SS2100 H
2
S Analyzer
1–6 4900002218 rev. C 11-6-15
Figure 1–3 Typical normalized absorption signal from
a laser diode absorption spectrometer
Note that contamination of the mirrors results solely in lower overall signal.
However, by tuning the laser off-resonance as well as on-resonance and
normalizing the data, the technique self calibrates every scan resulting in
measurements that are unaffected by mirror contamination.
Differential TDLAS
Similar to TDLAS, this SpectraSensors technology involves subtracting two
spectrums from one another. A “dry” spectrum, a response from the sample
when the analyte of interest has been completely removed, is subtracted from
the “wet” spectrum, a response from the sample when the analyte is present.
The remainder is a spectrum of the pure analyte. This technology is used for
very low or trace measurements and is also useful when the background matrix
changes over time. Refer to http://www.spectrasensors.com/spectral-
subtraction-in-tdlas-analysis/ for more information.
Wavelength Modulation Spectroscopy (WMS) Signal Detection
SpectraSensors takes the fundamental absorption spectroscopy concept a step
further by using a sophisticated signal detection technique called wavelength
modulation spectroscopy (WMS). When employing WMS, the laser drive
current is modulated with a kHz sine wave as the laser is rapidly tuned. A lock-
in amplifier is then used to detect the harmonic component of the signal that
is at twice the modulation frequency (2f), as shown in Figure 1–4. This phase-
sensitive detection enables the filtering of low-frequency noise caused by
turbulence in the sample gas, temperature and/or pressure fluctuations, low-
frequency noise in the laser beam or thermal noise in the detector.
1.0
0.99
0.98
0.97
0.96
0.95
Normalized Absorption Signal
Wavelength [a.u.]
Signal [a.u.]
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