In this work, a natural
deep eutectic solvent (NADES) composed
of l-menthol:acetic acid 1:1 (molar ratio) was applied for
the first time as the extraction solvent for the dispersive liquid–liquid
microextraction (DLLME) of a group of 9 phthalic acid esters (i.e.,
dipropyl phthalate, DPP; butyl benzyl phthalate, BBP; dibutyl phthalate,
DBP; diisopentyl phthalate, DIPP; di-n-pentyl phthalate,
DNPP; dicyclohexyl phthalate, DCHP; di(2-ethylhexyl) phthalate, DEHP;
diisononyl phthalate, DINP; and diisodecyl phthalate, DIDP) from different
water samples (i.e., tap and mineral water) and an apple juice beverage.
Dihexyl phthalate (DHP) and di-n-octyl phthalate
(DNOP) were used as internal standards. After addition of the DES,
agitation, and centrifugation, the NADES droplet was collected, dissolved
in the mobile phase, and injected in a high-performance liquid chromatograph
coupled to a UV detector. Relative recovery percentages were in the
range 71–120% with relative standard deviation values ≤20%,
while the limits of quantification of the method were in the range
3.6–22 μg/L for tap water, 3.8–20 μg/L for
mineral water, and 4.0–23 μg/L for an apple juice drink.
Infrared spectroscopy and differential scanning calorimetry analysis
of the DES drop after the DLLME procedure revealed the absence of
the DES after the extraction and the existence of an l-menthol-rich
phase as a result of an important transfer of acetic acid to the aqueous
phase. However, the use of l-menthol alone, which required
the previous thermal solubilization of l-menthol, was not
able to quantitatively extract some of the target analytes. The extraction
carried out after the thermal solubilization of l-menthol
and the addition of an acid like HCl suggested that the extraction
capacity of the DES is caused by a “transient network”
originated by the acidity of the medium, independently of the nature
of the acid. The optimized NADES-DLLME procedure was extremely simple,
quick, and also environmentally friendly, as a result of the biodegradability
and relatively low toxicity of both components of the DES.
