DESIGN, DEVELOPMENT AND APPLICATION OF ANALYTICAL APPARATUSES FOR LASER ELECTROSPRAY MASS SPECTROMETRY ANALYSIS

Abstract

With any analytical technique, there are inherent deficiencies that can be improved upon for optimizing sensitivity, ease of analysis, reproducibility, etc. The unifying goal of this work is the design and implementation of apparatuses to improve the capabilities of laser-electrospray hybrid based mass spectrometry techniques, with laser electrospray mass spectrometry (LEMS) serving as the experimental subject for projects discussed in this thesis. In LEMS, ~60 fs laser pulses centered at 800 nm are used to vaporize the analyte from a surface into the gas phase without the need for a matrix. The laser-vaporized analytes are then captured by an electrospray source and detected via a mass spectrometer. The dried droplet technique is the most common method of sample preparation for LEMS analysis where aliquots of sample are deposited onto a substrate followed by drying at room temperature. While the dried droplet technique is convenient, a spatially inhomogeneous distribution of analytes is deposited which results in high laser shot to shot signal variance, consequently resulting in high variances between replicate samples. To increase the homogeneity of the sample deposited, an electrospray deposition (ESD) device was built and used for deposition of samples of Victoria blue on stainless steel or indium tin oxide (ITO) slides. LEMS measurements of the ESD-prepared films on both substrates were comparable and revealed lower average relative standard deviations (RSD) for measurements within-film (20.9%) and between-films (8.7%) in comparison to dried droplets (75.5% and 40.2%, respectively). The mass spectral response for ESD samples on both substrates was linear (R2 > 0.99), enabling quantitative measurements over the selected range of 7.0 × 10−11 to 2.8 × 10−9 mol, as opposed to the dried droplet samples where quantitation was not possible (R2 = 0.56). Another major limitation in all laser-ESI hybrid systems is that the sample must be in close proximity to the mass spectrometer inlet. In order to transfer laser ablated materials via an electrospray source, the electrospray needle must be close enough to the high voltage inlet to produce charge separation, generate a Taylor cone and ultimately, charged droplets. This short distance (on the order of mm) restricts the size and geometry of the samples to be analyzed. This dissertation details the design of a novel remote sampling device for LEMS analysis. The vaporization process takes place inside a controlled gas flow compartment of a sample chamber where a nitrogen carrier gas is applied coaxially to the vaporization plume providing confined radial expansion dynamics of the particles. Vaporized particles are then transported through a tube and enter the nebulizing gas sheath of an electrospray ionization (ESI) needle where capture and post-ionization of the analyte occurs. Analysis of four selected pharmaceutical compounds revealed enhanced sensitivity and improved reproducibility of remote LEMS when compared to conventional LEMS measurements. This dissertation also explores the possibility of using a nanosecond laser as a means to vaporize samples from stainless steel and glass slides for LEMS analysis. Wet samples of myoglobin on stainless steel were successfully vaporized by ns laser pulses while irradiation of myoglobin on glass did not result in vaporization. In a comparison study, fs laser pulses were able to vaporize wet myoglobin from both substrates. To achieve vaporization from glass, surfactant-free gold nanostars (GNS) were successfully used as a matrix for desorption and detection of myoglobin using ns-LEMS. This dissertation also reports the first synthesis of gold nanostars (GNS) exploiting the conversion of dual microplasma generated Au clusters to GNS in aqueous solutions of KAuCl4 containing small amounts of AgNO3, without addition of a surfactant or a stabilizing agent. The single-cell dual microplasma source is also a novel apparatus. Primary experiments using dual microplasma processing yielded various sized spherical gold nanoparticles (AuNPs) with polydispersity index (PDI) of 0.29. By kinetic control of post microplasma reduction, monodispersed nanospheres were produced with PDI of 0.06. An important discovery was the excess amount of hydrogen peroxide produced during microplasma process, making the production of GNS possible.