SS2000 Operator’s Manual
H
2
S in Natural Gas 1–5
Due to their inherent structure, the molecules in the sample gas each have
characteristic natural frequencies (or resonances). When the output of the
laser is tuned to one of those natural frequencies, the molecules with that
particular resonance will absorb energy from the incident beam. That is, as the
beam of incident energy, I
0
(λ), passes through the sample, attenuation occurs
via absorption by the trace gas with absorption coefficient α(λ). According to
the Beer-Lambert absorption law, the energy remaining, I(λ), as measured by
the detector at the end of the beam path of length l (cell length × number of
passes), is given by
,(1)
where c represents the species concentration. Thus, the ratio of the absorption
measured when the laser is tuned on-resonance versus off-resonance is
directly proportional to the number of molecules of that particular species in
the beam path, or
.(2)
Figure 1–2 shows typical raw data from a laser absorption spectrometer scan
including the incident laser energy, I
0
(λ), and the signal, I(λ), for a clean system
and one with contaminated mirrors (shown to illustrate the systems relative
insensitivity to mirror contamination). The positive slope of the raw data results
from current tuning the laser, which not only increases the wavelength with
current, but also causes the corresponding output power to increase. By
normalizing the signal by the incident energy, any laser output fluctuations are
cancelled, and a typical, yet more pronounced, absorption profile results, as
shown in Figure 1–3. Note that contamination of the mirrors results solely in
lower overall signal. However, by tuning the laser off-resonance as well as on-
L
D
DETECTOR
LASER
MIRROR MIRROR
I
0
(λ)
I(λ)
TRACE GAS ABSORPTION
α(λ)
GAS IN
GAS OUT
Figure 1–1 Schematic of a typical laser diode
absorption spectrometer.
I λ() I
0
λ()exp αλ()lc–[]=
c
1–
αλ()l
--------------
I λ()
I
0
λ()
-------------
ln=
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