Quantifying Volatile Organic Compounds by GC-MS

PTR-MS analysis allows online VOC quantification at high temporal resolution without differentiation of isomeric compounds (Ghirardo et al., 2010 ). To identify the VOCs and separate different monoterpene and sesquiterpene isomers, GC-MS analysis was performed by trapping 4 l of air leaving the cuvettes onto polydimethylsiloxane-foam-adsorbent tubes (Gerstel, Mülheim an der Ruhr, Germany) at flow rates of 100ml min⁻¹. This procedure was optimized for determination of nonpolar compounds; some polar volatiles such as methanol, ethanol, and C6 LOX products were not included in this analysis. Samples were analysed after thermal desorption and cryofocusing by GC-MS, as described by Ghirardo et al. (2012) . VOCs from control experiments were used for background subtraction. For the quantification of VOCs, individual response factors were determined using the total ion count from calibration curves (R²>0.98) of pure standards (α-pinene, sabinene, 3-carene, p-cymene, limonene, linalool, trans-β-caryophyllene, α-farnesene, and nerolidol) at four different concentrations (1–100 pmol (l hexane)^–1). Other monoterpenes not present in the standard were quantified using sabinene; other monoterpene alcohols using linalool; other sesquiterpenes using (−)-β-caryophyllene; and other sesquiterpene alcohols using nerolidol. For the quantification of aliphatic and aromatic compounds, a response factor was calculated for each compound by using the response factor of sabinene (R²>0.99) and was normalized based on molecular weight in order to consider the changes of total ion count responses due to different molecular masses. The same procedure was used for the quantification of the aliphatic and aromatic alcohols, except that the response factor of the linalool standard (R²>0.99) was used as reference.