TGA200 Trace Gas Analyzer Overview
2.3 Concentration Calculation
The reference and sample detector signals are digitized, corrected for detector
offset and nonlinearity, and converted to absorbance. A linear regression of
sample absorbance vs. reference absorbance gives the ratio of sample
absorbance to reference absorbance. The assumption that temperature and
pressure are the same for the sample and reference gases is fundamental to the
design of the TGA200. It allows the concentration of the sample,
C
S
, to be
calculated by
S
R
R
S
L
D
L
C
C
=
Where
C
R
= concentration of reference gas, ppm
L
R
= length of the reference cell, cm
L
S
= length of the sample cell, cm
D
= ratio of sample to reference absorbance
3. Trace Gas Species Selection
The TGA200 can measure gases with absorption lines in the 3 to 10 micron
range by selecting appropriate lasers, detectors, and reference gas. Lead-salt
tunable diode lasers have a limited tuning range, typically 1 to 3 cm
-1
within a
continuous tuning mode. In some cases more than one gas can be measured
with the same laser, but usually each gas requires its own laser. The laser
dewar has two laser positions available (four with an optional second laser
mount), allowing selection of up to four different species by rotating the dewar,
installing the corresponding cable, and performing a simple optical
realignment.
The standard detectors used in the TGA200 are Peltier cooled and operate at
wavelengths up to 5 microns. These detectors are used for most gases of
interest, including nitrous oxide (N
2
O), methane (CH
4
), and carbon dioxide
(CO
2
). Some gases, such as ammonia (NH
3
), have the strongest absorption
lines at longer wavelengths and require the optional long-wavelength, liquid
nitrogen-cooled detectors. These detectors operate to wavelengths beyond 10
microns. The detector dewars have a liquid nitrogen capacity of 6.0 liters and
must be filled twice per week.
A prepared reference gas having a known concentration of the target gas must
flow through the reference cell. The beam splitter directs a small fraction of
the laser power through the reference cell to the reference detector. This gives
a reference signal proportional to the laser power, with the spectral absorption
signature of the reference gas. The reference signal provides a template for the
spectral shape of the absorption feature, allowing the concentration to be
derived without measuring the temperature or pressure of the sample gas, or
the spectral positions of the scan samples.
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