Below is a table of typical Mid IR absorbence unit values for certain gases that have been expressed as concentration units of ppm per meter. All the gases have been measured in a one meter pathlength gas cell at 25C temperature and made up to 1 atmosphere pressure with a nitrogen gas mix. The absorbence value given for the concentration of gas is expected at the indicated wavenumber position.
| Gas species | Formula | Band position (cm-1) | PPM/meter | Absorbance |
| Carbon dioxide | CO2 | 2360 | 100 | 0.40 |
| Carbon monoxide | CO | 2170 | 100 | 0.04 |
| Methane | CH4 | 3020 | 100 | 0.10 |
| C2 to C6 n-alkanes | 2960 | 100 | 0.10 | |
| Nitrogen dioxide | NO2 | 1630 | 100 | 0.15 |
| Nitric oxide | NO | 1900 | 100 | 0.015 |
| Sulfur dioxide | SO2 | 1370 | 100 | 0.09 |
| Hydrogen sulfide | H2S | 1300 | 1000 | 0.002 |
| Ammonia | NH3 | 960 | 100 | 0.12 |
| Hydrogen chloride | HCl | 2940 | 100 | 0.04 |
| Water | H2O | 1650 | 1000 | 0.20 |
| Vinyl chloride | CH2CHCl | 950, 900 | 100 | 0.06 |
| Acetaldehyde | CH3CHO | 2750 | 100 | 0.015 |
| Benzene | C6H6 | 670 | 10 | 0.09 |
| Toluene | C6H5CH3 | 730, 690 | 100 | 0.10 |
| Methanol | CH3OH | 1040 | 100 | 0.10 |
| Ethanol | CH3CH2OH | 1050 | 100 | 0.05 |
| Carbonyl sulfide | COS | 2070 | 100 | 0.40 |
| Nitrous oxide | N2O | 2235 | 100 | 0.15 |
| Sulfur hexafluoride | SF6 | 950 | 10 | 0.40 |
The table of values indicates that typically for 100ppm of carbon dioxide gas, a gas cell at 1 meter pathlength will give an absorbance value of 0.4 absorbance units at 2360cm-1. For a 10 meter pathlength cell, a 100ppm per meter concentration would measure 4 absorbance units. Therefore you can “trade” the pathlength advantage for sensitivity, so a 10 meter pathlength cell with carbon dioxide gas measuring 0.4 absorbance units at 2360cm-1 would be equivalent to 10ppm per meter of gas.
The other gas examples can be dealt with in exactly the same way. In comparison, carbon monoxide is ten times less sensitive than carbon dioxide to measure with an equivalent cell.
For low ppm measurements you need a long pathlength gas cell and also, very importantly, a fast responding, sensitive detector such as a liquid nitrogen MCT detection system to discriminate for the weak signals from certain gas concentrations.