Supported liquid extraction is a simple, yet effective sample preparation technique. It works like liquid-liquid extraction (LLE) but is supported on a solid surface. Aqueous samples are dispersed as small droplets on a high surface area material. The entire sample remains on the SLE column. Samples are eluted with a water-immiscible solvent, like ethyl acetate, hexane, or dichloromethane. The compounds of interest partition into the organic phase, like an LLE, while salts, phospholipids and other impurities remain on the column. The simple load, wait, elute protocol is fast and easy and provides excellent sample cleanup that can be automated.
Solid phase extraction is more involved. Samples are loaded onto a SPE column, and the sample flows through and is discarded. Analytes of interest (and some undesirable compounds) are retained by hydrophobic interaction or ion exchange. The column is washed with aqueous and organic wash solvents to remove interfering compounds. Then, the analytes of interest are eluted based on their retention mechanism. Organic solvent is used for hydrophobic compounds, acid or base for ionized compounds.
SLE is a great sample prep solution for many applications. In order for SLE to work, compounds must partition into the organic solvent. Ideally compounds should be neutral. Ionized compounds often don’t partition well by LLE or SLE. Hydrophilic compounds will remain on the SLE column and not be extracted by SLE. Acids, bases and neutrals can all be extracted by SLE, but sample pretreatment is usually required to get as many of the compounds in their neutral state as possible. Addition of 2-propanol, 2% methanol or acetonitrile can help with extraction of more hydrophilic compounds, but sample cleanliness may be compromised.
SPE is a stronger retention mechanism and allows greater sample cleanup because of the wash steps. An aqueous wash can be used to remove salts and other water-soluble impurities while retaining hydrophobic compounds. Non-polar interfering compounds can be partitioned into a moderately strong organic solvent prior to elution of the more hydrophobic analytes of interest. A strong organic wash can be used to remove interferences if compounds are retained by ion exchange. Compounds are eluted by “turning off the charge” of the desired compounds using acid or base. Care should be taken when using mixed mode SPE phases. Remember – know the properties of your compounds and how they are retained on the SPE phase.
So when would I choose SLE? When would I choose SPE? If the samples are more nonpolar, and they can be extracted by LLE, the method is a good candidate for SLE. Steroids and fat soluble vitamins are good examples. If they are ions but can be forced into a neutral state using pH, SLE might still be a good choice.
If the compounds are hydrophilic or zwitterionic and can be retained by ion exchange or you need maximum sensitivity and specificity for hydrophobic compounds, then the additional steps in SPE will provide cleaner samples. The method can be fine-tuned with careful control of the pretreatment, wash and elution solvents to separate interfering moieties from the analytes of interest. Take-home message: if SLE works – great! If you need additional cleanup, try SPE.