Teledyne 6200E - Sulfides Analyzer - Manual (Page 211)

Model 6200E Instruction Manual Theory Of Operation
M6200E Rev: A1 211
10.2.6.1. Direct Interference
Obviously, since the M6200E measures H
2
S by converting it to SO
2
, the most significant
interfering gas for this measurement would be ambient SO
2
that is present in the sample gas.
The M6200E circumvents this by passing the sample gas through a chemical scrubber that
removes all SO
2
from the sample gas before the H
2
S Æ SO
2
conversion takes place. This ensures
that the only SO
2
present in the sample chamber is the result of the H
2
S Æ SO
2
conversion.
Obviously to make sure that the analyzer operates correctly it is important to make sure that this
scrubber is functioning properly.
The second 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 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 6200E 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 reaches 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 6200E prevents this from occurring by having a very short light path between the area
where the SO
2
* fluorescence occurs and the PMT detector. Because the light path is so short, the
amount of O
3
needed to cause a noticeable effect would be much higher than could be reasonably
expected in any application for which this instrument is intended.
10.2.6.3. Dilution
Certain gases with higher viscosities can lower the flow rate though the critical flow orifice that
controls the movement of sample gas though the analyzer reducing the amount of sample gas in
the sample chamber and thus the amount of SO
2
available to react with the to the UV light. While
this can be a significant problem for some analyzers, the design of the Model 6200E is very
tolerant of variations in sample gas flow rate and therefore does not suffer from this type of
interference.
10.2.6.4. Third Body Quenching
While the decay of SO
2
* to SO
2
happens quickly, it is not instantaneous. Because it is not
instantaneous it is possible for the extra energy possessed by the excited electron of the SO
2
*
molecule to be given off as kinetic energy during a collision with another molecule. This in effect
heats the other molecule slightly and allows the excited electron to move into a lower energy orbit
without emitting a photon.