A chemometric approach for the quantification of free fatty acids in cottonseed oil by Fourier transform infrared spectroscopy

Figures & data

Table 1. FTIR spectral regions of cottonseed oil.

Identification number	Range cm^−1	Functional group	Nominal frequency	Mode of vibration
1	3029–2989	= C – H (cis)	3010	Stretching
2	2989–2946	–C – H (CH3)	2954	Stretching (asym)
3	2946–2881	–C – H (CH2)	2921	Stretching (asym)
4	2881–2782	–C – H (CH2)	2852	Stretching (sym)
5	1795–1677	–C = O (ester)	1743	Stretching
		–C = O (acid)	1710	Stretching
6	1670–1620	–C = C (cis)	1655	Stretching
7	1486–1446	–C – H (CH2)	1465	Bending (scissoring)
8	1446–1425	–C – H (CH3)	1456	Bending (asym)
9	1425–1409	=C – H (cis)	1416^[Citation27]	Bending (rocking)
10	1382–1371	–C – H (CH3)	1377^[Citation27]	Bending (sym)
11	1290–1211	–C – O –CH2–	1238^[Citation27]	Stretching
				Bending
12	1211–1147	–C – O	1160 [27]	Stretching
		–CH2–		Bending
13	1128–1106	–C – O	1117^[Citation27]	Stretching
14	1106–1072	–C – O	1098^[Citation27]	Stretching
15	1006–929	–HC = CH– (trans)	967^[Citation27]	Bending (out of plane)
16	929–885	–HC = CH– (cis)	916^[Citation27]	Bending (out of plane)
17	885–802	=CH2	843^[Citation28]	Wagging
18	754–701	–(CH2)n–	721^[Citation27]	Rocking bending (out of plane)
		–HC = CH– (cis)

Figure 1. FTIR spectra of cottonseed oil. A: containing high FFAs; and B: containing low FFAs.

Table 2. Prediction capabilities of PLS-FTIR and PCR-FTIR measurement model for the determination of FFA.

Parameters	PLS	PCR
Spectral region	1727–1690 cm^−1	1727–1690 cm^−1
Baseline type	None	Two points fixed-location
Factors/principal component	4 F	3 PC
Validation standards	3	3
RMSEC	0.467	0.904
RMSEP	0.266	0.654
R^2	0.9992	0.9973
Performance index (PI) of model	98.9	97.2

R² is a correlation coefficient of actual and calculated values of free fatty acids concentration in the calibration set.

RMSEC: root mean square error of calibration; RMSEP: root mean square error of prediction; F: factors; PC: principal component; PI: performance index.

Figure 2. Plot of actual FFAs versus predicted FFAs by FTIR A: PLS model; and B: PCR model.

Figure 3. Oleic acid spiked group spectra of calibration standards.

Table 3. FFAs content of cottonseed oil by titrimetric and FTIR (PLS and PCR).

Cottonseed oil varietiesVarieties	% FFAs titration	% FFAs FTIR
		PLS	PRD	PCR	PRD
Variety 1	23.7 ± 0.86	23.8 ± 0.8	–0.42	24.74 ± 0.51	–4.20
Variety 2	38.8 ± 1.10	38.5 ± 1.05	0.78	40.08 ± 0.91	–3.19
Variety 3	35.6 ± 1.05	35.4 ± 0.95	0.56	37.08 ± 1.0	–3.99
Variety 4	17.3 ± 1.11	17.1 ± 0.96	1.17	18.01 ± 0.98	–3.94
Variety 5	18.6 ± 0.88	18.3± 0.47	1.64	19.54 ± 0.54	–4.81

Percentage relative difference (PRD) = (Titration − FTIR) /FTIR × 100.

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