Copyright © Tony Burfield May 2005.
The combination of these two individual
instrumental techniques (Gas Chromatography and Mass Spectrometry) has
undoubtedly improved the quality and quantity of information available
about the composition of volatile components of many common essential
oils over the last few decades. It is becoming a standard tool (together
Thin Layer Chromatography and Infra-Red analysis) in the protocols for
essential oil analysis in many national pharmacopoeia’s, for example.
A basic description of the technique can be found in standard works such
as “The Chemistry of Essential Oils” by David Williams (Micelle
What the oil salesmen don’t tell
Aromatherapists should not be too over-awed by the claims of essential
oil traders, to the effect that GC-MS is the ultimate analytical tool
(“which tells you everything about an oil”). This simply is
not true. When properly used it is a certainly a powerful technique, but
when used sloppily by untrained operators, the interpretation of results
may be of limited value. Aromatherapist customers of oil traders should
try to learn to interpret the data presented for themselves, and be able
to critique the findings and take matters up with the supplier, if necessary.
Some of the limitations include the following
1 That the best GC detectors are
still inferior to the discriminatory ability and sensitivity of the
human nose. Many eluting compounds from GC columns can be distinguished
(and sometimes identified by a trained nose) when the detector does
not pick up any signal whatsoever. This is especially noticeable at
present where prevailing adverse weather conditions, over-logging etc.
in SE Asia, China, Indonesia etc. have lead to the water-logging of
some areas causing aromatic raw materials to rot: when distilled, the
corresponding oils smell disgusting
( the odour will not “air-off”) – but the standard
GC-MS runs looks fine!
2 The electronic detector does not respond uniformly to presented
materials, and for accurate determinations, has to be calibrated against
a ‘pure’ standard for each detected component. As you can
imagine this involves a lot of work. ‘Pure’ standards (where
available), as purchased from chemical companies standards, are often
not that pure – for example the purest analytical spec. citronellal
(major component of Eucalyptus citriodora etc.) may contain polymeric
material and have to be re-purified in the laboratory before use. Few
aroma trade analysts carry out this accurate & exacting work.
3 The mass spectra of many aromatic & terpinic substances
are quite similar, and so the interpretation of the spectra of unknown
materials may present a degree of challenge, or uncertainty, regarding
their correct identification (this becomes less of a problem where commonly
used essential oils are analysed routinely, and components are identified
by a standard “fingerprint”. However things can go wrong
even in this situation - the presence of two unusual minor unsaturated
paraffins (i.e. in the dissolved rose waxes) in a genuine rose oil caused
an egg-on-the face situation for an relatively inexperienced trade analyst
4 Running a standard analytical method protocol may not reveal
the presence of all the components present. For example, although it
is perfectly possible to reveal the presence of essential oil additives
such as mineral oils, vegetable oils, polyethylene glycols etc. by using
an appropriate methodology, these can be missed in a standard run (although
clues should be present – the total peak area should be diminished).
5 Some nitrogen and sulphur containing materials in essential
oils may require special detectors to reveal their presence (these are
often powerfully odoured materials).
6 The best GC-MS results still depend on the analyst’s
skills of interpretation, and adulteration is still rife in the essential
oil industry. Some recent (and largely unnoticed) fiddles have included
passing off cineol-rich Cinnamomum camphora fractions as Eucalyptus
globulus oil, mixing in illegally traded East African Sandalwood oil
from Osyris lanceolata with East Indian Sandalwood oil Santalum album
(both threatened/vulnerable species), mixing in Spinach absolute from
Spinacia oleracea with Violet Leaf absolute from Viola odorata, and
adding synthetic anethole to Aniseed oils (Illicium verum/Pimpinella
7 The GC-MS results may tell you little about the quality or
age of the oil. As oils age, they are subject to deterioration by both
oxidation and resinification. Hydroperoxide formation in the terpene
constituents of essential oils has been identified as a causative factor
in some types of sensitivity when oils have been applied to the dermis.
However these compounds often decompose on the GC column and are not
detected (although they can be estimated by other methods). Resinification
is an almost completely neglected area – the dimers, trimers and
polymers of essential oil components are almost never revealed in a
standard GC-MS analysis, although they may be present in significant
quantities in the oil.
I hope this helps to dispel any ‘trade
hype’ about GC-MS as a technique. Remember too, the common sense
approach if you are an essential oils customer presented with a GC-MS
print-out. Does this data relate to the batch number my order came from?
Is it up to date? (one prominent UK trader distributes GC-MS traces which
are several years old!). Does it show indicators of purity for the essential
oil, or just a few major components?