On-line laser desorption/ionization mass spectrometry of matrix-coated aerosols

Matrix-assisted laser desorption/ionization (MALDI) was used for the on-line analysis of single particles. An aerosol was generated at atmospheric pressure and particles were introduced into a time-of-flight (TOF) mass spectrometer through a single-stage differentially pumped capillary inlet. Prior to entering the mass spectrometer, a matrix was added to the particles using a heated saturator and condenser. A liquid matrix, 3-nitrobenzyl alcohol (NBA), and a solid matrix, picolinic acid (PA), were used. Particles were ablated with a 351 nm excimer laser and the resulting ions were mass-separated in a two-stage reflectron TOF mass spectrometer. Aerosol particles containing the biomolecules erythromycin and gramicidin S were analyzed with and without the matrix addition step. The addition of NBA to the particles resulted in mass spectra that contained an intact molecular ion mass peak. In contrast, PA-coated particles did not yield molecular ion peaks from matrix-coated particles.     

乙苯光氧化产生二次有机气溶胶的化学成分及反应机理分析

在自制的烟雾腔内,研究羟基自由基(OH·)启动的乙苯的光氧化反应和一系列后续反应,产生了二次有机气溶胶. 采用空气动力学直径粒谱分析仪分析了气溶胶粒子的尺寸分布;并用自制的气溶胶飞行时间质谱仪快速、实时地测量了单个二次有机气溶胶粒子的分子组分. 初步探讨了这些组分的可能反应机理.     

基质与分析物的摩尔比和激光能量对分析物离子信号的影响

A method of aerosol introduction for matrix-assisted laser desorption/ionization (MALDI) is described. The aerosol particles containing matrix and analyte enter directly into the aerosol time-of-flight mass spectrometer (ATOFMS) at atmospheric pressure. The scattered light signals from the aerosol particles are collected by a photomultiplier tube (PMT) and are passed on to an external electronic timing circuit, which determines particle size and is used to trigger a 266 nm pulsed Nd:YAG laser. The aerosol MALDI mass spectra and aerodynamic diameter of single particles can be obtained in real-time. Compared with other methods of liquid sample introduction, this method realizes detection of single particles and, more importantly, the sample consumption is lower. The effects of matrix-to-analyte ratio and laser pulse energy on analyte ion yield are examined. The optimal matrix-to-analyte ratio and laser energy are 50-110:1 and 200-400 μJ respectively.     

Coupling two-step laser desorption/ionization with aerosol time-of-flight mass spectrometry for the analysis of individual organic particles

Analysis of organic compounds in aerosol particles using real-time single particle techniques is difficult because of extensive fragmentation that occurs in the laser desorption/ionization step. In an effort to avoid such fragmentation processes, we coupled a “soft” two-step laser desorption/ionization technique (L2MS) with aerosol time-of-flight mass spectrometry (ATOFMS). In these studies, we find this combination preserves intact organic molecules while providing the real-time mass spectra of suspended aerosol particles. Mass spectra of particles analyzed by one-step desorption mass spectrometry and L2MS are presented for comparison. These include 2,4-dihydroxybenzoic acid as a test case and wood and cigarette combustion particles as real world examples. This is the first published demonstration of L2MS performed on single particles not deposited on a substrate prior to analysis.     

Chemical Composition and Reaction Mechanisms for Secondary Organic Aerosol from Photooxidation of Toluene

A laboratory study was carried out to investigate the secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon toluene. The experiments were conducted by irradiating toluene/CH₃ONO/NO/air mixtures in a home‐made smog chamber. The aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real‐time. According to a large number of single aerosol diameters and mass spectra, we obtained the size distribution and chemical composition of SOA statistically. Expeperimental results showed that aerosol created by toluene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm (i.e. PM2.5), and the predominant components of aerosol are furane, methyl glyoxylic acid, phenol, benzaldehyde, benzyl alcohol, cresol, 3‐hydroxy‐2,4‐dioxo‐pentanal, methyl nitrophenol, and 5‐hydroxy‐4,6‐dioxo‐2‐heptenal. The possible reaction mechanisms leading to these products were also discussed.     

Real-time analysis of secondary organic aerosol particles formed from cyclohexene ozonolysis using a laser-ionization single-particle aerosol mass spectrometer.

A real-time analysis of secondary organic aerosol (SOA) particles formed from cyclohexene ozonolysis in a smog chamber was performed using a laser-ionization single-particle aerosol mass spectrometer (LISPA-MS). The instrument obtains both size and chemical compositions of individual aerosol particles with a high time-resolution (approximately 2 s at the maximum). Both positive and negative-ion mass spectra are obtained. Standard particles generated from dicarboxylic acid solutions using an atomizer were also analyzed. For both standard and SOA particles, the negative-ion mass spectra provided information about the molecular weights of the organic compounds in the particles, since the intense ions in the negative-ion mass spectra are mainly attributable to the molecular-related ions [M-H]-. It was demonstrated that the real-time single-particle analysis of SOA particles by the LISPA-MS technique can reveal the formation and transformation processes of SOA particle in smog chambers.     

Application of single-particle laser desorption/ionization time-of-flight mass spectrometry for detection of polycyclic aromatic hydrocarbons from soot particles originating from an industrial combustion process

Combustion-related soot particles were sampled in situ from the stoker system of a 0.5 MW incineration pilot plant (feeding material was wood) at two different heights over the feed bed in the third air supply zone. The collected particles were re-aerosolized by a powder-dispersing unit and analyzed by a single-particle laser desorption/ionization (LDI) time-of-flight mass spectrometer (aerosol-time-of-flight mass spectrometry, ATOFMS). The ATOFMS instrument characterizes particles according to their aerodynamic size (laser velocimetry) and chemical composition (LDI mass spectrometry). Chemical species from the particles are laser desorbed/ionized by 266 nm Nd:YAG laser pulses. ATOFMS results on individual 'real world' particles in general give information on the bulk inorganic composition. Organic compounds, which are of much lower concentrations, commonly are not detectable. However, recent off-line laser microprobe mass spectrometric (LMMS) experiments on bulk soot aerosol samples have emphasized that organic compounds can be desorbed and ionized without fragmentation in LDI experiments from black carbonaceous matrices. This paper reports the successful transfer of the off-line results to on-line analysis of airborne soot particles by ATOFMS. The detection of polycyclic aromatic hydrocarbons from soot particles is addressed in detail. The results are interpreted in the context of the recent LMMS results. Furthermore, their relevance with respect to possible applications in on-line monitoring of combustion processes is discussed.     

Measurement of initial translational energies of peptide ions in laser desorption/ionization mass spectrometry

The initial kinetic energy distribution of [Arg]-vasopressin molecular ions generated by matrix-assisted UV laser desorption/ionization was measured using a delayed ion extraction, linear time-of-flight mass spectrometer. Energy distributions of the nicotinic acid matrix ions, with or without the presence of peptide, were also measured. These were compared with the kinetic energy distribution of gramicidin-S ions using IR laser desorption. The measured molecular ion kinetic energy distribution from vasopressin is much broader than that from gramicidin-S, and is characterized by a high-energy tail that most likely results from entrainment of anlayte ions in the higher velocity matrix ions and fragments as they leave the surface.     

Online aerosol mass spectrometry of single micrometer-sized particles containing poly(ethylene glycol)

The analysis of poly(ethylene glycol) (PEG)-containing particles by online single particle aerosol mass spectrometers equipped with laser desorption/ionization (LDI) is reported. We demonstrate that PEG-containing particles are useful in the development of aerosol mass spectrometers because of their ease of preparation, low cost, and inherently recognizable mass spectra. Solutions containing millimolar quantities of PEGs were nebulized and, after drying, the resultant micrometer-sized PEG-containing particles were sampled. LDI (266 nm) of particles containing NaCl and PEG molecules of average molecular weight<500 Da generated mass spectra reminiscent of mass spectra of PEG collected by other mass spectrometer platforms including the characteristic distribution of positive ions (Na+ adducts) separated by the 44 m/z units of the ethylene oxide units separating each degree of polymerization. PEGs of average molecular weight>500 Da were detected from particles that also contained the tripeptide tyrosine-tyrosine-tyrosine or 2,5-dihydroxybenzoic acid, which were added to nebulized solutions to act as matrices to assist LDI using pulsed 266 nm and 355 nm lasers, respectively. Experiments were performed on two aerosol mass spectrometers, one reflectron and one linear, that each utilize two time-of-flight mass analyzers to detect positive and negative ions created from a single particle. PEG-containing particles are currently being employed in the optimization of our bioaerosol mass spectrometers for the application of measurements of complex biological samples, including human effluents, and we recommend that the same strategies will be of great utility to the development of any online aerosol LDI mass spectrometer platform.     

Real-time,single-particle measurements of ambient aerosols in Shanghai

As one of the major components of the earth’s atmosphere, airborne particulate matter (or aerosol) has strong effects on air quality, regional and global climate, and human health. In ambient atmosphere, the different sources and complex evolutionary history of aerosol particles make the study of their chemical and physical properties extremely challenging. The invention of an online single-particle aerosol mass spectrometer provides a powerful technique to determine the size and chemical composition of individual aerosol particles in real time. We deployed an aerosol time-of-flight mass spectrometer (ATOFMS) to carry out single particle measurement in the urban area of Shanghai in the past few years. In this review paper, we summarize our recent work on the identification of particle type, mixing state and aging process, and the application of the individual particle information to the source apportionment of primary aerosol, and the investigation of the formation mechanism of secondary aerosol in Shanghai. The special capabilities of single particle mass spectrometry are proven essential to these studies. Multi-functional technique combinations of ATOFMS with other state-of-art aerosol instruments are also discussed for future studies.     

Low‐Molecular Weight and Oligomeric Components in Secondary Organic Aerosol from the Photooxidation of p‐Xylene

A laboratory study was performed to investigate the composition of secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon p‐xylene. The experiments were conducted by irradiating p‐xylene/CH₃ONO/NO/air mixtures in a home‐made smog chamber. The aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real‐time. According to a large number of single aerosol diameters and mass spectra, the size distribution and chemical composition of SOA were determined statistically. Experimental results showed that aerosol created by p‐xylene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm (i.e. PM2.5), and aromatic aldehyde, unsaturated dicarbonys, hydroxyl dicarbonys, and organic acid are major product components in the SOA after 2 hours photooxidation. After aging for more than 8 hours, about 10% of the particle mass consists of oligomers with a molecular mass up to 600 daltons. The possible reaction mechanisms leading to these products are also proposed.     

Optimization of matrix-assisted laser desorption/ionization time-of-flight collision-induced dissociation using poly(ethylene glycol)

Details of the optimization of the collision-induced dissociation (CID) process, using a collision cell on a matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometer, are described using poly(ethylene glycol) 1000 (PEG 1000) as a model analyte. The effects of collision gas identity (helium, air, and argon), as well as collision gas pressure, on the resulting MS/MS data were investigated. With PEG 1000, helium was found to give the best results with respect to signal-to-noise (S/N) ratio. The optimum pressure for each gas was found to be in the range where the precursor ion signal was attenuated to approximately 30-50% for helium and 40-60% for argon. The effect of cation choice (Li, Na, and K) on the CID of PEG was also studied. CID spectra were produced for each, but PEG cationized with lithium was found to produce the spectra with the highest S/N ratio. The MALDI-TOF CID spectra that were generated for PEG were compared with the high-energy and low-energy MS/MS spectra obtained from a sector mass spectrometer and from a triple quadrupole mass spectrometer, respectively. The results observed for PEG confirm that CID on a MALDI-TOF mass spectrometer is a high-energy MS/MS technique.     

Desorption electrospray ionization of aerosol particles

We have applied desorption electrospray ionization to aerosol particles. Ions were formed from aerosols by merging suspended dry particles with an electrospray of solvent in a modified ion trap mass spectrometer. Dry aerosol particles were generated using a fluidized bed powder disperser and directed toward the inlet of the mass spectrometer. A nanospray source was used to create a spray of solvent droplets directed at the inlet and at a right angle with respect to the aerosol. Ions generated by the interaction of the particles and electrospray were transferred into the ion trap mass spectrometer. Using this method, pure samples of caffeine and erythromycin A were analyzed. In addition, commonly available food and drug powders including instant cocoa powder, artificial sweetener and ibuprofen were analyzed.     

Real-time detection of individual secondary organic aerosol particle from photooxidation of toluene using aerosol time of flight mass spectrometer

Aromatic hydrocarbons are air pollutants in urban atmosphere and mainly from anthropogenic sources, i.e., emission from automotive exhaust and productive process of industry, and they constitute an important fraction of total volatile organic compounds (VOCs). Photochemical ozone and secondary organic aerosols (SOA) could be formed from the photochemical proc- esses of aromatic hydrocarbons. SOA may impact visibility of air, formation of clouds, change of the climate, and human health serio…     

337 nm matrix-assisted laser desorption/ionization of single aerosol particles.

Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained from single particles injected directly into a time-of-flight mass spectrometer. Aerosol particles were generated at atmospheric pressure using a piezoelectric single-particle generator or a pneumatic nebulizer and introduced into the mass spectrometer through a series of narrow-bore tubes. Particles were detected by light scattering that was used to trigger a 337 nm pulsed nitrogen laser and the ions produced by laser desorption were mass separated in a two-stage reflectron time-of-flight mass spectrometer. MALDI mass spectra of single particles containing bradykinin, angiotensin II, gramicidin S, vitamin B(12) or gramicidin D were obtained at mass resolutions greater than 400 FWHM. For the piezoelectric particle generator, the efficiency of particle delivery was estimated to be approximately 0.02%, and 50 pmol of sample were consumed for each mass spectrum. For the pneumatic nebulizer, mass spectra could be obtained from single particles containing less than 100 amol of analyte, although the sample consumption for a typical mass spectrum was over 400 pmol.     

Influence Factors on Particle Growth for On-line Aerosol Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectrometry

An evaporation/condensation flow cell was developed and interfaced with the matrix-assistedlaser desorption/ionization (MALDI) time-of-flight mass spectrometer for on-line bioaerosoldetection and characterization, which allows matrix addition by condensation onto the laboratory-generated bioaerosol particles. The final coated particle exiting from the con-denser is then introduced into the aerodynamic particle sizer spectrometer or home-built aerosol laser time-of-flight mass spectrometer, and its aerodynamic size directly effects on the matrix-to-analyte molar ratio, which is very important for MALDI technique. In order to observe the protonated analyte molecular ion, and then determine the classification of bi-ological aerosols, the matrix-to-analyte molar ratio must be appropriate. Four experimental parameters, including the temperature of the heated reservoir, the initial particle size, its number concentration, and the matrix material, were tested experimentally to analyze their influences on the final particle size. This technique represents an on-line system of detec-tion that has the potential to provide rapid and reliable identification of airborne biological aerosols.     

Influence of experimental conditions on the liquid secondary ion mass spectra of sulfonated azo dyes

Two monosulfonated and eight disulfonated azo dyes of varying relative molecular mass were examined by liquid secondary ion mass spectrometry (LSIMS). The effects of matrix, concentration, primary beam energy, and mode of operation were addressed in order to optimize sample ionization, whilst minimizing interference from matrix ions. Seven matrices were investigated: glycerol, thioglycerol, 3-nitrobenzyl alcohol, diethanolamine, 2-hydroxyethyl disulfide, a 1:1 (v/v) mixture of 2-hydroxyethyl disulfide and thioglycerol, and a 1 : 3 (v/v) mixture of dithioerythritol and dithiothreitol. Of these matrices, 3-nitrobenzyl alcohol produced LSIMS spectra that exhibited the most intense sample ions and the least inteiference from matrix ions. Minimum concentrations of 0.4 μg/μl and 4 μg/μl (dye in matrix) were necessary to produce useful full-scan spectra for monosulfonated azo dyes and disulfonated azo dyes, respectively; maximum sample ion intensities were obtained with concentrations ranging from 20 μg/μl to 60 μg/μl. A primary ion beam (cesium) of 10 to 15 kV produced the greatest secondary ionization efficiency, and a negative-ion analysis mode produced more useful spectra than those obtained in the positive-ion mode.     

Characterization of surgical aerosols by the compact single-particle mass spectrometer LAMPAS 3

A new method is presented using an optical particle counter and the compact mobile laser mass spectrometer LAMPAS 3 for in situ analysis of single particles generated by electrosurgical dissection of biological tissues. The instrumental performance is demonstrated for analysing aerosol particles formed during rapid thermal evaporation of porcine liver and porcine kidney tissues. Particle number concentrations of up to 5,000 particles per cubic centimetre were detected during surgical dissection. Chemical analysis of tissue particles was performed by bipolar time-of-flight mass spectrometry. The application of an online mass spectrometric particle analysis for surgical aerosols is reported here for the first time.     

Characterization of ice‐nucleating bacteria using on‐line electron impact ionization aerosol mass spectrometry

The mass spectral signatures of airborne bacteria were measured and analyzed in cloud simulation experiments at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facility. Suspensions of cultured cells in pure water were sprayed into the aerosol and cloud chambers forming an aerosol which consisted of intact cells, cell fragments and residual particles from the agar medium in which the bacteria were cultured. The aerosol particles were analyzed with a high‐resolution time‐of‐flight aerosol mass spectrometer equipped with a newly developed PM_2.5 aerodynamic lens. Positive matrix factorization (PMF) using the multilinear engine (ME‐2) source apportionment was applied to deconvolve the bacteria and agar mass spectral signatures. The bacteria mass fraction contributed between 75 and 95% depending on the aerosol generation, with the remaining mass attributed to agar. We present mass spectra of Pseudomonas syringae and Pseudomonas fluorescens bacteria typical for ice‐nucleation active bacteria in the atmosphere to facilitate the distinction of airborne bacteria from other constituents in ambient aerosol, e.g. by PMF/ME‐2 source apportionment analyses. Nitrogen‐containing ions were the most salient feature of the bacteria mass spectra, and a combination of C₄H₈N⁺ (m/z 70) and C₅H₁₂N⁺ (m/z 86) may be used as marker ions. Copyright © 2015 John Wiley & Sons, Ltd.     

Identification of flavonoids using liquid chromatography with electrospray ionization and ion trap tandem mass spectrometry with an MS/MS library

Searchable MS/MS spectra libraries, constructed using the results of liquid chromatography coupled with electrospray ionization (ESI) tandem mass spectrometry (LC/MS/MS) with data-dependent acquisition on an ion trap mass spectrometer, are presented with regard to the identification and confirmation of a variety of closely related flavonoids in a set of biological samples. Flavonoids were found to exhibit a maximum amount of structurally specific MS/MS spectra at 45% of normalized collision energy on the instrument used, without wideband activation. These MS/MS spectra were then searched automatically against a 297-substance MS/MS library that contains many previously acquired spectra of standard flavonoids. The possible applications of this powerful technique to biological samples are also discussed. Daidzein and genistein were identified through the MS/MS spectra library while searching through LC/MS/MS data for plant and microbial extracts. Moreover, these compounds proved completely distinguishable from other flavonoids of closely related structures in the MS/MS spectra library, using the NIST MS search program. The applicability of the library-searchable spectra at low concentrations was demonstrated by successful identification of daidzein and genistein at 0.05 and 0.5 microg/mL, respectively.