Evaluation of Fourier Transform-Near Infrared Spectroscopic Measurements for the Quantification of Curcumin in Turmeric Herbal Medicines

Abstract

In this investigation, Fourier transform-near infrared (FT-NIR) spectroscopy was used to determine the curcumin content in commercial turmeric herbal medicines. Two different sample presentations originating from a turmeric capsule and powder were evaluated and compared using an FT-NIR spectrometer equipped with a 30-capsule sample plate in the wavenumber region of 11,500–6000 cm−1 (transmittance mode) and a 6-vial sample plate in the wavenumber region of 10,000–4000cm−1 (reflectance mode), respectively. One hundred and forty samples of turmeric herbal medicines were divided into two sets: a training set with 110 samples and a prediction set with 30 samples. High-quality FT-NIR spectra for the turmeric powder containing the informative regions of curcumin (6850, 6000 and 4600 cm−1) were obtained while collecting FT-NIR data on the powder samples in the glass vials. A successful partial least-squares calibration model for curcumin content in commercial turmeric herbal medicine powders was then obtained by utilising the entire spectral region (10,000–4000 cm−1) as well as a second derivative spectral pretreatment containing seven principal components. The best model for predicting curcumin in 30 test samples returned a coefficient of determination ( r2) of 0.98, a root mean squares error of prediction ( RMSEP) of 0.13% that was only 1.35 times the standard error of laboratory and a ratio of RMSEP to the standard deviation of curcumin content in the prediction set ( RPD) of 7.76. These statistical values confirmed the high predictive performance of this model. All these results confirm that the use of turmeric herbal medicine powder and a wide informative wavenumber range of 10,000–4000 cm−1 can increase the efficacy of FT-NIR spectroscopy for the rapid and non-destructive analysis of curcumin content in commercial turmeric herbal medicines. By contrast, models developed for determining curcumin in turmeric contained in capsules were not suitable for analytical purposes.