Teledyne 6400A - Sulfur dioxide analyzer - Manual (Page 31)

TAI Model 6400A SO
2
Analyzer Operator Manual, 02164, Rev. G
4-1
4 THE 6400A SO
2
ANALYZER
4.1 Principle of Operation
The operation of the TAI Model 6400A Analyzer is based upon the well proven technology from the
measurement of fluorescence of SO
2
due to absorption of UV energy. Sulfur Dioxide absorbs in the
190 nm - 230 nm region free of quenching by air and relatively free of other interferences.
Interferences caused by PNA (poly-nuclear aromatics) are reduced by a "kicker"
1
which removes
PNA selectively through a membrane without affecting SO
2
sample gas.
The UV lamp emits ultraviolet radiation which passes through a 214 nm bandpass filter, excites the
SO
2
molecules, producing fluorescence which is measured by a PMT with a second UV bandpass
filter. The equations describing the above reactions are as follows:
* 2 SO hv SO Ia
1 2
⎯→ ⎯ +
(1)
The excitation ultraviolet light at any point in the system is given by:
( ) ( ) [ ]
2 0
SO ax exp 1 I Ia
− − =
(2)
Where I
0
is the UV light intensity, a is the absorption coefficient of SO
2
, x the path length, and (SO
2
)
the concentration of SO
2
. The excited SO
2
decays back to the ground state emitting a characteristic
fluorescence:
2 2
KF
2
hv SO * SO
+ ⎯→ ⎯
(3)
When the SO
2
concentration is relatively low, the path length of exciting light is short and the
background is air, the above expression reduces to:
( )
2
SO K F
=
(4)
Hence, the fluorescent radiation impinging upon the PMT is directly proportional to the
concentration of SO
2
.
The block diagram in Figure 4-1 illustrates the general operation principle of the Model 6400A.
1
Developed by Dr. Henk, J. Vande Wiel, Laboratory for Inorganic Chemistry, RIVM, National
Institute of Public Health and Environmental Protection, Biethoven, The Netherlands.