SPE is a column chromatographic analysis process bearing much similarity to high performance liquid chromatography (HPLC) in the mechanism of separation and choice of reagents and solvents. When a sample passes through the extraction column filled with a sorbent, a part or all of the constituents interact with the sorbent and are retained in the column. A solvent or mixture of solvents is then used to wash out interferences. The target analyte is then quickly eluted out of the column by a desired solution of reduced volume. The goal of fast separation, purification, and concentration is therefore achieved. SPE can be carried out with various types of sorbents to perform extraction and separation of different analytes. Depending on the nature of the sorbents, SPE extraction materials can be divided into normal phase sorbents, reverse phase sorbents and ion exchange sorbents.
It is important to control the flow rate when loading the sample. If the flow rate is too high, the pulling force from the mobile liquid could be stronger than the retention exerted by the sorbent especially in ion exchange interaction resulting in breakthrough. Nanomicro Tech sorbent is composed of monodisperse microspheres with very high retention ability. Even under high flow rate, it still maintains a strong interaction between the
Washing: Use a solvent with proper strength for washing the extraction column. The purpose is to rinse interferences through the column while leaving the target analyte in the column. To reach that goal the strength of the solvent should be strong to remove the absorbed interferences, but also weak enough to leave behind the retained analyte. A commonly used solvent for washing is water or a mixture of water and organic solvent.
Elution: Use a solvent or mixture of solvents capable of eluting the target analyte off the sorbents. This step has a strong effect on the recovery. It requires an accurate control of the amount and flow rate of the eluent to ensure the reproducibility of the separation. The strength of the eluent is also important. A strong eluent may remove some intentionally retained interference, whereas a weak eluent may not break up the retention of the target analyte on the sorbent. To increase the analyte concentration or change solvents, the collected eluent containing the target analyte can be dried by blowing nitrogen over it and then re-dissolve the analyte in an appropriate solvent.