Instant Notes: Analytical Chemistry

E14 – Mass spectrometry 273

opposite pairs of rods. This results in ions of one particular m/zvalue

passing straight through the space to a detector at the other end while all

others spiral in unstable trajectories towards the rods. By altering the DC

and RF signals applied to the rods, ions with different m/zratios can be

allowed to reach the detector in turn. The ion trapis a modified version

with a circular polarizable rod and end caps enclosing a central cavity

which is able to hold ions in stable circular trajectories before allowing

them to pass to the detector in order of increasing m/zvalue. A particular

feature of quadrupole and ion trap analyzers is their ability to scan through

a wide range of masses very rapidly, making them ideal for monitoring

chromatographic peaks (Section F).

(iv)Tandem mass analyzersincorporate several mass analyzers in series. This

enables ions selected from the first analyzer to undergo collision induced

dissociation(CID) with inert gas molecules contained in a collision cell

producing new ions which can then be separated by the next analyzer. The

technique, known as tandem mass spectrometry, MS-MSor (MS)n, is used

in the study of decomposition pathways, especially for molecular ions

produced by soft ionization techniques (vide infra). Collision-induced reac-

tions with reactive gases and various scan modes are also employed in

these investigations.

Mass spectrometers are designed to give a specified resolving power, the

minimum acceptable resolution being one mass unit. Two masses are consid-

ered to be resolved when the valley between their peaks is less than 10% of the

smaller peak height. For masses m 1 and m 2 , differing by Dm, resolving power is

defined as m 2 /Dm, and for unit mass resolution the requirement increases with

the magnitudes of m 1 and m 2 , for example,

Unit mass resolution for masses Required resolving power

99 and 100 100

499 and 500 500

4999 and 5000 5000

Ionizing

electron beam

dc and

rf voltages

Resonant

ion

Nonresonant

ion

Ion

collector

+Udc + V cos wt

• Udc – V cos wt

Fig. 4. Diagram of a quadrupole mass analyzer.