Measurement of enantiomer percentages for five monoterpenes from six conifer species by cartridge-tube-based passive sampling adsorption-thermal desorption (ps-ATD)

Many monoterpenes have at least two different stereochemical forms, and many biosynthetic pathways are known to favor one product over the other(s). A rapid method was developed and used in the determination of the (-/+)-enantiomeric distributions for alpha-pinene, beta-pinene, camphene, limonene, and beta-phellandrene as emitted by plant material from six conifer species. The six species included the two pine species Pseudotsuga menziesii and Pinus ponderosa, as well as the four cypress species Chamaecyparis lawsoniana, Thuja plilcata, Juniperus chinensis, and Thuja occidentalis. The method involved passive sampling adsorption-thermal desorption (ps-ATD). During sampling, the cartridge tube was placed in a 60 mL glass vial with plant material for 1 h. Sample analytes were thermally transferred to a chiral gas chromatography (GC) column. Detection was by mass spectrometry (MS). The six species exhibited different emission patterns for the five monoterpenes in the -/+ totals, although within a given species the distributions among the five monoterpenes were similar across multiple plants. beta-pinene dominated in P. menziesii and P. ponderosa, and alpha-pinene dominated in T. plicata and T. occidentalis. The chiral separations revealed differences in the -/+ enantiomeric distributions among the species. The (-)-enantiomers of alpha-pinene and beta-pinene dominated strongly in P. menziesii and P ponderosa; the (-)-enantiomer of beta-phellandrene dominated in C. lawsoniana. The dependence of the method precision on percent enantiomer abundance is discussed.