Fourier-transform ion cyclotron resonance mass spectrometry
(FT-ICR-MS) is often thought of as being the most complex method of
mass analysis and detection. When considered at first principles
though, it is fairly simple to describe - although this does ignore
many of the 3 dimensional ion perturbation effects which make the
technique far more complex. It is not necessary to discuss those in
any detail here. The technique of ICR-MS was first published in the
mid. 1950's [1] where it was demonstrated for measurement of very
small mass differences at very high precision. The technique remained
a largely academic tool until the application of FT methods [2] by
Alan Marshall and Melvin Comisarow in the early 1970's [3]. It is now
one of the most sensitive methods of ion detection in existence and
has almost unlimited resolution - >10
7 is possible, with
most experiments taking place in the 10
5 to 10
6
range.
In the basic FT-MS instrument, the ions are generated in the source
and pass through a series of pumping stages to increasingly high
vacuum. When the ions enter the cell (ion trap) pressures are in the
range of 10
-10 to 10
-11 mBar with temperatures
close to absolute zero. The cell is located inside a spatial uniform
static superconducting high field magnet (typically 4.7 to 13 Tesla)
cooled by liquid helium and liquid nitrogen. When the ions pass into
the magnetic field they are bent into a circular motion in a plane
perpendicular to the field by the Lorentz Force (see figure 1 and
equation 1). They are prevented from leaving the cell by the
trapping
plates at each end.
The frequency of rotation of the ions is dependent on their m/z
ratio (equation 2). At this stage, no signal is observed because the
radius of the motion is very small. Excitation of each individual m/z is achieved by a swept RF
pulse across the excitation plates
of the cell. Each individual excitation frequency will couple with
the ions natural cyclotron motion and excite them to a higher orbit
where they induce an alternating current between the detector
plates. The frequency of this current is the same as the
cyclotron frequency of the ions and the intensity is proportional to
the number of ions. When the RF goes off resonance for that
particular m/z value, the
ions drop back down to their natural orbit (relax) and the next m/z packet is excited. Although
the RF sweep is made up of a series of stepped frequencies, it can
be considered as all frequencies simultaneously. This results in the
measurement of all the ions in one go producing a complex frequency
vs. time spectrum (the convoluted frequency spectrum or FID)
containing all the signals. Deconvolution of this signal by FT
methods results in the deconvoluted frequency vs. intensity spectrum
which is then converted to the mass vs. intensity spectrum (the mass
spectrum) by equation 3 (the mass conversion). It is also usual to
correct for mass errors at this stage by applying a calibration.
|
- F is the
Lorentz Force observed by the ion upon entering the
magnetic field
- B is the
magnetic field strength (constant)
- v is the
incident velocity of the ion
- ωc
is the induced cyclotron frequency
- m is the
mass of the ion
- z is the
charge on the ion
|
Due to the ion-trap nature of FT-MS, it is possible to measure the
ions without destroying them, this enables further experiments to
performed on the ions. The most common of these would be some kind
of fragmentation study (MS/MS or MS
n) for structural
elucidation experiments, but also other gas-phase reactions and
studies can be performed - e.g. gas-phase basicity calculations,
gas-phase kinetics, ion dissociation studies as well as the study of
ion-molecule or ion-ion interactions. Alan Marshall has a published
a number of reviews of FT-ICR and its applications over the years
[4].
References:
[1] J.A. Hipple
et al.,
Physical Review, 76, 1949, p1877 and Physical Review, 82, 1951, p697.
[2] J.W. Cooley and J.W. Tukey, Mathematics of Computation, 19,
1965, p297.
[3] M.B. Comisarow and A.G. Marshall, Chemical Physics Letters, 25,
1974, p282 and Journal of Chemical Physics, 62, 1975, p293 and Journal
of Chemical Physics, 64, 1976, p110.
[4] A.G. Marshall, Accounts of Chemical Research, 18, 1985, p316 and
Accounts of Chemical Research, 29, 1996, p308.