Exploration of the effects of electrospray deposition spraying parameters and incident laser wavelength on matrix assisted laser desorption ionization time-of-flight mass spectrometry

Abstract

This thesis investigates the use of electrospray deposition (ESD) as a sample preparation technique for matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) and laser wavelength effects on the MALDI-TOFMS signal. A brief description of critical ESD parameters is provided and studies aimed at understanding the parameters that define the spraying diameter are presented. The experimental data suggest that the solvent and solute identity, solute concentration, spraying flow rate and spraying distance all affect the final sprayed sample diameter and the morphology of the amorphous particles deposited on the sample surface. The invention of a controlled ESD technique is described and both the gross sample morphology and sprayed particle morphology are discussed. Key spraying parameters of the controlled ESD technique are investigated using a combination of scanning electron microscopy, laser confocal microscopy and MALDI-TOFMS. Samples sprayed for longer time periods and sprayed from a larger spraying distance yielded MALDI samples with increased analyte sensitivity.A MALDI-TOFMS instrument equipped with two lasers operating at 355 and 337nm was used to investigate analyte response at the two wavelengths. Solid state and solution phase UV-visible absorption spectroscopy and a quartz-crystal micro-balance were also used to investigate the differences in absorption and desorption of MALDI matrix compounds at both wavelengths. Common MALDI matrix compounds were shown to have similar absorbance at both wavelengths in the solid state, however, the mass of desorbed material per laser shot was shown to be greater when using a 355nm laser. The absolute analyte signal at the optimal matrix-to-analyte molar ratio is greater when using an incidence wavelength of 355nm.The effect of dissolution solvent on MALDI signal was investigated using the ESD technique. The addition of acetonitrile into solutions of methanol was shown to decrease the observed analyte signal. A mixture of chloroform and methanol produced analyte signals greater than those generated by a solution of methanol alone. The data suggest that acetonitrile quenches the MALDI signal when using ESD as a sample preparation technique. The addition of chloroform as a co-solvent may affect the particle morphology of the sprayed samples leading to the increased analyte signal.%%%%Ph.D., Analytical Chemistry – Drexel University, 2012