PFA tubings with IDs between 0.5 and 1 mm were used as
extraction units in our previous m-EME experiments [21,22]. An
essential prerequisite for efficient preconcentration in m-EME is a
high ratio of donor vs. acceptor solution volume, i.e. at the
conditions of the narrow-bore PFA tubing, the donor solution plug
must be significantly longer than the acceptor solution plug. This
leads to an increased migration path for ionic species in donor
solutions during m-EMEs and consequently results in longer
extraction times. A different geometry of the extraction unit was
therefore considered and a PP micropipette tip with a conical
profile was examined. Fig. 1 depicts electric current values for
m-EMEs of 15 mL of 20 mM SPADNS (donor solution) using
extraction units made of the PP micropipette tip and the PFAtubing. Acceptor solution (DI water) and FLM (1-pentanol)
volumes were 1.0 mL in both units and extractions were carried
out at
150 V. Complete transfer of SPADNS from donor into
acceptor solution was visually observed after 6 and 15 min for the
micropipette tip and the PFA tubing, respectively and was
confirmed by UV–vis measurements of acceptor and donor
solutions. The completion of the extraction processes was also
deduced from the electric current curves depicted in Fig. 1, which
decreased gradually during the m-EMEs and then levelled off at 6
and 15 min, respectively.