Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication
Abstract
:1. Introduction
2. Chemical Classification of Essential Oil Components
2.1. Biosynthesis of Terpenes
2.2. Terpenes and Meroterpenes
2.3. Biosynthesis of Phenylpropanoids
2.4. Phloroglucinols and Phenylpropanoids
2.5. Parent ‘Skeleton’ and Character of Oxidation
2.6. Less Common and Rare Components
2.7. Colour and Viscosity of Essential Oils
3. Stereochemistry and Isomerism in Essential Oils
3.1. Diastereomers
3.2. Enantiomers (Chirality)
3.3. Fundamentals of Chirality (Enantiomers)
4. Chemical Analysis of Essential Oils
4.1. Gas Chromatography
4.2. Gas Chromatography Stationary Phases (Columns)
4.3. Mass Spectrometric Identification by Comparing to a Mass Spectral Library
4.4. What Name to Use from the NIST Search Results
- (1)
- Common name, which is the old convention where the person who first discovered the molecule typically named it with etymology related to the species from where it was isolated. For example, pinene was isolated from an essential oil produced from a member of the genus Pinus, and the ‘-ene’ in the name represents a double bond in the molecule.
- a.
- Common names are habitually provided with a stereochemical descriptor, i.e., α-pinene, γ-eudesmol, or δ-cadinene. As previously mentioned, these ‘isomers’ are usually a consequence of the position of a double bond, but on occasion they can also represent epimers, or an isomer that is not an enantiomer.
- b.
- These achiral isomers have slightly different mass spectral fragmentation patterns (signatures) and they also elute with different arithmetic indices, so they are usually validated by triangulation of these two metrics (arithmetic index and NIST match) with retention times of confirmed compounds eluting nearby on the chromatogram (within close retention times).
- c.
- A common name is usually short, and has no numbers in it, just English letters that are often combined with the Greek alphabet.
- (2)
- Common name hybridised with IUPAC nomenclature, which is a larger descriptor that is built on top of a common name.
- a.
- For example, germacrene D-4-ol, terpinen-4-ol, or methoxymyodesert-3-ene [18,80]. These are molecules that are very similar to another molecule that received a common name but are distinguished by a hydrogen deficiency (a double bond) or oxidation (an alcohol group or ketone, etc.). A number is used to specify the carbon number in the molecule where the difference occurs, relative to the namesake compound.
- b.
- Numeration of the derivatives of common name compounds does not follow IUPAC rules, it remains consistent with the original common name, i.e., methoxymyodesert-3-ene and myodesert-1-ene are numerated according to myodesertene, even though the numeration changes according to IUPAC rules.
- (3)
- Common name or hybrid common name that is given an enantiomeric descriptor.
- a.
- When the NIST library was created, authentic reference standards were used to build the database of mass spectral fragmentation patterns.
- b.
- It was common for authentic standards to be identified to the exact enantiomer, so when the reference was recorded the enantiomeric descriptors were included in the name, which was saved to the database.
- c.
- The enantiomeric descriptors, such as the plus sign ‘+’ or the R or S descriptors, are an artefact of the data entry process, when the data was being compiled into a library of mass spectral data (such as the NIST library).
- d.
- Enantiomeric descriptors, such as (+)-α-pinene, also known as 1R,5R-α-pinene, are determined using analytic methods, such as polarimetry or chiral GC, but are not determined using routine GC-MS.
- e.
- Thus, when a NIST match suggests a molecule with an enantiomeric descriptor, such as (+)-α-pinene, also known as 1R,5R-α-pinene, it is important to delete the descriptor and claim only the common name, or the hybrid common name, i.e., α-pinene, not (+)-α-pinene, or 1R,5R-α-pinene. This is because it is impossible to be this specific using only GC-MS.
- f.
- From a NIST library match, delete R or S, D or L, d- or l-, + or −, and simplify to the common or IUPAC (or systematic) nomenclature.
- (4)
- IUPAC nomenclature (also known as systematic nomenclature) is a convention established to standardise chemical names for an international audience. The long name is ‘International Union of Pure and Applied Chemistry’. The IUPAC nomenclature was established when common names became too frequent, causing some names to be used twice to describe different things.
- a.
- For example, the common name brevifolin can mean two different molecules, either brevifolin (geranium) which is 7,8,9-trihydroxy-1,2-dihydrocyclopenta[c]isochromene-3,5-dione in IUPAC nomenclature, or brevifolin carboxylic acid, which is 1-(2-Hydroxy-4,6-dimethoxyphenyl)ethan-1-one in IUPAC nomenclature.
- b.
- In IUPAC nomenclature, α-pinene is called 2,6,6-Trimethylbicyclo[3.1.1]hept-2-ene, and with an enantiomeric descriptor it is (1S)-, or (1R)-2,6,6-Trimethylbicyclo[3.1.1]hept-2-ene.
- c.
- Evidently the enantiomeric descriptor needs to be removed, i.e., delete (1S)-, or (1R)- and keep the remainder of the name, unless other work was done to confirm the chiral identity.
4.5. Calculation of Arithmetic Indices
- RT is retention time, i.e., RTa is retention time of n-alkane eluting before z,
- z is the essential oil component,
- a is the carbon number of the n-alkane eluting before z,
- b is the carbon number of the n-alkane eluting after z.
4.6. Other Techniques in Chromatographic Analysis
5. Authentication of Essential Oils
5.1. Analytical Methods Used for Authentication of Essential Oils and Natural Volatiles
5.2. Simplistic Methods for Authentication of Essential Oils
5.2.1. UV Absorbance Determination Using Spectrophotometry
5.2.2. Evaporation Ability
5.2.3. Thin Layer Chromatography
6. Suggestions and Concluding Remarks
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Non-Polar Columns | Polar Columns |
---|---|
ZB-1; DB-1; OV-1; SE-30; PB-1; OV-101; DB-5; DB-5MS; HP-5MS; BP-1; SPB-5; BPX-5; RTX-1 | PEG-20M; PEG 4000; Carbowax 20M; Carbowax 4000; HP-Wax; DB-Wax; Supelcowax; Supelcowax-10; Innowax |
Symbol | Cell ID | Excel Formula |
---|---|---|
RTz | A2–A13 | Retention time value obtained experimentally from GC-MS chromatogram |
RTa | B2–B13 | =@IF(A2 > H$13,H$13,IF(A2 > H$12,H$12,IF(A2 > H$11,H$11,IF(A2 > H$10,H$10,IF(A2 > H$9,H$9,IF(A2 > H$8,H$8,IF(A2 > H$7,H$7,IF(A2 > H$6,H$6,IF(A2 > H$5,H$5,IF(A2 > H$4,H$4,IF(A2 > H$3,H$3,IF(A2 > H$2,H$2,error)))))))))))) |
RTb | C2–C13 | =IF(B2 = H$2,H$3,IF(B2 = H$3,H$4,IF(B2 = H$4,H$5,IF(B2 = H$5,H$6,IF(B2 = H$6,H$7,IF(B2 = H$7,H$8,IF(B2 = H$8,H$9,IF(B2 = H$9,H$10,IF(B2 = H$10,H$11,IF(B2 = H$11,H$12,IF(B2 = H$12,H$13,IF(B2 = H$13,H$14)))))))))))) |
a | D2–D13 | =IF(B3 = H$2,8,IF(B3 = H$3,9,IF(B3 = H$4,10,IF(B3 = H$5,11,IF(B3 = H$6,12,IF(B3 = H$7,13,IF(B3 = H$8,14,IF(B3 = H$9,15,IF(B3 = H$10,16,IF(B3 = H$11,17,IF(B3 = H$12,18,IF(B3 = H$13,19)))))))))))) |
AI | E2–E13 | =100*(D4 + (A4-B4)/(C4-B4)) |
No. | G2–G13 | Carbon number of alkane from homologous series of n-alkanes |
RT | H2–H13 | Retention time of alkane from homologous series of n-alkanes |
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Sadgrove, N.J.; Padilla-González, G.F.; Phumthum, M. Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication. Plants 2022, 11, 789. https://doi.org/10.3390/plants11060789
Sadgrove NJ, Padilla-González GF, Phumthum M. Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication. Plants. 2022; 11(6):789. https://doi.org/10.3390/plants11060789
Chicago/Turabian StyleSadgrove, Nicholas J., Guillermo F. Padilla-González, and Methee Phumthum. 2022. "Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication" Plants 11, no. 6: 789. https://doi.org/10.3390/plants11060789