Carbon isotope ratio analysis of authentic and commercial essential oils of lemon balm

Abstract

Several commercial and authentic essential oils of lemon balm (Melissa officinalis L.) were investigated for the carbon isotope ratio of their main compounds (citral as its enantiomers geranial and neral, citronellal, β-caryophyllene, germacrene D and caryophyllene oxide) by GC-C(ombustion)-IRMS. Authentic samples of lemon balm were distilled in 2003, 2004 and 2007. In addition, commercially available samples of the individual main compounds of the oil with proclaimed synthetic and natural origins were investigated for their carbon isotopic composition by GC-C-IRMS and EA-C-IRMS in order to compare the investigated carbon isotopic values. All oils were analyzed via GC-MS and GC-FID to enable the identification of the compounds in the GC-C-IRMS chromatograms and to investigate the quantity of the single essential oil compounds. Data were interpreted regarding suitability for authenticity assessment of the analyzed essential oil samples. Synthetic citral showed δ13CPDB values of -28.68 and -29.00‰, whereas the natural labelled standard showed a value of -26.89‰. For the authentic oils, δ13CPDB values between -29.36 and -24.54‰ were determined. The commercial lemon balm oil samples indicated δ13CPDB values between -27.72 to -24.04‰ for citral. The synthetic citronellal standard showed a δ13C PDB value of -28.77 and the natural standard -12.91‰. The value of -12.91‰ of the natural citronellal standard indicates, that it was possibly produced by the distillation of a C4 plant (e.g. citronella), because the δ13CPDB value is more positive than usual for C3 plants (lemon balm). The same applies to four commercial lemon balm oils with δ13CPDB values for citronellal of -15.04 to -12.24‰. Authentic samples of lemon balm oils presented δ13CPDB values of -32.15 and -30.59‰ for citronellal. For the three minor compounds, β-caryophyllene, germacrene D and caryophyllene oxide, no clear differentiation could be made due to the lack of standards and the minor concentration of the compounds. The determination of the isotopic ratio of the stable carbon isotopes by GC-C-IRMS is a very powerful tool in authenticity assessment of essential oils for differentiation between C3 and C4 plants. Four of the investigated 19 lemon balm oils could be identified as oils of C4 plants (e.g. citronella oils), although they were labelled as lemon balm oils.