Teledyne 6200E - Sulfides Analyzer - Manual (Page 208)

Theory Of Operation Model 6200E Instruction Manual
208 M6200E Rev: A1
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, places 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 means 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 6200E uses a Photo Multiplier
Tube or PMT for this purpose (see 10.4.4 for more details regarding the electronic operation of
the PMT).
10.2.4. Optical Filters
The Model 6200E 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 that are not
needed to produce SO
2
*. The second stage protects the PMT detector from reacting to light not
produced by the SO
2
* returning to its ground state.
10.2.4.1. UV Source Optical Filter
Zinc-vapor lamps output light at other wavelengths beside the 214nm required for the SO
2
Æ
SO
2
* transformation including a relatively bright light of the same wavelength at which SO
2
*
fluoresces as it returns to its SO
2
ground state (330 nm). In fact, the intensity of the light emitted
by the UV lamp at 330nm is so bright, nearly five orders of magnitude brighter than that resulting
from the SO
2
* decay, it would drown out the SO
2
* fluorescence.