Biology Reference
In-Depth Information
Ions in the ICR cell irradiated with a pulse of RF at the same
frequency as ω
c
oscillate in stable ion cyclotron motions (Fig.
2c
).
Ions absorbing the energy increase the radius of their orbits induc-
ing thereby an oscillating charge in the walls of the ICR cell as they
precess. The frequency of this induced charge oscillation can be
detected and amplified. Because a wide variety of energies are
transmitted to the ICR cell, FT-ICR MS allows many ions to be
detected simultaneously. The advantages of detecting all ions
simultaneously derives from the “Fellgett advantage” [
25
] and is
derives from the expression that relates the orbital radius (
r
) with
the excitation electric field (
E
0
) and the excitation time (
t
excite
),
r
= (
E
0
t
excite
)/2
B
. The fact that the orbital radius of the excited ions
is independent of the
m
/
z
ratio, means that ions of different
m
/
z
ratios can be excited to the same ICR radius. In addition, because
frequency is an easily and accurately measurable parameter, FT-ICR
MS has the highest potential for mass accuracy determinations.
Pivotal to any mass spectrometry experiment is the mass resolv-
ing power of the mass analyzer,
m
/Δ
m
, where Δ
m
is the full peak
width at 50 % of maximum peak height. Mass resolving power is a
measure of how well two closely spaced peaks can be resolved. In
FT-ICR MS |
m
/Δ
m
| = |ω/Δω|. Because the frequency of ion cyclo-
tron precession is directly proportional to the strength of the mag-
netic field, increasing the strength of the magnet increases
resolution. In addition, Fourier transform MS is unique in that
increased measurement time also increases both sensitivity and res-
olution [
26
]. Increased sensitivity is derived from the fact that ions
are not consumed during the detection process. In a zero-pressure
limit, the frequency-domain mass spectral peak width is given by:
(
)
212060
π
.
T
∆ω
=
where
T
is the time-domain data acquisition period [
27
]. It follows that
the mass resolving power is directly proportional to the observation
period. In addition, the resolution at low pressure is given by:
(
)
1 274 10
7
.
×
zB T
m
m
0
=
∆
m
[
28
,
29
] clearly showing that the resolving power,
m
/Δ
m
, increases
linearly with the strength of the magnetic field (
B
0
) and the acqui-
sition time. Thus, resolution can be improved either by increasing
the strength of the magnet (in teslas) or by increasing the detec-
tion duration. Mass resolution (
R
) exceeding 1,500,000 have been
achieved by FT-ICR MS. However, high-performance instruments
use superconducting magnets that are large in size and expensive
to maintain from the standpoint of cryogenic cooling.
