Inlet Ionization Mechanistic Studies

Abstract

New ionization technology for use with mass spectrometry was first discovered at University of the Sciences by Trimpin et al. The method used laser ablation of a solid matrix at atmospheric pressure, similar to atmospheric pressure matrix assisted laser desorption ionization, but produced multiply charged ions from proteins and peptides similar to ESI. This method was termed laserspray ionization (LSI). Soon after, matrix assisted ionization (MAI), solvent assisted ionization (SAI), and electrospray inlet ionization (ESII) were discovered, and the techniques were collectively called inlet ionization. The main goal of this project is to characterize SAI and ESII in an effort to understand the ionization mechanism of these liquid introduction methods. An understanding of the fundamental aspects of SAI and ESII might aid in improving the methods.

In inlet ionization, the initial ionization event occurs in the sub-atmospheric pressure region of the heated inlet tube which is the interface between atmospheric pressure and the vacuum of the mass analyzer. Because a sample is introduced directly into the inlet in the solid or liquid states, gas phase ion losses associated with their transfer from atmospheric pressure into the vacuum of the mass analyzer, as occurs with commonly used atmospheric pressure ionization methods are reduced. SAI is a liquid introduction inlet ionization method which uses no voltages to produce ions, and a variant of SAI is ESII which applies a voltage to a solution that is introduced directly into the inlet. Similar to electrospray ionization, these methods produce highly charged ions, with or without the use of voltage.

The fundamentals of SAI were first explored through the calculation of internal energy distributions of benzylpyridinium ions (so called thermometer ions) in relation to ESI. Thermometer ions, initially introduced by DePauw, have well characterized simple fragmentation patterns which were used to determine the survival yield of ions created by SAI and ESI. Charged species in SAI and ESII are believed to derive from droplets similar to ESI. The difference between ESI and SAI may be how charged droplets are formed.

To further examine the fundamentals of SAI, several surface activity investigations were conducted. Surface activity is the tendency to find a molecule on the surface of a droplet relative to the interior. First, ionization efficiency study of simple and complex mixtures were explored by comparing SAI, ESII, and ESI using infusion and liquid chromatography sample introduction. Second, how salts and surfactants responded in ESI, ESII, and SAI was examined. Third, ionization of low polarity mixture by ESI, ESII, and SAI was studied. The mechanism proposed for SAI ion production has the exterior of a droplet being negatively charged, unlike ESI where the droplet exterior is positive. We postulated that in SAI a thermal gradient between the surface and interior of the droplet causes a charge separation where the exterior of the droplet is negatively charged due to autoprotolysis. Highly charged prodigy droplets are produced by removal of surface charge through bubble formation. If our assumption is correct then SAI should ionize hydrophilic molecules more efficiently than ESI.

An alternative gas phase ionization mechanism for SAI was also explored. In this study, compounds known to ionize well with atmospheric pressure chemical ionization (APCI) were explored using ESI and SAI. Finally, MAI ionization mechanism was inspected by examining the ionization of angiotensin II incorporated into dihydroxybenzoic acid isomers.