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- Differential mobility analysis (DMA) (1)
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Infrared matrix-assisted desorption and ionization (IR-MALDI) enables the transfer of sub-micron particles (sMP) directly from suspensions into the gas phase and their characterization with differential mobility (DM) analysis. A nanosecond laser pulse at 2940 nm induces a phase explosion of the aqueous phase, dispersing the sample into nano- and microdroplets. The particles are ejected from the aqueous phase and become charged. Using IR-MALDI on sMP of up to 500 nm in diameter made it possible to surpass the 100 nm size barrier often encountered when using nano-electrospray for ionizing supramolecular structures. Thus, the charge distribution produced by IR-MALDI could be characterized systematically in the 50-500 nm size range. Well-resolved signals for up to octuply charged particles were obtained in both polarities for different particle sizes, materials, and surface modifications spanning over four orders of magnitude in concentrations. The physicochemical characterization of the IR-MALDI process was done via a detailed analysis of the charge distribution of the emerging particles, qualitatively as well as quantitatively. The Wiedensohler charge distribution, which describes the evolution of particle charging events in the gas phase, and a Poisson-derived charge distribution, which describes the evolution of charging events in the liquid phase, were compared with one another with respect to how well they describe the experimental data. Although deviations were found in both models, the IR-MALDI charging process seems to resemble a Poisson-like charge distribution mechanism, rather than a bipolar gas phase charging one.
The doctoral thesis presented provides a comprehensive view of laser-based ablation techniques promoted to new fields of operation, including, but not limited to, size, composition, and concentration analyses. It covers various applications of laser ablation techniques over a wide range of sizes, from single molecules all the way to aerosol particles. The research for this thesis started with broadening and deepening the field of application and the fundamental understanding of liquid-phase IR-MALDI. Here, the hybridization of ion mobility spectrometry and microfluidics was realized by using IR-MALDI as the coupling technique for the first time. The setup was used for monitoring the photocatalytic performance of the E-Z isomerization of olefins. Using this hybrid, measurement times were so drastically reduced that such photocatalyst screenings became a matter of minutes rather than hours. With this on hand, triple measurements screenings could not only be performed within ten minutes, but also with a minimum amount of resources highlighting its potential as a green chemistry alternative to batch-sized reactions. Along the optimizing process of the IR-MALDI source for microfluidics came its application for another liquid sample supply method, the hanging drop. This demarcated one of the first applications of IR-MALDI for the charging of sub-micron particles directly from suspensions via their gas-phase transfer, followed by their characterization with differential mobility analysis. Given the high spectral quality of the data up to octuply charged particles became experimentally accessible, this laid the foundation for deriving a new charge distribution model for IR-MALDI in that size regime. Moving on to even larger analyte sizes, LIBS and LII were employed as ablation techniques for the solid phase, namely the aerosol particles themselves. Both techniques produce light-emitting events and were used to quantify and classify different aerosols. The unique configuration of stroboscopic imaging, photoacoustics, LII, and LIBS measurements opened new realms for analytical synergies and their potential application in industry. The concept of using low fluences, below 100 J/cm2, and high repetition rates of up to 500 Hz for LIBS makes for an excellent phase-selective LIBS setup. This concept was combined with a new approach to the photoacoustic normalization of LIBS. Also, it was possible to acquire statistically relevant amounts of data in a matter of seconds, showing its potential as a real-time optimization technique. On the same time axis, but at much lower fluences, LII was used with a similar methodology to quickly quantify and classify airborne particles of different compositions. For the first time, aerosol particles were evaluated on their LII susceptibility by using a fluence screening approach.
The quantification and identification of aerosols in industry plays a key role in process monitoring and control and lays the foundation for process automation aspired by the industry 4.0 initiative.
However, measuring particulate matter's mass and number concentrations in harsh environments poses great analytical constraints.
The presented approach comprises a comprehensive set of light-and imaging-based techniques, all contactless, in-line, and real-time. It includes, but is not limited to, stroboscopic imaging, laser-induced breakdown spectroscopy (LIBS) and laser-induced incandescence (LII). Stroboscopic imaging confirmed the particles sphericity and was used to measure the particle number density. A phase-selective LIBS setup with low fluence and 500 Hz repetition rate was used to classify each particle with a single-pulse and in real time. Simultaneously, the created plasma was captured by CCD imaging to determine the detection volume and hit rate of the LIBS setup.
Both data sets combined were converted to a particle number density, which was consistent with the particle number density of the stroboscopic measurements. Furthermore, using a photodiode and microphone in parallel to the LIBS setup allowed for the photoacoustic normalization of the spectral line intensity at the laser repetition rate of 500 Hz.
This was done as a partial photoacoustic normalization method with the cut-off based on the coefficient of variation (CV), reducing it by 25%. Aside from that photodiode and microphone were proven to be valuable event counting with the advantage of the less spatially constricted. A second laser setup was used for laser -induced incandescence (LII) making it possible to classify the particles based on their incandescence tendency. Given its larger probing volume, LII could be employed at very low particle number densities.
With respect to the current literature, this is the first approach of using LII as an in-line, real-time analytical technique for the compositional classification of metal-bearing aerosols.