Introduction
Principle of Operation
1-2
Model 43i Trace Level-Enhanced Instruction Manual
Thermo Fisher Scientific
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high standards of quality, performance, and workmanship. Service
personnel are available for assistance with any questions or problems that
may arise in the use of this analyzer. For more information on servicing, see
the “Servicing” chapter.
The Model 43
i
Trace Level-Enhanced operates on the principle that SO
2
molecules absorb ultraviolet (UV) light and become excited at one
wavelength, then decay to a lower energy state emitting UV light at a
different wavelength. Specifically,
The sample is drawn into the Model 43
i
Trace Level-Enhanced through
the SAMPLE bulkhead, as shown in
Figure 1–1
. The sample flows through
a hydrocarbon “kicker,” which removes hydrocarbons from the sample by
forcing the hydrocarbon molecules to permeate through the tube wall. The
SO
2
molecules pass through the hydrocarbon “kicker” unaffected.
The sample then flows into the fluorescence chamber, where pulsating UV
light excites the SO
2
molecules. The condensing lens focuses the pulsating
UV light into the mirror assembly. The mirror assembly contains eight
selective mirrors that reflect only the wavelengths which excite SO
2
molecules.
As the excited SO
2
molecules decay to lower energy states they emit UV
light that is proportional to the SO
2
concentration. The bandpass filter
allows only the wavelengths emitted by the excited SO
2
molecules to reach
the photomultiplier tube (PMT). The PMT detects the UV light emission
from the decaying SO
2
molecules. The photodetector, located at the back
of the fluorescence chamber, continuously monitors the pulsating UV light
source and is connected to a circuit that compensates for fluctuations in the
UV light.
As the sample leaves the optical chamber, it passes through a flow sensor, a
capillary, and the “shell” side of the hydrocarbon kicker. The sample then
flows to the pump and is exhausted out the EXHAUST bulkhead of the
analyzer. The Model 43
i
Trace Level-Enhanced outputs the SO
2
concentration to the front panel display and the analog outputs, and also
makes the data available over the serial or Ethernet connection.
Principle of
Operation
