The quantification of citral in lemongrass and lemon oils by near-infrared spectroscopy.

Previous work has demonstrated the capability of near-infrared (NIR) spectroscopy to determine the cineole content (not less than 70% w/w) of eucalyptus oil with an accuracy comparable with that of the British Pharmacopoeia (BP) assay method. The aim of the present study was to determine if the same method was capable of quantifying other chemical constituents at similar levels in essential oils and also to ascertain if NIR spectroscopy can accurately quantify compounds present at much lower levels in essential oils. Lemongrass oil contains citral at concentrations of approximately 65-85% w/w, and lemon oil contains citral at a concentration of approximately 2-5% w/w. A total of 26 samples of pure lemongrass oil and 35 samples of pure lemon oil (both including samples that were "spiked" with citral to increase the calibration range) were scanned on the FOSS NIRSystems 6500 Rapid Content Sampler using a reflectance vessel as sample presentation method. The reference method for both types of oil was the BP monograph titration assay for the citral content of lemon oil and calibrations were constructed using these reference data. For the lemongrass oils, the mean accuracy was found to be 1.00% or less and the mean bias was 0.09% or less. For the lemon oils, the mean accuracy was found to be 4.28% or less and the mean bias was -0.71% or less. The NIR method developed was rapid, simple and non-destructive and may prove beneficial for the accurate determination of the citral content of lemongrass oils and for the approximate citral content of lemon oils.