Quantitative detection of aromatic compounds in single aerosol particle mass spectrometry

Laser-Pulse-Length Effects in Ultrafast Laser Desorption

Shortening the laser pulse length opens up new opportunities for laser desorption (LD) of molecules, with benefits for mass spectrometry (MS) sampling and ionization. The capability to ablate any material without the need for an absorbing matrix and the decrease of thermal damage and molecular fragmentation has promoted various applications with very different parameters and postionization techniques. However, the key issues of the optimum laser pulse length and intensity to achieve efficient and gentle desorption of molecules for postionization in MS are not resolved, although these parameters determine the costs and complexity of the required laser system. Here, we address this research gap with a systematic study on the effect of the pulse length on the LD of molecules. Keeping all other optical and ionization parameters constant, we directly compared the pulses in the femtosecond, picosecond, and nanosecond range with respect to LD-induced fragmentation and desorption efficiency. To represent real-world applications, we investigated the LD of over-the-counter medicaments naproxen and ibuprofen directly from tablets as well as the LD of retene and ship emission aerosols from a quartz filter. With our study design, we excluded interfering effects on fragmentation and LD efficiency from, for example, collisional cooling or postionization by performing the experiments in vacuum with resonance-enhanced multiphoton ionization as the postionization technique. Regarding LD-induced fragmentation, we already found benefits for the picosecond pulses. However, the efficiency of LD was found to continuously increase with decreasing pulse length, pointing to the application potential of ultrashort pulses in trace analytics. Because many interfering effects beyond the LD pulse length could be excluded in the experiment, our results may be directly transferable to the LD applied in other techniques.

Aerosol mass spectrometry of neutral species based on a tunable vacuum ultraviolet free electron laser

A vacuum ultraviolet free electron laser (VUV-FEL) photoionization aerosol mass spectrometer (AMS) has been developed for online measurement of neutral compounds in laboratory environments. The aerosol apparatus is mainly composed of a smog chamber and a reflectron time-of-flight mass spectrometer (TOF-MS). The indoor smog chamber had a 2 m³ fluorinated ethylene propylene film reactor placed in a temperature- and humidity-controlled room, which was used to generate the aerosols. The aerosols were sampled via an inlet system consisting of a 100 μm orifice nozzle and aerodynamic lenses. The application of this VUV-FEL AMS to the α-pinene ozonolysis under different concentrations reveals two new compounds, for which the formation mechanisms are proposed. The present findings contribute to the mechanistic understanding of the α-pinene ozonolysis in the neighborhood of emission origins of α-pinene. The VUV-FEL AMS method has the potential for chemical analysis of neutral aerosol species during the new particle formation processes.

Laser acceleration of absorbing particles

A Yb-doped fiber laser is used to accelerate and evaporate absorbing particles in air. Optical intensities of 1MW/cm² and 2MW/cm² illuminate stainless steel particles. These particles are accelerated to velocities of tens of meters per second before evaporating within a few tenths of a millisecond. Position measurements are made using direct imaging with a high-speed camera. A fundamental system of coupled differential equations to track particle momentum, velocity, mass, radius, temperature, vapor opacity, and temperature distribution is developed and shown to accurately model the trajectories and lifetimes of laser heated particles. Atoms evaporating from the particle impart momentum to the larger particle, which accelerates until it is slowed by drag forces. Heat transfer within the evaporating particles is dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars.

Chemical compositions of black carbon particle cores and coatings via soot particle aerosol mass spectrometry with photoionization and electron ionization

Black carbon is an important constituent of atmospheric aerosol particle matter (PM) with significant effects on the global radiation budget and on human health. The soot particle aerosol mass spectrometer (SP-AMS) has been developed and deployed for real-time ambient measurements of refractory carbon particles. In the SP-AMS, black carbon or metallic particles are vaporized through absorption of 1064 nm light from a CW Nd:YAG laser. This scheme allows for continuous "soft" vaporization of both core and coating materials. The main focus of this work is to characterize the extent to which this vaporization scheme provides enhanced chemical composition information about aerosol particles. This information is difficult to extract from standard SP-AMS mass spectra because they are complicated by extensive fragmentation from the harsh 70 eV EI ionization scheme that is typically used in these instruments. Thus, in this work synchotron-generated vacuum ultraviolet (VUV) light in the 8-14 eV range is used to measure VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra of commercially available carbon black, fullerene black, and laboratory generated flame soots were obtained. Small carbon cluster cations (C(+)-C5(+)) were found to dominate the VUV-SP-AMS spectra of all the samples, indicating that the corresponding neutral clusters are key products of the SP vaporization process. Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and SP-AMS spectra are used to confirm spectral features that could be used to distinguish between different types of refractory carbon particles. VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing cores are also examined and found to display less thermally induced decomposition and fragmentation than spectra obtained with thermal vaporization at 200 °C (the minimum temperature needed to quantitatively vaporize ambient oxidized organic aerosol with a continuously heated surface). The particle cores tested in these studies include black carbon, silver, gold, and platinum nanoparticles. These results demonstrate that SP vaporization is capable of providing enhanced organic chemical composition information for a wide range of organic coating materials and IR absorbing particle cores. The potential of using this technique to study organic species of interest in seeded laboratory chamber or flow reactor studies is discussed.

Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids

A new method of electrospray-assisted laser desorption/ionization (ELDI) mass spectrometry, which combines laser desorption with post-ionization by electrospray, was applied to rapid analysis of solid materials under ambient conditions. Analytes were desorbed from solid metallic and insulating substrata using a pulsed nitrogen laser. Post-ionization produced high-quality mass spectra characteristic of electrospray, including protein multiple charging. For the first time, mass spectra of intact proteins were obtained using laser desorption without adding a matrix. Bovine cytochrome c and an illicit drug containing methaqualone were chosen in this study to demonstrate the applicability of ELDI to the analysis of proteins and synthetic organic compounds.

Depth profiling of sterically-stabilised polystyrene nanoparticles using laser ablation/ionisation mass spectrometric methods

An aerodynamic lens has been used to introduce polystyrene nanoparticles into an apparatus that combines laser vaporisation with mass spectrometric measurements of ion intensity. The particles have a mean diameter of 129 nm and are sterically-stabilised with poly(ethylene glycol) which coats the surface to a depth of approximately 5 nm. Measurements have been made at wavelengths of 266, 355, and 523 nm, and over a range of laser powers. The results provide clear evidence that depth profiling can be achieved by changing the wavelength of the ablating radiation, but that changes in power at a single wavelength have little influence on the range of ions observed. At 523 nm the mass spectra are predominantly derived from surface-bound material, whilst at 266 nm the dominant contribution is from ions related to the polystyrene core of the particles. It is proposed that these differences in behaviour can be equated with existing models of the laser ablation process.

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.

Time-resolved studies of the interactions between pulsed lasers and aerosols

Studies of the interaction between a pulsed CO2 laser and micrometer-sized aqueous and organic particles by use of light-scattering methods and step-scan Fourier-transform infrared (FTIR) spectroscopy are reported. Visible two-color extinction experiments indicate primary particle shattering, accompanied by a high fraction of vaporization, followed by secondary particle evaporation. The extent of the latter depends on the pulse intensity and particle composition. Angle-resolved light-scattering investigations provide insight into the aerosol size distribution and temperature following the pulsed heating event. The time dependence of the vapor plume, monitored with step-scan FTIR spectroscopy, confirms that a large fraction of the initial particle is quickly evaporated during the shattering event, followed by secondary fragment evaporation and thermal expansion.