A.W. Colburn

Threshold fluences for production of positive and negative ions in matrix-assisted laser desorption/ionisation using liquid and solid matrices

Abstract Dependences of positive and negative analyte-ion and matrix-ion signals upon pulse energy (at 266 nm) have been measured in matrix-assisted laser desorption/ionization (MALDI), for a liquid matrix (3-nitrobenzyl alcohol) and for a solid matrix (sin-apinic acid). The analyte was the protein bovine insulin. The results suggest that the threshold fluences for the formation of positive and of negative ions are the same as each other, for a given matrix. The threshold fluences are lower in the case of the liquid matrix as compared to the solid matrix.

Initial velocities of positive and negative protein molecule-ions produced in matrix-assisted ultraviolet laser desorption using a liquid matrix

Abstract The initial axial velocities for the positive and negative cytochrome c molecule-ions desorbed from a liquid matrix (3-nitro-benzyl alcohol) using a 17 ns ultraviolet laser (266 nm) have been determined. The method employed was based upon measurement of ion flight times through a field-free path co-axial with the ion optical axis. The possible interferences, such as energy deficits, due to the presence of electric fields were avoided by using a grid-electrode placed at a very short distance from the sample stage. The potential of this electrode was maintained at the same potential as the sample stage, thereby creating a narrow field-free region within which ion generation took place. Systematic variations of the potential at a second grid-electrode caused a gradual shift of the ion flight times, which could be related to the initial ion velocities. The ion velocities were obtained by correlating the measured time-shifts with values from theoretical analysis. The positive and negative molecule-ions were found to have axial velocities of 840±70 and 750±40 ms −1 , respectively.

Design considerations in energy resolved time-of-flight mass spectrometry

Abstract We report the design, construction and performance of an energy resolved time-of-flight (ER-TOF) mass spectrometer. The system consists of a linear TOF coupled in parallel to the electric sector of a large double-focussing magnetic-sector mass spectrometer. The use of lensing between the TOF and the electric sector is discussed with respect to the required beam conditions. Ion transmission through the electric sector is treated theoretically, especially with respect to time-of-flight. Preliminary experimental results from the instrument using matrix-assisted laser desorption—ionisation are presented and discussed, with particular regard to the fragmentation of large ions.

Collisional excitation and fragmentation of alkali-metal halide cluster ions

Caesium iodide cluster ions have been formed by bombardment of the solid salt in vacuo with xenon atoms possessing kiloelectron volt (keV) kinetic energies, and the ions have been mass-selected on the basis of magnetic deflection. The mass-selected cluster ions with translational energies of ca. 10 keV have been allowed to collide with thermal inert and other gases, and the translational energy spectra of surviving cluster ions and of fragment ions have been measured using a cylindrical electric sector analyser. Cluster ions suffered large translational energy losses ΔE(e.g. 100 eV) in those collisions leading to fragmentation. Different fragment ions were associated with translational energy losses of different magnitudes. For a given fragment ion, the magnitude of the translational energy loss tended to be related to the mass of the collision gas, typically being larger for lighter gases. Translational energy losses tended to be greater in the case of larger incident cluster ions. The translational energy spectra are discussed in terms of direct momentum transfer in an impulsive collision.

Computer-controlled activation of 25/μm diameter tungsten wire for field desorption

Abstract An attempt has been made to translate the “art” of growing carbon microneedles on tungsten wires into an automated procedure by means of microcomputer control. The concept of a power-resistance isotherm is introduced, as a means of characterizing the activation process for the purpose of automation. As well as freedom from the requirement for an operator to monitor activation, high degrees of reproducibility for the activation processes and easier optimization of conditions when, for example, different diameter wires are used have been found to be important benefits resulting from automation.

Peter J Derrick and the Grand Scale ‘Magnificent Mass Machine’ mass spectrometer at Warwick

The value of the Grand Scale ‘Magnificent Mass Machine’ mass spectrometer in investigating the reactivity of ions in the gas phase is illustrated by a brief analysis of previously unpublished work on metastable ionised n-pentyl methyl ether, which loses predominantly methanol and an ethyl radical, with very minor contributions for elimination of ethane and water. Expulsion of an ethyl radical is interpreted in terms of isomerisation to ionised 3-pentyl methyl ether, via distonic ions and, possibly, an ion-neutral complex comprising ionised ethylcyclopropane and methanol. This explanation is consistent with the closely similar behaviour of the labelled analogues, C3H7CH2CD2OCH3+. and C3H7CD2CH2OCH3+., and is supported by the greater kinetic energy release associated with loss of ethane from ionised n-propyl methyl ether compared to that starting from directly generated ionised 3-pentyl methyl ether.