The quadrupole ion trap (QIT) mass analyser was
developed in parallel with the quadrupole mass analyser by the third
Nobel prize winning mass spectrometry pioneer, Wolfgang Paul
[1,2]. His work in the early 1950's lead to the development of the
basic
parameters of today's benchtop instruments, however it took
breakthroughs in design at Finnigan MAT in the 1980's [3] to make the
QIT-MS the simple to use practical instrument it is today. Commercial
QIT instruments are today very common in Chemical, Biochemical and
Forensics laboratories and they are very amenable to being couple with ESI and MALDI
ionisation as well as being coupled with liquid chromatography.
A schematic of the
basic set up of a QIT mass analyser is shown in Fig.1. The ions,
produced in the source of the instrument, enter into the trap through
the inlet and are trapped through action of the three hyperbolic
electrodes: the ring electrode and the entrance and exit endcap
electrodes. Various voltages are applied to these electrodes which
results in the formation of a cavity in which ions are trapped.
The ring electrode RF potential, an a.c. potential of constant
frequency but variable amplitude, produces a 3D quadrupolar potential
field within the trap. This traps the ions in a stable oscillating
trajectory. The
exact motion of the ions is dependent on the voltages applied
and their individual mass-to-charge (m/z)
ratios. For detection of the ions, the potentials are altered to
destabilise the ion motions resulting in ejection of the ions through
the exit endcap. The ions are usually ejected in order of increasing m/z by a gradual change in the
potentials. This 'stream' of ions is focussed onto the detector of the
instrument to produce the mass spectrum
Fig. 1: A Schematic (cutaway view) of a Quadrupole Ion Trap Mass
Analyser.
The very nature of
trapping and ejection makes a quadrupolar ion trap especially suited to
performing MSn experiments in structural elucidation
studies. It is possible to selectively isolate a particular m/z in the trap by ejecting all the
other ions from the trap. Fragmentation of this isolated precursor ion
can then be induced by CID experiments. The isolation and fragmentation
steps can be repeated a number or times and is only limited by the
trapping efficiency of the instrument. MS5 experiments are
fairly routine with this set-up as is the coupling of liquid
chromatography to perform LC-MSn studies.
References:
[1] W. Paul & H. Steinwedel; Zeitschrift für Naturforschung,
8A;
1953, p448.
[2] W. Paul; Agewandte
Chemie - International Edition, 29; 1990, p739.
[3] G. C. Stafford et al.; International
Journal of Mass Spectrometry and Ion Processes, 60; 1984, p85 and Analytical Chemistry, 59; 1987, p1677.
