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C
HEMILUMINESCENCE
THE PHOTO MULTIPLIER TUBE (PMT)
The analyzer uses a special kind of vacuum tube, called a photo-multiplier tube (PMT), to
detect the amount of light created by the NO and O
Photons enter the PMT and strike a negatively charged photo cathode causing it to emit
electrons. These electrons are accelerated by an applied high voltage and multiplied
through a sequence of similar acceleration steps (dynodes) until a useable current signal is
generated (see Section 6.5 for a more detailed description). The more light present (in this
case photons given off by the chemiluminescent reaction described above), the more
current is produced. Therefore the more NO present in the reaction cell the more current is
produced by the PMT.
The current produced by the PMT is converted to a voltage and amplified by the
preamplifier board and then communicated to the analyzer's CPU via the Aà D converter
circuitry on the analyzer.
OPTICAL FILTER
A high pass optical filter, only transparent to wavelengths of light above 645nm, placed
between the reaction cell and the PMT (see Figure 6-1) in conjunction with the response
characteristics of the PMT creates a very narrow window of wavelengths of light to which
the analyzer will respond.
The narrowness of this band of sensitivity allows the analyzer to ignore extraneous light
and radiation that might interfere with the analyzer's measurement (more on interferents
in Section 6.1.5). For instance, some oxides of sulfur can also be chemiluminescent
emitters when in contact with O
around 260nm to 480nm).
178
Teledyne API T200 NO/NO
D
ETECTION IN THE
NO
NO+O
Optical
Filter
PMT
PMT HOUSING
Figure 6-1. Reaction Cell with PMT Tube and Optical Filter
but give off light at much shorter wavelengths (usually
3
/NO
Analyzer with NumaView™ Software
2
x
R
C
EACTION
reaction in the reaction cell.
3
Reaction
O
3
Cell
3
ELL
083730200A DCN7962
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