MoO-03


ION-MOLECULE REACTION STUDIES ON A PENTAQUADRUPOLE MASS SPECTROMETER

Peter D. Thomas and Richard J. S. Morrison

Department of Chemistry, Monash University, Clayton, VIC 3168, Australia


At previous ANZSMS conferences we have reported on the construction of a pentaquadrupole mass spectrometer for performing multiple-stage mass spectrometric experiments up to MS3. We now present some recent data obtained with our instrument.

The kinetic method1 provides access to the relative affinities of two species for a third via the fragmentation of a cluster ion. For example the dissociation of the proton-bound dimer AHB+ gives information on the relative proton affinities of species A and B. The method has also been applied to electron affinities and affinities for other moieties such as Cl+, SiCl3+ and OCNCO+. These experiments are often performed on tandem instruments with the relevant dimer being produced from a mixed sample introduced into a chemical ionisation source. The pentaquadrupole can provide a more flexible route to interesting dimers via its ability to create these species in ion-molecule reactions.

We are currently investigating the proton-bound dimers of a number of substituted pyridines and pyrimidines using the kinetic method; figure 1 shows a typical experiment.

Figure 1
Fig. 1. left: product spectrum of the reaction AH+ + B, right: CID spectrum of the dimer AHB+.
A = pyridine, B = 4,6 dimethyl pyrimidine.


Another facet of our work concerns the proton-bound dimers of chiral species such as 2-butanol and 2-butanamine. Experiments with enantio-selectively deuterated tartrates have shown that proton-bound dimers containing the same enantiomer are formed in preference to those with opposite enantiomers.2,3 This has been explained by increased hydrogen bonding in the homochiral dimers.3 In our experiments we generate the [M+H]+ ion of a pure enantiomer in the source and react this with a mixture of deuterated and non-deuterated enantiomers in the first reaction region. The structures of the hetero- (mixed, "meso") and homo- dimers are then probed by CID in the second reaction region.

1. R.G.Cooks, J.S.Patrick, T.Kotiaho, S.A.McLuckey, Mass Spectrom. Rev., 13, 287
2. H.M.Fales, G.J.Wright, J. Am. Chem. Soc., 99, 2339
3. F.J.Winkler, D.Stahl, F.Maquin, Tetrahedron Lett., 27, 335