3.1.2 Accelerator Mass Spectrometer
Ions which dissociate on fast timescales can be studied at a single-pass instrument
such as an accelerator mass spectrometer (also known as a sector instrument as
magnetic and electrostatic sectors follow each other in space) (Fig. 3.15). In the
illustrated instrument ions are produced by electrospray, and passed through a
heated capillary and tube-lens/skimmer region into an octopole which can be
operated as a trap by proper pulsing of the lens directly following it. When the
octopole is used in trapping mode, ion-molecule reactions can occur between the
ions and an injected gas to form complexes of interest. The following linear ion trap
can be used to bunch and thermalise the ions when the octopole is used as an ion
guide, to thermalise the ion bunch formed in the octopole through collisions with
helium buffer gas, or simply as an ion guide. Ions are then accelerated to kinetic
energies of 50 keV times charge state, and those of interest selected by an electromagnet and subsequently irradiated with laser light. Irradiation can be done in
Fig. 3.13 Fragmentation can be measured at ELISA by switching the voltages at a particular time
to store a potential daughter ion with a different kinetic energy, and subsequently dumping any
stored ions on the detector [17–19]. Mass spectra are obtained by repeating this process for
different potential daughter ions. (a) Voltages are switched at the same time as laser irradiation
to measure prompt decay. (b) Prompt mass spectrum. (c) The parent ions continue to be stored
after laser irradiation, and voltages are only switched at a later time to measure delayed dissociation. (d) Delayed mass spectrum. The delayed and prompt mass spectra may be different from one
another, e.g., hot ions (two-photon absorption) dissociate faster than colder ions (one-photon
absorption)
3 Experimental Techniques
33
Ions which dissociate on fast timescales can be studied at a single-pass instrument
such as an accelerator mass spectrometer (also known as a sector instrument as
magnetic and electrostatic sectors follow each other in space) (Fig. 3.15). In the
illustrated instrument ions are produced by electrospray, and passed through a
heated capillary and tube-lens/skimmer region into an octopole which can be
operated as a trap by proper pulsing of the lens directly following it. When the
octopole is used in trapping mode, ion-molecule reactions can occur between the
ions and an injected gas to form complexes of interest. The following linear ion trap
can be used to bunch and thermalise the ions when the octopole is used as an ion
guide, to thermalise the ion bunch formed in the octopole through collisions with
helium buffer gas, or simply as an ion guide. Ions are then accelerated to kinetic
energies of 50 keV times charge state, and those of interest selected by an electromagnet and subsequently irradiated with laser light. Irradiation can be done in
Fig. 3.13 Fragmentation can be measured at ELISA by switching the voltages at a particular time
to store a potential daughter ion with a different kinetic energy, and subsequently dumping any
stored ions on the detector [17–19]. Mass spectra are obtained by repeating this process for
different potential daughter ions. (a) Voltages are switched at the same time as laser irradiation
to measure prompt decay. (b) Prompt mass spectrum. (c) The parent ions continue to be stored
after laser irradiation, and voltages are only switched at a later time to measure delayed dissociation. (d) Delayed mass spectrum. The delayed and prompt mass spectra may be different from one
another, e.g., hot ions (two-photon absorption) dissociate faster than colder ions (one-photon
absorption)
3 Experimental Techniques
33