Instant Notes: Analytical Chemistry

E14 – Mass spectrometry 271

The principal components are

● a sample inlet,which facilitates the controlled introduction of gaseous or

vaporized liquid samples via a molecular leak(pinhole aperture) and solids

via a heated probeinserted through a vacuum lock;

● an ion sourceto generate ions from the sample vapor;

● a mass analyzerwhich separates ions in space or time according to their

mass-to-charge ratio. Ions generated in the source are accelerated into the

analyzer chamber by applying increasingly negative potentials to a series of

metal slits through which they pass.

100

50

Relative abundance %

15

15

18

30

m/z

29

31

32

m/z Relative m/z Relative

abundance % abundance %

12 0.33 28 6.3

13 0.72 29 64

14 2.4 30 3.8

15 13 31 100

16 0.21 32 66

17 1.0 33 0.73

18 0.9 34 ~0.1

Fig. 1. Electron impact ionization (EI) mass spectrum of methanol.

Sample Pressure, 10–1 – 10–8 Nm–2

Inlet

system

Ion

source

Mass

analyzer

Vacuum

system

Detector

Signal

processor

Read-out

Fig. 2. Block diagram of a mass spectrometer.