Instant Notes: Analytical Chemistry

272 Section E – Spectrometric techniques

There are several types of mass analyzer.

(i) A single focusing magnetic mass analyzer(Fig. 3) generates a field at right

angles to the rapidly moving ions, causing them to travel in curved traject-

ories with radii of curvature, r, determined by their mass-to-charge ratio,

m/z, the magnetic field strength, B, and the accelerating voltage, V, as given

by the relation



The majority of ions carry a charge of +1, hence mis directly proportional to

r^2. For ions of a particular mass there is a specific combination of values of B

and Vthat allows them to pass along the center of a curved analyzer tube

to a detector positioned at the end. Progressive variation of the field or the

accelerating voltage allows ions of different mass to pass down the center of

the analyzer tube, be detected and a mass spectrum recorded.

(ii) A double focusing mass analyzeremploys an electrostatic separatorin

addition to a magnetic analyzer to improve the mass resolution. Ions of the

same mass inherently acquire a range of kinetic energies when accelerated

and this leads to overlapping signals from those with similar masses.

Application of an electrostatic field to the moving ions allows the selection

of those with the same kinetic energy so eliminating this problem.

(iii) A quadrupole mass analyzerconsists of a set of four parallel metal rods

positioned very closely together, but leaving a small space through the

center (Fig. 4). Ions are accelerated into the space between the rods at one

end and a DC potential and a high frequency RF signal is applied across

B^2 r^2




2 V

m




z

To

pump

10 –5 torr

Sample

reservoir

Ionization

chamber

Hot

filament

electron

source

Inlet

Anode

To pump

Slit A

Slit B

Path of

lighter ions

Path of

heavier ions

Metal

analyzer

tube

Ion collector

10 –7 torr Exit

slit

Output to

amplifier and

recorder

Fig. 3. Diagram of a single focusing magnetic analyzer mass spectrometer. From Principles of
Instrumental Analysis, 2nd edn, by D.A. Skoog & D.M. West © 1980. Reprinted with permission of
Brooks/Cole an imprint of the Wadsworth Group, a division of Thomson Learning.