2.3 Cluster Reaction Apparatus
21
2.3.1 Ion Traps and Tandem Quadrupole/Hexapole Reactors
Three-dimensional and two-dimensional linear quadrupole ion traps (developed by
Wolfgang Paul) [159–161] generate a RF quadrupole field to store ions within defined
boundaries, combined with mass spectrometry, enabling to probe metal cluster reactions up to approximate 10 Pa pressures. Similar to ion traps, tandem quadrupole
or hexapole reactors are also a kind of customized cluster systems that use dynamic
RF/DC electric fields to host and guide charged clusters allowing for gas-phase reactions at low pressures [162–164]. Typically a RF-octupole ion trap is combined with a
time-of-flight (TOF) mass spectrometer [165], such as those built in Bernhardt group
[163] and He group [166]. When running reactions, small cluster ions are concentrated in a helium-filled ion trap with tunable experimental conditions allowing for
reactions for ~0.1 s or even longer [163]. The ions of both reactants and products are
then released from the ion trap for mass analysis [167–171].
2.3.2 Selected Ion Flow Tube (SIFT)
In the past, the flowing afterglow technique [172] and other related flow reactors
[173, 174], have provided a wealth of information on general ion-molecule reactions although limited attention was paid to cluster systems [175, 176]. Selected
ion flow tube (SIFT) is a technique termed by Adams and Smith in 1976 [161] as
shown in Fig. 2.6, where a mass-selected positive ion beam derived from a certain
source is injected into a flowing gas. A combination of SIFT with mass spectrometry, also abbreviated as SIFT-MS, has been known as a convincing quantitative
mass spectrometric technique for trace gas analysis, which involves the chemical
ionization of trace volatile compounds by selected positive precursor ions during a
well-defined time period along a flow tube. Reasonable interest is attracted on the
subsequent reaction of the ions with neutral molecules which are introduced into the
carrier gas downstream of the injection point. In addition to the extensive studies to
explore ion-molecule reaction kinetics, crucial advances had been made on its application to ionospheric and interstellar ion chemistry [147], SIFT-MS was also used in
human breath analysis and showed promise as a non-invasive tool for physiological
monitoring and correlated disease diagnosis [152].
Among others, a typical SIFT system in Schwarz group allow to study reactions
of ions produced via one of the three sources, i.e., electron or chemical ionization
(EI/CI), electrospray ionization (ESI) or glow-discharge ionization (GDI) [148, 149].
Before going through the flow tube, the resulting ions from these optional sources are
mass-analyzed or selected using the first quadrupole mass filter (Q 1 ). Next, there are
options to choose either all ions or just a single-mass ion to be directed towards the
flow tube. A pressure of ~0.5 mbar with buffer gas helium in the flow tube is kept under
SIFT conditions for ion-molecular reactions. The ion products extracted from the flow
tube are then directed towards the second quadrupole analyzer Q 2 , pass an octupole
21
2.3.1 Ion Traps and Tandem Quadrupole/Hexapole Reactors
Three-dimensional and two-dimensional linear quadrupole ion traps (developed by
Wolfgang Paul) [159–161] generate a RF quadrupole field to store ions within defined
boundaries, combined with mass spectrometry, enabling to probe metal cluster reactions up to approximate 10 Pa pressures. Similar to ion traps, tandem quadrupole
or hexapole reactors are also a kind of customized cluster systems that use dynamic
RF/DC electric fields to host and guide charged clusters allowing for gas-phase reactions at low pressures [162–164]. Typically a RF-octupole ion trap is combined with a
time-of-flight (TOF) mass spectrometer [165], such as those built in Bernhardt group
[163] and He group [166]. When running reactions, small cluster ions are concentrated in a helium-filled ion trap with tunable experimental conditions allowing for
reactions for ~0.1 s or even longer [163]. The ions of both reactants and products are
then released from the ion trap for mass analysis [167–171].
2.3.2 Selected Ion Flow Tube (SIFT)
In the past, the flowing afterglow technique [172] and other related flow reactors
[173, 174], have provided a wealth of information on general ion-molecule reactions although limited attention was paid to cluster systems [175, 176]. Selected
ion flow tube (SIFT) is a technique termed by Adams and Smith in 1976 [161] as
shown in Fig. 2.6, where a mass-selected positive ion beam derived from a certain
source is injected into a flowing gas. A combination of SIFT with mass spectrometry, also abbreviated as SIFT-MS, has been known as a convincing quantitative
mass spectrometric technique for trace gas analysis, which involves the chemical
ionization of trace volatile compounds by selected positive precursor ions during a
well-defined time period along a flow tube. Reasonable interest is attracted on the
subsequent reaction of the ions with neutral molecules which are introduced into the
carrier gas downstream of the injection point. In addition to the extensive studies to
explore ion-molecule reaction kinetics, crucial advances had been made on its application to ionospheric and interstellar ion chemistry [147], SIFT-MS was also used in
human breath analysis and showed promise as a non-invasive tool for physiological
monitoring and correlated disease diagnosis [152].
Among others, a typical SIFT system in Schwarz group allow to study reactions
of ions produced via one of the three sources, i.e., electron or chemical ionization
(EI/CI), electrospray ionization (ESI) or glow-discharge ionization (GDI) [148, 149].
Before going through the flow tube, the resulting ions from these optional sources are
mass-analyzed or selected using the first quadrupole mass filter (Q 1 ). Next, there are
options to choose either all ions or just a single-mass ion to be directed towards the
flow tube. A pressure of ~0.5 mbar with buffer gas helium in the flow tube is kept under
SIFT conditions for ion-molecular reactions. The ion products extracted from the flow
tube are then directed towards the second quadrupole analyzer Q 2 , pass an octupole