GC-MS signals were recorded for each calibration standard, with blanks placed after every second measurement to avoid cross-contamination. For the calibration standard at 0.45 mol l -1 concentration two independent replicates were recorded. GC-MS peak areas for nopinone were detected at a retention time RT ≈ 6.8 min. The raw calibration data recorded for the various calibration standards is shown in Figure and Table . By inspection, the relationship between GC-MS response and nopinone concentration in Figure appears to be roughly linear at concentrations beneath 0.05 mol l -1 . As such, linear least squares regression was employed to establish a calibration function for this region.