In HPLC, the sample is introduced via elution development (Figure 2.44a) in which ‘‘the mixture is applied as a small quantity at the head of the col- umn ... and the individual components are separated by being transported along the stationary phase by the continuous addition and movement of the mobile phase’’ [120]. Sample introduction in SPE is conducted as frontal chromatography (Figure 2.44b) in which there is ‘‘the continuous addition of the dissolved mixture to the column, with the result that the least sorbed compound is obtained in a pure state’’ [120]. Linear chromatography is dis- tinguished from nonlinear chromatography by the di¤erent way in which the sample is fed into the sorbent. Therefore, SPE results in greater concentra- tion of the analyte in the final elution volume than in the original sample, while HPLC, for example, dilutes the sample in the e¿uent relative to the original sample.