Generally speaking, the volume of these sample solutions such the extracts and the eluents is too large, so only a small portion of sample solutions can be injected in order to avoid volume overload of LC, especially when the sample solvents have higher eluent strength than the mobile phase. Thus, the sample loss decreases the sensitivity dramatically. The analytes are sometimes not welldistributed in a sample stream especially for the eluent stream and hence, the time of fraction collection is crucial for analytical sensitivity and precision. In this case, a “heart-cutting” method is necessary. A real-time monitoring strategy using a detector has been proposed to select the “cutting” time window of the elution
stream [23]. Following this strategy, the SBSE eluent fraction containing the highest concentration of target analytes can be precisely heart-cut introduced into the HPLC system, and both the reproducibility and the sensitivity are improved. In addition, the heart-cutting method can be used in online SPE-HPLC systems for clean-up of the sample from interfering matrix components. It can be also applied to the elimination of residue sample (or wash) remaining in a SPE column.