The following procedure, which permits to obtain a SUPRAS volume
(6 mL) able to treat 30 food samples, was routinely followed
(Ruiz et al., 2006). Octanoic acid (5 g) was mixed with Bu4NOH,
40% (11.25 mL) and distilled water (40 mL) in a 100 mL glass centrifuge
tube. The mixture was centrifuged for 5 min at 3000 rpm
(2000g). The SUPRAS formed was less dense than water and was
separated as a new liquid phase at the top of the solution. Then,
it was withdrawn with a syringe and stored in a closed glass vial
until use. Under these conditions, it was stable at room temperature
for at least one month. The volume of solvent obtained (Vs)
from aqueous solutions of octanoic acid and Bu4NOH, at an