Page 109 - Tandem Techniques

Page 90

obviously requires considerable modification to the probe. However, the probe design for tandem use
will be discussed later with other interfaces.

Mass Spectrometry

Mass spectrometry, as opposed to the other spectrometric methods that have been discussed so far, is
not involved with the absorption of electromagnetic radiation and thus is an entirely different type of
analytical technique. Basically, mass spectrometry involves first, the production of ions from the
sample and these ions can be molecular ions, ion fragments or ion complexes, depending on the
ionization process that is used. Second, the ions are then accelerated to high velocities in a vacuum and,
by applying a range of different physical and electrical techniques, the ions are separated into their
individual masses and each mass-group sensed and identified. The advantages of this type of analytical
approach and its value as a tandem instrument are very obvious.

There are three basic types of mass spectrometer, the sector mass spectrometer, the quadrupole mass
spectrometer (which includes the mass analyzer) and the time-of-flight mass spectrometer. All three
have been used (and indeed are still used) in tandem configuration, and consequently the basic
principles of all three will be described. However, the quadrupole mass spectrometer in one of its forms
is by far the most popular mass spectrometer in tandem use.

The Sector Mass Spectrometer.

The sector mass spectrometer functions on the combined mass selection of an electrostatic field and a
magnetic field. An instrument that utilizes both an electric and a magnetic field to analyze the ions is
often called a double focusing sector mass analyzer. A diagram of the basic double focusing mass
analyzer is shown in Figure 2. 29.

Consider an ion, mass (m) and charge (z) entering the electric field (E) at velocity (v). The ion will
describe an arc of radius (R ) and then enter the magnetic sector. Now the electrostatic deflecting force
1
will equal the centrifugal force of any ion that will enter the magnetic sector entry slit at