Gas spectroscopy is a measure of the intensity of light absorbed or reflected by a sample as a function of wavelength or frequency (“absorption/reflection spectrum”). Absorption and reflection spectra are measured to characterise and quantise the materials and concentrations present in the sample.
Absorption occurs when a photon is incident on a sample and the photon energy matches an allowable electronic transition within the sample. When this occurs, the energy of the incident photon excites a molecule in the sample into a higher energy state. The resulting light is imaged using a spectrometer and compared with that used to excite the sample and areas of absorbtion identified.
The molecular species present in a gaseous sample can be determined by looking at the gas’s absorption spectrum.
Molecules have a unique set of allowable transitions which is defined by their atomic structure. As a result, the absorbtion profile for all molecules of one specific atomic arrangement is unique and differs from that of molecules of another, and this can be used to identify the molecule present
By comparing the measured absorption spectrum with known reference spectra, the materials present in the sample can be identified. The intensity of the absorption lines is used to quantify the relative concentrations of each material.
Reflection spectroscopy is used to measure the intensity of light reflected by a sample as a function of wavelength. Reflections are caused by scattering between photons and molecules in the sample.
Photons that are inelastically scattered will undergo a shift in wavelength as energy is lost or gained during the scattering event. Elastically scattered photons do not lose energy, and therefore don’t undergo a shift in wavelength during the scattering event. Elastic scattering can only take place at allowed energy states which are defined by the moelculr structure, and which are unique to molecules with that structure. As a result, the wavelength shift (or lack of it) is indicative of the molecules present in the sample, and the spectrum of the elastically scattered photons is soecific to molecules of that structure
By analysing the varying degree to which individual wavelengths are reflected, it is possible to determine unique scattering signatures, and hence the sample’s molecular composition.