When using WMS, the sensitivity of a spectrometer
is often determined by taking the ratio of the peak to peak
amplitude of the WMS signal of an absorption
line to that of the noise level.
Using an absorption line’s amplitude to detect a
species, however, neglects the width of the line
and, as a result, gives the same intensity for both
broad and narrow lines with the same amplitude.
For example, even though a 25ppm sample of NH3 at 600mbar has 12 times more molecules than the same sample at 50 mbar, its absorption spectrum
shows only about a 25% enhancement in peak absorption:
The majority of additional absorption manifests
itself in the broadening of the lines [10,11]. As a
result, when dealing with broadened lines, a more accurate
measure of the absorption intensity can be
achieved by integrating over the absorption line.
Assuming that the absorbance a (A) is small
(as is typically the case with trace gas detection),
the integrated absorption may be written as
Since the absorption coefficient , where
is the cross section and N is the concentration,
the integrated absorption signal for a single line is
proportional to the concentration of the species.
Based on this, it has been shown that the sum of
the areas of a set of absorption lines varies linearly
with the concentration, and conducting trace gas detection
by integrating multiple absorption lines can
enhance the sensitivity of a detector by over 1 order
of magnitude [11].
This concept may be extended to further enhance
the sensitivity of a detector by integrating the absolute
value of the wavelength modulation spectra of a
species over multiple lines. Since the nth Fourier
amplitude HnðνÞ is proportional to the absorption
coefficient α, integrating the absolute value of the