Theory Of Operation Model 100E Instruction Manual
170 045150102 Rev XB1
A lens located between PMT and the sample chamber collects as much of the fluoresced
UV created there as possible and focuses it on the most sensitive part of the PMT’s photo
cathode.
Another lens located between the excitation UV source lamp and the sample chamber
collimates the light emitted by the lamp into a steady, circular beam and focuses that
beam directly onto the reference detector. This allows the reference detector to
accurately measure the effective intensity of the excitation UV by:
• Eliminating the effect of flickering inherent in the plasma arc that generates the
light.
• Making sure that all of the light emitted by the source lamp, passed though the
214 nm filter and not absorbed by the SO
2
reaches the reference detector.
Conversely, this also makes sure that the volume of sample gas affected by the
excitation beam is similar to the volume of fluorescing SO
2
* being measured by
the PMT, eliminating a possible source of measurement offset.
10.2.6. Measurement Interferences
It should be noted that the fluorescence method for detecting SO
2
is subject to
interference from a number of sources. The M100E has been successfully tested for its
ability to reject interference from most of these sources.
10.2.6.1. Direct Interference
The most common source of interference is from other gases that fluoresce in a similar
fashion to SO
2
when exposed to UV Light. The most significant of these is a class of
hydrocarbons called poly-nuclear aromatics (PNA) of which xylene and naphthalene are
two prominent examples. Nitrogen oxide fluoresces in the a spectral range near to SO
2
.
For critical applications where high levels of NO are expected an optional optical filter is
available that improves the rejection of NO (contact customer service for more
information).
The Model 100E Analyzer has several methods for rejecting interference from these
gasses.
• A special scrubber (kicker) mechanism removes any PNA chemicals present in the
sample gas before it the reach the sample chamber.
• The exact wavelength of light needed to excite a specific non-SO
2
fluorescing gas
is removed by the source UV optical filter.
• The light given off by Nitrogen Oxide and many of the other fluorescing gases is
outside of the bandwidth passed by the PMT optical filter.
10.2.6.2. UV Absorption by Ozone
Because ozone absorbs UV Light over a relatively broad spectrum it could cause a
measurement offset by absorbing some of the UV given off by the decaying SO
2
* in the
sample chamber. The Model 100E prevents this from occurring by having a very short
light path between the area where the SO
2
* fluorescence occurs and the PMT detector.
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