Proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) as a tool for studying animal volatile organic compound (VOC) emissions

Abstract

Chemical sensing in vertebrates is crucial in their lives, and efforts are undertaken towards deciphering their chemical language. Volatile organic compounds (VOCs) are a group of chemicals believed to play an essential role in a wide variety of animal interactions. Therefore, understanding what animals sense themselves and untangling the ecological role of their volatile cues can be accomplished by analysing VOC emissions. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) is an instrument that measures VOCs in real-time in an air sample. Since this technique acts as a hyper-sensitive ‘nose’ it has a similar potential in deciphering the chemical language of vertebrates. Here, we validate the use of PTR-TOF-MS as a tool to measure VOCs from vertebrates, which in turn will help resolve vertebrate interactions through VOCs. The instrument monitors and records the full spectrum of VOCs emitted by an individual with a high accuracy and low detection limit, including transient VOC emissions. We propose and test diverse measuring configurations that allow for measurement of VOC emissions from different vertebrates and their exudates: full body, specific parts of the body, urine and femoral pores. In addition, we test configurations for sudden and short-lasting processes as VOCs emitted during adder skin shedding as well as the emissions of skin secretions upon mechanical and physiological stimulation in amphibia. Our configurations work in tandem with gas chromatography–mass spectrometry (GC-MS) to allow compound structure verification. We discuss the configurations and methodologies used and conclude with recommendations for further studies, such as the choice of chamber size and flow. We also report the results of the measurements on vertebrates—that are novel to science—and discuss their ecological meaning. We argue that PTR-TOF-MS has a high potential to resolve important unanswered questions in vertebrate chemical ecology with great adaptability to a wide range of experimental set-ups. If combined with a structure verification tool, such as GC-MS, the creative deployment of PTR-TOF-MS in various future study designs will lead to the identification of ecologically relevant VOCs.