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The Reflex III MALDI-TOF mass spectrometer is our workhorse instrument
for mass spectrometry of polymers. With a theoretically unlimited mass
range we are able to obtain molecular weight information for a wide
range of polymer types, in some case for synthetic polymers >100
kDa. The resolution is sufficiently high in the lower mass range that
it is possible to obtain not only absolute molecular weight
information, but also to determine repeat unit, end group information,
and to analyze copolymer composition.
The process of MALDI works by introducing a sample containing a
mixture of low concentration of synthetic polymer with a matrix
material that absorbs light at the frequency of the N2
laser (337nm). The , laser shot vaporizes the material at the same time
the matrix acts as a photo-acid, causing ionization of the analyte.
Ions are then accelerated by a high electric potential, and separated
in the time-of-flight chamber by the time taken to reach the detector.
Time of flight is directly proportional to the mass-to-charge (m/z) ratio of the ion so this can readily be converted to a mass spectrum.
Theory of Time of Flight mass spectrometry
| Under the influence of a potential V, kinetic energy of an ion:
| E = zeV
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| Kinetic energy of a particle of mass m:
| E = ½mv2 |
| Equating:
| zeV = ½mv2
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| Rearranging:
| v = (m/2zeV)½
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| Now, time of flight (t) for length L:
| t = L/v
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| So:
| t = L(m/2zeV)½
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| Finally, rearranging we obtain :
| m/z = 2eVt2/L2
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So by measuring the time of flight, so long as we know the length of
the flight path and the acceleration potential applied, we can obtain m/z directly. In practice we use calibration compounds to allow for thermal expansion of the flight tube and voltage variations.
As well as synthetic polymer research this instrument is also used by
other research groups for analysis of peptides and proteins, in
particular small-scale proteomics projects.
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