Thin-Layer Chromatography

24 2 Physical Methods of Detection
a)
100
50
b)
100
^50
200 300 100 500
10
200 300 (00
X [nml
Fig. 16: Relative intensity distribution of a mercury NK 4/4 low pressure lamp (A) and of a
mercury St 41 or St 48 lamp (B).
Detectors
The commercial instruments employ detectors of various types. Their utility depends
fundamentally on
• the constancy with time of the photocurrent at constant radiation levels and
constant external conditions,
• the proportionality of the photocurrent to the intensity of illumination and
• the signal to noise ratio of the photodetector.
A role is also played by the temperature and frequency dependence of the
photocurrent, the variable surface sensitivity at various parts of the cathode and
the vector effect of polarized radiation [40]. All the detectors discussed below are
electronic components whose electrical properties vary on irradiation. The effects
depend on external (photocells, photomultipliers) or internal photo effects (photoelements,
photodiodes).
500
Z.Z ueieciwn OJ Aosurum% ouusiurwa ij
Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly
employed in photometry. These are detectors with an "external photo-effect".
Photocells: The basic construction of a photocell is illustrated in Figure 17. A
photocurrent flows when the photocathode is illuminated, this is proportional to
the intensity of illumination if the supply potential has been chosen to be higher
than the saturation potential. A minimal potential is required between the
photocathode and the anode in order to be able to "collect" the electrons that are
emitted. The sensitivity is independent of frequency up to 10 7 Hz. The temperature
sensitivity of evacuated photocells is very small. The dark current (see below) is
ca. 10~ n A[l].
Cathode / Vs -
l
- 0
r 0
( fV 1 3-100 V
Anode \T>J
r 1
Fig. 17: The construction of a photocell, schematic [1].
Samples of analytical units with photocells :
• TURNER fluorimeter, model III (CAMAG)
• Quick Scan R & D densitometer (HELENA)
• Fiber optic densitometer, model 800 (KONTES)
Photomultipliers: Secondary electron multipliers, usually known as photomultipliers,
are evacuated photocells incorporating an amplifier. The electrons emitted
from the cathode are multiplied by 8 to 14 secondary electrodes (dynodes). A
diagramatic representation for 9 dynodes is shown in Figure 18 [5]. Each electron
impact results in the production of 2 to 4 and maximally 7 secondary electrons at
each dynode. This results in an amplification of the photocurrent by a factor of 10 6
to 10 8 . It is, however, still necessary to amplify the output of the photomultiplier.