Ion Cyclotron Resonance ICR Mass Spectrometry

1
Ion Cyclotron Resonance (ICR) Mass Spectrometry

• Mass Spectrom. Rev. 1996, 15, 163.
• Mass Spectrom. Rev. 1998, 17, 1.
• Ion Cyclotron Resonance Spectrometry
  Lehman/Bursey
• QD96I54L44 (1976)

In the absence of an electric field.
2

• the force is perpendicular to v and B. For very
  large B, circular motion results.

where r is the radius of the ion path
wc, the angular frequency, is given by,
therefore,
The basic cyclotron equation. Note that wc
is independent of v.
3
The cyclotron frequency, nc , is more important
for detection,
or for an ion with z charges,
4
The frequency of CO in a typical magnetic flux
density of 1.0 T is,
Due to the presence of the electric field, ions
drift from the source to the resonance region.
5
The drift velocity is,
In a typical cell, the distance between the
plates is 1 cm and the potential difference 0.5
V. Therefore,
For a distance from the electron beam to the far
end of the analyzer of about 8.5 cm (d), the time
the ion spends in the cell is
6
The length of the ions path (including cyclotron
motion) is a product of the ions speed and drift
time. The ion has thermal energy unless it is
absorbing energy from an rf oscillator.
kB Boltzmann constant
Therefore the distance traveled by the ion is,
7
At about 10-5 to 10-4 torr, the mean free path is
about 0.1 to 0.01 m. There will, therefore, be
many collisions with neutral gases in the ICR
cell or many opportunities to react.
The diameter of the cyclotron motion is
Since v decreases with the square root of
increasing mass, and nc decreases with increasing
mass the diameter of cyclotron motion increases
with the square of the mass of the ion. (show
this!!)
During one revolution the linear distance
traveled (drift distance) is,
Therefore the orbits are nearly circular.
8
Cyclotron Resonance Detection (Past)
Ions were detected using a marginal oscillator
Ions at resonance with the RF oscillator (ie.
ion cyclotron frequency is identical to the rf
frequency) will absorb energy and change the
capacitive impedance of the RLC circuit. The RF
oscillator frequency was held fixed and the
magnetic field strength, B, was scanned in order
to bring the ions into resonance.
9
It was convenient to choose an rf oscillation
frequency (and therfore cyclotron frequency of
the ions) such that a change in B of 0.01 T
corresponded to a change in 1 mass
unit. Later the magnetic field strength
was held fixed and the rf oscillation frequency
was scanned to bring ions into resonance.
10
Ion Trapping ICR and Fourier Transform ICR (FTICR
or FTMS)
With the rapid development of superconducting
magnets and Fourier transform techniques for NMR
in the 70s, Comisarow and Marshall (in Canada)
quickly learned to apply these techniques to the
ICR experiment.

• ions enter the cell (or are created internally)
  and they begin their cyclotron motion, orbiting
  around the centre of the magnetic field
• since the magnetic field is quite high (typical
  minimum of 4.7 T, but this is increasing) the
  ions are trapped in the radial (x,y) direction.

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• by applying small, equal potentials to the two
  end or trapping electrodes, the ions are
  confined in the z or axial direction.
• ions can be confined for very long periods of
  time such that ion/molecule reactions or even
  slow unimolecular dissociation processes can be
  observed and monitored.

12
Fridgen/McMahon J. Chem. Phys. A. 2001,105,
1011. Fridgen/McMahon J. Am. Chem. Soc. 2001,
123, 3980.
13
Fridgen/McMahon J. Phys. Chem. A. 2002, 106, 1576.
14
FTICR Detection
In FT detection, all ions, regardless of their
mass are detected at the same time.
Once ions are trapped inside the ICR cell they
are excited by a fast sweep of all the RF
frequencies, exciting the ions to cyclotron
motion with a larger radius.
15
lower m/z
higher m/z
All ions are resonantly excited for the same
amount of time. Each ion retains its
characteristic cyclotron frequency (depending on
m/z) but their radii of orbit increase. After
excitation all ions have the same radii of motion
since they were irradiated with rf of the same
amplitude for the same amount of time.
16
Once the rf is turned off, each ion packet,
consisting of ions of the same m/z value induces
an image current on two sets of receiver plates
which are part of the ion cell.
17
When a packet of ions (ve) approaches an
electrode, electrons are attracted from ground
and accumulate in that electrode causing a
temporary current.
e e e
As the ions continue to orbit, the electrons
accumulate in the other electrode. The flow of
electrons in the external circuit represents an
image current. The amplitude of the current is
proportional to the number of ions in the packet.
e e e
18

• the frequency of the image current oscillation
  is the same as the frequency of the ions
  cyclotron motion which is related to mass. A
  small AC voltage is created across a resistor and
  is amplified and detected.

19

• using FT techniques all ion packets, each
  containing ions of the same mass, are detected.
  The decay of the image current (as the excited
  cyclotron orbit radius decays) is detected in
  time and transformed into a frequency domain
  signal by a Fourier transform. The frequencies
  are related to m/q inversely.

• cyclotron frequencies can be measured with very
  high accuracy and precision leading to ultra high
  resolving power and high accuracy mass
  measurements.

20
Resolution
In all types of Fourier transform spectroscopy
the resolution is usually defined as the full
width of a spectral peak at half-maximum peak
height namely Dw50 or Dm50 for frequency or
mass domain FT-ICR spectra. Resolving power is
defined as
The first derivative of the following with
respect to m, is
21
From which we obtain the useful relation,
or
Resolution is constant in Dw50 but decreases
with increasing mass. For a normal Lorentzian
peak shape,
where t is the signal relaxation time (ie. ions
dampen back to their thermal energy cyclotron
radius of orbit). Therefore,
and the longer it takes the ions excited radius
of orbit to dampen back to the thermal energy
radius, the longer one can observe the time
domain signal and the better the resolution. At
higher pressures, t is smaller and resolution
diminishes.
22
Resolution vs Transient Duration
The top mass spectrum is obtained from a 1.2 s
transient and exhibits a resolution of 90 000
(FWHM) while the bottom mass spectrum is obtained
in an eighth of the time but the resolution is
obviously poorer, roughly 12 000 (FWHM). (Figure
from Marshall, Hendrickson and Jackson, Mass
Spectrometry Reviews, 1998, 17, 1-35.)
23
High Resolution mass spectrum of the proton-bound
dimer of 35Cl- and the chloride-bound water
dimer, both with a nominal mass of 71 amu.
24
Upper Mass Limit Since the ion trap must contain
side electrodes in order to provide for
excitation and/or detection, the upper mass limit
is the mass at which the ion cyclotron radius of
a thermal ion reaches the radius of the trap.
Since
and
then
or
r(m), B(T), m(amu), T(K), z(multiples of
elementary charge)
25
Thus for a 4.7 T magnet and a cell with a 4 cm
radius, the upper mass limit for a singly charged
ion at 298 K is,
26
FTICR experiments are pulsed,
rf events
27

• can use rf events to isolate specific ions, ie.
  eject unwanted ions to follow reaction of a
  particular ion.
• the resulting mass spectra can be accumulated to
  make use of the Fellget advantage to increase the
  signal to noise (S/N) ratio,
• where n is the number of spectra accumulated and
  added (or averaged for some types of spectroscopy
  such as FTIR).