Teledyne 100E, Instruction Manual, Page: 169 / 258

Model 100E Instruction Manual Theory Of Operation
045150102 Rev XB1 167
mercury-vapor lamps for this application because they produce very strong emission
levels at the wavelength required to convert SO
2
to SO
2
*, 213.9 nm (see Figure 10-4).
The lamp used in the Model 100E is constructed with a vacuum jacket surrounding a
double-bore lamp element (Figure 10-3). The vacuum jacket isolates the plasma arc from
most external temperature fluctuations. The jacket also contains the thermal energy
created by the lamps operation thereby helping the lamp heat up to and maintain proper
vaporization temperature. Light is emitted through a 20 mm x 5 mm portal.
Light Output
Portal
Zinc-Vapor
Plasma Arc
Vacuum
Jacket
Dual Bore
Figure 10-3: Source UV Lamp Construction
10.2.2.
The Reference Detector
A vacuum diode, UV detector that converts UV light to a DC current is used to measure
the intensity of the excitation UV source lamp. It’s location, directly across from the
source lamp at the back of a narrow tube-shaped light trap, placers it directly in the path
of the excitation UV light. A window transparent to UV light provides an air-proof seal
that prevents ambient gas from contaminating the sample chamber. The shape of the
light trap and the fact that the detector is blind to wavelengths other than UV no extra
optical filtering is needed.
10.2.3.
The PMT
The amount of fluoresced UV produced in the sample chamber is much less than the
intensity of excitation UV source lamp (see Figure 10-4). Therefore a much more
sensitive device is needed to detect this light with enough resolution to be meaningful.
The Model 100E uses a Photo Multiplier Tube or PMT for this purpose.
A PMT is typically a vacuum tube containing a variety of specially designed electrodes.
Photons enter the PMT and strike a negatively charged photo cathode causing it to emit
electrons. These electrons are accelerated by a high voltage applied across a series of
special electrodes called dynodes that multiply the amount of electrons until a useable
current signal is generated. This current increases or decreases with the amount of
detected light (see Section 10.4.4for more details regarding the electronic operation of
the PMT).
10.2.4.
Optical Filters
The Model 100E analyzer uses two stages of optical filters to enhance performance. The
first stage conditions the UV light used to excite the SO
2
by removing frequencies of light