2.5. Determination of TFA by PE film-based procedure
Approximately 100lL of sample was deposited onto the surface
of the film using a micropipette and subsequently spread uniformly
using the tip of the micropipette. The viscosity of oil samples
was reduced by mixing samples with hexane, which aided
deposition with the oil film forming after the solvent evaporated.
Films prepared in this manner were maintained in a horizontal
position, and the spectra of the oil film were obtained using a PE
film background spectrum. The spectra of oils were recorded using
these PE film. An effective pathlength was determined from each
spectrum and then used to normalize the spectrum to a fixed pathlength.
This effective pathlength could be used to compare
quantitatively results obtained from different films or different
sample loadings. The effective pathlengths of oil film spectra were
normalized to a fixed pathlength of 0.15 mm by a pathlength calibration
equation relating the effective path length to the absorbance
at 4334 cm1 (corresponding to a maximum of the broad
absorption band that oils exhibit in the CH combination band
region (4500–4100 cm1
)) relative to a single-point baseline at
504 L. Xu et al. / Food Chemistry 185 (2015) 503–508
3850 cm1 (Dong et al., 2015; Sun et al., 2014). Spectral data processing
and statistical analysis were carried out using TQ Analyst
7.2 (Thermo Electron Inc., Madison, WI) and OriginPro7.5
(Originlab, Northampton, MA).