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Teledyne API - T100 UV Fluorescence SO2 Analyzer Principles of Operation
281
x = The distance between the UV source and the SO
2
molecule(s) being affected
(path length).
The second stage of this reaction occurs after the SO
2
reaches its excited state (SO
2
*).
Because the system will seek the lowest available stable energy state, the SO
2
* molecule
quickly returns to its ground state (Equation 10-3) by giving off the excess energy in the
form of a photon (h). The wavelength of this fluoresced light is also in the ultraviolet
band but at a longer (lower energy) wavelength centered at 330nm.
nm33022
hvSO*SO
(Equation 13-3)
The amount of detectable UV given off by the decay of the SO
2
* is affected by the rate
at which this reaction occurs (k).
*SO
k
F
2
(Equation 13-4)
Where:
F = the amount of fluorescent light given off.
k = The rate at which the SO
2
* decays into SO
2.
SO
2
*
= Amount of excited-state SO
2
in the sample chamber.
Therefore:
nm
kF
hvSOSO
33022
*
(Equation 13-5)
Finally, the function (k) is affected by the temperature of the gas. The warmer the gas,
the faster the individual molecules decay back into their ground state and the more
photons of UV light are given off per unit of time.
Given that the absorption rate of SO
2
(a) is constant, the amount of fluorescence (F) is a
result of:
The amount of SO
2
* created which is affected by the variable factors from
(Equation 13-2) above: concentration of SO
2
; intensity of UV light (I
0
); path length
of the UV light(x) and;
The amount of fluorescent light created which is affected by the variable factors
from (Equation 13-5): the amount of SO
2
* present and the rate of decay (k) which
changes based on the temperature of the gas.
When and the intensity of the light (I
0
) is known; path length of excited light is short (x);
the temperature of the gas is known and compensated for so that the rate of SO
2
*decay
is constant (k). and; no interfering conditions are present (such as interfering gases or
stray light); the amount of fluorescent light emitted (F) is directly related to the
concentration of the SO
2
in the Sample Chamber.
06807C DCN6650