Internal energy deposition in chemical ionization/tandem mass spectrometry

Abstract

The efficiency of the collision-induced dissociation (CID) process as a function of the internal energy deposited into the ion during the ionization event was evaluated. (M ϩ H) ϩ ions of pyrrole, pyrrolidine, pyridine and piperidine (five and six-membered ring heterocyclics) were generated by chemical ionization (CI). The internal energy of the ions was varied by using different reagent gases. Both high-energy (keV) and low-energy (eV) CID were performed on these ions. The experiments showed that the (M ϩ H) ϩ ions of the five-membered ring compounds, pyrrole and pyrrolidine, have higher fragmentation efficiencies than the sixmembered ring compounds, pyridine and piperidine. Fragmentation efficiencies in highenergy CID clearly correlate with the internal energy deposited by the ionization technique. Experiments showed that the low-energy CID process is more sensitive than high-energy CID to changes in internal energy. C hemical ionization is one of the most widespread ionization techniques used in mass spectrometry. In chemical ionization, collisions between reagent gas ions and sample molecules produce abundant pseudo-molecular ions characteristic of the sample [1]. Ionization in CI can occur by different mechanisms such as charge exchange, electrophilic addition, proton transfer, hydride abstraction, or other anion abstraction mechanism [1][2][3]. Proton transfer from a reagent gas ion (B ϩ H) ϩ to a sample molecule M is the most common process and is represented by the equation:The internal energy (⌬H) of the (M ϩ H) ϩ ion produced by proton transfer depends on the proton affinities (PA) of the species involved and is described by the equation:Fragmentation of the (M ϩ H) ϩ ion in CI spectra depends on its internal energy, which can be controlled by varying the reagent gas. In general, fragmentation increases when the exothermicity of the proton transfer reaction increases. It is possible to predict which proton transfer reactions will occur in a particular system, since values of PA of a large number of compounds are tabulated [4]. CI spectra are often simple, frequently giving only molecular weight information; therefore, activation techniques such as collision with a neutral gas are commonly used to increase the internal energy of the ion and promote fragmentation. It is well known that fragmentation observed in collision-induced dissociation depends upon the internal energy present in the ions prior to collision as well as the energy deposited during collisional activation. There are limited publications on the role of precollisional internal energy on CID spectra and they show conflicting results. Some studies indicate that precursor ion internal energy has a negligible effect on the CID spectrum, except for product ions formed through processes with the lowest activation energy [5][6][7][8]. In contrast, other studies indicate that CID spectra can be highly dependent on initial energy of the precursor ion and/or angular momentum [9 -12]. The effect of internal energy on the relative rates of the reacti...

The mechanisms of collisional activation of ions in mass spectrometry

“…This large discrepancy between internal energy transfer efficiencies between the two groups should not come as a surprise as there will be a strong dependence on molecular properties, as illustrated by Equation (8). Ospina, Powell, and Yost (2003) demonstrated that both low and high energy CID can be affected by the initial, precollision internal energy of small ions. Forming ions by CI using reagents with different proton affinity values, they were able to generate protonated N-containing heterocyclic ions with different internal energy content.…”

Chemical Characterization and Quantitation of Phenols in Fuel Extracts by Using Gas Chromatography/Methane Chemical Ionization Triple Quadrupole Mass Spectrometry

“…However, nitrogen-containing compounds are polar and may co-elute with phenols during SPE but they usually have very low concentrations . As protonation is the dominant CI mechanism, nitrogen-containing compounds ionized upon methane CI would have even m / z values while the protonated natural phenols have odd m / z values. Therefore, they would have been readily distinguished from the phenols if present in the fuel samples.…”

Nanosecond Pulsed Dielectric Barrier Discharge Ionization Mass Spectrometry

“…An important characteristic of an ion source is the extent of energy that is deposited during ionization. 45 Benzylamonium thermometer ion measurements were used to obtain the extent of internal energy deposition in nanosecond pulsed plasma ionization, conventional microsecond pulsed plasma ionization, and electrospray ionization (refer to the Supporting Information). Electrospray ionization was used as a comparison because is it considered one of the "softest" known ion sources.…”