Further interference peaks in mass analysed ion kinetic energy spectra

Abstract

It is shown that interference peaks in mass analysed ion kinetic energy spectra can also occur from ions decomposing in the accelerating region of the ion source. INTRODUCHONRecently, Beynon and co-workers reported' on interference peaks in mass analysed ion kinetic energy (MIKE) spectra. We have been confronted with this problem too,' and arrived principally at the same explanations concerning the origins of certain interference peaks in MIKE spectra.' However, in addition to the cases mentioned by Ast et ul.,' there are often a lot of sharp peaks observed in MIKE spectra which do not fit the equations given by these authors and which arise from decompositions of metastable ions in the acceleration field. These interference peaks are easily distinguished from normal signals in a MIKE spectrum by their much smaller peak width (see Fig. 2). Decompositions occurring within the accelerating region of the ion source have also been considered by Lacey and M a~d o n a l d .~ Their generalized treatment of ion fragmentations within the various regions of a mass spectrometer of conventional geometry is not directly related to MIKES. however. ~ _ _ _~ RESULTS AND DISCUSSION Considering the fragmentation(1) ions ml+ continuously decompose along their flight path through the mass spectrometer. The local point of decomposition is determined by the rate constants of reaction (1) and by instrumental parameters. Those ions m2+ which are formed within the accelerating region of the ion source will obtain a continuum of kinetic energies, given by Eqn Similarly, the electric sector voltage Ef at which these ions pass the electric sector is given by Eqn (4) derived from Eqn (2)From Eqns (3 and 4) the range of apparent masses mf and the range of the electric sector voltages can be evaluated for reaction (1) occurring in the accelerating region of the ion source. Setting f = 0 is equivalent to regarding ions m2+ generated in the ion source before acceleration, i.e. mf = m2 and Ef = E,. The other extreme, f = 1, is equivalent to observing ions m2+ formed in the first field free region, i.e. Thus, by preselecting mf values in between these extremes the fraction f of V, [Eqn (2)] at which the observed m2+ is formed is predetermined and can be calculated explicitly from Eqn ( 5 )Having determined f the corresponding E, can be calculated [Eqn (4)] and the interference peak can be recorded. This is most easily done on a mass spectrometer with 'reversed geometry' by setting the magnet current to the appropriate mf/z value and scanning E over a small region around Ef In this way any slow fragmentation reaction can be followed step by step from the ionization chamber to the 1st field free region (FFR).~ The procedure outlined above is illustrated by the reactions shown in Scheme 1. The ranges of m, and Ef values are also given in Scheme 1. Three regions of mf values can be distinguished in this case: (i) 1 4 9 3 mf 3 145, only ions b are expected to be transmitted, (ii) 145 2 mf 3 135, both ions a and ions b can be transmitted, (iii) 1 3 4 ...

Influence of instrumental parameters and sample preparation on mass‐analysed ion kinetic energy spectra with fast atom bombardment exemplified with the oxonium ion of peracetylated ribopyranose

“…They could, however, be assigned to interference peaks resulting from processes in the field-free region (FFR) preceding the magnetic sector23 or even in the accelerating region of the ion source. 24 The remarkable peak at m/z 217, which is hardly observable in the MIKE spectrum of glycerol [ Fig. l(d)] and is also a minor peak in the MIKE spectrum of Rp [ Fig.…”