Adulteration can be found in several industries. In the agricultural sector, for example, mixing low-grade crops with premium ones or adding artificial colors to enhance the appearance of fruits are common adulteration practices. In the realm of luxury goods, counterfeit products made of subpar materials often deceive unsuspecting consumers seeking genuine articles. Particularly with liquid articles which are often easier to adulterate such practices are quite common. So it comes at no surprise that also the essential oil industry suffers a vast adulteration challenge. Experts are estimating the amount of adulterated product on the market as high as 80%.
While it is maybe not too important to go into too much of detail, all professionals in the essential oil business should understand which speciality testing methods exist and have a basic idea about how they work.
What is the Standard in Testing?
The oldest Method of quality control for essential oils is called organoleptic testing. This refers particularly to appearance, colour and odour of an oil. Instrumental testing includes certain tests like the Refractive Index, which measures how light rays bend while passing through a substance. Also Optical Rotation which is the direction of rotation of light rays while passing through a substance and a series of other standard tests, which we are speaking about in more depth in the blog article ‘Physiochemical Testing for Essential oils’. Today’s Standard Testing also includes routine GC–MS testing.
More specialised Tests on the Market
GC-IRMS
GC-IRMS is an abbreviation for Gas Chromatography Isotope Ratio Mass Spectrometry and is particularly utilised to determine the relative abundance of isotopes within a given sample to analyse the ratios of light stable isotopes of carbon, hydrogen, nitrogen, or oxygen within individual compounds of often complex mixtures. For such testing the compounds in the sample must be volatile and thermally stable. Samples are separated into individual hydrocarbons by gas chromatography after which the output is introduced into an isotope ratio mass spectrometer, to determine isotope ratios of individual compounds through combustion and catalytic conversion. The advantages of conducting compound-specific isotope measurements are quite persuasive despite the process being labor-intensive and expensive.
SNIF-NMR
NMR is an abbreviation for Nuclear Magnetic Resonance while NIF stands for Natural Isotopic Fractionation. The method enables precise quantification of isotopic variations at each site of a molecule for in depth isotopic analysis. It allows for the measurement of specific natural isotope fractionation occurring at various positions within the molecule. It was specifically developed to identify adulteration techniques of sugaring in wine in the early 80s and has been used for essential oil testing since the early 90s.
When chemically produced, nature-identical molecules are used to adulterate essential oils as cheaper substitutes for natural flavours, various Isotopic Methods (such as 13C, 2H, and 14C) have emerged. While simpler and more common methods like 14C counting and 13C SIRA analyses can be easily circumvented by adulterators through the addition of isotopically enriched compounds, the SNIF-NMR method has proven to provide a reliable and non counterfeit-able analysis of natural molecules by providing isotopic ratios for each molecular position within a compound.
HPLC
Is an abbreviation for High-performance liquid chromatography (HPLC) and has become significantly important in food analysis, evident from its extensive range of applications. It is utilised for the separation, identification, and quantification of individual components within a mixture. Pressurised liquid solvent containing the sample is passed through a column filled with a solid adsorbent material by a pump. As the individual components within the sample interact differently with the adsorbent material, they exhibit distinctly different flow rates, which allows them to separate as they exit the column.
HPTLC
Is an abbreviation for High-Performance Thin-Layer Chromatography and is also an excellent alternative or complement to GC (Gas Chromatography) for testing essential oils' authenticity. While not all potential components of essential oils like menthofurane, camphene, a-pinene, g-terpinene or β-pinene are detectable with this method, it increases the chance to detect adulteration.
It is an improved method to TLC (Thin Layer Chromatography) in which a plate consisting of a non-reactive solid layer coated with a thin film of adsorbent material, is used. The sample is applied to the plate and then subjected to elution using a solvent or solvent mixture known as the mobile phase. The solvent travels up the plate through capillary action. As in any chromatography process, certain compounds exhibit stronger attraction to the mobile phase, while others favor the stationary phase. As a result, various compounds ascend the TLC plate at different rates, leading to their separation.
The primary distinction between HPLC and HPTLC lies in their capabilities for component separation in a sample. HPLC enables quantitative separation of components, whereas HPTLC does not.
GC×GC
Is an abbreviation for Comprehensive Two-dimensional gas chromatography and is a multidimensional gas chromatography technique. In GC×GC, two different columns with distinct stationary phases are employed. The effluent from the first dimension column is directed to the second dimension column through a modulator. The modulator rapidly traps and then "injects" the effluent from the first dimension column onto the second dimension. This process results in the creation of a retention plane that combines the 1st dimension separation with the 2nd dimension separation.
GC×GC overcomes several limitations associated with conventional multidimensional gas chromatography when analysing highly complex samples or samples with overlapping peaks of different polarity.
In one study the analysis of peppermint (Mentha piperita) and spearmint (Mentha spicata) was achieved. One-dimensional gas chromatography and two-dimensional gas chromatography were compared. Through GC×GC testing, peppermint essential oil revealed 89 identifiable peaks, while only 30 peaks were observed in the GC chromatogram. Similarly, spearmint essential oil exhibited 68 peaks in the GC×GC chromatogram, compared to 28 peaks in GC. 52 common compounds between the two oils using GC×GC were identified, in contrast to only 18 matches obtained by one-dimensional GC. (1)
IRMS
Is an abbreviation for Isotope-ratio mass spectrometry and is a specialised form of mass spectrometry that utilises various mass spectrometric techniques to determine the relative abundance of isotopes in a given sample.
Two significant advantages of isotope-ratio mass spectrometry are its ability to be configured for multiple-collector analysis and its capacity to produce high-quality 'peak shapes'. These attributes play a crucial role in enabling isotope-ratio analysis at exceptionally high levels of precision and accuracy.
Conclusion
Many of these novel and complex methods are quite expensive, however due to the increasing prevalence of adulteration in the essential oil industry they have gained significant importance. As chemical analysis techniques advance, so do the methods of adulteration, making it crucial to adopt more refined and foolproof approaches to address this issue effectively.
Sources
(1) Application of Comprehensive Two-Dimensional Gas Chromatography (GC×GC) to the Qualitative Analysis of Essential Oils Jean-Marie D. Dimandja, Stephen B. Stanfill, James Grainger, Donald G. Patterson Jr.