6 P-Cymene Formation by Oxidation of Cyclohexadiene Monoterpenes – a Pitfall in Air Sampling of Terpenes

Abstract

Abstract It has been known for a long time that certain terpenes are sensitive to oxidation. Especially in the presence of small amounts, as is the case in the environment or indoors, changes occur due to UV light or ozone exposition. Various oxidized products have also been detected when sampling low terpene concentrations using Tenax® tubes and subsequent thermal desorption. Here we show that carrying out common air sampling of monoterpenes in the lower ppm range on charcoal tubes, the atmospheric oxygen is sufficient to oxidize the most sensitive terpenes to a considerable extent, forming almost exclusively p-cymene. As expected, the extent of aromatization was strongest in the terpenes with a double unsaturated ring system such as α- or γ-terpinene or α-phellandrene while p-menthene or limonene were not affected. We studied the influence of sampling time, temperature, humidity and storage time of the coated tubes. Simultaneously conducted experiments with thermal desorption in the ppb range showed a significantly reduced p-cymene formation compared to the activated charcoal results. Our findings were confirmed by air samples of terpenes taken during the grinding of nutmegs. The concentrations of p-cymene determined were significantly higher than the terpene pattern of nutmeg oil would have suggested. Thus, the detection of p-cymene during air sampling may be an artifact caused by the presence of cyclohexadiene monoterpenes. This is particularly important because p-cymene is often used as an internal standard in terpene analysis thus preventing its artifact formation from being noticed.