E C O P H Y S I C S Practical hints
CLD 780 TR / July 2000 94
7.3 Interferences
If an analytical method gives a response not only to the analyte of inter-
est, but also to other compounds, these are normally referred to as inter-
ferences. In this case it means that some interference compounds produce
an artificial NO or a NO.c (NO
2
resp.) signal.
When using NO/NO
x
analyzers two different kinds of interferences must
be distinguished:
a) Some compounds give chemiluminescent emission when reacting with
ozone. Certain unsaturated hydrocarbons (e.g. ethylene, propylene),
and sulfides are examples. Since the kinetics of these reaction are slow
and the optical emission spectrum is different from that of NO-O
3
che-
miluminescence, they do not produce serious interference unless their
concentrations are of the order of 100 times higher than the NO con-
centration in the sample gas. Their effect does become significant du-
ring the measurement of very low (ppt level) concentrations of NO.
The CLD 780 TR analyzer eliminates more than 90 % of the signal inc-
rease due to this effect through the use of an integrated pre-chamber
(see Chap. 3).
b) A second kind of interference occurs when other nitrogen compounds
in addition to NO
2
are converted to NO in the NO
2
-to-NO converter.
The most important substances in this context are peroxyacetyl nitrate
(PAN) and nitric acid (HNO
3
).
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