UV Spectrophotometry with Chemometric Analysis as a Means to
Authenticate Thyme (Thymus vulgaris)
Reviewed: Gad HA, El-Ahmady SH, Abou-Shoer MI,
Al-Azizi MM.
A
modern approach to the authentication and quality assessment of thyme using UV
spectroscopy and chemometric analysis. Phytochem Anal.
2013;24(6):520-526.
Thyme is one of the botanicals where a number of
different species are accepted as having medicinal benefits. The thyme
monograph of the European Pharmacopoeia (Ph. Eur.)
lists two species, thyme (Thymus vulgaris,
Lamiaceae) and Spanish thyme (T. zygis), as
acceptable sources. In addition, wild thyme (T. serpyllum)
has its own monograph in the Ph. Eur.1 According to the Handbook of herbs and spices, references
to thyme usually mean T. vulgaris, but
most thyme traded is a mixture of T.
capitatus, T. serpyllum, and
T. vulgaris.2 To make matters worse, a number of chemotypes and genetic
variations exist that cause changes in the chemical composition and provide a
challenge for quality control of Thymus species. The authors indicate that
adulteration of T. vulgaris with many other similar herbs either belonging to Lamiaceae
(mint family)
or
other families is common and still unresolved in the Egyptian market where the
commercial samples were obtained.
The UV spectra (200-400 nm) of
methanol extracts from 30 authenticated samples of T. vulgaris,
and samples of T. serpyllum, T. capitatus, Origanum syriacum
(Lamiaceae), Satureja montana (Lamiaceae), Plectranthus
amboinicus
(Lamiaceae), and Eriocephalus
africanus
(Asteraceae), were used to build up a library. The absorption readings obtained
over the spectral points of all the samples were converted into a data matrix
using Microsoft Excel software with the spectral
points as variables represented by columns and the corresponding spectral
absorption measurements represented by rows. This data matrix was then subjected
to statistical evaluation using principal component analysis (PCA) and
hierarchical cluster analysis (HCA). In the model construction, PCA was
followed by the supervised technique of soft independent modeling of class
analogy (SIMCA) for confirmation. A cluster analysis of the 12 commercial samples from
the Egyptian markets resulted in nine samples clustering around the
authenticated T. vulgaris samples, while two
samples clustered around winter savory (S. montana).
The last sample was not close to any of the authenticated plant clusters and
was considered a non-thyme species of unknown identity.
Comment:
UV/Vis spectrophotometry may not be an obvious choice for identity testing of
botanicals since it lacks the resolving power of many other techniques.
However, the results of this study suggest that it can be a viable option if
the parameters are correctly chosen and an adequate statistical evaluation is
performed. The
discriminatory power of UV/Vis could be improved if second derivatives of the
spectra are calculated. Spectra that are apparently similar may reveal
significant differences in the derivative mode, which is useful for
identification. Based on a discussion with one manufacturer in the
United States, UV/Vis with subsequent statistical analysis is already in use
for authentication of botanicals. This technique could prove to be a low-cost
alternative to infrared or near-infrared spectroscopy for companies with a very
tight budget. Still, it is unclear to what degree the method can detect
admixture of an adulterant; for example, how much winter savory can be added to
the thyme before the method is capable of picking it up. As with all
spectrophotometric methods, the key to using it in a scientifically valid
manner for identification purposes is to have a large enough number of samples
of the varying species. This allows to determine both intra- and interspecies
variations and to capture the inherent variability in the target and
adulterating plants. For correct identification, multivariate statistical data analysis
needs to be applied.
References
1. European Directorate for the
Quality of Medicines & Healthcare. European Pharmacopoeia (EP 7.4). Strasbourg, France: Council of
Europe; 2011.
2. Muggeridge
M, Clay M. Quality
specifications for herbs and spices. In: Peter KV, ed. Handbook
of herbs and spices. Cambridge, UK:
Woodhead Publishing Ltd; 2001:13-21.
