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Theory Of Operation Teledyne API M100E Analyzer Operation Manual
224
The reference detector offset is stored as and viewable via the front panel as the test function DRK LMP.
The PMT offset is stored as and viewable via the front panel as the test function DRK PMT.
11.2.5. OPTICAL FILTERS
The M100E analyzer uses two stages of optical filters to enhance performance. The first stage conditions the UV
light used to excite the SO
2
by removing frequencies of light that are not needed to produce SO
2
*. The second
stage protects the PMT detector from reacting to light not produced by the SO
2
* returning to its ground state.
UV SOURCE OPTICAL FILTER
Zinc-vapor lamps output light at other wavelengths beside the 214nm required for the SO
2
SO
2
*
transformation including a relatively bright light of the same wavelength at which SO
2
* fluoresces as it returns to
its SO
2
ground state (330 nm). In fact, the intensity of light emitted by the UV lamp at 330nm is so bright, nearly
five orders of magnitude brighter than that resulting from the SO
2
* decay, it would drown out the SO
2
*
fluorescence.
LAMP OUTPUT
(Arbitrary Untis)
10
5
10
4
10
2
202.5
307.6
481.1
10
3
10
1
1
100 200 300 400 500
WAVELENGTH (nm)
0
213.9
330.3
275.6
SO
2
*
Fluorescent
Spectrum
LAMP OUTPUT
(Arbitrary Untis)
10
5
10
4
10
2
10
3
10
1
1
100
200
300 400
500
WAVELENGTH (nm)
0
213.9
330.3
SO
2
* FLUORESCENT
SPECTRUM
UV SOURCE OPTICAL FILTER
BANDWIDTH
BEFORE AFTER
Figure 11-4: Excitation Lamp UV Spectrum Before/After Filtration
To solve this problem, the light emitted by the excitation UV lamp passes through a band pass filter that screens
out photons with wavelengths outside the spectrum required to excite SO
2
into SO
2
* (refer to Figure 11-4).
PMT OPTICAL FILTER
The PMT use
d in the M100E reacts to a wide spectrum of light which includes much of the visible spectrum and
most of the UV spectrum. Even though the 214 nm light used to excite the SO
2
is focused away from the PMT,
some of it scatters in the direction of the PMT as it interacts with the sample gas. A second optical band pass
filter placed between the sample chamber (refer to Figure 11-2) and the PMT strips away light outside of the
fluore
s
cence spectrum of decaying SO
2
* (refer to Figure 11-5) including reflected UV form the source lamp and
other stray light.
04515F DCN6048