Alex W. Colburn

In-series combination of a magnetic-sector mass spectrometer with a time-of-flight quadratic-field ion mirror

A tandem mass spectrometer consisting of a double-focusing magnetic-sector mass analyzer in series with a time-of-flight (TOF) mass analyzer has been designed and constructed. The TOF analyzer was a quadratic-field ion mirror. The method of ionization used was matrix-assisted laser desorption/ionization. Precursor ions were mass selected with the magnetic-sector analyzer, and time focused by ion bunching prior to fragmentation in a collision cell. The fragment ions were mass analyzed with the TOF analyzer, which possessed the property that residence times (i.e., times of flight) in the mirror were independent of ion velocity. The theoretical background to the instrumental design is presented. Experimental results are presented, showing resolutions of 4000 in fragment ion spectra and demonstrating effective high-energy collision-induced decomposition of peptide molecule ions.

The Ion Conveyor. An ion focusing and conveying device.

The control and transmission of ions or small charged droplets in the intermediate to high-pressure regime is of primary importance in areas such as atmospheric pressure ionisation. Where small apertures separate differentially pumped vacuum regions in the inlet systems to mass spectrometers, a large proportion of the available ion current is lost to the surrounding electrode structures. A new ion-optical device, named the ion conveyor, incorporating electrodynamic focusing and conveying of charged entities is described. Results from ion-optical simulations are presented demonstrating the performance of the device in various operating modes and electrode configurations.

A tandem time-of-flight mass spectrometer: combination of a multi-turn time-of-flight and a quadratic-field mirror.

A tandem time-of-flight (ToF) mass spectrometer consisting of a multi-turn ToF and a quadratic-field ion mirror has been designed and constructed. The instrument combines the unique capabilities of both ToF instruments, namely high-resolution and monoisotopic precursor ion selection from the multi-turn ToF and temporal focus for fragment ions with different kinetic energies from the quadratic-field mirror. The first tandem mass spectra for this unique combination of ToF systems are presented.

A Quadratic-Field Reflectron Time-of-Flight Mass Spectrometer Incorporating Intermediate Temporal Focusing:

A time-of-flight mass spectrometer, using a two-electrode cone-plus-hyperboloid geometry to produce a quadratic reflecting field, has been designed, constructed and tested. The included angle of the cone was 90°. Matrix-assisted laser desorption/ionisation and direct laser desorption/ionisation were the methods used to produce ions. Physical constraints imposed by the geometry of the ion mirror were overcome by the use of ion-bunching to form an intermediate temporal focus. Experimental and ion-optical simulation results are presented, showing the large energy-bandwidth of the quadratic-field reflectron and the effectiveness of ion-bunching as a method of producing a temporal focus at the entrance to the ion mirror.

The mechanism of water loss from the oxonium ions CH3CH2CH2+OCH2 and (CH3)2CH+OCH2

Extensive new 2H-labelling results are reported, which pertain to the mechanism of water expulsion from metastable CH3CH2CH2+OCH2 and (CH3)2CH+OCH2 ions. Detailed mechanisms, involving ion–neutral complexes comprising incipient propyl cations coordinated to formaldehyde, propene attached to protonated formaldehyde, or propene and formaldehyde attached to a common proton, are discussed in the light of the labelling data. Loss of positional integrity of the hydrogen and deuterium atoms within the original propyl groups occurs; it is proposed that this takes place via interconversion of the ion–neutral complexes. The crucial step in water elimination appears to be irreversible reorganization of the proton-bound complex (or an ion–neutral complex of protonated formaldehyde and propene) to the open-chain carbonium ion CH3+CHCH2CH2OH.

Unimolecular reactions of ionized methyl butenyl ethers

The reactions of metastable C4H7OCH·+3 radical cations generated by ionization of methyl ethers derived from stable butenols have been investigated by means of 2H- and 13C-labelling experiments, kinetic energy release measurements and analysis of collision-induced dissociation spectra. The reactions shown by the ionized allylic ethers, CH3CHCHCH2OCH·+3 and CH2C(CH3)CH2OCH·+3, are almost identical, especially at low internal energy. However, the behaviour of these isomers of C4H7OCH·+3 differs subtly from that of CH2CHCH(CH3)OCH·+3. All these species undergo mainly loss of a methyl radical is slow dissociations, usually with formation of CH2CHCHO+CH3 as the product ion. The specificity of hydrogen atom selection shown in methyl radical elimination from 2H-labelled analogues of CH2CHCH(CH3)OCH·+3 is distinct from that displayed by the two other allylic isomers of C4H7OCH·+3. In contrast to the three ionized allylic ethers, metastable CH2CHCH2CH2OCH·+3 undergoes a sizeable amount of two other reactions — expulsion of a hydrogen atom or a formaldehyde molecule — in addition to undergoing methyl radical elimination. The specificity of methyl radical loss from this ion is different from those found for the analogous fragmentation of the ionized allylic ethers; moreover, the resultant C4H7O+ product ion is mainly CH2CH2CH2CHO+, rather than CH2CHCHO+CH3.

DESIGN AND PERFORMANCE OF AN ELECTROSPRAY ION SOURCE FOR MAGNETIC-SECTOR MASS SPECTROMETERS

An electrospray ion (ESI) source capable of operating at accelerating potentials of up to 11 kV has been designed and fabricated. The ESI source has been shown to deliver ion beams with a total current up to 20 pA and an emittance of 2–3 mm mrad in analysis of the peptide gramicidin S (molecular mass 1140.7 Da) and the protein bovine insulin B chain (molecular mass 3495.9 Da). Coupled to a two-sector tandem mass spectrometer, the ESI source produced efficiently the multiply charged ions of proteins, such as bovine ubiquitin (molecular mass 8564.8 Da) and cytochrome c (molecular mass 12327 Da). The high ion currents and high kinetic energies of the electrospray ions (up to 200 keV) characterize this ESI source as a powerful tool to be used in structural analysis of macromolecules by collision-induced dissociation.

The mechanism of formaldehyde loss from the oxonium ions CH3CH2CH2CHO+CH3 and CH3CH2CH2CH2+OCH2

In contrast to CH3CH2(CH3)CO+CH3 and (CH3)2CHCHO+CH3, which in slow dissociations lose mainly CH3OH, metastable CH3CH2CH2CHO+CH3 exples predominantly CH2O by isomerising to CH3CH2CH2CH2+OCH2, probably via two 1,2-H shifts and a subsequent 1,5-H shift; CH3CH2CH2+OCH2 undergoes limited interconversion with CH3+CHCH2CH2OCH3, prior to CH2O elimination, via a 1,5-H shift.