Fatty acids are of key importance to the food industry. Especially long chain polyunsaturated fatty acids (LC PUFA) are receiving more and more attention due to their positive influence on human health. LC PUFA refined from natural oils are frequently added to food products to gain a positive health effect. Since LC PUFA are rather unstable and prone to oxidation, the quality of oils and fats has to be controlled. Certain aldehydes, ketones and other compounds are markers for oil and fat quality. Some of these have unpleasant odors and/or tastes (e.g. fishy) which are not acceptable to consumers. The compound 4-heptenal, for example, adds a detectable fishy odor and taste to food products at concentrations as low as 10 ng/g. This and other marker compounds therefore have to be monitored in the ng/g range by the food industry.