Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 15)

1.2.3 Concentration Calculation
The reference and sample detector signals are digitized and averaged over 50 consecutive scans. The average reference
and sample scans are then 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 TGA100. It allows the concentration of the sample, C
S
, to be calculated by:
) 1 (
) )( )( (
D L L
D L C
C
A S
R R
s − +
=
Where C
R
= concentration of reference gas, ppm
L
R
= length of the short reference cell, cm
L
S
= length of the short sample cell, cm
L
A
= length of the long sample cell, cm
D = ratio of sample to reference absorbance
1.3 Trace Gas Species Selection
The TGA100 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 TGA100 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.
They require filling with liquid nitrogen once each day.
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.
1.4 Dual Ramp Mode
The TGA100 can be configured to measure two gases simultaneously by alternating the spectral scan wavelength
between two nearby lines. This technique requires that the two absorption lines be very close together (within about 1
cm
-1 ), so it can be used only in very specific cases. The dual ramp mode is used to measure isotope ratios in carbon
dioxide or water by tuning each ramp to a different isotopomer.
The dual ramp mode may also be used to measure some other pairs of gases, such as carbon monoxide and nitrous
oxide, or nitrous oxide and methane, but the measurement noise will be higher than if a single gas is measured. For
measurements of a single gas, the laser wavelength is chosen for the strongest absorption lines of that gas. Choosing a
laser that can measure two gases simultaneously involves a compromise. Weaker absorption lines must be used in order
to find a line for each gas within the laser’s narrow tuning range.
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