As a general trend, it could be observed that the extraction yield decreased towards higher extraction flows and that the effect was stronger at lower analyte concentrations.
Jochmann et al. suggested diffusion into the sorbent pores to be the rate-limiting effect at higher extraction flows [ 3], as it can also be observed by increasing plate heights in gas chromatography [ 22].
Although most authors used a method with a high extraction flow and a large number of extraction strokes [ 3, 19, 20, 18, 15], the measurement of different combinations of extraction strokes and flows has so far only been reported, recently [ 21, 4].
For all systems, the results showed droplet diameter decreasing with an increase of volume flow rate, and higher extraction efficiency due to the increase in specific interfacial area and decrease in diffusion distance.
The results demonstrated that NTDs packed with small particles possess higher extraction capacity and efficiency but much higher resistances to flow as well.
The largest influence was observed for ethyl acetate, where the extraction yield at an extraction flow of 10 μL s−1 was almost twice as high as at 100 μL s−1, whereas no significant influence on the extraction yield could be observed for geraniol and vinyl chloride.
Total CRRT dose (sum of substitution, dialysis, citrate, and extraction flows) per kilogram was higher in the citrate group, but the difference was not statistically significant (69 versus 59 mL/kg/hour; P = 0.154).
Four combinations which result in a constant extraction time are compared in Fig. 6c; here, the higher number of fast extraction strokes by far outperforms the lower extraction flows.
Maximum allowable steam extraction flows can constitute further constraints.
As expected, the peak areas increased towards lower extraction flows and also towards more extraction strokes.
