Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 28

Syringe

Quadrupole MS

GC

Chromatogram

Ion source

Detector

Oven

Computer

Column

Inferface

To vacuum system

Figure 2.19 Gas chromatography coupled to a quadrupole mass spectrometer.

the typical mass range required may extend from 0 up to 400 amu. This is the
most commonly used mass spectrometer in forensic science.

2.6.5.2 Ion Trap MS
An ion trap MS traps ions of specific m/z ratios within three cylindrically
symmetrical electrodes consisting of two caps and a ring electrode. By apply-
ing increasing RF voltages to the electrodes, ions of increasing m/z ratio leave
the ion trap and are detected. GC ion trap MS is being used in forensic sci-
ence in the areas of toxicology and fire debris analysis.

2.6.5.3 Time-of-Flight (TOF) MS
A TOF MS separates ions, based on their m/z ratio, according to their veloc-
ity. As each ion has a different molecular weight, it will travel at a different
velocity when a voltage is applied. Separation is achieved in this type of MS
by allowing the ions to travel over a distance. Often in a TOF, MS presepara-
tion is required; this can be done using a quadrupole MS.

2.6.5.4 Detection
The MS separated ions are detected using an electron multiplier tube (EMT).
The ion of a specified m/z ratio strikes the surface of a semiconductor, where
it is converted to an electron. Each electron generated is then cascaded toward
an anode. On the way, however, an electron will strike the internal surface
of the EMT, creating additional electrons. The cascade of electrons generated
is collected as an electric current at the anode; the electric current is then
converted to a signal and visualised using appropriate software as either a
chromatogram or mass spectrum.