It is one of the most used extraction method due to its simplicity and efficacy. It is also used for cleaning up and concentrating the required analytes. SPE is used to semivolatile or nonvolatile analytes from both liquid and solid samples. It is also cheap and easy to perform and to automate. The theory of SPE is to retain the compound of interest by different ways of molecule attractions (hydrophobic, hydrophilic, electrostatic or a combination) and thus separating the compound of interest from the extract.
The general process is to find the proper application to suit the sample. Due to the big variety of chemistries, adsorbents and sizes the proper method can be established and optimized depending on the chemistry of the compound.
To collect the compound you can either choose the SPE sorbent to bind to the compound or the impurities and then either collect the compound of interest through elution or dispose of the impurities, this is called selective extraction. Another way of using SPE is selective washing where the compound of interest and the impurities bind to the adsorbent then using a wash solution that is strong enough to elute the impurities but it should have weak effect on the compound. The third way is selective elution, which is the opposite of selective washing. In this case the elute should be strong enough to elute the compound yet leave the impurities still retained in the tube.