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Teledyne API M100E Analyzer Operation Manual Theory Of Operation
227
11.2.7. 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.
11.2.7.1. 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 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 M100E 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.
11.2.7.2. 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 M100E 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.
11.2.7.3. 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 M100E is very tolerant of variations in sample gas flow rate and therefore
does not suffer from this type of interference.
11.2.7.4. 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.
The most significant interferents in this regard are nitrogen oxide (NO), carbon dioxide (CO
2
), water vapor (H
2
O)
and molecular oxygen (O
2
). In ambient applications the quenching effect of these gasses is negligible. For stack
applications where the concentrations of some or all of these may be very high, specific steps MUST be taken to
remove them from the sample gas before it enters the analyzer.
04515F DCN6048