Determination of the dissociation kinetics of a transient intermediate.

Tandem mass spectrometry provides information on the dissociation pathways of gas-phase ions by providing a link between product ions and parent ions. However, there exists a distinct possibility that a parent ion does not dissociate directly to the observed product ion, but that the reaction proceeds through unobserved reaction intermediates. This work describes the discovery and kinetic analysis of an unobserved reaction intermediate with a quadrupole ion trap. [a4 - NH3] ions formed from [YG beta FL + H] ions dissociate to [(F*YG - NH3) - CO] ions. It is expected, however, from previous results, that [F*YG - NH3] ions should form prior to [(F*YG - NH3) - CO] ions. Double-resonance experiments are used to demonstrate the existence of intermediate [F*YG - NH3] ions. Various kinetic analyses are then performed using traditional collision-induced dissociation kinetics and double-resonance experiments. The phenomenological rates of formation and decay of peptide rearrangement ion dissociation products are determined by curve fitting decay and formation data generated with the kinetics experiments. The data generated predict an observable level of the intermediate in a time frame accessible but previously not monitored. By examining early product-ion formation, the intermediate ions, [F*YG - NH3]+, are observed.

Collision-induced signal enhancement: a method to increase product ion intensities in MS/MS and MSn experiments.

Collision-induced signal enhancement (CISE), a new technique to enhance the MSn capabilities of the quadrupole ion trap, is demonstrated. CISE is based on the chemistry, i.e., the dissociation pathways, of the analyte examined. Polysaccharides up to hexamers are used to demonstrate the capabilities of CISE to enhance signal in two distinct functional modes. Mode 1 CISE is designed to enhance the signal of an ion desired for MSn analysis. Mode 2 CISE is designed to enhance structurally significant product ions in an MS/MS spectrum. Two different approaches can be utilized to effect the two functional modes of CISE. Both approaches use conventional resonant excitation techniques to effect dissociation, which is performed nonanalytically, i.e., without isolation of the ions to be dissociated. The two approaches are (1) single-frequency resonance excitation, and (2) broad-band wave form resonant excitation. Experimental results for Mode 1 CISE analysis demonstrate up to a 17.3-fold signal increase for the single-frequency approach and 5.3-fold using broad-band excitation. Mode 2 CISE analysis shows up to a 16.3-fold increase in signal strength with single-frequency excitation and 3.3-fold using broad-band excitation.

Reaction from isomeric parent ions in the dissociation of dimethylpyrroles.

The major dissociation pathways of the [M-H](+) (loss of NH3 or CH4) and the [M+H](+) (loss of NH3 or CH3) ions from dimethylpyrroles have been determined to occur from isomeric parent ions. For the [M-H](+) ion (formed by loss of a methyl hydrogen), loss of NH3 leads to the formation of the phenylium ion and is preceded by consecutive carbon ring expansions followed by a ring contraction to form protonated aniline. Loss of CH4 occurs after the first carbon ring expansion, which forms protonated picoline. The relative partitioning between the two dissociation paths depends upon the internal energy content of the parent ion; the highest point on the potential energy surface is the second ring expansion step. The [M+H](+) ion reacts through a similar pathway via dihydro analogs of picoline and aniline. The proposed reaction pathways are supported by results of semiempirical molecular orbital calculations.