Particle formation in ambient mass spectrometry
A particle sizing system was developed that couples a light scattering particle sizer with a differential mobility analyzer for particle detection and elucidation with the capability for measuring particle sizes in the range of 10 nm to 20 μm. The particle sizing system was used to investigate particle formation that is associated with several methods of atmospheric pressure ion formation used in mass spectrometry: atmospheric pressure matrix-assisted laser desorption ionization (MALDI) and inlet ionization. The methods for particle formation were 1) ultraviolet (UV) laser ablation, 2) infrared (IR) laser ablation, and 3) shock-generated particle formation. Particles formed by UV laser irradiation of solid MALDI matrix materials were analyzed at laser fluences from 300 J/m2 to 1100 J/m2. It was discovered that a large number of ejected particles were nanoparticles in the range of 40 nm to 200 nm. These particles were attributed to hydrodynamic sputtering of melted matrix with additional contribution from agglomeration of smaller particles and clusters. A relatively large mass of particulate was observed between 500 nm and 2 μm and was attributed to spallation and matrix melting. A third local maximum of particle size was observed between 10 and 30 nm and attributed to direct ejection of clusters. Particle size measurements were made at IR laser wavelengths between 2.8 and 3.0 μm. The laser fluence and wavelength dependence of particle sizes from IR irradiation of glycerol and three solid matrices were investigated. The distribution of particles was characterized by a large concentration of clusters with diameters near 20 nm and large fraction of the ejected mass as coarse particle with diameters greater than 1 μm. The wavelength dependence revealed a shift for the maximum particle production with respect to the IR absorption of the matrix compounds that is attributed to heating and disruption of the hydrogen bonds in the matrix that shifts the absorption to shorter wavelengths. Particle formation under inlet ionization conditions was investigated with the goal of elucidating the ionization mechanism of this technique. In inlet ionization, ions are formed from particles directed toward the inlet of mass spectrometer either directly from powder or by laser ablation of particles. The particle size measurements revealed a high concentration of particles with diameters near 10 nm was observed in both shock-generated particle formation and transmission mode laser ablation. Matrix compounds with high efficiency for inlet ionization had a high concentration of clusters. The high concentration of clusters is consistent with postulated mechanisms for inlet ionization.
Ionization, mass spectrometry, particle formation, laser ablation.
Networked Digital Library of Theses and Dissertations