SS2100 TDLAS H
2
S Gas Analyzer BA02191C
1-8 Endress+Hauser
Differential TDLAS
Similar to TDLAS, this 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.
Wavelength modulation spectroscopy (WMS) signal detection
Endress+Hauser 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 on
page 1–8. 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.
With the resulting low-noise signal and use of fast post-processing algorithms,
reliable parts per million (ppm) or parts per billion (ppb) detection levels are
possible (depending on target and background species) at real-time response
rates (on the order of 1 second).
Figure 1–4 Typical normalized 2f signal;
species concentration is proportional to the
peak height
Normalized
2f Signal
Wavelength [a.u.]
Signal [a.u.]
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