Kathleen A. Cox

Mass shifts and local space charge effects observed in the quadrupole ion trap at higher resolution

Mass measurements made using the quadrupole ion trap mass spectrometer under high resolution conditions display non-random errors which are attributed to local and general space charge conditions. Each ion in the trap experiences a degree of charge interaction which depends on (i) the total number of charges in the trap, (ii) the number of ions of the same m/z value, and (iii) the abundances and mass differences of neighboring ions. The net field formed by the sum of these ion/ion interactions produces delayed ion ejection. Mass assignments are observed to shift on simply increasing the ionization time, so increasing the total number of trapped ions. The effect of the presence of other ions on mass shifts is amplified as the mass difference between the two populations of ions decreases. As ions of a particular m/z value are isolated by gradual elimination of neighboring ions of lower and, more particularly, higher mass, the observed mass of the ion of interest shifts to lower mass due to the reduction of space charge. The different environments experienced by calibrant and analyte ions constitute a source of error in mass calibration methods which employ external mass standards. Current calibration procedures which involve the construction of a calibration line based on the observed masses of CsI cluster ions are in error because these ions have much higher abundances than the analyte ions. However, extrapolation of experimental data to the point of zero space (i.e. zero ion abundance) eliminates the differences in the charge environments experienced by each ion. Clear trends are observed, even for high resolution experiments on peptide mixtures, between the number and masses of ions in the trap and the observed ejection times of ions of different masses.

Conformer selection of protein ions by ion mobility in a triple quadrupole mass spectrometer

Electrospray mass spectra of multiply charged protein molecules show two distinct charge state distributions proposed to correspond to a more highly charged, open conformational form and a lower charged, folded form. Elastic collisions carried out in the radiofrequency-only collision cell of a triple quadrupole mass spectrometer have dramatic effects on the appearance of the mass spectra. The different cross sectional areas of the conformers allow preferential selection of one charge state distribution over the other on the basis of ion mobility. Preferential selection is dependent on the nature and pressure of the target gas as well as the nature of the protein. In the case of positively charged horse heart apomyoglobin (MW 16,951 da), a high charge state distribution centered around (M + 20H)20+ predominates at low target gas pressures and a second distribution centered around (M + 10H)10+ predominates at high target gas pressures. Bimodal distributions are observed at intermediate pressures and, remarkably, charge states between the two distributions are not effectively populated under most of the conditions examined. Hard sphere collision calculations show large differences in collision frequencies and in the corresponding kinetic energy losses for the two conformational states and they demonstrate that the observed charge state selectivity can be explained through elastic collisions.

Rapid Determination of Pharmacokinetic Properties of New Chemical Entities: In vivo Approaches

There is a continuing need for increased throughput in the evaluation of new chemical entities in terms of their pharmacokinetic (PK) parameters as part of new drug discovery. This review summarizes various approaches that have been used to increase throughput in this area. The article divides the approaches into two areas: assay enhancement and sample reduction.

High Precision Mass Spectrometry with the Ion Trap Mass Spectrometer

The quadrupole ion trap was first described by Wolfgang Paul (Paul et al., 1959). It was developed as a means of performing gas chromatography/mass spectrometry by Stafford and coworkers (Stafford et al., 1984) who introduced a mass selective method of ion ejection which provided a convenient method of scanning a mass spectrum. This instrument, introduced commercially in 1983, utilized electron impact ionization to convert gaseous samples into ions. Refinements such as chemical ionization (Brodbelt et al.,1984), automatic gain control (Stafford et al., 1987) and axial modulation (Grabau et al., 1983) increased the dynamic range and sensitivity of the ion trap for the analysis of ions which have a mass-to-charge ratio < 650 Da/charge.