A look at infra red gas detection and the promise of
more advances through the use of spectrophotometric
techniques
Introduction
Mid-infrared absorption technology continues to be the most widely used method for gas measuring. Mid-infrared is typically defined as wavelengths from
3 to 8 μ. Mid-Infrared gas measuring involves illuminating
a gas sample with infrared light at a selected frequency,
and measuring optical energy loss across the gas sample.
Energy loss at certain wavelengths indicates absorption
due to molecular excitation of the gas sample. The atomic
construction of the gas molecules determines where in the
spectrum absorption occurs. By monitoring the proper
wavelengths, a particular gas can be identified and quantified. This area of the spectrum contains fundamental
molecular resonances for many gases of interest.
The mechanism for mid-infrared gas measuring involves
exciting molecules to higher energy levels (vibrational)
using photons. The energy contained in the photon is
transferred to the gas molecule and the photon is gone.
This energy transfer occurs when the photon energy1
matches the energy required to initiate the molecular
vibration. Since the energy contained in the photon is a
function of wavelength and the energy required to initiate the vibration is a function of the type of bond, only
certain wavelengths of light are absorbed by the target
gas molecule. The gas molecule continues to vibrate until
it loses the energy by emitting a photon or transferring
the energy to another molecule. In order to be affected by
light in this manner, the gas molecule needs to be polar
(i.e. contain a dipole moment2). Homonuclear molecules
(N2, O2, He, Ne…) are not polar, therefore generally do not
exhibit mid-infrared absorption.
History
Historically there have been two distinct types of mid-infrared gas measuring devices:
Spectrophotometer3—This type of device illuminates
the gas sample with a broad spectrum light source, sepa-
rates the light into many narrow bands and measures
the absorption at each band. A scan with gas present
is then compared to a reference scan and analyzed for
differences due to absorption.
The mid-infrared area of the
spectrum contains fundamental
molecular resonances for many
gases of interest.
There are two generally used methods using the spec-
trophotometer:
Illuminate the sample with an impulse excitation and
perform a Fourier Transform conversion to measure indi-
vidual frequency bands. (FTIR)
Illuminate the sample with wideband source and
separate out light bands using an interferometer or an
optical grating.
This type of device gives excellent resolution and is
able to precisely identify and quantify gas molecules, but
generally has delicate optics and requires very precise
setup and significant maintenance. It is not suitable for
an industrial environment and is usually restricted to
laboratory usage. The expense of a spectrophotometer
is also prohibitive for many applications often costing an
order of magnitude more than a gas detector.
Gas Detector—This type of device usually monitors
two light bands, one to measure the analyte (target gas)
and one to compensate for environmental conditions
and light source aging. These devices are typically very
robust and require minimal maintenance. Gas Detectors
are used to monitor a single gas and are able to operate
reliably in extreme environments. They are often rated
explosion proof or intrinsically safe for use in hazardous
locations.
