Surface area-to-volume (SA/vol) rat

Surface area-to-volume (SA/vol) ratios play a key role in the overall recovery of extract residues. The potential volatility of extractable components also requires the residue be obtained in a controlled and low-energy manner, when possible. We have found that in certain circumstances, increasing SA/vol ratios actually decreases the recovery of extract residue; Figure 1 shows the residues obtained when the same total amount of material was extracted in a given volume after subdividing into finer pieces, thus increasing surface area. The SA/vol ratios tested bracket the USP recommendation that is given for samples of thickness > 0.5 mm and ≤ 1 mm. These results may vary with sample thickness (non-exhaustive extraction may be diffusion- and therefore thickness-limited), but are very relevant for the sample thicknesses seen with typical pharmaceutical tubing.