Two immiscible liquid plugs of several microliters moving inside a capillary induce circulating flows in each plug that facilitate efficient extraction at the liquid–liquid interface. In their Communication on page 14124 ff., Z. Ouyang and co-workers present a direct, quantitative mass spectrometric method based on this slug-flow microextraction for analysis of biomarkers in 5 μL samples of biofluids. Real-time monitoring of the enzymatic functions of blood samples can also be performed.