Biomedical Engineering Reference
In-Depth Information
ions of charge Ze take to y through the tube of length D is proportional
to the square root of mass:
1
2
1
2 mv 2 ;
2Ze
m
E k = Ze =
v =
:
(1)
Using vt = D, we nd:
1
2
m
2Ze
t =
D:
(2)
Modern ionization methods like laser desorption ionization (LDI), ESI,
etc., enable ions to be generated eciently from liquid or solid samples. The
consequences are that increasingly heavier ions like peptides and proteins
can now be generated from a very small amount of sample. For example,
it has been reported that MALDI may achieve a detection limit as low as
zeptomoles (10 21 mol) 18 . Together with fast electronics that can work at
nanosecond or sub-nanosecond sample rates to record the mass spectrum,
this has led to rapid developments in instrumentation and applications of
TOF-MS. It is now widely used in chemistry, biochemistry, biology and
biomedical science.
Ideally, ions of a specic m=z would hit the detector after the same time
of ight, resulting in a sharp peak lineshape like a delta function of certain
height. In reality, however, because of the nite time during which the
energy source acts on the sample, sample surface morphology and complex
physical and chemical reactions that occur when energy is deposited onto
the sample, ions of the same m=z are formed at dierent times and positions
according to some initial time distribution and spatial distribution. They
also come o the sample surface with an initial velocity distribution of nite
width. Though there are ion optics strategies that attempt to correct for
these eects, such as time-lag extraction or reectrons, ions still enter the
free drift region with velocity and time distributions of nite width, which
results in a nite peak width for ions of a specic m=z. We refer to the sum
of total ions of a given mass peak (integrated intensity) as the intensity of
that peak.
Very often, in a TOF instrument, such as MALDI-MS and secondary ion
mass spectrometry (SIMS), laser/primary ions which are very well focused
can be rastered over the sample surface for a number of irradiations in a
certain pattern. For each irradiation, only a small portion of the scanned
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