The recorded GC-MS peak areas (y-variable) were linearly regressed against the respective standard nopinone concentrations (x-variable), assuming that errors in calibration standard preparation and concentration are negligible compared to random errors in GC-MS instrument response (i.e. that errors in x are insignificant compared to errors in y), and that all errors in instrument response (errors in y) are distributed normally with a constant standard deviation. Least squares linear regression analysis performed in MS Excel™ yielded the calibration function: 𝑦 = 1662436506 𝑥 + 1692018 (rounded values) Eq. S1