Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 52)

Finding the Absorption Line
The TGA’s spectral scan must be locked onto a selected absorption line. When the TGA is restarted, its spectral scan
position is set by the parameters stored in the default parameter file. After the laser temperature has stabilized, the
previously selected absorption line should be visible in the detector response. Note that it may take a minute or two for
the actual laser temperature to stabilize completely after the displayed temperature is stable.
If the absorption line is not visible, small adjustments in the DC current will often bring it into view. Increasing or
decreasing the DC current will increase or decrease the laser’s emission frequency. It may be helpful to think of the
reference detector display as a viewing window looking upon a portion of the absorption spectrum. Increase the DC
current to move the window to the right and decrease the DC current to move the window to the left. It may also be
helpful to temporarily increase the width of the spectral scan by increasing the modulation current.
After the absorption line is found, adjust the DC current up or down to find nearby absorption lines. Compare the
spacing and relative depth of the observed absorption lines to the absorption spectrum provided in the user manual to
verify it is the desired absorption line. Adjust the DC current to position the selected absorption line near the center of
the spectral scan. Readjust the modulation current (see section 4.4.5) and initiate the line locking function.
If the expected absorption line is not found easily, it may be helpful to map the laser’s output systematically as a
function of temperature and current, as described in section 4.3.
For dual ramp operation, ramp A and ramp B must be locked onto different absorption lines. For example, the two lines
may correspond to different isotopomers of CO
2
for CO
2
isotope ratio measurements. To find the absorption lines in
dual ramp mode, first select the “RefDet A & B” detector display option. Then lock ramp A onto its absorption line as
described above. If the TGA has already been set up for the pair of lines, then as the ramp A line is centered, the ramp
B line should also be close to the center. Press alt-I to start the ramp B line lock. If the ramp B offset and the reference
transmittance for ramp A and ramp B are consistent with previous values, this is a good indication that the TGA is
locked onto the correct absorption lines.
below illustrates ramp A locked onto a 13 CO
2
line and ramp B
locked onto a 12 CO
2
line. The 12 CO
2
line is at a lower wavenumber (not shown), so the ramp B offset must be negative,
i.e., the laser current during ramp B is less than during ramp A.
Figure 4-5
Figure 4-5. Dual Ramp Laser Scan Sequence
Laser
Current
CO
2
13
CO
2
12
Ramp B Offset
Ramp A Ramp B
Detector
Response
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