Page 116 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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computer control which sequentially tune to analytical wavelengths, greatly increased the flexibility
of such systems. More recently, highresolution echelle monochromators have been used in conjunction
with array detectors which can be used to monitor much of the spectrum simultaneously.
Q. What are the main ionization and excitation mechanisms that occur in the ICP?
Q. What is the short-hand notation for atom and ion lines?
Q. What desirable properties should a monochromator possess if it is to be used for atomic emission
spectrometry?
Q. Could a coloured filter be used successfully for AES—if not, why not?
Q. Name two ways of increasing the resolution of a Czerny-Turner monochromator.
Q. What is the advantage of blazing the grating?
Q. Why is an echelle monochromator ideal for use with an array detector?
4.4.6 Detectors
The traditional means of detection has been through the use of a photo-multiplier tube, which converts
the photon flux into electron pulses which are then amplified. It consists of a partially evacuated
transparent envelope containing a photocathode which ejects electrons when struck by electromagnetic
radiation. The ejected electrons are accelerated towards a dynode, which then ejects approximately five
electrons for every one that strikes. These electrons then strike another dynode, ejecting more electrons
and so on until between 9 and 16 dynode stages have been reached. Up to 106 secondary electrons may
be ejected from the action of one photon striking the photocathode of a nine-dynode PMT. The
electrical current measured at the anode is directly proportional to the radiation reaching the PMT.
Figure 4.14 shows schematically the amplification produced by a PMT. The correct choice of
photosensitive material used to coat the cathode is important. Usually it is a semiconductive material
containing an alkali metal. Different cathode materials have different response curves, as can be seen
from Fig. 4.15. It can be seen that caesium-antimony cathodes work well up to about 550 nm, but their
response at the red end is poor. For these longer wavelengths, a trialkali cathode, sodium-potassium-
antimony with a trace
