Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 46)

4.1.4 Air Gap Purge
For isotope ratio applications, the air gap between the dewar and lens and the short sample cell should be purged as
shown in Figure 4-1. This is not required for most trace gas applications, where the ambient concentration is very low,
and there is very little absorption. The sample cell is at low pressure, making the sample absorption very narrow
compared to the pressure-broadened ambient absorption. Thus the concentration measurement is relatively insensitive
to trace gases in the ambient-pressure air gap. However, for CO
2
or water isotope measurements, the ambient
concentration is relatively high, it may change rapidly (especially if the TGA100 cover is off and someone exhales
nearby), and this application requires extremely high accuracy. For these reasons the air gap should be purged to
prevent absorption. This requires that the optional air gap purge boot be installed between the dewar and sample cell. A
tank of compressed nitrogen should be connected to the inlet of the short sample cell, and the outlet of the short sample
cell should be connected to the inlet of the air gap purge collar on the laser dewar. A flow of approximately 10 ml/min
is recommended. The regulator on the user's tank may be connected to the short sample cell using the flow meter,
needle valve, and tubing included in PN 15837, Reference Gas Connection, or similar hardware.
Figure 4-1. TGA100 Optical Layout, Including Air Gap Purge
4.1.5 Polyethylene Sample Cell Liner
The standard sample cell for the TGA100 is a stainless steel pipe. For water vapor measurements this sample cell
should be lined with polyethylene to reduce the problem of water sticking to it. The tubing inside the analyzer is also
changed to polyethylene. This modification is also used for ammonia measurements. Contact Campbell Scientific for
details.
46