A critical evaluation of Raman spectroscopy for the analysis of lipids: Fatty acid methyl esters

Abstract  The work presented here is aimed at determining the potential and limitations of Raman spectroscopy for fat analysis by carrying out a systematic investigation of C4−C24 FAME. These provide a simple, well-characterized set of compounds in which the effect of making incremental changes can be studied over a wide range of chain lengths and degrees of unsaturation. The effect of temperature on the spectra was investigated over much larger ranges than would normally be encountered in real analytical measurements. It was found that for liquid FAME the best internal standard band was the carbonyl stretching vibration ρ(C=O), whose position is affected by changes in sample chain length and physical state; in the samples studied here, it was found to lie between 1729 and 1748 cm−1. Further, molar unsaturation could be correlated with the ratio of the ρ(C=O) to either ρ(C=C) or δ(H−C=) with R 2>0.995. Chain length was correlated with the δ(CH2)tw/ρ(C=O) ratio, (where “tw” indicates twisting) but separate plots for odd- and even-numbered carbon chains were necessary to obtain R 2>0.99 for liquid samples. Combining the odd- and even-numbered carbon chain data in a single plot reduced the correlation to R 2=0.94–0.96, depending on the band ratios used. For molal unsaturation the band ratio that correlated linearly with unsaturation (R 2>0.99) was ρ(C=C)/δ(CH2)sc (where “sc” indicates scissoring). Other band ratios show much more complex behavior with changes in chemical and physical structure. This complex behavior results from the fact that the bands do not arise from simple vibrations of small, discrete regions of the molecules but are due to complex motions of large sections of the FAME so that making incremental changes in structure does not necessarily lead to simple incremental changes in spectra.