3.2.5. Magnetic stirring rate and time
In SDME, the change of stirring rate was expected to affect extraction dynamics since the thickness of the interfacial layer surrounding the droplet would influence the mass transfer of analytes. In general, mass transfer is a time dependent process and only when the system is at equilibrium can the maximum absorbance be attained. But the achievement of equilibrium is really a time-consuming process. In generally, complete equilibrium needs not to be attained for accurate and precise analysis. The result of stirring rate was shown inFig. 6. The absorbance increased with the increase of stirring rate of 240rpm, and then the signal became lower and kept constant with the further increase of the stirring rate. For the higher stirring rate, the higher risk of release from the syringe or dissolving in the solution would occur to the drop. Therefore, 240rpm of stirring rate was used. For the stirring time, the absorbance increased dramatically as the time increased from 5 to 10min, and after 10min, no obvious increasing was achieved with the longer time. Considering the efficiency and drop dissolution, the extraction time of 10min was selected for all subsequent experiments.