Interference peaks in mass analysed ion kinetic energy spectra

When mass analysed ion kinetic energy spectroscopy is performed on ions of low abundance and low mass compared with the rest of the ions present in the ion chamber, severe interference may be observed. The major form taken by the interference is of narrow peaks at non-integral mass numbers. Many such peaks can be observed in mass analysed ion kinetic energy spectra and they are mainly attributed to decompositions of metastable ions occurring in the first field free region of the reversed geometry double focusing mass spectrometer.     

The observation of broad metastable ion peaks in the surface-induced dissociation mass spectra of protonated and cationated peptides

The product ion mass spectra of protonated and cationated peptides of relative molecular mass (RMM) 555–574 Da have been obtained by surface-induced dissociation of MH⁺ and [M + Cat]⁻ ions in a four-sector tandem mass spectrometer equipped with a specially designed collision cell. A linked scan of the electric and magnetic sector field strengths of the second mass spectrometer was used to transmit the fragment ions arising from collisions with a stainless steel surface. The resulting mass spectra contained broad metastable ion peaks produced by the dissociation of MH⁺ and [M + Cat]⁺ ions before the second magnetic sector, in the fourth field-free region of the instrument.     

Further interference peaks in mass analysed ion kinetic energy spectra

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.     

Metastable ion Characteristics. XXXXIII—separation and comparison of collisional activation spectra and metastable ion spectra

Additional ion acceleration in a separate collision chamber in the field free drift region makes it possible to add extra kinetic energy to those ions undergoing decomposition in the chamber. In this way product ions formed in the collision chamber (the [collisional activation] spectrum) can be distinguished from ions formed from decompositions in the low pressure portion of the field free drift region (the [metastable ion] spectrum). This technique is used to show that often a relatively small proportion of the ions in the [pure] collisional activation spectrum arises from processes which produce the metastable ion spectrum.     

Metastable ion studies: XIII—the measurement of appearance energies of metastable peaks

A simple, accurate and reproducible method is described for measuring the appearance energy of a metastable peak. The method involves the comparison of the metastable peak intensity with that of a ‘standard’ metastable peak, over a small range of ionizing electron energies above their onsets. Six fragmentation processes whose reaction energetics are well established were selected to provide suitable calibrant metastable peaks. Results are also presented for fragmentations having large kinetic shifts and for some primary and secondary ion decomposition of current interest.     

Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

Composite metastable peaks are generated in the unimolecular fragmentations (i) [C₃H₅]⁺ → [C₃H₃]⁺ + H₂ (flat-top upon flat-top) and (ii) [C₄H₉]⁺ → [C₃H₅]⁺ + CH₄ (flat-top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert- and sec-butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C₃H₄]⁺· → [C₃H₃]⁺ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C₆H₈]²⁺ ions. The second component in (iii) then arises from the reaction [C₆H₈]²⁺ → [C₆H₆]²⁺ + H₂.     

Three-dimensional representation of metastable peaks at high mass resolution

Isobaric daughter ions formed from a common metastable ion have been analysed at high mass resolution. A three-dimensional representation is used to summarize the data for competitive CO and C₂H₄ losses and competitive CHO˙ and C₂H₅˙ losses from metastable molecular ions of cyclohexanone.     

Relative intensities of metastable peaks measured with B/Elinked scan and metastable defocusing techniques

The ratios (A) of metas table- and parent-ion peak heights measured by me testable defocusing and the B/E inked scan technique have the same order of magnitude. This was established for the M⁺ → [M - 15]⁺ and M⁺ → [M ? OR¹]⁺ (R¹ = H, Me, Ac) transitions in the mass spectra of diterpene alkaloids with a licoctonine skeleton. The A values were demonstrated to depend on the position of the eliminated group in the parent molecule. The A values can also be employed as a criterion for the identification of daughter-ion reaction configurations.     

Frequency shifts due to the interference of resolved peaks in magnitude-mode Fourier-transform ion cyclotron resonance mass spectra

We have obtained relationships for frequency shifts resulting from the interference of spectral components for the magnitude mode Fourier transform. The approximation of a weak perturbation of well resolved peaks has been used. Both the low- and high-pressure limits for Fourier-transform ion cyclotron resonance (FTICR) operation have been considered. We have found that the shifts can be either negative or positive, depending on the initial phase and/or the choice of the time-domain interval. The magnitude of shifts generally does not exceed the peak width. In the approximation of small perturbations the shifts produced by multiple peaks are additive. We have compared theoretical results with experimental shifts for isotopic clusters of multiply charged insulin. Up to 1 ppm frequency variations were experimentally observed for the insulin 5+ charge state, consistent with theoretical estimates. The peak interference is of particular significance in the case of bio-molecular mass spectra having a large number of peaks and covering a considerable dynamic range (i.e., relative abundance). We conclude that the common mass measurement procedure based on the location of the magnitude mode maxima of well resolved peaks can result in systematic mass measurement errors. The relationships obtained provide corrections for the frequency shifts and thus improve the mass measurement accuracy.     

The main beam correction term in kinetic energy release from metastable peaks

The correction term for the precursor ion signal width in determination of kinetic energy release is reviewed, and the correction term is formally derived. The derived correction term differs from the traditionally applied term. An experimental finding substantiates the inaccuracy in the latter. The application of the “T ‐value” to study kinetic energy release is found preferable to kinetic energy release distributions when the metastable peaks are slim and simple Gaussians. For electronically predissociated systems, a “borderline zero” kinetic energy release can be directly interpreted in reaction dynamics with strong curvature in the reaction coordinate.     

A three-dimensional representation of metastable peaks from double focusing mass spectrometers

The ion current transmitted by the collector slit of a double focusing mass spectrometer is considered as a function of two variables which independently reflect the effects of the instrument's electric and magnetic sectors. The ionic products of reactions occurring in the source and in various regions of the flight tube give rise to peaks or ridges with distinctive shapes and positions in the surface so derived. Volumes under peaks are proportional to abundances of ions. The surface provides a unified view of the alternative methods of scanning and facilitates understanding of the contrasting perspectives of metastable peaks which they reveal. The method, which may be applied with instruments of either conventional or reversed geometry, is illustrated for the former by reference to that part of the surface for toluene which relates to the loss of H from the molecular ion.     

High-sensitivity measurement of metastable ion abundances

Measurement of metastable ion abundances over a dynamic range of 10⁵ is possible with a specially designed mass spectrometer.     

The effect of ion molecule reactions on peaks in ion mobility spectrometry

The peak width, shape and position are heavily affected by ion–molecule reactions which are inevitable in the drift region of an ion mobility spectrometer. This paper discusses three major types of reactions occurring in drift tube and their effects on the shape and displacement of the peaks. The first reaction is the dissociation of dimer ions during their flight time creating a tail for the monomer peak. The second one is the reaction between the monomer ions with molecules of the same kind in the drift region. Such a process shifts the peak to longer drift times and causes a tail for the peak as well as its broadening. The last one is the reaction of ions with molecules of different types such as dopant which may exist in the drift region. Depending on the reactivity of the ions, this kind of reaction displaces the peaks differently so that peak-to-peak resolution is lost or, in some cases, gained.     

The use of two linked scanning modes in alternation to analyse metastable peaks

Analyses of masses and relative kinetic energies for metastable peaks may be achieved by the use in alternation of two types of linked scans of the electric sector voltage and magnetic field. The method is demonstrated by an appraisal of hydrogen exchange in molecular ions of 2-methyl-[²H₃]-benzoic acid.     

The metastable spectra of cis- and trans-butenes

A new type of ion detector has been developed that can be made to operate in two ways. It can detect all ions, or it can be made to detect only those within a preselected energy range. Its use on a mass spectrometer to examine the metastable spectra of cis- and trans-butenes is described.     

The Use of two scanning modes for the analysis of metastable peaks

Two scanning modes can be used sequentially to determine the relative abundances of a series of daughter ions formed from the same parent ion in the field free region preceding the first sector of a double focusing mass spectrometer. Hydrogen exchange reactions in the molecular ions of benzoic acid and anisole have been analysed by these procedures, and a study of diphenylmethane demonstrates that the usual procedure of single scans gives rise to peaks that might be interpreted erroneously.     

Mass spectrometric studies of peptides—VI:. Applications of metastable ion and collisional activation spectra

An unambiguous differentiation between leucine and isoleucine residues in peptides is possible with unimolecular metastable ion (MI) and collisional activation (CA) spectra. The immonium ion \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm [Y}\!\mathop {\rm N}\limits^{\rm + } {\rm H = CHC}_{\rm 4} {\rm H}_{\rm 9} {\rm ]} $\end{document}, which is present in the normal mass spectra of all leucine or isoleucine containing peptides, yields MI spectra with which this differentiation can be made with high sensitivity. The MI and CA spectra of other peptide fragments are also applicable in particular cases. MI and CA spectra can also be used to obtain information on the amino acide sequences of individual ions. In general more extensive sequece information is available from CA than from MI spectra. This information appears to be particularly promising for the sequencing of oligopeptide mixtures or of ions produced by field or chemical ionization.     

Deconvolution of composite metastable peaks: A new method for the determination of metastable transitions occurring in the first field free region

The masses of daughter ions resulting from metastable transitions in the first field free region of a “reversed geometry” mass spectrometer can be accurately determined through a mass analysed ion kinetic energy scan at the apparent mass at which these daughter ions travel through the magnetic sector. A combination of these mass analysed ion kinetic scans with scanning the magnetic flux allows the resolution of overlapping metastable diffuse peaks.