Liquid-Liquid-Extraction (LLE) is used to separate substances based on their solubility, using two different immiscible liquids (usually water and an organic solvent). After vigorous shaking of the mixture and allowing the liquids to settle, a two-phase system has been established. Either the lower or the upper phase contains the extracted substance.
To aspirate the upper phase, the tip starts at the liquid surface and then follows the liquid level (Level tracking). If the phase boundary can be detected, the tip is positioned a few millimeters above and stays at this level, until the aspiration is complete.
Transferring the lower phase can be conducted in a similar way, by aspirating just below the phase boundary and tracking the liquid phase to the bottom. If no detection is possible, the tip can also start directly at the lowest position.
Approaches like Liquid Level Detection, Image Analysis and Height calculation are available to determine the position of the phase boundary.