Time-Dependent Oxidation during Nano-Assisted Laser Desorption Ionization Mass Spectrometry: A Useful Tool for Structure Determination or a Source of Possible Confusion?

Ultrafast Photocatalytic Reaction Screening by Mass Spectrometry

Here we report a semiconductor-assisted laser desorption/ionization mass spectrometry (SA-LDI MS) platform to monitor photocatalytic reactions online and apply it for ultrafast reaction screening. In this method, we use photocatalytic nanomaterials as the substrate for LDI and then initiate and monitor the reactions simultaneously. The features of our method include the following: (i) It has a reaction acceleration effect: only seconds are needed in our interfacial reactions vs hours in conventional bulk phase. (ii) The reaction trend in our system agrees with that in bulk phase. (iii) By adding a stable analogue of reactant as internal standard, a quantification of the reaction can be achieved. (iv) The sensitivity is high: for 500 amol of reactant, the photocatalytic reaction can still be initiated and detected. This platform has advantages in ultrafast reaction screening (e.g., screening of nine catalysts requires 24 h by the UPLC-MS system but only 10 min by SA-LDI MS). Furthermore, the high specificity of MS enables the screening of catalytic selectivity of A-TiO₂ nanoparticles for a methyl red (MR) and acid yellow (AY) mixture, whose absorption wavelengths are overlapped and thus cannot be discriminated by conventional optical methods. Furthermore, by using SA-LDI MS, we also monitored reductive debrominations during the degradation process of polybrominated diphenyl ethers (PBDEs), which is a type of important pollutant that is difficult to degrade and detect in liquid phase, and the photocatalytic reduction of CO₂. Overall, SA-LDI MS realizes ultrafast photocatalytic reaction screening for the first time and provides practical analytical value in the field of catalyst screening.

Mass spectrometry imaging of illicit drugs in latent fingerprints by matrix-free and matrix-assisted desorption/ionization techniques

Compared with classical matrix-assisted laser-desorption ionization mass spectrometry (MALDI), the matrix free-based strategies generate a cleaner background, without significant noise or interference coming from an applied matrix, which is beneficial for the analysis of small molecules, such as drugs of abuse. In this work, we probed the detection efficiency of methamphetamine, heroin and cocaine in nanostructure-assisted laser desorption-ionization (NALDI) and desorption electrospray ionization and compared the sensitivity of these two matrix-free tools with a standard MALDI mass spectrometry experiment. In a typical mass spectrometry imaging (MSI) setup, papillary line latent fingerprints were recorded as a mixture a common skin fatty acid or interfering cosmetics with a drug. In a separate experiment, all drugs (1 µL of 1 μM standard solution) were detected by all three ionization techniques on a target. In the case of cocaine and heroin, NALDI mass spectrometry was the most sensitive and revealed signals even from 0.1 μM solution. The drug/drug contaminant (fatty acid or cosmetics) MSI approach could be used by law enforcement personnel to confirm drug abusers of having come into contact with the suspected drug by use of fingerprint scans at time of apprehension which can aid in reducing the work of lab officials.

Investigation and Applications of In-Source Oxidation in Liquid Sampling-Atmospheric Pressure Afterglow Microplasma Ionization (LS-APAG) Source

A liquid sampling-atmospheric pressure afterglow microplasma ionization (LS-APAG) source is presented for the first time, which is embedded with both electrospray ionization (ESI) and atmospheric pressure afterglow microplasma ionization (APAG) techniques. This ion source is capable of analyzing compounds with diverse molecule weights and polarities. An unseparated mixture sample was detected as a proof-of-concept, giving complementary information (both polarities and non-polarities) with the two ionization modes. It should also be noted that molecular mass can be quickly identified by ESI with clean and simple spectra, while the structure can be directly studied using APAG with in-source oxidation. The ionization/oxidation mechanism and applications of the LS-APAG source have been further explored in the analysis of nonpolar alkanes and unsaturated fatty acids/esters. A unique [M + O - 3H]⁺ was observed in the case of individual alkanes (C₅-C₁₉) and complex hydrocarbons mixture under optimized conditions. Moreover, branched alkanes generated significant in-source fragments, which could be further applied to the discrimination of isomeric alkanes. The technique also facilitates facile determination of double bond positions in unsaturated fatty acids/esters due to diagnostic fragments (the acid/ester-containing aldehyde and acid oxidation products) generated by on-line ozonolysis in APAG mode. Finally, some examples of in situ APAG analysis by gas sampling and surface sampling were given as well. Graphical Abstract ᅟ.

Surface oxidation under ambient air--not only a fast and economical method to identify double bond positions in unsaturated lipids but also a reminder of proper lipid processing

A simple, fast approach elucidated carbon-carbon double bond positions in unsaturated lipids. Lipids were deposited onto various surfaces and the products from their oxidation in ambient air were observed by electrospray ionization (ESI) mass spectrometry (MS). The most common oxidative products, aldehydes, were detected as transformations at the cleaved double bond positions. Ozonides and carboxylic acids were generated in certain lipids. Investigations of the conditions controlling the appearance of these products indicated that the surface oxidation depends on light and ambient air. Since the lipid oxidation was slower in a high concentration of ozone, singlet oxygen appeared to be a parallel oxidant for unsaturated lipids. The 3-hydroxyl group in the sphingoid base of sulfatides offered some protection from oxidation for the Δ4,5-double bond, slowing its oxidation rate relative to that of the isolated double bond in the N-linked fatty acyl chain. Direct sampling by thin-layer chromatography (TLC)-ESI-MS provides a powerful approach to elucidate detailed structural information on biological samples. Co-localization of the starting lipids and their oxidation products after TLC separation allowed assignment of the native unsaturation sites. Phosphatidylserine and N,N-dimethyl phosphatidylethanolamine isomers in a bovine brain total lipid extract were distinguished on the basis of their oxidation products. Meanwhile, the findings reported herein reveal a potential pitfall in the assignment of structures to lipids extracted from TLC plates because of artifactual oxidation after the plate development.

1,8-bis(dimethylamino)naphthalene/9-aminoacridine: a new binary matrix for lipid fingerprinting of intact bacteria by matrix assisted laser desorption ionization mass spectrometry

The effectiveness of a novel binary matrix composed of 1,8-bis(dimethylamino)naphthalene (DMAN; proton sponge) and 9-aminoacridine (9AA) for the direct lipid analysis of whole bacterial cells by matrix assisted laser desorption ionization mass spectrometry (MALDI MS) is demonstrated. Deprotonated analyte signals nearly free of matrix-related ions were observed in negative ion mode. The effect of the most important factors (laser energy, pulse voltage, DMAN/9AA ratio, analyte/matrix ratio) was investigated using a Box-Behnken response surface design followed by multi-response optimization in order to simultaneously maximize signal-to-noise (S/N) ratio and resolution. The chemical surface composition of single or mixed matrices was explored by X-ray photoelectron spectroscopy (XPS). Moreover, XPS imaging was used to map the spatial distribution of a model phospholipid in single or binary matrices. The DMAN/9AA binary matrix was then successfully applied to the analysis of intact Gram positive (Lactobacillus sanfranciscensis) or Gram negative (Escherichia coli) microorganisms. About fifty major membrane components (free fatty acids, mono-, di- and tri-glycerides, phospholipids, glycolipids and cardiolipins) were quickly and easily detected over a mass range spanning from ca. 200 to ca. 1600 m/z. Moreover, mass spectra with improved S/N ratio (compared to single matrices), reduced chemical noise and no formation of matrix-clusters were invariably obtained demonstrating the potential of this binary matrix to improve sensitivity.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Comparison of NIMS and MALDI platforms for neuropeptide and lipid mass spectrometric imaging in C. borealis brain tissue

Nanostructure-initiator mass spectrometry (NIMS) is a recently developed matrix-free laser desorption/ionization technique that has shown promise for peptide analyses. It is also useful in mass spectrometric imaging (MSI) studies of small molecule drugs, metabolites, and lipids, minimizing analyte diffusion caused by matrix application. In this study, NIMS and matrix-assisted laser desorption/ionization (MALDI) MSI of a crustacean model organism Cancer borealis brain were compared. MALDI was found to perform better than NIMS in these neuropeptide imaging experiments. Twelve neuropeptides were identified in MALDI MSI experiments whereas none were identified in NIMS MSI experiments. In addition, lipid profiles were compared using each ionization method. Both techniques provided similar lipid profiles in the m/z range 700 - 900.

Evaluation of matrix-assisted laser desorption/ionization mass spectrometry for second-generation lignin analysis

Matrix-Assisted Laser Desorption/Ionization time-of-flight (MALDI-TOF) mass spectrometry is evaluated as an elucidation tool for structural features and molecular weights estimation of some extracted herbaceous lignins. Optimization of analysis conditions, using a typical organic matrix, namely α-cyano-4-hydroxycinnamic acid (CHCA), in combination with α-cyclodextrin, allows efficient ionization of poorly soluble lignin materials and suppression of matrix-related ions background. Analysis of low-mass fragments ions (m/z 100–600) in the positive ion mode offers a “fingerprint” of starting lignins that could be a fine strategy to qualitatively identify principal inter-unit linkages between phenylpropanoid units. The molecular weights of lignins are estimated using size exclusion chromatography and compared to MALDI-TOF-MS profiles. Miscanthus (Miscanthus x giganteus) and Switchgrass (Panicum Virgatum L.) lignins, recovered after a formic acid/acetic acid/water process or aqueous ammonia soaking, are selected as benchmarks for this study.

Evaluation of nanoporous gold with controlled surface structures for laser desorption ionization (LDI) analysis: surface area versus LDI signal intensity

The structural effect of a nanoporous gold (NPG) surface on the signal intensities of laser desorption ionization-mass spectrometry (LDI-MS) were investigated using NPG surfaces with controlled structures. The relationship between surface area and LDI efficiency was compared and evaluated. Comparisons between bare flat gold and NPG surfaces show that nanostructures increased LDI efficiency. We also found that the LDI signal decreased with increasing depth of nanoporous layers, thus increasing the surface area. This result agrees with a previous report (Shin J. A. et al., J. Am. Soc. Mass Spectrom. 2010, 21, 989) in which the LDI efficiency of small molecules decreased for ZnO wires with longer lengths. This observation was explained by the penetration and deposition of samples into locations inaccessible to photons because of structural screening. The LDI-MS analysis of oils with NPG surfaces (but without matrix) showed the same trend whereby the NPG with about a 200 nm depth of porous area showed the highest sensitivity. This study clearly shows that the active surface area for solution chemistry can differ from LDI-MS and that NPGs can function as a substrate for LDI oil analysis.

Using ambient ozone for assignment of double bond position in unsaturated lipids

Unsaturated lipids deposited onto a range of materials are observed to react with the low concentrations of ozone present in normal laboratory air. Parent lipids and ozonolysis cleavage products are both detected directly from surfaces by desorption electrospray ionisation mass spectrometry (DESI-MS) with the resulting mass spectra providing clear evidence of the double bond position within these molecules. This serendipitous process has been coupled with thin-layer chromatography (TLC) to provide a simple but powerful approach for the detailed structural elucidation of lipids present in complex biological extracts. Lipid extracts from human lens were deposited onto normal phase TLC plates and then developed to separate components according to lipid class. Exposure of the developed plates to laboratory air for ca. 1 h prior to DESI-MS analysis gave rise to ozonolysis products allowing for the unambiguous identification of double bond positions in even low abundant, unsaturated lipids. In particular, the co-localization of intact unsaturated lactosylceramides (LacCer) with products from their oxidative cleavage provide the first evidence for the presence of three isomeric LacCer (d18:0/24:1) species in the ocular lens lipidome, i.e., variants with double bonds at the n-9, n-7 and n-5 positions.