Volatile organic compounds emitted after leaf wounding: On-line analysis by proton-transfer-reaction mass spectrometry

Volatile organic compounds (VOCs) released from vegetation, including wound -induced VOCs, can have important effects on atmospheric chemistry. The ana lytical methods for measuring wound-induced VOCs, especially the hexenal fa mily of VOCs (hexenals, hexenols, and hexenyl esters), are complicated by t heir chemical instability and the transient nature of their formation after leaf and stem wounding. Here we demonstrate that formation and emission of hexenal family compounds can be monitored on-line using proton-transfer-re action mass spectrometry (PTR-MS), avoiding the need for preconcentration o r chromatography. These measurements allow direct analysis of the rapid emi ssion of the parent compound, (Z)-3-hexenal, within 1-2 s of wounding of as pen leaves and then its disappearance and the appearance of its metabolites including (E)-2-hexenal, hexenols, and hexenyl acetates. Similar results w ere seen in wounded beech leaves and clover. The emission of hexenal family compounds was proportional to the extent of wounding, was not dependent on light, occurred in attached or detached leaves, and was greatly enhanced a s detached leaves dried out. Emission of (Z)-3-hexenal from detached drying aspen leaves averaged 500 mu g C g(-1) (dry leaf weight). Leaf wound compo unds were not emitted in a nitrogen atmosphere but were released within sec onds of reintroduction of oxygen; this indicates that there are not large p ools of hexenyl compounds in leaves. The PTR-MS method also allows the simu ltaneous detection of less abundant hexanal family VOCs including hexanal, hexanol, and hexyl acetate and VOCs formed in the light (isoprene) or durin g anoxia (acetaldehyde), PTR-MS may be a useful tool for the analysis of VO C emissions resulting from grazing, herbivory, and other physical damage to vegetation, from harvesting of crops, and from senescing leaves.