Quadrupole Mass Analysis
The quadrupole mass analyser was developed in parallel with the
quadrupole ion trap by the third Nobel prize winning mass spectrometry
pioneer, Wolfgang Paul [1,2]. A quadrupole mass analyser consists of
four parallel rods (see figure) that have fixed DC and alternating RF
potentials applied to them. Ions produced in the source of the
instrument are then focussed and passed along the middle of the
quadrupole rods. Their motion will depend on the electric fields so
that only ions of a particular
m/z
will be in resonance and thus pass through to the detector. The RF is
varied to bring ions of different
m/z
into focus at the detector and thus build up a mass spectrum. The
trajectory of the ions through the quadrupole is actually very complex
- the figure shows a simplified schematic.
The two opposite rods in the quadrupole
have a potential of +(U+Vcos(ωt)) (labelled '+' on thefigure) and
the other two -(U+Vcos(ωt)) where 'U' is the fixed potential and
Vcos(ωt) is the applied RF of amplitude 'V' and frequency 'ω'. The
applied potentials on the opposed pairs of rods varies sinusoidally
as cos(ωt) cycles with time 't'. This results in ions being able to
traverse the field free region along the central axis of the rods
but with oscillations amongst the poles themselves. These
oscillations result in complex ion trajectories dependent on the m/z of the ions. Specific
combinations of the potentials 'U' and 'V' and frequency 'ω' will
result in specific ions being in resonance creating a stable
trajectory through the quadrupole to the detector. All other m/z values will be non-resonant
and will hit the quadrupoles and not be detected (see figure). The
mass range and resolution of the instrument is determined by the
length and diameter of the rods.
Quadrupole mass spectrometers generally have two configurations in
the modern laboratory. They are very commonly used in conjunction
with either gas-chromatography (GC/MS) or liquid-chromatography
(LC/MS) as a simple high throughput screening system. Quadrupoles
can also be placed in tandem to enable them to perform fragmentation
studies - the most common set-up is the triple quadrupole (QQQ) [3]
mass spectrometer which enables basic ion fragmentation studies
(tandem mass spectrometry MS/MS) to be performed.
References:
[1] W. Paul & H. Steinwedel, Zeitschrift für Naturforschung,
8A; 1953, p448.
[2] W. Paul, Agewandte Chemie - International Edition, 29; 1990,
p739.
[3] R.A. Yost and C.G. Enke, Journal of the American Chemical
Society, 100; 1978, p2274.