Application of vibrational spectroscopy in the quality assessment of Buchu oil obtained from two commercially important Agathosma species (Rutaceae)

• Published in: South African journal of botany Vol. 76; no. 4; pp. 692 - 700
• Main Authors: Sandasi, M., Kamatou, G.P.P., Baranska, M., Viljoen, A.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2010
• Subjects: Agathosma betulina; Agathosma crenulata; Chemometrics; Essential oils; MIR; NIR; OPLS-DA; PLS regression; Raman; South Africa; Agathosma crenulata; PLS regression; Essential oils; NIR; MIR; Agathosma betulina; OPLS-DA; Raman; Chemometrics
• ISSN: 0254-6299; 1727-9321
• DOI: 10.1016/j.sajb.2010.07.013

Quality assessment of natural raw materials and derived consumer products is often done using conventional analytical techniques such as liquid and gas chromatography which are expensive and time consuming. This paper reports on the use of vibrational spectroscopy techniques as possible alternatives for the rapid and inexpensive assessment of the quality of ‘buchu oil’ obtained from two South African species; Agathosma betulina and Agathosma crenulata belonging to the Rutaceae family. Samples of A. betulina (55) and A. crenulata (16) were collected from different natural localities and cultivation sites in South Africa. The essential oil was obtained by hydrodistillation and scanned on Near infrared (NIR), mid infrared (MIR) and Raman spectrometers. The spectral data obtained was processed using chemometric techniques and orthogonal partial least squares discriminant analysis (OPLS-DA) was used to clearly differentiate between A. betulina and A. crenulata. The OPLS-DA technique also proved to be a useful tool to identify wave regions that contain biomarkers (peaks) that contributed to the separation of the two species. The three spectroscopy techniques were also evaluated for their ability to accurately predict the percentage composition of seven major compounds that occur in A. betulina ‘buchu’ oil. Using GC–MS reference data, calibration models were developed for the MIR, NIR and Raman spectral data to predict/profile the major compounds in ‘buchu oil’. A comparison of the three spectroscopy techniques showed that MIR together with PLS algorithms produced the best model (R 2X = 0.96; R 2Y = 0.88 and Q 2Ycum = 0.85) for the quantification of six of the seven major oil constituents. The MIR model showed high predictive power for pseudo-diosphenol (R 2 = 0.97), isomenthone (R 2 = 0.97), menthone (R 2 = 0.90), limonene (R 2 = 0.91), pulegone (R 2 = 0.96) and diosphenol (R 2 = 0.85). These results illustrate the potential of MIR spectroscopy as a rapid and inexpensive alternative to predict the major compounds in buchu oil. ► Vibrational spectroscopy is shown to be a reliable alternative method in the quality assessment of buchu oil. ► MIR was the best method compared to NIR and Raman spectroscopy for discrimination of Agathosma species and producing the best calibration model. ► Agathosma crenulata was characterised by a high level of pulegone, while A. betulina is dominated by diosphenol, pseudo-diosphenol, isomenthone and limonene.