Date of Award
2006
Degree Type
Thesis
Degree Name
Doctor of Philosophy
Program
Chemistry
Supervisor
Dr. Robert H. Lipson
Abstract
Matrix Assisted Laser Desorption Ionization (MALDI) is an analytical technique that has been used extensively to vaporize and ionize non-volatile biological samples. MALDI has been used successfully to “soft” ionize peptides, proteins and other large molecules. Concurrent detection of neutrals and ions in the MALDI process has shown that the degree of ionization is on the order of ~0.01%. Laser desorption followed by laser post-ionization should significantly improve the sensitivity of the MALDI technique. Coherent VUV sources generated by non-linear optical methods such as third harmonic generation are excellent soft-ionization sources able to ionize most organic molecules having ionization potentials between 7 and 13 eV in a single non-resonant step with little or no fragmentation. Coherent VUV light at a wavelength of 118 nm was introduced into a MALDI ion source coupled to a triple quadrupole mass spectrometer through an LiF window and used to post-ionize the MALDI plume created by a conventional MALDI N2 laser. Only the ions of interest were focused into the triple quadrupole mass spectrometer, eliminating signals due to matrix interference. Experiments are described using MALDI mass spectrometry to detect aspirin in a DHB matrix for the first time. While protonated aspirin was not observed sodium and potassium aspirin adducts were found with a limit of detection on the order of 0.4 nmol. The presence of alkali metals in the MALDI spectrum was ascribed to impurities in the matrixes. This relatively poor limit of detection could be improved by an order of magnitude by using heavier alkali salt iii additives. The best signals were obtained by using RbCl and CsCl as cationization source. Furthermore, the formation of salt analyte adducts shifted the resulting spectral signals away from the m/z region of maximum matrix interference. Density functional calculations were carried out to determine the structure and energetics of the alkali-aspirin adducts. Despite extensive research over the past two decades few MALDI studies have been done using visible wavelength lasers. Most biological molecules have : I negligible absorption in this region, and therefore fragmentation is expected to be minimal. Organic dye molecules absorb visible light with extremely high efficiency making them promising MALDI matrices. Experiments are described using the organic laser dyes Coumarin 510 and Coumarin 519 to record the visible wavelength MALDI spectra of the peptide Dalargin in both positive and negative ionization modes. Detection limits are presented in both cases.
Recommended Citation
Lacey, Diane, "Experiments in VUV and Visible Wavelength MALDI Using a Triple Quadrupole Mass Spectrometer" (2006). Digitized Theses. 5099.
https://ir.lib.uwo.ca/digitizedtheses/5099