of the ions is through a sector of the circular poles of the magnet, hence the term
magnetic sector.
Figure 9.9 shows several possible trajectories for a given ion in the magnetic field.
Only one set of ions will be focussed on the detector. If the field is changed, these ions
will be defocussed because they will not be deflected to the correct extent. A new set of
ions will be deflected and collected at the detector. By starting at either end of the
magnet range the ions can be scanned from high to low mass or from low to high mass.
Thismagnetic scanningis the most commonly used type of analysis in this instrument.
Alternatively, the mass spectrum can be scanned electrically by varying the voltage, V,
while holding the magnetic field,B, constant. This type of instrument is called atwo-
sectorordouble-focussingmass spectrometer and resolving power to parts per million
may be obtained.9.3.6 Accelerator mass spectrometry
Accelerator mass spectrometry(AMS) has proved to be extremely useful for quanti-
fying rare isotopes and has had a major impact in archaeology (to measure^14 C) and
geochronology. AMS can also measure radioisotopes such as^3 H,^10 Be,^26 Al,^36 Cl and(^41) Ca with attomole (10 (^18) ) to zeptomole (10 (^21) ) levels of sensitivity and very high
precision. AMS has found increasing application in human microdosing studies in
drug development. This enables metabolites to be measured in human plasma or urine
after administration of low, pharmacologically relevant doses of labelled drugs.
Among the many applications of AMS are long-term pharmacokinetic studies to
determine low-dose and chronic effects and the analysis of molecular targets
of neurotoxins (see Section 18.3.1).
Ion source
Detector
Magnet
Electrostatic
analyser
Beam of ions
m/z 3
m/z 1
m/z 2
Fig. 9.9Double-focussing magnetic sector mass spectrometer. The figure shows the ‘forward geometry’
arrangement where the electrostatic analyser is before the magnetic sector (known as EB; E for electric,
B for magnetic). Similar results may be obtained if the reverse geometry (BE) type is used. The radial path
followed by each ion is shown by scanning the magnetic field, B, and each ion of a particularm/zcan be
brought into the detector slit in turn.
365 9.3 Mass analysers