Application of infrared laser desorption vacuum-UV single-photon ionization mass spectrometry for analysis of organic compounds from particulate matter filter samples

A new built instrument suitable for laser desorption-single photon ionization time-of-flight mass spectrometry (LD-SPI-TOFMS) with use of Vacuum-UV photons with a wavelength of 118 nm was used for the analysis of organic compounds. Fragmentation-free analysis of a variety of substances was achieved for desorption experiments with pure compounds desorbed from quartz glass filters applying low desorption energies. It was further found that the rate of fragmentation is strongly dependent on the desorption energy. Matrix effects were investigated by desorption experiments utilizing soot spiked with several organic compounds.The characteristics of the desorption process are assessed in more detail and the impact on the analysis of ambient particulate matter (PM) samples on filters are discussed. First results obtained from the application of the technique to the analysis of organic compounds from ambient PM are presented. Furthermore, possibilities of future developments of the method, in particular for analysis of ambient PM, are discussed.     

Online laser desorption-multiphoton postionization mass spectrometry of individual aerosol particles: molecular source indicators for particles emitted from different traffic-related and wood combustion sources

Direct inlet aerosol mass spectrometry plays an increasingly important role in applied and fundamental aerosol and nanoparticle research. Laser desorption/ionization (LDI) based techniques for single particle time-of-flight mass spectrometry (LDI-SP-TOFMS) are a promising approach in the chemical analysis of single aerosol particles, especially for the detection of inorganic species and distinction of particle classes. However, until now the detection of molecular organic compounds on a single particle basis has been difficult due to the high laser power densities which are required for the LDI process as well as due to the inherent matrix effects associated with this ionization technique. By the application of a two-step approach, where an IR desorption laser pulse is applied to perform a gentle desorption of organic material from the single particle surface and a second UV-laser performs the soft ionization of the desorbed species, this drawback of laser based single particles mass spectrometry can be overcome. The postionization of the desorbed molecules has been accomplished in this work by resonance enhanced multiphoton ionization (REMPI) using a KrF excimer laser (248 nm). REMPI allows an almost fragmentation free trace analysis of polycyclic aromatic hydrocarbons (PAHs) and their derivatives from individual single particles (laser desorption-REMPI postionization-single particle-time-of-flight mass spectrometry or LD-REMPI-SP-TOFMS). Crucial system parameters of the home-built aerosol mass spectrometer such as the power densities and the relative timing of both lasers were optimized with respect to the detectability of particle source specific organic signatures using well characterized standard particles. In a second step, the LD-REMPI-SP-TOFMS system was applied to analyze different real world aerosols (spruce wood combustion, gasoline car exhaust, beech wood combustion, and diesel car exhaust). It was possible to distinguish the particles from different sources by their molecular signature. Finally, exemplary ambient aerosol measurements have been carried out, which demonstrate the potential of the method for investigating urban aerosol and making contributions to source attribution studies.     

Observation of low molecular weight poly(methylsilsesquioxane)s by graphite plate laser desorption/ionization time-of-flight mass spectrometry

Mass spectra of polystyrene and poly(methylsilsesquioxane)(PMSSQ) derived from methyltriethoxysilane(MTES) were obtained in the 100-1,000 Da range by laser desorption/ionization time-of-flight mass spectrometry using a graphite plate without a matrix. Clean mass spectra were obtained without interference from carbon clusters or other low molecular weight compounds. Initial reaction products derived from condensation of partially hydrolyzed MTES were observed. Upon 30 min of heating at 30 degrees C, the ethoxy groups were fully hydrolyzed to hydroxy groups. Many PMSSQ species consistent with predictable polymerization reaction pathways involving intermolecular condensation and intramolecular dehydration were observed. Thus, laser desorption/ionization time-of-flight mass spectrometry using a graphite plate, without added matrix materials, is shown to provide valuable information on low molecular weight polymer not available by MALDI-TOF-MS.     

Thin layer chromatography/plasma assisted multiwavelength laser desorption ionization mass spectrometry for facile separation and selective identification of low molecular weight compounds

A novel plasma assisted multiwavelength (1064, 532, and 355 nm) laser desorption ionization mass spectrometry (PAMLDI-MS) system was fabricated and applied in the analysis of low molecular weight compounds through combination with thin layer chromatography (TLC). The TLC/PAMLDI-MS system successfully integrated TLC, the multiwavelength laser ablation, and the excitated state plasma from direct analysis in real time (DART) and was proved to be effective in the facile separation and selective identification of low molecular weight compounds. An automated three-dimensional platform was utilized to facilitate the analysis procedures with all the parameters of the TLC/PAMLDI-MS systematically optimized, and the desorption/ionization mechanisms were discussed. The successful combination of three-wavelength laser with DART based system extended the range of the analytes and provided broad possibilities for the compound desorption from the TLC. The experimental results clearly showed that the laser desorption was wavelength dependent. The PAMLDI-MS system was successfully applied in the detection of low molecular weight compounds from different kinds of samples separated on a normal-phase silica gel, such as dye mixtures, drug standards, and tea extract, with the detection level of 5 ng/mm(2).     

Detection of native protein ions in aqueous solution under ambient conditions by electrospray laser desorption/ionization mass spectrometry

Liquid electrospray laser desorption/ionization (ELDI) mass spectrometry allows desorption and ionization of proteins directly from aqueous solutions and biological fluids under ambient conditions. Native protein ions such as those of myoglobin, cytochrome c, and hemoglobin were obtained. A droplet (ca. 5 microL) containing the protein molecules and micrometer-sized particles (e.g., carbon graphite powder) is irradiated with a pulsed UV laser. The laser energy adsorbed by the inert particles is transferred to the surrounding solvent and protein molecules, leading to their desorption; the desorbed gaseous molecules are then postionized within an electrospray (ESI) plume to generate the ESI-like protein ions. With the use of this technique, we detected only the protonated protein ions in various biological fluids (including human tears, cow milk, serum, and bacterial extracts) without interference from their corresponding sodiated or potassiated adduct ions. In addition, we rapidly quantified the levels of glycosylated hemoglobin present in drops of whole blood obtained from diabetic patients without the need of sample pretreatment.     

Electrospray-assisted laser desorption/ionization mass spectrometry for continuously monitoring the states of ongoing chemical reactions in organic or aqueous solution under ambient conditions

Electrospray-assisted laser desorption/ionization (ELDI) combined with mass spectrometry allows chemical and biochemical compounds to be characterized directly from hydrophilic and hydrophobic organic solutions mixed with carbon powders under ambient conditions. Organic and inorganic compounds dissolved in polar or nonpolar solvent such as methanol, tetrahydrofuran, ethyl acetate, toluene, dichloromethane, or hexane can be detected using this ambient ionization technique without prior pretreatment. We have used this technique to monitor the progress in several ongoing reactions: the epoxidation of chalcone in ethanol, the chelation of ethylenediaminetetraacetic acid with copper and nickel ions in aqueous solution, the chelation of 1,10-phenanthroline with iron(II) in methanol, and the tryptic digestion of cytochrome c in aqueous solution. Liquid-ELDI analyses simply require irradiation of the surface of the sample solution with a pulsed ultraviolet laser; the laser energy is adsorbed by the carbon powder presuspended in the sample solution; the absorbed laser energy is then transferred to the surrounding solvent and to the analyte molecules in the solution, leading to their desorption; the desorbed gaseous analyte molecules are then postionized within an electrospray (ESI) plume to generate ESI-like analyte ions.     

Laser desorption and matrix-assisted laser desorption/ionization mass spectrometry of 29-kDa Au:SR cluster compounds

Positive and negative ions generated by laser-based ionization methods from three gold:thiolate cluster compounds are mass analyzed by time-of-flight mass spectrometry. The three compounds have similar inorganic core masses ( approximately 29 kDa, approximately 145 Au atoms) but different n-alkanethiolate ligands associated with each cluster compound (Au:SR, R = butane, hexane, dodecane). Irradiation of neat films (laser desorption/ionization) and films generated by dilution of the cluster compounds in an organic acid matrix (matrix-assisted laser desorption/ionization) with a nitrogen laser (337 nm) produced distinct ion abundances that are relevant to different structural aspects of the cluster compound. Laser desorption/ionization of neat Au:SR compound films produces ions consistent with the inorganic core mass (i.e., devoid of original hydrocarbon content). Matrix-assisted laser desorption/ionization produces either ions with m/z values consistent with the core mass of the cluster compounds or ions with m/z values consistent with the approximate molecular weight of the cluster compounds, depending on ionization conditions. The ion abundances, and ionization conditions under which they are detected, provide insight into desorption/ionization processes for these unique cluster compounds as well as other analytes typically studied by matrix-assisted laser desorption/ionization.     

Graphite/Liquid mixed matrices for laser desorption/ionization mass spectrometry

A combination of liquid matrix and graphite particulates (2 μm) has been proposed as a method suitable for the laser desorption/ionization mass spectrometry of peptides and proteins (Sunner, J.; et al. Anal. Chem. 1995, 67, 4335). Here we demonstrate the potential of this approach as a straightforward, and very general, method of achieving the ultraviolet laser desorption/ionization of a broad range of intermediate weight analytes. The desorption/ionization mechanism, the influence of preparative procedures, and the breadth of application of this methodology have been investigated. A simple and robust preparative procedure is presented for the analysis of proteins, oligosaccharides, and synthetic polymers. Detection sensitivities are in the femtomole region for lower molecular weight peptides and oligosaccharides. The graphite acts as an energy transfer medium by absorbing the UV radiation, leading to thermal desorption of the liquid matrix and analyte. The liquid matrix was observed to fulfill several important roles. In the case of peptides and proteins, which preferentially form protonated molecular ions, it acts as a protonating agent. It also enhances the signal intensities of cationized species (e.g., polysaccharides and polar polymers) by assisting their desorption. An excess of liquid matrix serves to cool the analyte during the desorption step and minimize decomposition. The presence of liquid matrices increases the sample lifetime at a particular desorption spot, minimizing the time-consuming search for "hot spots". The addition of cationizing salts has been shown to improve the quality of mass spectra obtained for polar polymers and extend the range of materials that can be investigated to include apolar synthetic polymers.     

On-line analysis of organic components in fine and ultrafine particles by photoionization aerosol mass spectrometry

A new method, photoionization aerosol mass spectrometry (PIAMS), is described for real-time analysis of organic components in airborne particles below approximately 300 nm in diameter. Particles are focused through an aerodynamic lens assembly into the mass spectrometer where they are collected on a probe in the source region. After a sufficient amount of sample has been collected, the probe is irradiated with a pulsed infrared laser beam to vaporize organic components, which are then softly ionized with coherent vacuum ultraviolet radiation at 118 nm (10.5 eV). Since the photon energy is close to the ionization energies of most organic compounds, fragmentation is minimized. Both aliphatic and aromatic compounds of atmospheric relevance are detected and quantified in the low- to midpicogram range. The photoionization signal intensity increases linearly with the amount of material sampled and is independent of particle size. The fragmentation induced by laser desorption is greater than that observed with thermal vaporization, suggesting that the internal energy imparted by the former is greater. Although some molecular fragmentation is observed, mass spectra from common sources of ambient organic aerosol are distinguishable and consistent with previous off-line measurements by gas chromatography/mass spectrometry. These results illustrate the potential of PIAMS for molecular characterization of organic aerosols in ambient and smog chamber measurements.     

Synthesis and Laser Desorption Fourier Transform Mass Spectrometry of Massive Fullerenes

Synthesis of buckminsterfullerene (C₆₀) is highly simple and the mechanism of the molecule formation is quite well understood. It makes easier the search for the optimum conditions necessary for formation of massive fullerenes (more than 100 carbon atoms). The most popular graphite arc method has been used for fullerene synthesis in our laboratories. Other available methods, like laser ablation and E-gun evaporation, were not suitable for achieving of the convenient conditions. Results of the synthesis have been analyzed by laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD FT/ICR-MS).

The conditions for massive fullerenes synthesis are discussed in the paper. The special care has been given to the LD FT/ICR-MS analysis. The importance of cooling gas during fullerene nucleation and growth in the very specific vacuum conditions is analyzed. The necessity for a thorough dynamic plasma diagnostic during the giant fullerenes growth is suggested.     

不同吸附量条件下氯化锶混合吸附剂-氨的导热系数与渗透率研究

氯化锶作为一种化学吸附剂,其主要特征是在吸附与解吸过程中会出现膨胀与结块现象,这种现象会进一步影响到吸附剂的导热系数与渗透率。为了确定在这种条件下氯化锶的导热系数与渗透率变化规律,研究中先选择膨胀石墨为基质对氯化锶进行了传热强化。即按照3种不同比例、2种不同固化密度配制了氯化锶/膨胀石墨固化混合吸附剂,然后在不同吸附量的条件下采用稳态法以及Ergun模型原理对样品进行了导热系数和渗透率测试。测试结果表明,随着制冷剂含量的升高,氯化锶化学吸附剂导热系数值增加幅度偏大。当配制比例为83%,固化密度为450 kg/m3时,其导热系数可从0.88W/(m?K)最高上升至1.61W/(m?K),同时氯化锶混合吸附剂渗透率随着制冷剂含量的升高而有所下降。     

Interpretation of mass spectra from organic compounds in aerosol time-of-flight mass spectrometry

Organic compounds containing a variety of functional groups have been analyzed using aerosol time-of-flight mass spectrometry. Both positive and negative laser desorption/ionization mass spectra have been acquired for compounds of relevance to ambient air particulate matter, including polycyclic aromatic hydrocarbons, heterocyclic analogues, aromatic oxygenated compounds such as phenols and acids, aliphatic dicarboxylic acids, and reduced nitrogen species such as amines. In many cases, positive ion mass spectra are similar to those found in libraries for 70-eV electron impact mass spectrometry. However, formation of even-electron molecular ions due to adduct formation also plays a major role in ion formation. Negative ion mass spectra suggest that organic compounds largely disintegrate into carbon cluster fragments (C(n)- and C(n)H-). However, information about the heteroatoms present in organic molecules, especially nitrogen and oxygen, is carried dominantly by negative ion spectra, emphasizing the importance of simultaneous analysis of positive and negative ions in atmospheric samples.     

Using volatile solvents for ion formation in liquid molecular beam expansion mass spectrometry

Cocrystallization between analyte and matrix is required by matrix-assisted laser desorption/ionization and can represent a significant limitation of the technique. A molecular beam expansion, mass spectrometric method has been developed to explore the possibility of using pure solvents as matrix to avoid cocrystallization. Two kinds of solvent, liquid CS2 and liquid or supercritical CO2, have been studied with 266-nm UV laser irradiation. We successfully ionized a number of compounds, including caffeine, guanine, cholesterol, and mixed fullerenes. Under some conditions, the mass spectra reflect parent radical cations formed by photoionization. Under other conditions, protonated, sodiated (and with CS2 even sulfated) ions are seen reflecting a nonunimolecular ionization process. When UV-transparent CO2 is used as a solvent, only analyte molecules with a UV chromophore are detected. However, with UV-absorbing CS2, we demonstrate ionization of molecules lacking a UV chromophore. This work provides strong evidence that one can form solvent clusters containing analyte, that laser photoionization of the solvent precedes ionization of the analyte, and that solvent evaporation along with the indirect ionization leads to reduced parent ion fragmentation. The exploration of this now demonstrated concept with other solvents would appear fruitful for future work.     

Using electrospray-assisted laser desorption/ionization mass spectrometry to characterize organic compounds separated on thin-layer chromatography plates

Electrospray-assisted laser desorption/ionization (ELDI), an ionization method that combines laser desorption and electrospray ionization (ESI), can be used under ambient conditions to characterize organic compounds (including FD&C dyes, amines, extracts of a drug tablet) separated in the central track on a thin-layer chromatography (TLC) plate coated with either reversed-phase C18 particles or normal-phase silica gel. After drying, the TLC plate was placed on an acrylic sample holder set in front of the sampling skimmer of an ion trap mass analyzer. The chemicals at the center of the TLC plate were analyzed by pushing the sample holder into the path of a laser beam with a syringe pump. The molecules in the sample spot were desorbed by continuously irradiating the surface of the TLC plate with a pulsed nitrogen laser. Then, the desorbed sample molecules entered an ESI plume where they were ionized through the reactions with the charged species (including protons, hydronium ions and their cluster ions, solvent ions, and charged droplets) generated by electrospraying a methanol/water solution. MS/MS analyses were also performed to further characterize the analytes. The detection limit of TLC/ELDI/MS is approximately 10(-6) M. This was evaluated by using FD&C red dye as the standard. A linear relationship was found for the calibration curve with the concentration of FD&C red dye ranged from 10(-3) to 10(-6) M.     

Building blocks for the development of an interface for high-throughput thin layer chromatography/ambient mass spectrometric analysis: a green methodology

Interfacing thin layer chromatography (TLC) with ambient mass spectrometry (AMS) has been an important area of analytical chemistry because of its capability to rapidly separate and characterize the chemical compounds. In this study, we have developed a high-throughput TLC-AMS system using building blocks to deal, deliver, and collect the TLC plate through an electrospray-assisted laser desorption ionization (ELDI) source. This is the first demonstration of the use of building blocks to construct and test the TLC-MS interfacing system. With the advantages of being readily available, cheap, reusable, and extremely easy to modify without consuming any material or reagent, the use of building blocks to develop the TLC-AMS interface is undoubtedly a green methodology. The TLC plate delivery system consists of a storage box, plate dealing component, conveyer, light sensor, and plate collecting box. During a TLC-AMS analysis, the TLC plate was sent to the conveyer from a stack of TLC plates placed in the storage box. As the TLC plate passed through the ELDI source, the chemical compounds separated on the plate would be desorbed by laser desorption and subsequently postionized by electrospray ionization. The samples, including a mixture of synthetic dyes and extracts of pharmaceutical drugs, were analyzed to demonstrate the capability of this TLC-ELDI/MS system for high-throughput analysis.     

High-reactivity matrices increase the sensitivity of matrix enhanced secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is a desorption/ionization method in which ions are generated by the impact of a primary ion beam on a sample. Classic matrix assisted laser desorption and ionization (MALDI) matrices can be used to increase secondary ion yields and decrease fragmentation in a SIMS experiment, which is referred to as matrix enhanced SIMS (ME-SIMS). Contrary to MALDI, the choice of matrices for ME-SIMS is not constrained by their photon absorption characteristics. This implies that matrix compounds that exhibit an insufficient photon absorption coefficient have the potential of working well with ME-SIMS. Here, we evaluate a set of novel derivatives of the classical MALDI matrices α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) for usability in ME-SIMS. This evaluation was carried out using peptide mixtures of different complexity and demonstrates significant improvements in signal intensity for several compounds with insufficient UV absorption at the standard MALDI laser wavelengths. Our study confirms that the gas-phase proton affinity of a matrix compound is a key physicochemical characteristic that determines its performance in a ME-SIMS experiment. As a result, these novel matrices improve the performance of matrix enhanced secondary ion mass spectrometry experiments on complex peptide mixtures.     

Easy ambient sonic-spray ionization mass spectrometry combined with thin-layer chromatography

On-spot detection and analyte characterization on thin-layer chromatography (TLC) plates is performed via ambient desorption/ionization and (tandem) mass spectrometry detection, that is, via easy ambient sonic spray ionization mass spectrometry (EASI-MS). As proof-of-principle cases, mixtures of semipolar nitrogenated compounds as well as pharmaceutical drugs and vegetable oils have been tested. The technique has also been applied to monitor a chemical reaction of synthetic importance. EASI is the simplest and gentlest ambient ionization technique currently available, assisted solely by N2 (or air). It uses no voltages, no electrical discharges; no UV or laser beams, and no high temperature and is most easily implemented in all API mass spectrometers. TLC is also the simplest, fastest, and most easily performed chromatographic technique. TLC plus EASI-MS therefore provide a simple and advantageous combination of chromatographic separation and sensitive detection of the TLC spots as well as on-spot MS or MS/MS characterization. The favorable characteristics of TLC-EASI-MS indicate advantageous applications in several areas such as drug and oil analysis, phytochemistry and synthetic chemistry, forensics via reliable counterfeit detection, and quality control.     

Desorption atmospheric pressure photoionization

An ambient ionization technique for mass spectrometry, desorption atmospheric pressure photoionization (DAPPI), is presented, and its application to the rapid analysis of compounds of various polarities on surfaces is demonstrated. The DAPPI technique relies on a heated nebulizer microchip delivering a heated jet of vaporized solvent, e.g., toluene, and a photoionization lamp emitting 10-eV photons. The solvent jet is directed toward sample spots on a surface, causing the desorption of analytes from the surface. The photons emitted by the lamp ionize the analytes, which are then directed into the mass spectrometer. The limits of detection obtained with DAPPI were in the range of 56-670 fmol. Also, the direct analysis of pharmaceuticals from a tablet surface was successfully demonstrated. A comparison of the performance of DAPPI with that of the popular desorption electrospray ionization method was done with four standard compounds. DAPPI was shown to be equally or more sensitive especially in the case of less polar analytes.     

Analysis of a GC/MS thermal desorption system with simultaneous sniffing for determination of off-odor compounds and VOCs in fumes formed during extrusion coating of low-density polyethylene

A thermal desorption equipment introducing volatile organic compounds (VOCs) into the gas chromatographic/ mass spectrometric system (GC/MS) with simultaneous sniffing (SNIFF) is a suitable method for identifying the volatile organic off-odor compounds formed during the extrusion coating process of low-density polyethylene. Fumes emitted during the extrusion coating process of three different plastic materials were collected at two different temperatures (285 and 315 degrees C) from an outgoing pipe and near an extruder. The VOCs of fumes were analyzed by drawing a known volume of air through the adsorbent tube filled with a solid adsorbent (Tenax GR). The air samples were analyzed by using a special thermal desorption device and GC/MS determination. The simultaneous sniffing was carried out to detect off-odors and to assist in the identification of those compounds that contribute to tainting and smelling. The amounts of off-odor carbonyl compounds and the total content of the volatile organic compounds were determined. The most odorous compounds were identified as carboxylic acids while the majority of the volatile compounds were hydrocarbons. The detection and quantification of carboxylic acids were based on the characteristic ions of their mass spectra. The higher the extrusion temperature the more odors were detected. An important observation was that the total concentration of volatiles was dependent not only on the extrusion temperature but also on the plastic material.     

Infrared laser ablation sample transfer for MALDI imaging

An infrared laser was used to ablate material from tissue sections under ambient conditions for direct collection on a matrix assisted laser desorption ionization (MALDI) target. A 10 μm thick tissue sample was placed on a microscope slide and was mounted tissue-side down between 70 and 450 μm from a second microscope slide. The two slides were mounted on a translation stage, and the tissue was scanned in two dimensions under a focused mid-infrared (IR) laser beam to transfer material to the target slide via ablation. After the material was transferred to the target slide, it was analyzed using MALDI imaging using a tandem time-of-flight mass spectrometer. Images were obtained from peptide standards for initial optimization of the system and from mouse brain tissue sections using deposition either onto a matrix precoated target or with matrix addition after sample transfer and compared with those from standard MALDI mass spectrometry imaging. The spatial resolution of the transferred material is approximately 400 μm. Laser ablation sample transfer provides several new capabilities not possible with conventional MALDI imaging including (1) ambient sampling for MALDI imaging, (2) area to spot concentration of ablated material, (3) collection of material for multiple imaging analyses, and (4) direct collection onto nanostructure assisted laser desorption ionization (NALDI) targets without blotting or ultrathin sections.