Molecule of the Month

“Ts” (18a(H)-22,29,30-trisnorneohopane) By Paul Farrimond

The compound often referred to simply as “Ts” is a C₂₇ rearranged hopane with a near-ubiquitous occurrence in oils and mature source rocks.  Although the structure of this compound was established in the early 1970s, the commonly-used abbreviation “Ts” dates from a paper by Seifert & Moldowan (1978) that noted its high thermal stability and termed it “Terpane, stable” (Ts) as opposed to the less stable non-rearranged C₂₇ 17a(H)-trisnorhopane (“Terpane, maturable”, Tm).

In geochemical data reports both Ts and Tm are identified and quantified in the m/z 191 mass chromatogram, and are routinely used in a ratio of Ts/Tm (more recently commonly reported as the index Ts/[Ts+Tm]).  This ratio is best known as a maturity parameter, which increases with thermal stress as indicated above, but it can also be higher in clay-rich samples (and oils from clay-rich sources), and Ts has a greater resistance to biodegradation.  A C₂₉ rearranged hopane having related structure with an ethyl side chain at C-21 is usually referred to as “29Ts”, and has similar characteristics.  A “30Ts” can sometimes also be found in very low relative abundance.

Reference:

Seifert W.K. & Moldowan J.M. (1978). Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochimica et Cosmochimica Acta 42, 77-95.