Heart-cutting GC stops citrus fakery

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  • Published: Jun 1, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Heart-cutting GC stops citrus fakery

Adulteration of citrus products is a challenge

Citrus fruits and the various products derived from them, such as their essential oils, are important commercial items, used in a vast range of foodstuffs, household goods, perfumes, and more. However, adulteration by cheaper synthetic compounds or less expensive natural extracts is common in some countries and may be hard to prove. Since some of the compounds in citrus fruits are mixtures of two enantiomers, with one favoured over the other in a constant manner, measurement of the enantiomeric excess (i.e. the difference between the percentage of the major enantiomer and that of the minor enantiomer, abbreviated as ee) provides a means of detecting adulteration. Any significant amount of adulterant will inevitably alter the ee values, so can be detected if a suitable analytical method is available.

The scientists from South Korea and Pakistan looked at detecting the ee values of four terpenoids, limonene, camphene, sabinene and β-phellandrene, in three citrus fruits: lime, lemon and yuzu (Citrus junos, a sour fruit widely used in Korea, Japan and China). To analyse the enantiomeric mixtures, heart-cutting MDGC was used. In this technique, portions of the output from one GC column are transferred onto another for further separation. In this case, a non-chiral GC column was used for the initial separation, followed by a chiral column to separate enantiomers.

Heart-cutting MDGC used to detect enantiomeric excess in citrus compounds

Citrus fruit were peeled, homogenised and mixed with deionised water, which was then adjusted to pH 6.5. n-Butyl benzene was added as an internal standard. The resulting slurry was extracted with a solvent mixture of n-pentane and diethyl ether using a simultaneous distillation-extraction (SDE) apparatus. The extract was dried with sodium sulphate and concentrated by distillation through a Vigreux column.

GC profiles for each fruit were obtained for quantitative analysis using conventional GC-MS. An Agilent 7890 GC instrument was used, with a J & W Scientific DB-5MS column. The temperature gradient went from 40 to 250 °C in a series of ramps. Mass spectroscopy was carried out with an Agilent 7000 mass spectrometer using electron ionisation (EI). The profiles for the three types of fruit were similar to those reported in the literature, with the terpenoid limonene being the most abundant compound in all three fruits.

MDGC was then carried out using an Agilent double oven MDGC-MS. The initial separation employed a Phenomenex RTX-5MS column, which was non-chiral, with a gradient from 40 to 200 °C. A flame ionisation detector (FID) was used to detect the compounds, with those of interest being taken onto the chiral column (heart-cutting), a Supelco β-Dex™ 225, which was run with a gradient from 60 to 200 °C. Final detection by mass spectrometry gave the ee values, with the pairs of enantiomers being clearly separated by the second column.

The ee values for the four compounds of interest, limonene, camphene, sabinene and β-phellandrene, were determined for each of the three types of fruit. The ee values ranged from 59% to almost 100%. They were in line with previous literature values where available.

Heart-cutting MDGC gives ee values in complex mixtures

The developed MDGC method allows for the accurate determination of ee values, which are a powerful means of detecting adulteration of citrus products. The technique allows for accurate ee measurements in cases where a single chiral column would be unlikely to succeed, since other peaks would probably interfere with one or other of the two enantiomeric peaks. The technique could also be applied to many other complex mixtures of natural products.

Related Links

Phytochemical Analysis, 2017, Early View Paper. Hong et al. Determination of volatile flavour profiles of Citrus spp. fruits by SDE-GC–MS and enantiomeric composition of chiral compounds by MDGC–MS.

Flavour and Fragrance Journal, 2001, 16, 136-148. Chaintreau. Simultaneous distillation–extraction: from birth to maturity—review.

Analytica Chimica Acta, 2012, 716, 66-75. Tranchida et al. Heart-cutting multidimensional gas chromatography: A review of recent evolution, applications, and future prospects.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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