A novel dynamic flush method to reduce column-related carryover

Column-related carryover affects both accuracy and precision in liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. In this work, a novel straightforward dynamic flush method to reduce the column-related carryover was developed by alternating the column flow direction of liquid chromatographic separation with a Valco switching valve and pristine instrument control software. By alternating the column flow direction, a fresh inlet is always in line to accommodate sample injection and stacking. In addition, the contaminated column inlet from the previous run is switched to the outlet position for a flush with a gradient during the next sample run. In this way, the column-related carryover can be reduced effectively without additional blank runs. It also minimizes the carryover risk between the adjacent unknown samples. The column-related carryover of the tested "sticky" compounds was reduced by 52.3-94.4% compared with the non-dynamic flush method under the same experimental conditions. The performance and reproducibility of high-performance liquid chromatography (HPLC) separation in terms of the retention time shift and peak shape are not compromised under the dynamic flush even after over 300 consecutive injections. The described novel method is simple and easy to implement for compounds with column-related carryover.