Biomedical Engineering Reference
In-Depth Information
From the pattern of masses detected, or from the exact mass, important
clues to the structure of the original molecule may be determined.
Molecules may travel through the instrument relatively intact or may
fragment into smaller parts. A skilled mass spectrometrist is often able to
infer the chemical structure of the original molecule by studying the
fragments produced.
Some methods of mass spectrometry produce highly fragmented
spectra, which represent 'fi ngerprints' characteristic of a particular
molecule so searching against a stored library is possible. Modern
multistage instruments are also able to deliberately fragment individually
selected molecules by collision with low-pressure gas. This is known as
MS/MS or sometimes MS
n
.
Rather surprisingly to an outsider, mass spectrometers often give poorly
reproducible results between instruments, so spectra acquired from one
instrument may not exactly match those from another. It is common
practice to run large numbers of known molecules on a particular
instrument to build a library specifi c to that instrument. The reason for
the poor reproducibility is that ionisation of compounds is dependent on
many factors and subtle changes between instruments can result in drastic
differences in fragmentation and instrument response. Indeed ionisation
of different molecules is unpredictable so that there is no direct relationship
between the composition of a mixture and the response of the spectrometer.
For quantitative work careful calibration of the mass spectrometer is
required to determine the response of each chemical component.
With the newest high-resolution spectrometers, the precise mass of the
compound can yield molecular formula information [3]. This is commonly
termed 'accurate mass measurement'. How can the mass of a compound
lead to its molecular formula? To understand this, fi rst a little theory is
required.
4.2.1 Spectrometer resolution and accuracy
The resolution of a spectrometer is defi ned as the mass number of the
observed mass divided by the difference between two masses that can be
separated. For instance a resolution of 100 would allow a m/z of 100 to
be distinguished from a m/z of 101.
The mass accuracy is defi ned in ppm (parts per million) units, and
expresses the difference between an observed m/z and theoretical m/z.
Today sub-ppm accuracy is not uncommon.
mass accuracy = [m/z (observed - exact)/exact × 1 000 000].
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