Internal energy effects in mass spectrometry

Vicki Hopper Wysocki, Purdue University

Abstract

The distribution of internal energies of a population of ions controls its unimolecular fragmentation. However, little is known about the actual energy distributions of ions after activation. A simple method of estimating the energy distributions of fragmenting ions was applied to a number of methods, including electron ionization, low-energy or high-energy collisions with a neutral atomic target, charge exchange, and collision with a surface (surface-induced dissociation). Data for the ions Fe(CO)$\sb5\sp+\cdot$, W(CO)$\sb6\sp+\cdot$, (C$\sb2$H$\sb5)\sb4$Si$\sp+\cdot$, Fe(CO)$\sb4\sp+\cdot$, Fe(CO)$\sb4\sp-\cdot$, Cr(CO)$\sb6\sp+\cdot$, Cr(CO)$\sb5\sp+\cdot$, and Cr(CO)$\sb5\sp-\cdot$ allow some generalizations to be drawn. The results show that the average energy deposited in keV and eV collisional activation can be comparable, that keV collisions result in a low-probability, high-energy tail which is absent in eV collisions, that collision energy and target pressure have striking effects on the energy distributions in the low-energy regime, and that surface-induced dissociation can deposit higher average energies than the other methods examined. The data explain some general trends previously seen in mass spectrometry and may be utilized to help solve particular ion structural and analytical problems. Acid-catalyzed cyclization of straight chain $\alpha$,$\omega$-amino acids and $\alpha$,$\omega$-amino alcohols was studied as a function of reaction conditions in the gas phase. Competitive dehydration and deamination from the protonated amino alcohols and amino acids was found to depend on the reaction region (ion source vs. collision region) in three types of mass spectrometers. The ratio of deamination to dehydration for both types of compounds were found to show dramatic variation with varying chain length in the collision region of the mass spectrometer but not in the ion source. These effects were rationalized with calculated thermochemical data. Attempts were made to induce remote site fragmentation in a series of functionalized long-chain alkyl ions. Activation was by low-energy collisions with a neutral target gas or with a surface. The energy required to promote this unusual fragmentation is shown to be compound dependent, which suggests that the mechanism is more complex than previously reported.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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