Liquified petroleum gas, LPG
Liquified petroleum gas (LPG) is a mixture of hydrocarbons mostly propane and buthane, with formulae C3H8 and C4H10, respectively. In large concentrations LPG may cause asphyxiation.
Propane has a chain molecule with three carbon atoms.
Two variants of buthane, n-buthane and methyl-propane (iso-buthane).
In addition to any LPG mixture there is always an odorant so that a leakage can be discovered. Often the odorant is a terribly smelling thiol, an organic molecule with a -SH group which has a very low threshold for human detection (on the level of ppb).
Ethanethiol a common odorant, this molecule model is from Wikipedia. The -SH group is rather similar to hydroxyl groups in alcohols.
Liquified petroleum gas is extremely flammable and thus suitable for burners, heating appliances, and vehicles. Also, it is used as aerosol propellants and as refigrerants instead of freons.
IR spectrum of liquefied petroleum gas, LPG
Regardless of the exact composition of a LPG mixture the hydrocarbons have many C - H bonds which stretching modes strongly absorb IR radiation at 2900 cm−1, or 3.4 µm. The C –C stretchings from the alkane chains absorb in broad bands between 800 and 1300 cm−1 corresponding to 12.5 - 7.7 µm.
Spectrum of n-buthane from the NIST database at http://webbook.nist.gov/cgi/