John F. J. Todd

Recent improvements in and analytical applications of advanced ion trap technology

Abstract A new method of ion trap operation is disclosed in which the trap functions in a mass selective instability mode. Relatively high pressures of a light gas are used to damp the ionic motion and increases in resolution, sensitivity and detection limit have been found. The use of an ion trap as a sensitive, compound-specific gas chromatographic detector is discussed.

Resonance excitation of ions stored in a quadrupole ion trap. Part 1. A simulation study

Abstract Preliminary findings are presented of a simulation study of resonance excitation of gaseous ions stored within a quadrupole ion trap. The ion trap was operated in the common mode wherein a radio-frequency drive potential was applied to the ring electrode while the end-cap electrodes were grounded or held at a small potential. The arrangement chosen for computer simulation of resonance excitation discussed here is that in which an auxiliary potential is applied to both end-cap electrodes such that these electrode potentials are in phase with each other. The effects of resonance excitation are manifested in the following ways: temporal variation of ion kinetic energy; temporal variations of axial and radial displacements from the trap centre; direction of ion ejection; and emergence of new frequency components of ion motion. The extent of resonance excitation has been examined as a function of ion mass, trapping parameters a u and q u , ion initial position and velocity, drive and auxiliary potential phase angles, delayed imposition of auxiliary potential, auxiliary potential amplitude and frequency, and mass of an inert collision gas. Four principal findings are reported. Irradiation of trapped ions at the two fundamental secular frequencies, axial and radial, does not lead exclusively to ejection of ions in the axial and radial directions, respectively: there is clearly a degree of perturbation of ion radial motion upon irradiation at the axial fundamental secular frequency, and vice versa, which can become dominant under certain conditions and lead to a change in direction of ion ejection. Excitation at theoretically predicted secular frequencies other than the fundamental frequencies is virtually negligible, as expected from the magnitudes of the C 2 n coefficients. Strong resonance absorption occurs at a series of frequencies which are not present in the motion of the unexcited ions; these frequencies are designated as new frequencies. The motion of ions subjected to resonance excitation often becomes characterized by additional frequency components which we have designated as induced frequencies.

Resonance excitation of ions stored in a quadrupole ion trap Part II. Further simulation studies

Abstract Further simulation studies have been carried out on the behaviour of ions stored in a quadrupole ion trap and subjected to small auxiliary potentials oscillating at frequencies related to the secular frequencies of ion motion. Quadrupolar excitation at frequencies of β r Ω and β z Ω, that is, twice the fundamental radial and axial secular frequencies, respectively, results in parametric resonance which induces rapid excitation of ion motion. Frequency analysis of ion axial and radial motions has been carried out to determine the effects of collisions on ion motion, and of auxiliary potential amplitude and frequency for several working points in the stability diagram. A systematic survey was carried out of the variation, as a function of working point, of ion kinetic energy averaged over the final three r.f. cycles prior to ejection. The chosen working points lay on the locus of β r Ω = β z Ω, the two main areas of the stability diagram for which β z Ω r Ω and β r Ω z Ω, the q z axis, and working points for which a z was varied at constant q z . Ion kinetic energies averaged over the final three r.f. cycles are interpreted in terms of potential well-depths, while the irradiation times required for ion ejection indicate relative efficiencies for energy absorption by the subject ion from the resonance radiation.

Investigation of the complex reactions of SiH4 and GeH4 in the ion trap using dynamically programmed scanning

Abstract The rate constants of the gas-phase reactions of primary ions in a 1:1 SiH 4 /GeH 4 mixture have been determined by ion trap mass spectrometry and compared with those obtained for SiH 4 and for GeH 4 alone. The main reaction pathways are hydride or hydrogen transfer, formation of ions containing two silicon or two germanium atoms and ions containing silicon and germanium together. Experimental rate constants have been compared with the calculated collision rate constants and efficiencies have been determined. The formation of mixed ions, important in the radiolytical preparation of materials of interest in photovoltaic technology, is more efficient when SiH + m ( m = 0, 1) ions react with GeH 4 than when GeH + reacts with SiH 4 , and occurs at comparable rates to SiH + 2 and GeH + 2 reacting with GeH 4 and SiH 4 , respectively.

The quadrupole ion store (quistor) part IX. space-charge and ion stability. a theoretical background and experimental results

Abstract Stability diagrams for a series of ionic species trapped within the QUISTOR are obtained under various experimental conditions. The extent to which the boundaries of the diagrams are displaced from their theoretical positions is discussed in terms of existing models for the influence of space charge upon ion stability. Whilst space-charge within the trapped ion cloud does appear to exert an effect there is evidence of some, as yet unexplained, additional factor causing destabilisation.

The quadrupole ion store (quistor). part VII. simultaneous positive/negative ion mass spectrometry

Abstract A system is described which permits the simultaneous storage, analysis and detection of positive and negative ions. Scanning techniques have been developed which optimize the ion signal and which can specify the polarity of the species being stored.

The quadrupole ion store (QUISTOR). Part xii. The trapping of ions injected from an external source: A description in terms of phase-space'dynamics

Abstract The methods of phase-space dynamics have been employed to predict the trapping efficiency for ions injected into the QUISTOR from an external source. Broad agreement with previously published work has been obtained and, of the three geometric arrangements which have been considered, the most effective appears to be radial injection through the ring electrode.

The quadrupole ion store (QUISTOR). Part X. Space charge and ion stability. B. On the theoretical distribution and density of stored charge in RF quadrupole fields

Abstract The method of phase-space dynamics has been extended to provide equations for the ion density distributions theoretically expected in the cases of one-, two- and three-dimensional radio-frequency quadrupole electric fields. The method, which ignores space charge, has been employed to predict the ion distribution within the mass filter and within the ion trap under specified conditions, and has been found to agree in each case with published experimental results where these are available. For the ion trap, predictions of oscillations in the trapped ion plasma have been obtained, and a model which closely replicates the “total pressure” curve of the QUISTOR has been developed.

Resonance excitation of ions stored in a quadrupole ion trap Part III. Introduction to the field interpolation simulation method

Abstract Additional simulation studies have been carried out on the behaviour of single ions stored in a quadrupole ion trap and then subjected to auxiliary potentials of small amplitude oscillating at frequencies related to the secular frequencies of ion motion. The ion trap was operated in the common mode wherein an r.f. drive potential was applied to the ring electrode while the end-cap electrodes were grounded or held at a small potential. A new method of field interpolation has been developed for the calculation of ion trajectories; this method has been tested by direct comparison with the analytic solution of the Mathieu equation. The field interpolation method has been employed to calculate trajectories of ions excited resonantly by application of an auxiliary oscillating potential in each of three possible modes. Each ion trajectory is calculated to the point of ejection from the ion trap or for a pre-selected time period. The continual absorption of resonance radiation by an ion stored within a trap leads to increasing ion kinetic energy until the trapping potential is exceeded and the ion is ejected from the trap. It has been shown earlier that ion ejection requires a constant fluence, where fluence is the product of auxiliary potential amplitude and the duration of irradiation. At a given frequency, where the absorption of energy over a period of time is proportional to the coefficient of absorption at that frequency, a small coefficient of absorption will require a large fluence for ion ejection. Thus the coefficient of absorption is proportional to the reciprocal of the fluence required, at a given frequency, for ion ejection. Relative absorption coefficients have been obtained in this manner for the three types of irradiation mode and at a number of auxiliary potential frequencies; these relative absorption coefficient are compared with experimental results of the abundance of ions ejected under conditions of constant fluence.

Some operational characteristics of a quadrupole ion storage mass spectrometer having cylindrical geometry

Abstract Established techniques have been employed to determine the stability diagrams for a number of ionic species stored within three ion traps having cylindrical geometries. The results obtained are in broad agreement with other published theoretical work allowing for possible distortions arising from space charge. The mass spectrum of n-hexane recorded with one of the traps has been studied in detail and the conditions necessary to improve performance have been found to parallel those required for the QUISTOR, having quadrupole geometry. The first analysis of the mass-selective storage mass spectrum of a compound (n-hexane) has been reported.