Collisional activation experiments

In the ion-source El spectrum of ClSi(CH3)3, ions of the formal composition [H2,C2,Si]+ have been observed171. Collisional activation experiments performed with these ions indicated that they possess a cyclic structure 49170. The successful neutralization... 

These results from collisional activation experiments have been augmented by ab initio calculations [MP2/6-311 G(df) CCSD(T) level] on the neutral and cationic [Si2,02] surface196,214. The theoretical results point to the existence of three low-lying cationic isomers 60, 61 and 62. 

It is of interest to mention some detailed mass spectrometric results that were obtained by performing distinct experiments (a) ion-molecule reactions in mixtures of haloben-zenes and ammonia under chemical ionization conditions (b) ion-molecule reactions of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell of a hybrid tandem mass spectrometer (c) ion-molecule reactions of phenyl diazonium cations with ammonia in the same quadrupole collision cell and, finally, (d) electrospray ionization of anilines in a hybrid quadrupole-time of flight mass spectrometer (QTof). Characterization of the product ions relies on collisional activation experiments in the low or high kinetic energy regime98. 

Fischer carbenes can be generated from diazomalonates in the gas phase, as reported by Beauchamp and co-workers. Suitable cations for study were obtained by electrospray of solutions of Ag or Cu salts with the diazoma-lonate 21 in methanol water acetonitrile (80 20 0.1). The further reaction steps were conducted in a quadrupole ion trap spectrometer. Complexes formed via electrospray first undergo Fischer carbene formation and subsequently Wolff rearrangement upon collisional activation. Experiments with isotopically labeled diazomalonates suggest the underlying mechanism shown in Scheme 8. 

Fast Atom Bombardment-Fourier Transform-Mass Spectrometry (FAB-FT-MS) and low-energy collisional activation experiments proved that the oligomers of j3-ketolactone generally have a macrocyclic ring structure rather than a catenane ring. ... 

In neutralization-reionization mass spectrometric experiments on CH2Si+ formed by electron-impact dissociative ionization of ClCH2SiH3, Srinivas, Stilzle and Schwarz found evidence for the formation of a viable neutral molecule whose fragmentation pattern and collisional activation mass spectrum were in accord with a H2C=Si structure422. These authors suggested that their experiments supported electron-capture by CH2Si+" as a mechanism for the formation of H2C=Si in interstellar space. Various models have predicted that H2C=Si is one of the most abundant forms of silicon in dense interstellar clouds423. 

Improvements in SIMS and FAB which lead to more intense ion beams are highly desireable because that will permit the information to be obtained for smaller samples and for lower abundance constituents in complex mixtures such as surfactants. Moreover, consecutive activation steps (MS/MS/MS) should be important in investigations of mixtures of complex lipids and related materials. Here one step of collisional activation is necessary to liberate the fatty acid carboxylate and a second step is required to activate the anions. These experiments also require intense ion beams. It is our hope that the analytical possibilities raised by the chemistry discusses here will stimulate further research to improve FAB and SIMS. 

In addition, several approaches to ion activation that are not well-suited to conventional types of mass spectrometers, are very well-suited for use with FTMS. These include photodissociation and ion-electron collisions [46-49]. In this paper, we would like to present some applications of ion-electron collisions to MS/MS, and show that the method may be a suitable alternative to collisional activation for some applications. We will also discuss some of the desirable characteristics of the dual cell geometry, as applied to MS/MS experiments. 

In addition to the EIEIO experiments, the dual cell has also been employed for collisional activation (CA) experiments. Selected parent ions may be transferred to the analyzer cell for CA. With pulsed valve introduction of the collision gas (argon) into the analyzer cell, it is possible to obtain very high daughter ion resolution. Figure 13 shows a C6H5C0+/C8H9+ doublet from CA of acetophenone and mesitylene, detected at a resolution of 500,000. 

In the studies outlined below, collisionally activated dissociation (MS/MS) experiments were attempted as a means of helping establish the identities of a number of anions. In every case tried, however, the parent anions appeared to undergo electron detachment with no daughter ion production observed. 

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