Maldı-Kütle Spektrometresi’nde İyon Oluşma Mekanizmalarına Etki Eden Bazı Faktörlerin Belirlenmesi
Özet
MALDI mass spectrometry has important advantages such as fast sample preparation and working time, high sensitivity, analysis of complex samples without requiring serious separation preliminary steps. Despite this fact, its usage is limited, especially for quantitative analysis. The reasons for this include the low reproducibility and the poor understanding of ionization mechanisms. By enlightening the ionization mechanisms, standard analysis methods can be developed and the use of the technique can be expanded.
For this purpose, various parameters that affect ion formation in MALDI were examined within the scope of the thesis. LDI ion formation profiles of matrices were investigated before MALDI studies of different sample types. In positive mode, it was observed that matrices other than 3-HPA and PNA were ionized both as radical cation and by protonation. It was determined that all the matrix molecules ionized in the form of radical cations also tend to be reduced by hydrogen atom transfer. It was found that the presence of acid did not affect the ionization character of both matrices and samples, but only changed the crystallization behavior. It was observed that 3-HPA and PNA matrix were ionized by disproportionation reaction. In the study of ionic organometallic compounds, it was determined that matrices do not affect the ionization character of these species, which are already in the form of ions in the solid. However, as a result of the instability of the highly charged structures formed upon removal of more than one ion from the metal center in MALDI, differences due to matrices were observed in the transition of these ions to single charged state. It was observed that CHCA, SA, and DHB matrices tend to drop to single charge state by transferring electrons to the sample, while others tend to take protons from the sample. In another ionic organometallic compound that has a proton that it can give easily, charge reduction was not observed as a result of electron uptake. Ionization in positive mode in organic molecules of basic perylene diimide derivative with different side groups was mostly due protonation. While no radical cation formation was observed for the molecule with chlorine in its side groups, radical cation formation with different relative intensities was observed with the others depending on the electron transfer properties of the matrix. Accordingly, it was found that the tendency of the matrices to induce radical cations by electron transfer follows the order of DCTB > PNA > 3-HPA. Isotopic distribution variations caused by hydrogen transfer were also observed in the same molecules. Examining the relative intensity ratios of [M + 2H]+/[M + H]+ peaks, hydrogen atom transfer tendencies of acidic matrices were determined as DHB > SA > CHCA. In MALDI analysis of phthalocyanine complexes with different metal centers, it was observed that ionization in the form of radical cation and protonation competed. While the manganese complex was completely ionized as a radical cation, it was found that nickel and cobalt phthalocyanine complexes were ionized both by protonation and as radical cation. It was seen that + 3 / + 2 reduction potentials of manganese and cobalt phthalocyanine complexes were compatible with these data. As nickel is not active in electron transfers in nickel phthalocyanine complexes, it was concluded that the observed ionization character is based on phthalocyanine.
It was found that for zinc, which does not have a known +3 oxidation state complex, ionization takes place through phthalocyanine with interactions similar to nickel, and these interactions increase the tendency of ionization in the form of radical cation by facilitating electron transfer stronger compared to nickel. While these findings partially agree with the proton affinity, ionization energy and electrochemical parameters of the matrices and samples, it was concluded that all parameters should be evaluated together and more reliable physicochemical measurements were needed to explain the ionization character.
