Size-selected 2, 5, and 10 nm gold nanoparticles for laser desorption/ionization mass spectrometry
The analytical utility of gold nanoparticles (AuNPs) for laser desorption/ionization mass spectrometry (LDI-MS) is examined here. An evaluation of the parameters that affect desorption/ionization show that careful treatments of AuNPs is needed, as subtle changes in the solution environment can result in subsequent changes in the mass spectra. A thorough evaluation of the parameters that affect desorption/ionization of peptides is presented here, and these parameters include: (i) AuNP-to-analyte ratio, (ii) AuNP size, (iii) solvent, (iv) AuNP surface composition, (v) pH and buffer effects, (vi) amino acid sequence, and (vii) additives such as fructose or glycerol. Specifically, controlling the AuNP-to-analyte ratio, pH, peptide composition, and AuNP size are important parameters for ionization. Additionally, effects of passivating the AuNP surface with halides or oxyanions was investigated. The presence of NaF, NaCl, NaBr, and NH4X (X = F, Cl, Br, I) were shown to not significantly affect analyte ion abundances, whereas addition of NaI strongly suppressed analyte ion yields. Further physical characterization of the NPs showed that etching had occurred, which suggests that the surface chemistry of the NPs is important for desorption/ionization. Throughout these investigations, questions remain as to what the internal energies of peptides are after the desorption/ionization event, and how energy is deposited. Peptide ion fragmentation is examined under different solution conditions to evaluate the relative internal energies of peptides, and the fragmentation pattern examined for insight into fragmentation mechanisms. The data suggest that radical species are important for fragmentation of peptides when using AuNPs. However, it is likely that multiple processes are actually directing the fragmentation. Finally, based on the data presented in this dissertation, a thermal desorption mechanism of pre-formed ions is proposed. This fundamental research is intended to lay foundations for optimizing the use of nanoparticles in routine LDI-MS analysis as well as giving insight into nanoparticle ionization mechanisms. Since very little work has been done in this area, this dissertation investigates, in detail, many of the subtle characteristics that affect desorption/ionization of biomolecules when using NPs.