Detailed description of LLE

LLE is a very common method to pre-concentrate organics. Here, a small volume of a water-immiscible organic solvent (e.g., n-hexane, dichloromethane, or toluene) is added to a much larger volume of the aqueous sample. The two phases are shaken until partition equilibrium is obtained. (For details on phase partitioning refer back to chapter III.3 ).

LLE aims for high extraction efficiencies . In other words, the analyte should ideally transfer 'quantitatively' from the water to the organic solvent. For a given solvent, s, and analyte, i, (i.e. for a given solvent-water partition constant K_{i SW} ), the extraction efficiency (i.e., the fractional amount of total analyte that is transferred from the aqueous to the solvent phase expressed in percent) may be increased by extracting in several consecutive steps rather than in one single step (see Problem IV.3.3 and Figure 49 ). Addition of salt to the water sample also is helpful: it shifts the partition coefficient further to the organic solvent (see chapter V.1 ) and it helps to prevent the formation of emulsions.

Emulsions make a phase separation between the solvent and the water phase difficult. They frequently form when there is a lot of biological material (e.g., bacteria, algae) or humic substances in the water sample. In such cases another problem arises because not only the targeted pollutants but also other organic compounds are extracted into the solvent. This problem can be solved if the water sample is separated from the organic solvent during extraction through a semi-permeable membrane that cannot be passed by large organic molecules.

Obviously it is useful for the design of a liquid-liquid extraction procedure if we know the partition constants of our target compounds between water and various solvents. We can calculate these partition constants if the solvation parameters of the target compounds are known (see script). One has to keep in mind though that these equations are valid for clean water and not for water that contains humic acids, bacteria or other types of organic matter. In order to extract organic ions from water it is usually necessary to convert them to a non-ionic form either by adjusting the pH-value (see chapter VIII ) or by ion-pair formation.

While LLE has traditionally been performed using separatory funnels, recent technical advances have lead to the development of mini-, micro-, and continuous LLE (for details, please refer to sample preparation textbooks ). The main advantage of LLE is its simplicity. The disadvantages of LLE include practical problems such as formation of emulsions and the use of relatively large volumes of organic solvents (which often are toxic and environmentally hazardous).