The spectra in the region between 2800 and 3200 cm–1 were used. This
region is responsible for the C–H bond stretching vibrations of methyl groups and the cyclic hydrocarbon vibrations, which
are characteristic of cholesterol [23]. The samples from different stages of lactation were analyzed using the ATR-FTIR
technique with the comparable coeffi cients of variation. There were no differences between the cholesterol concentrations in
the samples from the groups of early (1–3 months), medium (4–6 months), and late (> 6 months) lactation (p = 0.096968).
The cholesterol concentration obtained by the FTIR-ATR method ranged from 4.30 to 21.77 mg/100 cm3 (Table 3). Such a
broad range is due to the differences between the samples from different women (p = 0.000184). The minimum and maximum
values of the cholesterol concentration in the fi rst group (1–3 months of lactation) were 4.94 and 19.75 mg/100 cm3, respectively.
In the second group (4–6 months of lactation) these values were 4.30 and 14.92 mg/100 cm3, and in the third group
(> 6 months of lactation) 4.68 and 21.77 mg/100 cm3 (Table 4).
Validation of the method by gas chromatography. Gas chromatography (GC) was used as a reference method. The
results are shown in Table 5. There were no statistically signifi cant differences between the results of the methods used
in the analysis (p = 0.772251). A good correlation was observed between the data on the fat content and the cholesterol
concentration obtained by gas chromatography and the proposed method