip and subsequently analyzing the sample archive using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Resulting peak widths are found to be significantly reduced compared to standard continuous flow spotting technologies as well as in comparison to a conventional nano-LC-electrospray ionization-mass spectrometry interface. Moreover, we demonstrate the advantage of our high-definition nanofractionation device by applying two different MALDI matrices to all collected fractions in an alternating fashion. Since the information that is obtained from a MALDI-MS measurement depends on the choice of MALDI matrix, we can extract complementary information from neighboring spots containing almost identical composition but different matrices.

Fluorometric beam profiling of UV MALDI lasers

The photon distribution (beam profile) of the laser as projected onto the sample is an important variable in matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Measurement of the beam profile is, therefore, an important factor within MALDI-MS. In this study a simple, low-cost fluorometric laser beam profiling technique is presented and applied in conjunction with MALDI-MS experiments. A comparison of the beam profile information afforded by a commercial system and the fluorometric method is carried out to determine the variation of beam profile for an Nd:YVO4 laser operated between 1 and 25 kHz. The beam profile information can be used, in conjunction with corresponding ion yields, to inform MALDI-MS experiments. The fluorometric beam profiling technique is used to obtain information about the beam dimensions as incident upon the MALDI-MS sample plate in-source. These values are compared with equivalent information obtained from ablation of thin film α-cyano-4-hydroxycinnamic acid (CHCA). In this study, area estimation by ablation provided a value 1.6 times smaller than that obtained by the fluorometric method, demonstrating the need for caution when measuring beam profile and, therefore, fluence, in MALDI-MS. ᅟ