Analytical techniques for sesquiterpene emission rate studies in vegetation enclosure experiments

Abstract

Sesquiterpene (SQT) compounds (C 15 H 24 ) and their oxygenated alcohol and ketone derivatives are biogenic volatile organic compounds that have been identified in emissions from vegetation. SQT emission rates and landscape flux estimates are highly uncertain. Reliable ambient flux measurements have not been possible because of low-ambient concentrations, rapid atmospheric reactions (prohibiting ambient tower flux measurements), and analytical challenges and uncertainties that stem from the low volatility of SQT. Standards from an in situ capillary diffusion system with 18 SQT compounds and four other organic compounds (geranylacetone, 1,3,5-tri-isopropylbenzene, diphenylmethane, nonylbenzene) were used to thoroughly investigate experimental procedures for SQT emission rate studies by vegetation enclosure techniques. Recovery rates in tubing materials, sampling bags, leaf cuvettes, on six solid adsorbent materials (Tenax TA, Tenax GR, Carbotrap, Carbotrap C, Unibeads, Glass Beads) for gas chromatography analysis, and gas chromatography retention indices and mass spectral fragmentation patterns were determined. SQT compounds were found to exhibit a high degree of stickiness to all materials tested. However, the non-oxygenated SQT can be recovered in enclosure experiments for quantitative emission rate determination after careful consideration of the analytical conditions. It is utmost important to allow sufficient purging and equilibration times for all materials in contact with the sample air. In contrast oxygenated SQT were irreversibly lost in enclosure experiments which made their quantitative measurement prohibitive. Results for the other organic compounds were similar and indicate that these data mostly stem from the volatility of these compounds. Consequently, the findings of this study provide guidelines for the analysis of a wide range of volatile organic compounds in the ∼C13–C17 volatility range.