In order to understand how these techniques work it is essential to have some small appreciation of the complex processes occurring during oxidative rancidity:
Oxidative Rancidity:
Unsaturated lipids + O2 → Primary Products → Secondary Products
The primary products are transient molecules that subsequently decompose/ react further and they include peroxides and conjugated dienes as monitorable species.
The secondary products are more stable and typically include ketones, aldehydes, carboxylic acids, alcohols and hydrocarbons as monitorable species. It is these secondary products that cause the off flavours and odours in rancid oils.