An FTIR spectrometer (A Nicolet 5700 from Thermo Nicolet
Corp.) was used that was equipped with a deuterated triglycine
sulfate (DTGS) as a detector. IR spectra were recorded in the
400–4000 cm−1 range with a resolution of 4 cm−1. The room was
kept at a controlled ambient temperature (25 ◦C) and relative
humidity (30%).
Precisely weighed, dry KBr powder (200 mg) was made into
two pieces of transparent blank KBr tablets that were approximately
5 mm in diameter and approximately 1 mm in thickness.
Volatile oils (2 L) were coated on the KBr tablets to form thin
liquid films for infrared spectrometry analysis. The sample measurements
were replicated 5 times with 4 scans each for a total
of 20 spectra, and then the average chart was taken as a last sample
spectrum. The background air spectrum, water vapor and CO2
interference were subtracted from these spectra. After baseline
correction and smoothing were performed using the OMNIC8.0
software, the spectrum data were imported in Unscrambler 9.7
software to standardize the normal variations.