Sample preparation by supercritical fluid extraction for quantification. A model based on the diffusion-layer theory for determination of extraction time

A mathematical model based on the diffusion-layer theory was elaborated in order to calculate the extraction time in dynamic supercritical fluid extraction required to reach a predefined level of extraction recovery. The goodness of the model is demonstrated by application to the extraction of the main neutral cannabinoids from marihuana and hashish samples. For monitoring of the cannabinoid content of extracts normal-phase HPLC was applied. To obtain reliable quantitative results, the extraction time ensuring a predefined level of recovery should be calculated for each individual sample according to the model because the extraction recovery depends on the sample matrix. The systematic error caused by the unextracted compounds can be eliminated by correction of the experimental data. For semi-quantitative determinations, where a knowledge of the correct value of the extraction recovery is not important, as a rule of thumb the extraction of marihuana with carbon dioxide of density 0.9 g/ml at 40 degrees C for 34 min and of hashish for 18 min can be suggested. The application of the proposed extraction times ensured at least a 95% recovery for the main neutral cannabinoids.