Rapid screening and determination of designer drugs in saliva by a nib‐assisted paper spray‐mass spectrometry and separation technique

A method for the rapid screening and determination of amphetamine‐type designer drugs in saliva by a novel nib‐assisted paper spray‐mass spectrometry procedure is described. Under optimized conditions, the limit of detections for amphetamine derivatives (model samples: o‐, m‐, p‐chloroamphetamine and o‐, m‐, p‐fluoroamphetamine, respectively) were determined to 0.1 μg/mL by the nib‐assisted paper spray‐mass spectrometry method. This method is easier and has a higher sensitivity than similar methodologies, including atmospheric pressure/matrix‐assisted laser desorption ionization mass spectrometry and electrospray‐assisted laser desorption ionization/mass spectrometry. Data obtained using more classical separation methods, including liquid chromatography and capillary electrophoresis, are also reported.     

Silver‐109‐based laser desorption/ionization mass spectrometry method for detection and quantification of amino acids

A new methodology applicable for both high‐resolution laser desorption/ionization mass spectrometry and mass spectrometry imaging of amino acids is presented. The matrix‐assisted laser desorption ionization‐type target containing monoisotopic cationic ¹⁰⁹Ag nanoparticles (¹⁰⁹AgNPs) was used for rapid mass spectrometry measurements of 11 amino acids of different chemical properties. Amino acids were directly tested in 100,000‐fold concentration change conditions ranging from 100 μg/mL to 1 ng/mL which equates to 50 ng to 500 fg of amino acid per measurement spot. Limit of detection values obtained suggest that presented method/target system is among the fastest and most sensitive ones in laser mass spectrometry. Mass spectrometry imaging of spots of human blood plasma spiked with amino acids showed their surface distribution allowing optimization of quantitative measurements.     

Platinum Nanoflowers on Scratched Silicon by Galvanic Displacement for an Effective SALDI Substrate

We report a new and facile method for synthesizing 3D platinum nanoflowers (Pt Nfs) on a scratched silicon substrate by electroless galvanic displacement and discuss the applications of the Pt Nfs in surface‐assisted laser desorption/ionization‐mass spectrometry (SALDI‐MS). Surface scratching of n‐type silicon is essential to induce Pt Nf growth on a silicon substrate (to obtain a Pt Nf silicon hybrid plate) by the galvanic displacement reaction. The Pt Nf silicon hybrid plate showed excellent SALDI activity in terms of the efficient generation of protonated molecular ions in the absence of a citrate buffer. We propose that the acidity of the Si? OH moieties on silicon increases because of the electron‐withdrawing nature of the Pt Nfs; hence, proton transfer from the Si? OH groups to the analyte molecules is enhanced, and finally, thermal desorption of the analyte ions from the surface occurs. Signal enhancement was observed for protonated molecular ions produced from a titania nanotube array (TNA) substrate on which Pt nanoparticles had been photochemically deposited. Moreover, surface modification of the Pt Nf silicon hybrid plate by perfluorodecyltrichlorosilane (FDTS) (to obtain an FDTS‐Pt Nf silicon hybrid plate) was found to facilitate soft SALDI of labile compounds. More interestingly, the FDTS‐Pt Nf silicon hybrid plate acts 1) as a high‐affinity substrate for phosphopeptides and 2) as a SALDI substrate. The feasibility of using the FDTS‐Pt Nf silicon hybrid plate for SALDI‐MS has been demonstrated by using a β‐casein digest and various analytes, including small molecules, peptides, phosphopeptides, phospholipids, carbohydrates, and synthetic polymers. The hybridization of Pt Nfs with a scratched silicon substrate has been found to be important for achieving excellent SALDI activity.     

碳纳米管为基质对氨基酸的MALDI-FT MS分析

Twenty common amino acids have been analyzed successfully by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using carbon nanotubes as matrix. From the spectra, little or no background interference or fragmentation of the analytes has been observed. This method was also applied to the analysis of amino acid mixture successfully. Carbon nanotubes have some features such as large surface area to disperse the analyte molecules sufficiently and prevent the sample aggregation and strong ultraviolet absorption to transfer energy easily to the analyte molecules. The present method has potential application for the rapid and sensitive analysis of amino acids and their mixture.     

Mass spectrometry imaging of surface lipids on intact Drosophila melanogaster flies

The spatial distribution of neutral lipids and semiochemicals on the surface of six‐day‐old separately reared naive Drosophila melanogaster flies has been visualized and studied using matrix‐assisted laser desorption/ionization‐time of flight (MALDI‐TOF) mass spectrometry and laser‐assisted desorption/ionization (LDI)‐TOF imaging (MSI). Metal targets were designed for two‐dimensional MSI of the surface of 3‐D biological objects. Targets with either simple grooves or profiled holes designed to accurately accommodate the male and female bodies were fabricated. These grooves and especially holes ensured correct height fixation and spatial orientation of the flies on the targets after matrix application and sample drying. For LDI‐TOF to be used, the flies were arranged into holes and fixed to a plane of the target using fast‐setting glue. In MALDI‐TOF mode, the flies were fixed as above and sprayed with a lithium 2,5‐dihydroxybenzoate matrix using up to 100 airbrush spray cycles. The scanning electron microscopy images revealed that the deposits of matrix were homogenous and the matrix formed mostly into the clusters of crystals (40–80 µm) that were separated from each other by an uncovered cuticle surface (30–40 µm). The MSI using target with profiled holes provided superior results to the targets with simple grooves, eliminating the ion suppression/mass deviation due to the 3‐D shape of the flies. Attention was paid to neutral lipids and other compounds including the male anti‐attractant 11‐cis‐vaccenyl acetate for which the expected distribution with high concentration on the tip of the male abdomen was confirmed. The red and blue mass shift (PlusMinus1 colour scale) was observed associated with mass deviation predominantly between ±0.2 and 0.3 Da. We use in‐house developed software for mass recalibration, to eliminate the mass deviation effects and help with the detection of low‐intensity mass signals. Copyright © 2014 John Wiley & Sons, Ltd.     

Matrix pre‐coated targets for high throughput MALDI imaging of proteins

We have developed matrix pre‐coated targets for imaging proteins in thin tissue sections by matrix‐assisted laser desorption/ionization mass spectrometry. Gold covered microscope slides were coated with sinapinic acid (SA) in batches in advance and were shown to be stable for over 6 months when kept in the dark. The sample preparation protocol using these SA pre‐coated targets involves treatment with diisopropylethylamine (DIEA)‐H₂O vapor, transforming the matrix layer to a viscous ionic liquid. This SA‐DIEA ionic liquid layer extracts proteins and other analytes from tissue sections that are thaw mounted to this target. DIEA is removed by the immersion of the target into diluted acetic acid, allowing SA to co‐crystallize with extracted analytes directly on the target. Ion images (3–70 kDa) of sections of mouse brain and rat kidney at spatial resolution down to 10 µm were obtained. Use of pre‐coated slides greatly reduces sample preparation time for matrix‐assisted laser desorption/ionization imaging while providing high throughput, low cost and high spatial resolution images. Copyright © 2014 John Wiley & Sons, Ltd.     

Combining MALDI‐2 and transmission geometry laser optics to achieve high sensitivity for ultra‐high spatial resolution surface analysis

A transmission geometry optical configuration allows for smaller laser spot size to facilitate high‐resolution matrix‐assisted laser/desorption ionization (MALDI) mass spectrometry. This increase in spatial resolution (ie, smaller laser spot size) is often associated with a decrease in analyte signal. MALDI‐2 is a post‐ionization technique, which irradiates ions and neutrals generated in the initial MALDI plume with a second orthogonal laser pulse, and has been shown to improve sensitivity. Herein, we have modified a commercial Orbitrap mass spectrometer to incorporate a transmission geometry MALDI source with MALDI‐2 capabilities to improve sensitivity at higher spatial resolutions.     

The role of physical and chemical properties of Pd nanostructured materials immobilized on inorganic carriers on ion formation in atmospheric pressure laser desorption/ionization mass spectrometry

Fundamental parameters influencing the ion‐producing efficiency of palladium nanostructures (nanoparticles [Pd‐NP], nanoflowers, nanofilms) during laser irradiation were studied in this paper. The nanostructures were immobilized on the surface of different solid inorganic carrier materials (porous and mono‐crystalline silicon, anodic porous aluminum oxide, glass and polished steel) by using classical galvanic deposition, electroless local deposition and sputtering. It was the goal of this study to investigate the influence of both the nanoparticular layer as well as the carrier material on ion production for selected analyte molecules. Our experiments demonstrated that the dimensions of the synthesized nanostructures, the thickness of the active layers, surface disorders, thermal conductivity and physically or chemically adsorbed water influenced signal intensities of analyte ions during surface‐assisted laser desorption/ionization (SALDI) while no effects such as plasmon resonance, photoelectric effect or catalytic activity were expected to occur. Excellent LDI abilities were seen for Pd‐NPs immobilized on steel, while Pd nanoflowers on porous silicon exhibited several disadvantages; viz, strong memory effects, dependency of the analytical signal on amount of physically and chemically adsorbed water inside porous carrier, reduced SALDI activity from unstable connections between Pd and semiconductor material, decrease of the melting point of pure silicon after Pd immobilization and resulting strong laser ablation of metal/semiconductor complex, as well as significantly changed surface morphology after laser irradiation. The analytical performance of Pd‐NP/steel was further improved by applying a hydrophobic coating to the steel surface before galvanic deposition. This procedure increased the distance between Pd‐NPs, thus reducing thermal stress upon LDI; it simultaneously decreased spot sizes of deposited sample solutions. Copyright © 2014 John Wiley & Sons, Ltd.     

Mixed iridium(III) and ruthenium(II) polypyridyl complexes containing poly(ε‐caprolactone)‐bipyridine macroligands

A hydroxy‐functionalized bipyridine ligand was polymerized with ε‐caprolactone utilizing the controlled ring‐opening polymerization of ε‐caprolactone in the presence of stannous octoate. The resulting poly(ε‐caprolactone)‐containing bipyridine was characterized by ¹H NMR and IR spectroscopy, and gel permeation chromatography, as well as matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, revealing the successful incorporation of the bipyridine ligand into the polymer chain. Coordination to iridium(III) and ruthenium(II) precursor complexes yielded two macroligand complexes, which were characterized by NMR, gel permeation chromatography, matrix‐assisted laser desorption/ionization time‐of‐flight MS, cyclic voltammetry, and differential scanning calorimetry. In addition, both photophysical and electrochemical properties of the metal‐containing polymers proved the formation of a trisruthenium(II) and a trisiridium(III) polypyridyl species, respectively. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4153–4160, 2004     

Laser desorption/ionization mass spectrometric analysis of small molecules using fullerene‐derivatized silica as energy‐absorbing material

In spite of the growing acceptance of matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry for the analysis of a wide variety of compounds, including polymers and proteins, its use in analyzing low‐molecular‐weight molecules (<1000m/z) is still limited. This is mainly due to the interference of matrix molecules in the low‐mass range. Here the derivatized fullerenes covalently bound to silica particles with different pore sizes are applied as thin layer for laser desorption/ionization (LDI) mass spectrometric analysis. Thus, an interference of intrinsic matrix ions can be eliminated or minimized in comparison with the state‐of‐the‐art weak organic acid matrices. The desorption/ionization ability of the developed fullerene–silica materials depends on the applied laser power, sample preparation and pore size of the silica particles. Thus, fullerene–silica serves as an LDI support for mass spectrometric analysis of molecules (<1500 Da). The performance of the fullerene–silica is demonstrated by the mass analysis of variety of small molecules such as carbohydrates, amino acids, peptides, phospholipids and drugs. Copyright © 2010 John Wiley & Sons, Ltd.     

Direct profiling of saccharides,organic acids and anthocyanins in fruits using electrospray droplet impact/secondary ion mass spectrometry

Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from atmospheric‐pressure electrospray are accelerated in vacuum by several kV and impact on the sample deposited on the metal substrate. In this study, we applied EDI/SIMS directly to fruits, such as bananas, strawberries, grapes and apples. The major components in the fruits – fructose, glucose, sucrose and organic acids – could be observed with strong signal intensities. EDI/SIMS was also applied to the analysis of different regions of strawberries and apples. Compared with matrix‐assisted laser desorption/ionization (MALDI), ion signals with lower background signals could be obtained, particularly for the low molecular weight analytes. Copyright © 2010 John Wiley & Sons, Ltd.     

Trypsin and MALDI matrix pre‐coated targets simplify sample preparation for mapping proteomic distributions within biological tissues by imaging mass spectrometry

Prefabricated surfaces containing α‐cyano‐4‐hydroxycinnamic acid and trypsin have been developed to facilitate enzymatic digestion of endogenous tissue proteins prior to matrix‐assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS). Tissue sections are placed onto slides that were previously coated with α‐cyano‐4‐hydroxycinnamic acid and trypsin. After incubation to promote enzymatic digestion, the tissue is analyzed by MALDI IMS to determine the spatial distribution of the tryptic fragments. The peptides detected in the MALDI IMS dataset were identified by Liquid chromatography‐tandem mass spectrometry/mass spectrometry. Protein identification was further confirmed by correlating the localization of unique tryptic fragments originating from common parent proteins. Using this procedure, proteins with molecular weights as large as 300 kDa were identified and their distributions were imaged in sections of rat brain. In particular, large proteins such as myristoylated alanine‐rich C‐kinase substrate (29.8 kDa) and spectrin alpha chain, non‐erythrocytic 1 (284 kDa) were detected that are not observed without trypsin. The pre‐coated targets simplify workflow and increase sample throughput by decreasing the sample preparation time. Further, the approach allows imaging at higher spatial resolution compared with robotic spotters that apply one drop at a time. Copyright © 2016 John Wiley & Sons, Ltd.     

Charge assisted laser desorption/ionization mass spectrometry of droplets

We propose and evaluate a new mechanism to account for analyte ion signal enhancement in ultraviolet-laser desorption mass spectrometry of droplets in the presence of corona ions. Our new insights are based on timing control of corona ion production, laser desorption, and peptide ion extraction achieved by a novel pulsed corona apparatus. We demonstrate that droplet charging rather than gas-phase ion-neutral reactions is the major contributor to analyte ion generation from an electrically isolated droplet. Implications of the new mechanism, termed charge assisted laser desorption/ionization (CALDI), are discussed and contrasted with those of the laser desorption atmospheric pressure chemical ionization method (LD-APCI). It is also demonstrated that analyte ion generation in CALDI occurs with external electric fields about one order of magnitude lower than those needed for atmospheric pressure matrix assisted laser desorption/ionization or electrospray ionization of droplets.     

Effects of substrate surfaces in matrix‐assisted laser desorption/ionization mass spectrometry

For matrix‐assisted laser desorption/ionization (MALDI) mass spectra, undesirable ion contamination can occur due to the direct laser excitation of substrate materials (i.e., laser desorption/ionization (LDI)) if the samples do not completely cover the substrate surfaces. In this study, comparison is made of LDI processes on substrates of indium and silver, which easily emit their own ions upon laser irradiation, and conventional materials, stainless steel and gold. A simultaneous decrease of ion intensities with the number of laser pulses is observed as a common feature. By the application of an indium substrate to the MALDI mass spectrometry of alkali salts and alkylammonium salts mixed with matrices, 2,5‐dihydroxybenzoic acid (DHB) or N‐(4‐methoxybenzylidene)‐4‐butylaniline (MBBA), the mixing of LDI processes can be detected by the presence of indium ions in the mass spectra. This method has also been found to be useful for investigating the intrinsic properties of the MALDI matrices: DHB samples show an increase in the abundance of fragment ions of matrix molecules and cesium ions with the number of laser pulses irradiating the same sample spot; MBBA samples reveal a decrease in the level of background noise with an increase in the thickness of the sample layer. Copyright © 2009 John Wiley & Sons, Ltd.     

On‐target separation of analyte with 3‐aminoquinoline/α‐cyano‐4‐hydroxycinnamic acid liquid matrix for matrix‐assisted laser desorption/ionization mass spectrometry

3‐Aminoquinoline/α‐cyano‐4‐hydroxycinnamic acid (3AQ/CHCA) is a liquid matrix (LM), which was reported by Kumar et al. in 1996 for matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry. It is a viscous liquid and has some advantages of durability of ion generation by a self‐healing surface and quantitative performance. In this study, we found a novel aspect of 3AQ/CHCA as a MALDI matrix, which converges hydrophilic material into the center of the droplet of analyte‐3AQ/CHCA mixture on a MALDI sample target well during the process of evaporation of water derived from analyte solvent. This feature made it possible to separate not only the buffer components, but also the peptides and oligosaccharides from one another within 3AQ/CHCA. The MALDI imaging analyses of the analyte‐3AQ/CHCA droplet indicated that the oligosaccharides and the peptides were distributed in the center and in the whole area around the center of 3AQ/CHCA, respectively. This 'on‐target separation' effect was also applicable to glycoprotein digests such as ribonuclease B. These features of 3AQ/CHCA liquid matrix eliminate the requirement for pretreatment, and reduce sample handling losses thus resulting in the improvement of throughput and sensitivity. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluation of a new matrix-free laser desorption/ionization method through statistic studies: comparison of the DIAMS (desorption/ionization on self-assembled monolayer surface) method with the MALDI and TGFA-LDI techniques

This work demonstrates that the desorption/ionization on self-assembled monolayer surface (DIAMS) mass spectrometry, a recent matrix-free laser desorption/ionization (LDI) method based on an organic target plate, is as statistically repeatable and reproducible as matrix assisted laser desorption ionization (MALDI) and thin gold film-assisted laser desorption/ionization (TGFA-LDI) mass spectrometries. On lipophilic DIAMS of target plates with a mixture of glycerides, repeatability/reproducibility has been estimated at 15 and 30% and the relative detection limit has been evaluated at 0.3 and 3 pmol, with and without NaI respectively. Salicylic acid and its d(6)-isomer analysis confirm the applicability of the DIAMS method in the detection of compounds of low molecular weight.     

Novel ionization processes for use in mass spectrometry: ‘Squeezing’ nonvolatile analyte ions from crystals and droplets

Together with my group and collaborators, I have been fortunate to have had a key role in the discovery of new ionization processes that we developed into new flexible, sensitive, rapid, reliable, and robust ionization technologies and methods for use in mass spectrometry (MS). Our current research is focused on how best to understand, improve, and use these novel ionization processes which convert volatile and nonvolatile compounds from solids or liquids into gas‐phase ions for analysis by MS using e.g. mass‐selected fragmentation and ion mobility spectrometry to provide reproducible, accurate, and improved mass and drift time resolution. In my view, the apex was the discovery of vacuum matrix‐assisted ionization (vMAI) in 2012 on an intermediate pressure matrix‐assisted laser desorption/ionization (MALDI) source without the use of a laser, high voltages, or any other added energy. Only exposure of the matrix:analyte to the sub‐atmospheric pressure of the mass spectrometer was necessary to initiate ionization. These findings were initially rejected by three different scientific journals, with comments related to ‘how can this work?’, ‘where do the charges come from?’, and ‘it is not analytically useful’. Meanwhile, we and others have demonstrated analytical utility without a complete understanding of the mechanism. In reality, MALDI and electrospray ionization are widely used in science and their mechanisms are still controversially discussed despite use and optimization of now 30 years. This Perspective covers the applications and mechanistic aspects of the novel ionization processes for use in MS that guided us in instrument developments, and provides our perspective on how they relate to traditional ionization processes.     

Measurement of laser activated electron tunneling from semiconductor zinc oxide to adsorbed organic molecules by a matrix assisted laser desorption ionization mass spectrometer

Measurement of light induced heterogeneous electron transfer is important for understanding of fundamental processes involved in chemistry, physics and biology, which is still challenging by current techniques. Laser activated electron tunneling (LAET) from semiconductor metal oxides was observed and characterized by a MALDI (matrix assisted laser desorption ionization) mass spectrometer in this work. Nanoparticles of ZnO were placed on a MALDI sample plate. Free fatty acids and derivatives were used as models of organic compounds and directly deposited on the surface of ZnO nanoparticles. Irradiation of UV laser (λ = 355 nm) with energy more than the band gap of ZnO produces ions that can be detected in negative mode. When TiO₂ nanoparticles with similar band gap but much lower electron mobility were used, these ions were not observed unless the voltage on the sample plate was increased. The experimental results indicate that laser induced electron tunneling is dependent on the electron mobility and the strength of the electric field. Capture of low energy electrons by charge-deficient atoms of adsorbed organic molecules causes unpaired electron-directed cleavages of chemical bonds in a nonergodic pathway. In positive detection mode, electron tunneling cannot be observed due to the reverse moving direction of electrons. It should be able to expect that laser desorption ionization mass spectrometry is a new technique capable of probing the dynamics of electron tunneling. LAET offers advantages as a new ionization dissociation method for mass spectrometry.     

Analysis of three different types of fullerene derivatives by laser desorption/ionization time‐of‐flight mass spectrometry with new matrices

Three different types of fullerene derivatives, namely methano[60]fullerene dicarboxylate esters, [60]fulleropyrrolidines, and imino[60]fullerenes, were analyzed by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry using trans‐4‐tert‐butyl‐4′‐nitrostilbene (TBNS), 1,8,9‐anthracenetriol (dithranol), 6‐aza‐2‐thiothymine (ATT), 2,5‐dihydroxybenzoic acid (DHB) and carbazole as matrices. Unit mass resolution (sufficient to clearly resolve isotopic peaks), high signal‐to‐noise ratio, and clean mass spectra for all analytes were acquired by the optimization of experimental parameters and choice of optimal solvent for the matrix and molar matrix‐to‐analyte ratio. The new matrix, TBNS, gave the best results in the positive‐ion mode, as it can provide higher yields of analyte molecular ions at a lower laser threshold than the other four matrices, together with a very low degree of unwanted fragmentations. In the negative‐ion mode dithranol was better than TBNS, and the other three matrices gave relatively poor mass spectra for these fullerene derivatives. Copyright © 2005 John Wiley & Sons, Ltd.