Matrix-free Laser Desorption Ionization Research Articles

Matrix-free laser desorption/ionization mass spectrometry imaging for rapid evaluation of wood biomass conversion.

This study overcomes traditional biomass analysis limitations by introducing a pioneering matrix-free laser desorption/ionization (LDI) approach in mass spectrometry imaging (MSI) for efficient lignin evaluation in wood. The innovative acetic acid-peracetic acid (APA) treatment significantly enhances lignin detection, enabling high-throughput, on-site analysis. Wood slices, softwood from a conifer tree (Japanese cypress) and hardwood from a broadleaf tree (Japanese beech), were analyzed using MSI with a Fourier transform ion cyclotron resonance mass spectrometer. The developed APA treatment demonstrated effectiveness for MSI analysis of biomass. Our imaging technique successfully distinguishes between earlywood and latewood and enables the distinct visualization of lignin in these and other wood tissues, such as the radial parenchyma. This approach reveals significant contrasts in MSI. It has identified intense ions from β-O-4-type lignin, specifically in the radial parenchyma of hardwood, highlighting the method's precision and utility in wood tissue analysis. The benefits of matrix-free LDI include reduced peak overlap, consistent sample quality, preservation of natural sample properties, enhanced analytical accuracy, and reduced operational costs. This innovative approach is poised to become a standard method for rapid and precise biomass evaluation and has important applications in environmental research and sustainable resource management and is crucial for the effective management of diverse biomass, paving the way towards a sustainable, circular society.

Preparation and evaluation of in situ photocleavable mass tags with facile mass variation for matrix-free laser desorption ionization mass spectrometry.

Mass tags have been used for the precise identification, quantification, and characterization of macrobiomolecules and small organic molecules. Existing research has not yet demonstrated the preparation of a series of trityl-based photocleavable mass tags (PMTs) with similar structures but different molecular weights and mass variability. Herein, we introduce the design and synthesis of trityl-based in situ PMTs that generate heterolytic photocleavable cationic species upon laser irradiation. Mass variation of the PMTs was achieved via a simple conjugation reaction in the final step of synthesis. We prepared a series of PMTs with similar structures but different molecular weights and performed organic matrix-free laser desorption/ionization mass spectrometry (LDI MS) analysis. The practical applicability of the PMTs was evaluated by conjugating PMTs to oligonucleotides and utilizing them for detecting specific oligonucleotide targets combined with a mass signal amplification strategy. Quantitative aspects were also evaluated to verify the capability of the mass tags for multiplexed detection and the quantification of targets. The LDI MS analysis clearly demonstrated in situ heterolytic photocleavage that formed trityl cation peaks with high S/N ratios and high sensitivity. We strongly believe that the developed mass tags and LDI MS are useful alternatives to conventional signal transduction methods used for biosensors, such as surface plasmon resonance, electrochemical redox, and fluorescence.

Matrix free laser desorption ionization assisted by 13C NMR dereplication: A complementary approach to LC-MS2 based chemometrics

The identification of bioactive constituents from complex mixtures of Natural Products (NPs) is an important field of applied chemometrics. It requires the thorough chemical analysis of sample material, which is generally facilitated by liquid chromatography coupled to high-resolution tandem mass spectrometry (LC-MS2). Providing a simple and time efficient complementary approach, the current work presents the use of matrix free laser desorption ionization mass spectrometry (LDI-MS) within a chemometric model that allows the identification of activity markers in complex mixtures. As a working example the correct prediction and consecutive isolation of NPs with notable anti-AGEs effects from crude bark extracts of Garcinia parvifolia is discussed. Results were thoroughly compared to those obtained by a concurrently performed LC-MS2 analyses. In addition, the 13C NMR dereplication tool MixONat was successfully used to confirm the structures of high yield activity markers directly from complex mixtures prior to their isolation. Overall LDI-MS yielded comparable results to LC-MS2 and should be considered as complementary technique in applied chemometrics.

Fractionation and characterization of dissolved organic matter using solid-phase extraction followed by Fourier transform ion cyclotron resonance mass spectrometry with electrospray, atmospheric pressure photoionization, and laser desorption ionization.

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with different ionization techniques provides a powerful means to characterize dissolved organic matter (DOM) at the molecular level. Solid-phase extraction (SPE) is currently the most widely utilized method for extracting the DOM, but one-step elution using methanol does not provide a comprehensive picture of DOM. The development of efficient extraction and enrichment methods as well as characterization techniques from water samples remains a priority for DOM research, which is investigated in this study. The DOM was extracted from lake water by SPE using one-step elution (methanol or dichloromethane) and multistep elution (water, methanol, acetone, and dichloromethane). A combination of electrospray ionization (ESI), atmospheric pressure photoionization (APPI), and matrix-free laser desorption ionization (LDI) was utilized for FT-ICR MS analysis in both positive (+) and negative (-) ion modes. The total recovery of the multistep elution was 23.5% higher as compared to the investigated one-step elution procedure (85% vs. 61.6%); however, a comparison of the observed molecular species and the range of diversity under different ionization techniques along with the statistical analyses showed that proper selection of solvent and ionization method was required to explore specific compounds from the sample. For DOM species containing different heteroatoms, a combination of ESI, APPI, and LDI can offer a comprehensive profile of DOM in aquatic ecosystems. The specific molecular formulae of each ionization technique are characterized as follows: ESI- mode exhibited strong selectivity for lignin-like and tannins-like species with high oxygen content, as well as organosulfates. ESI+ favored lipid species and peptide/protein compounds. Unsaturated and condensed aromatic hydrocarbons with low oxygen were preferably ionized by both APPI and LDI.

Tissue Imprinting on 2D Nanoflakes-Capped Silicon Nanowires for Lipidomic Mass Spectrometry Imaging and Cancer Diagnosis.

Spatially resolved tissue lipidomics is essential for accurate intraoperative and postoperative cancer diagnosis by revealing molecular information in the tumor microenvironment. Matrix-free laser desorption ionization mass spectrometry imaging (LDI-MSI) is an emerging attractive technology for label-free visualization of metabolites distributions in biological specimens. However, the development of LDI-MSI technology that could conveniently and authentically reveal molecular distribution on tissue samples is still a challenge. Herein, we present a tissue imprinting technology by retaining tissue lipids on 2D nanoflakes-capped silicon nanowires (SiNWs) for further mass spectrometry imaging and cancer diagnosis. The 2D nanoflakes were prepared by liquid exfoliation of molybdenum disulfide (MoS2) with nitrogen-doped graphene quantum dots (NGQDs), which serve as both intercalation agent and dispersant. The obtained NGQD@MoS2 nanoflakes were then decorated on the tip of vertical SiNWs, forming a hybrid NGQD@MoS2/SiNWs nanostructure, which display excellent lipid extraction ability, enhanced LDI efficiency and molecule imaging capability. The peak number and total ion intensity of different lipids species on animal lung tissues obtained by tissue imprinting LDI-MSI on NGQD@MoS2/SiNWs were ∼4-5 times greater than those on SiNWs substrate. As a proof-of-concept demonstration, the NGQD@MoS2/SiNWs nanostructure was further applied to visualize phospholipids on sliced non small cell lung cancer (NSCLC) tissue along with the adjacent normal tissue. On the basis of selected feature lipids and machine learning algorithm, a prediction model was constructed to discriminate NSCLC tissues from the adjacent normal tissues with an accuracy of 100% for the discovery cohort and 91.7% for the independent validation cohort.

Matrix Free Laser Desorption Ionization Assisted by 13C NMR Dereplication: An Alternative Approach to LC-MS2 Based Chemometrics

Matrix Free Laser Desorption Ionization Assisted by 13C NMR Dereplication: An Alternative Approach to LC-MS2 Based Chemometrics

Molecular Mapping of Neutral Lipids Using Silicon Nanopost Arrays and TIMS Imaging Mass Spectrometry.

We demonstrate the utility of combining silicon nanopost arrays (NAPA) and trapped ion mobility imaging mass spectrometry (TIMS IMS) for high spatial resolution and specificity mapping of neutral lipid classes in tissue. Ionization of neutral lipid species such as triglycerides (TGs), cholestryl esters (CEs), and hexosylceramides (HexCers) from biological tissues has remained a challenge for imaging applications. NAPA, a matrix-free laser desorption ionization substrate, provides enhanced ionization efficiency for the above-mentioned neutral lipid species, providing complementary lipid coverage to matrix-assisted laser desorption ionization (MALDI). The combination of NAPA and TIMS IMS enables imaging of neutral lipid species at 20 μm spatial resolution while also increasing molecular coverage greater than 2-fold using gas-phase ion mobility separations. This is a significant improvement with respect to sensitivity, specificity, and spatial resolution compared to previously reported imaging studies using NAPA alone. Improved specificity for neutral lipid analysis using TIMS IMS was shown using rat kidney tissue to separate TGs, CEs, HexCers, and phospholipids into distinct ion mobility trendlines. Further, this technology allowed for the separation of isomeric species, including mobility resolved isomers of Cer(d42:2) (m/z 686.585) with distinct spatial localizations measured in rat kidney tissue section.

A thorough evaluation of matrix-free laser desorption ionization on structurally diverse alkaloids and their direct detection in plant extracts.

Alkaloids represent a major group of natural products (NPs), derived from highly diverse organisms. These structurally varied specialized metabolites are widely used for medicinal purposes and also known as toxic contaminants in agriculture and dietary supplements. While the detection of alkaloids is generally facilitated by GC- or LC-MS, these techniques do require considerable efforts in sample preparation and method optimization. Bypassing these limitations and also reducing experimental time, matrix-free laser desorption ionization (LDI) and related methods may provide an interesting alternative. As many alkaloids show close structural similarities to matrices used in matrix-assisted laser desorption ionization (MALDI), they should ionize upon simple laser irradiation without matrix support. With this in mind, the current work presents a systematic evaluation of LDI properties of a wide range of structurally diverse alkaloids. Facilitating a direct comparison between LDI and ESI-MS fragmentation, all tested compounds were further studied by electrospray ionization (ESI). Moreover, crude plant extracts of Atropa belladonna, Cinchona succirubra, and Colchicum autumnale were analyzed by LDI in order to evaluate direct alkaloid detection and dereplication from complex mixtures. Finally, dose-dependent evaluation of MALDI and LDI detection using an extract of Rosmarinus officinalis spiked with atropine, colchicine, or quinine was conducted. Overall, present results suggest that LDI provides a versatile analytical tool for analyzing structurally diverse alkaloids as single compounds and from complex mixtures. It may further serve various potential applications ranging from quality control to the screening for toxic compounds as well as the build up of MS databases. Graphical abstract.

Single Cell Metabolomics: A Future Tool to Unmask Cellular Heterogeneity and Virus-Host Interaction in Context of Emerging Viral Diseases.

Viral emergence is an unpredictable but obvious event, particularly in the era of climate change and globalization. Efficient management of viral outbreaks depends on pre-existing knowledge and alertness. The potential hotspots of viral emergence often remain neglected and the information related to them is insufficient, particularly for emerging viruses. Viral replication and transmission rely upon usurping the host metabolic machineries. So altered host metabolic pathways can be exploited for containment of these viruses. Metabolomics provides the insight for tracing out such checkpoints. Consequently introspection of metabolic alteration at virus-host interface has evolved as prime area in current virology research. Chromatographic separation followed by mass spectrometry has been used as the predominant analytical platform in bulk of the analyses followed by nuclear magnetic resonance (NMR) and fluorescence based techniques. Although valuable information regarding viral replication and modulation of host metabolic pathways have been extracted but ambiguity often superseded the real events due to population effect over the infected cells. Exploration of cellular heterogeneity and differentiation of infected cells from the nearby healthy ones has become essential. Single cell metabolomics (SCM) emerges as necessity to explore such minute details. Mass spectrometry imaging (MSI) coupled with several soft ionization techniques such as electrospray ionization (ESI), laser ablation electrospray ionization (LAESI), matrix assisted laser desorption/ionization (MALDI), matrix-free laser desorption ionization (LDI) have evolved as the best suited platforms for SCM analyses. The potential of SCM has already been exploited to resolve several biological conundrums. Thus SCM is knocking at the door of virus-host interface.

Mass spectrometry imaging of triglycerides in biological tissues by laser desorption ionization from silicon nanopost arrays.

Mass spectrometry imaging (MSI) is used increasingly to simultaneously detect a broad range of biomolecules while mapping their spatial distributions within biological tissue sections. Matrix-assisted laser desorption ionization (MALDI) is recognized as the method-of-choice for MSI applications due in part to its broad molecular coverage. In spite of the remarkable advantages offered by MALDI, imaging of neutral lipids, such as triglycerides (TGs), from tissue has remained a significant challenge due to ion suppression of TGs by phospholipids, e.g. phosphatidylcholines (PCs). To help overcome this limitation, silicon nanopost array (NAPA) substrates were introduced to selectively ionize TGs from biological tissue sections. This matrix-free laser desorption ionization (LDI) platform was previously shown to provide enhanced ionization of certain lipid classes, such as hexosylceramides (HexCers) and phosphatidylethanolamines (PEs) from mouse brain tissue. In this work, we present NAPA as an MSI platform offering enhanced ionization efficiency for TGs from biological tissues relative to MALDI, allowing it to serve as a complement to MALDI-MSI. Analysis of a standard lipid mixture containing PC(18:1/18:1) and TG(16:0/16:0/16:0) by LDI from NAPA provided an ~49 and ~227-fold higher signal for TG(16:0/16:0/16:0) relative to MALDI, when analyzed without and with the addition of a sodium acetate, respectively. In contrast, MALDI provided an ~757 and ~295-fold higher signal for PC(18:1/18:1) compared with NAPA, without and with additional Na+ . Averaged signal intensities for TGs from MSI of mouse lung and human skin tissues exhibited an ~105 and ~49-fold increase, respectively, with LDI from NAPA compared with MALDI. With respect to PCs, MALDI provided an ~2 and ~19-fold increase in signal intensity for mouse lung and human skin tissues, respectively, when compared with NAPA. The complementary coverage obtained by the two platforms demonstrates the utility of using both techniques to maximize the information obtained from lipid MS or MSI experiments.

Mass Spectrometry Imaging of Lipids in Human Skin Disease Model Hidradenitis Suppurativa by Laser Desorption Ionization from Silicon Nanopost Arrays

Neutral lipids have been implicated in a host of potentially debilitating human diseases, such as heart disease, type-2 diabetes, and metabolic syndrome. Matrix-assisted laser desorption ionization (MALDI), the method-of-choice for mass spectrometry imaging (MSI), has led to remarkable success in imaging several lipid classes from biological tissue sections. However, due to ion suppression by phospholipids, MALDI has limited ability to efficiently ionize and image neutral lipids, such as triglycerides (TGs). To help overcome this obstacle, we have utilized silicon nanopost arrays (NAPA), a matrix-free laser desorption ionization (LDI) platform. Hidradenitis suppurativa (HS) is a chronic, recurrent inflammatory skin disease of the apocrine sweat glands. The ability of NAPA to efficiently ionize lipids is exploited in the analysis of human skin samples from sufferers of HS. Ionization by LDI from NAPA allows for the detection and imaging of a number of neutral lipid species, including TGs comprised of shorter, odd-chain fatty acids, which strongly suggests an increased bacterial load within the host tissue, as well as hexosylceramides (HexCers) and galabiosyl-/lactosylceramides that appear to be correlated with the presence of HS. Our results demonstrate that NAPA-LDI-MSI is capable of imaging and potentially differentiating healthy and diseased human skin tissues based on changes in detected neutral lipid composition.

Polydopamine-Modified Substrates for High-Sensitivity Laser Desorption Ionization Mass Spectrometry Imaging.

Mass spectrometry imaging (MSI) serves as a powerful tool for biological research, and laser desorption ionization (LDI) is used as a major sampling ionization method. Study of materials for LDI represents a major field in the MSI research, either for matrices in matrix-assisted LDI (MALDI) or sample substrates allowing matrix-free LDI. In this study, we developed a composite substrate using polydopamine (PDA) film to coat an antireflection (AR) surface for LDI-MSI. The AR material has been previously shown to confine UV energy within the micro-/nanostructures, leading to a highly localized temperature rise to facilitate analyte thermal desorption. PDA coating on the AR material further enhances the light-to-heat conversion and improves the contact between the substrate surface and the biological sample materials. With this substrate, desorption and ionization of lipids from raw human plasma samples and biological tissue sections have been achieved. Matrix-free LDI-MSI of around 30 lipid species in mouse brain sections was achieved with a significantly simplified MSI procedure at a spatial resolution of 50 μm. This method was applied to determine mouse fatty liver disease through monitoring the abundances and distributions of triacylglycerols and glycerophospholipids. Dramatic differences in the lipid profiles were subsequently identified between the liver tissues from the wild-type and obese mice.

Study of the structural ligand effects on the fragmentation pattern of some Schiff base complexes of V (IV), Cu(II) and Ni (II) against the IR laser radiation using Matrix-free LDI-TOF technique

In this work, the time of flight (TOF) mass spectra of several Schiff base complexes of V(IV), Ni (II) and Cu(II) were recorded using matrix-free laser desorption-ionization (Direct-LDI) technique at λ = 1064 nm. Two different substrates including simple aluminum foil and its anodized form were used. The change in the mass spectra patterns due to the structural variation of the ligand for the complexes with similar central metal atoms was studied. It was found that specific substitutions in the structure of ligand decrease the fragmentation of the complex against the laser radiation, so that the peak related to the precursor ion appears in the mass spectra. It was also observed that the type of substrate affects the fragmentation pattern of the complexes.

Digital interference microscopy and density reconstruction of picosecond infrared laser desorption at the water-air interface

Material ablation and evaporation using pulsed infrared lasers pose promising approaches for matrix-free laser desorption ionization and in laser surgery. For the best results, key parameters such as laser wavelength, pulse duration, and pulse energy need to be carefully adjusted to the application. We characterize the dynamics at the water-air interface induced by a 10 ps infrared laser tuned to the water absorption band at 3 μm, a parameter set facilitating stress confined desorption for typical absorption depths in biological samples and tissue. By driving the ablation faster than nucleation growth, cavitation induced sample damage during the ablation process can be mitigated. The resultant explosive ablation process leads to a shock front expansion and material ejection which we capture using off-axis digital interference microscopy, an interference technique particularly useful for detecting the phase shift caused by transparent objects. It is demonstrated that the method can yield local density information of the observed shock front with a single image acquisition as compared to the usually performed fit of the velocity extracted from several consecutive snapshots. We determine the ablation threshold to be (0.5±0.2) J cm−2 and observe a significant distortion of the central parts of the primary shock wave above approximately 2.5 J cm−2. The differences in plume shape observed for higher fluences are reflected in an analysis based on shock wave theory, which shows a very fast initial expansion.

Matrix-free laser desorption ionization mass spectrometry as a functional tool for the analysis and differentiation of complex phenolic mixtures in propolis: a new approach to quality control.

Matrix-free laser desorption ionization (LDI) is a rapid and versatile technique for the ionization of small, UV-light-absorbing molecules. Indeed, many natural products such as polyphenols exhibit inherent LDI properties, potentially facilitating their detection from highly complex samples such as crude extracts. With this in mind, the present work thoroughly evaluated the potential of LDI as an analytical tool for the chemical profiling and differentiation of propolis samples obtained from different global regions. Propolis is a complex bee product containing, among others, significant amounts of phenolic constituents that may show LDI effects. The present work will demonstrate that LDI not only provides reproducible and highly specific fingerprint spectra for each of the tested samples, it further allows their clear differentiation by principal compound analysis (PCA). Contrary to classical analytical approaches such as LC- or GC-MS, LDI does not require time-consuming sample preparation and method optimization procedures. Thus, the technique represents a most interesting analytical tool and potent supplement to classic LC-MS for quality control of herbal pharmaceuticals and dietary supplements. Present results clearly support this approach and further suggest the use of LDI as a versatile tool for the automated analysis of large sample batches on an industrial scale. Graphical abstract ᅟ.

Fabrication of polyaniline/silver composite coating as a dual-functional platform for microextraction and matrix-free laser desorption/ionization

Polyaniline/silver (PANI/Ag) composites were prepared with a one-step synthesis method, and fabricated into solid-phase microextraction fiber coatings through three different means, i.e. in situ growth, gluing and chemical bonding. This work explored the utility of the as-fabricated PANI/Ag SPME fibers as dual-functional platforms for microextraction and matrix-free laser desorption/ionization (LDI) of the target analyte. The results showed that after incorporating Ag into the PANI polymer, the LDI efficiency was improved by over two times, which was 92.8% the LDI efficiency of the commercial LDI plate. In addition, the in situ grown fiber coatings were observed the most sensitive as dual platforms for the analysis of bifenthrin. The signal intensity was increased over twenty times by using in situ grown fiber, comparing to direct analysis with commercial LDI plate, which were attributed to the excellent extraction ability of PANI backbone, as well as the high LDI efficiency of the PANI/Ag composites. Low detection limit (lower than 10ngL−1) and good fiber-to-fiber repeatability (relative standard deviation 11.0%, n=3) were obtained by using the in situ grown coating for the analysis of bifenthrin, and bifenthrin in real water samples was successfully analyzed. The work successfully coupled SPME with matrix-free LDI, which greatly simplified the analytical process and improved the analytical sensitivity.

Rapid Surface Heating Promotes Laser Desorption Ionization of Thermally Labile Molecules from Surfaces

In recent years, matrix-free laser desorption ionization (LDI) for mass spectrometry of thermally labile molecules has been an important research subject in the pursuit of new ionization methods to serve as alternatives to the conventional matrixassisted laser desorption ionization (MALDI) method. While many recent studies have reported successful LDI of thermally labile molecules from various surfaces, mostly from surfaces with nanostructures, understanding of what drives the LDI process still requires further study. This article briefly reviews the thermal aspects involved in the LDI mechanism, which can be characterized as rapid surface heating. The thermal mechanism was supported by observed LDI and postsource decay (PSD) of peptide ions produced from flat surfaces with special thermal properties including amorphous Si (a-Si) and tungsten silicide (WSix). In addition, the concept of rapid surface heating further suggests a practical strategy for the preparation of LDI sample plates, which allows us to choose various surface materials including crystalline Si (c-Si) and Au tailorable to specific applications.

Design and synthesis of new mass tags for matrix-free laser desorption ionization mass spectrometry (LDI-MS) based on 6,11-dihydrothiochromeno[4,3-b]indole

We have rationally designed and synthesized new mass tags that are heterolytically cleavable upon UV-irradiation; these tags are based on a 6,11-dihydrothiochromeno[4,3-b]indole skeleton. Unless exposed to UV light, the dithioacetal group maintained its stability under normal conditions. After chemical conjugation of the mass tags with biomolecules of interest, such as proteins, the resulting conjugates efficiently and selectively generated the corresponding mass-tag fragment ions without the aid of a matrix under laser desorption/ionization (LDI) conditions. We envision that these new dithioacetal-based tags would provide a new platform of the so-called matrix-free laser desorption ionization mass spectrometry (LDI-MS), that would allow multiple detection of biomarkers with high sensitivity and selectivity. The limit of detection (LOD) of these tags was measured to be 5 fmol in the case of non-conjugated mass tag themselves and 2.8 fmol in the case of mass-tag-conjugated myoglobin.

A technique for relative quantitation of cancer biomarkers in formalin-fixed, paraffin-embedded (FFPE) tissue using stable-isotope-label based mass spectrometry imaging (SILMSI).

We developed a novel technique for the relative quantitation of pairs of cancer biomarkers in formalin-fixed paraffin-embedded (FFPE) tissue. The method utilizes stable isotope labeled (SIL) chromogens deposited during the standard immunohistochemistry (IHC) tissue staining process. The labeled chromogens are precipitated on tissue enzymatically using the standard IHC protocols. The tissue is then imaged with matrix-free laser desorption ionization time-of-flight mass spectrometry, and peak intensities of reporter ions are used to estimate the relative quantitation of protein biomarkers across the tissue. The relative abundance of two breast cancer biomarkers, estrogen receptor (ER) and progesterone receptor (PgR), were quantitated using their ratio of expression in xenograft models, and the ratios were found to be reproducible both within and across serial sections. The relative quantification of multiple biomarkers in situ across a single tissue section adds an additional dimension in cancer histological evaluation by allowing a visual and statistical assessment of tumor heterogeneity. Copyright © 2015 John Wiley & Sons, Ltd.

Spatial distribution of theobromine--a low MW drug--in tissues via matrix-free NALDI-MS imaging.

The ability of nano-assisted laser desorption-ionization mass spectrometry imaging (NALDI-IMS) to provide selective chemical monitoring with appropriate spatial distribution of a low molecular drug in a biological tissue was investigated. NALDI-IMS is a matrix-free laser desorption ionization (LDI) protocol based on imprinting of tissue constituents on a nanostructured surface. Using the accumulation of theobromine in rat kidney as a model, NALDI-IMS was found to provide well-resolved images of the special distribution of this low molecular weight (MW) drug in tissue.