Fundamentals and applications of DESI-MS and investigations on the formation and chiral amplification of the serine octamer
Abstract
Ambient ionization for mass spectrometry emerged several years ago when the first ambient ionization technique, DESI, was introduced. By gaining access to ambient samples (i.e. condensed, minimally processed samples and objects in their own environment), mass spectrometry entered the area of high throughput and direct analysis, unapproachable before. The first part of the thesis is focused on the development of novel applications for DESI (in the fields of industrial polymers and lipids in biological tissue) and on studying the underlying mechanisms that govern the production of gas-phase ions from solid samples. The projects described here are part of a larger team effort to realize the full potential of this new ionization technique. In the second part of the thesis ionization at atmospheric pressure (sublimation/APCI, ESSI, and SSI) was used to investigate serine clustering. The study focuses on the protonated serine octamer, the most intriguing of the clusters due to its unusual stability and preferred homochiral composition. It is proposed that [Ser8+H]+ is produced through the dissociation of larger serine cluster ions during the ionization step. Charged serine metaclusters are produced from both the solid during sublimation followed by corona discharge ionization and the concentrated solution by ESSI and SSI. At concentrations in excess of 1 mM and low droplet charge density (i.e. in SSI) formation of charged solid residues by complete droplet evaporation dominates over formation of monomer and dimer ions through field evaporation from the charged droplets. This, rather than the softer ionization conditions explains why SSI is more efficient than ESSI in producing [Ser8+H]+. Formation of [Ser8+H]+ from metaclusters explains the similarity between the mass spectra and the chiral preference of [Ser8+H] + obtained from two different condensed phases (solution and solid phase) by two different ionization methods. Based on this mechanism, an attempt is made to explain the phenomenon of chiral amplification observed upon ionization and mass spectrometric analysis of non-racemic serine solutions and solids.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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