Equation (9.28) shows that for ions carrying a single positive charge, the mass is directly proportional
to the square of the radius of curvature. At a particular value of the accelerating voltage V fragment ions
of a given mass will pass along the analyser tube and through an exit slit to impinge on a collector
plate. A continuous stream of ions of this mass will be registered as an ion current which can be
amplified and fed to a recording system. By continuously varying the accelerating voltage, fragment
ions of different masses can be 'focused' through the exit slit in turn and an entire mass spectrum
recorded. The resolution, or ability to discriminate between ions of a similar mass, can be improved
considerably by 'double focusing', an arrangement incorporating separate electrostatic and magnetic
fields in the analyser tube. Resolution is defined as m/∆m where m and (m + ∆m) are the m/z values of
two adjacent peaks of equal height and separated by a valley which is 10% of the peak height.
Resolving power decreases with increasing mass but spectrometers having a resolution of 1000, i.e. the
ability to discriminate between m/z values of 1000 and 1001, or between 100 and 99.9, are adequate for
many applications. Double-focusing instruments may be capable of resolutions of 20 000–50 000 or
more.
Characteristics and Interpretation of Molecular Mass Spectra
The recorded mass spectrum of a compound is usually represented graphically in the form of a line
diagram by plotting the abundance of each fragment ion expressed as a percentage of the most abundant
one against mass-to-charge ratio, m/z; in most cases z = +1. The peak derived from the most abundant
ion is called the base peak. The fragmentation or cracking pattern for a single substance is uniquely
characteristic and may be used for qualitative identification purposes. The mass spectrum of n-
hexadecane is shown in Figure 9.53.
Figure 9.53
Mass spectrum of n-hexadecane.