Chemistry Reference
In-Depth Information
fields; this is an approximation, however, as there are electric fields from both the trapping
plates and other ions within the analyzer cell; calibration equations have been developed to
correct for these electric field effects].Thus, if the frequency of the ion cyclotron motion (
f
)
can be determined then the mass-to-charge ratio (
m/z
m/q
) can be derived based on
the relationship of Equation (7.6). The cyclotron frequency
f
is measured by excitation of
the cyclotron motion of the trapped ions; this is achieved through application of an RF
field to the excite plates of the analyzer cell that is in resonance with the cyclotron motion
of the ions. The ions absorb energy from this RF field and move to a larger radius, with
all ions of a given mass-to-charge ratio moving coherently (Figure 7.8a). This collection
of ions induces an image current that can be measured and amplified as shown in Figure
7.8b. The ions continue to move in a coherent packet at this larger radius until they are
eventually knocked out of phase with each other, caused by collisions with molecules of
the background neutral gas, and the image current thus decays with time.
=
(a)
(b)
Excite
Detect
+
+
+
+
+
+
+
B
×
+
+
+
RC
+
+
+
+
+
+
Figure 7.8
Excitation (a) and detection (b) of the ion cyclotron motion within an
FTMS mass analyzer cell. Reprinted from Marshall, A.G. and Hendrickson, C.L., Fourier
transform ion cyclotron resonance detection: principles and experimental configurations.
International Journal of Mass Spectrometry
,
215
, 59-75. Copyright (2002), with permission
from Elsevier.
Some typical mass spectra for a peptide are shown in Figure 7.9b and d. Excitation
and detection of ions of more than one mass-to-charge ratio leads to a time domain signal
(i.e. signal intensity as a function of time) that consists of a superposition of sinusoidal
waveforms for each mass-to-charge ratio present within the mass analyzer cell (Figure 7.9a
and c). Fourier transformation of the time domain signal yields a spectrum that shows
the signal intensity as a function frequency, also known as the frequency domain signal.
As described above, the mass-to-charge ratio (
m/z
m/q
) can then be derived using
Equation (7.6) to generate the corresponding mass spectrum (Figure 7.9). The time domain
signal decay is directly dependent on the pressure in the mass analyzer cell. For slow signal
decay, necessary to achieve both high resolution and highmass accuracy, ultra-high vacuum
(UHV) is required in the analyzer cell of FTMS instruments. If the pressure is high in the
mass analyzer cell then a more rapid decay of the signal occurs due to collisions with the
abundant background gas and poor resolution and peak shape in the mass spectrum result. If
lower pressuremass analyzer cell conditions are achieved, then the transient signal response
=
