In general, as the mixture is too complex, the identification and
quantification of wax esters involve a preliminary fractionation of the neutral lipid
fraction from the source studied on a column of silicic
acid.
The neutral lipid fraction is then separated into classes on another silicic acid column.
The glass column is prepared with 1g (for 10-20 mg lipids) of silicic acid suspended in
dichloromethane/hexane (2/3, v/v).
The lipid sample is dried and dissolved in the same solvent (1 ml) and placed on top of
the packing. When the solvent is drained, wax esters are eluted by the same solvent (the
needed volume is to be determined according to the lipid mass and the wax composition,
about 5-10 ml).A larger volume elutes triglycerides.
A pure wax fraction may be isolated by a further TLC in toluene or xylene (Rf: 0.65-0.70)
and identified with authentic samples.
Another TLC system may be used to isolate was esters from several chemical
derivatives : silica gel plates are developed with a mixture of hexane/diethyl
ether (94/6, v/v). Wax had a Rf of about 0.64 (Graille J et al., JAOCS 1986,
63, 111).
To analyze the wax content of vegetable oils, a simple procedure of purification
based on liquid chromatography on a double-adsorbent layer of silica gel and
silver nitrate-impregnated silica gel was proposed (Hénon G et al., JAOCS
2001, 78, 401). The hydrocarbons and waxes were then analyzed by gas
chromatography.
Analysis of wax esters
1. It is sometimes advantageous to fractionate
wax esters into individual molecular species according to chain length using GLC with
capillary column (SE 30 or OV1 type) at high temperature (between 250 and 320°C). The column is
standardized for chain length identification with synthetic esters covering the desired
range. An example of gas chromatogram of jojoba wax esters is given below.
2. Wax esters may be transformed into fatty acid ethyl esters and fatty alcohols
by an acid catalyzed ethanolysis which is complete after refluxing 1.5 hr with
an anhydrous solution of 1N HCl in ethanol (made in adding acetyl chloride in
ethanol).
After recovery in hexane, the mixture is fractionated by TLC or by silicic column chromatography
with an elution by dichloromethane/ether (35/1, v/v) which purifies the fatty
acid derivatives and an
elution by dichloromethane/ether (9/1, v/v) which purifies the residual wax alcohols.
Analysis of the fatty acid ethyl esters are performed as usual by GLC on a polar column.
Analysis of fatty alcohols is performed by GLC on the same column either directly or
better as acetate or trimethylsilyl esters.
A reliable and simple method to analyze the composition of jojoba wax by column capillary
gas chromatography has been described (Graille J et al., JAOCS 1986, 63, 111).
The analysis of the wax ester fraction of vegetal oils by gas chromatography and
mass spectrometry was also described in details (Reiter B et al., JAOCS 2001,
78, 881). The use of high temperature gas chromatography and mass
spectrometry allowed the detection and identification of a wide range of
components which was needed for the characterization of several waxes used in
works of art (Regert M et al., J Chromatogr A 2005, 1091, 124).
Characterization of waxes used in pictorial artworks was also carried out by
chromatographic analysis of fatty acids and hydrocarbons (Peris-Vicente
J et al., J Chromatogr A 2006, 1101, 254).
A simple procedure for the determination of waxes present in vegetal oils (from
34 to 46 carbon atoms) has been described (Perez-Camino MC et al., J
Chromatogr A 2003, 983, 283). It involves a solid-phase extraction on
silica-gel cartridges and a separation by GLC on a dimethyl -
diphenylpolysiloxane capillary column. The retention data of higher wax esters
within the range C24 to C44 were calculated after gas chromatography, thus
allowing the determination of number of carbon atom and the number and position
of double bonds in acid and in alcohol moieties of esters (Stransky K et al.,
J Chromatogr A 2006, 1128, 208).
A more complex equipment is needed when complex mixtures must be analyzed. Thus,
the wax ester fraction of
edible oils were analyzed by on-line NPLC-GC-MS and by comprehensive
two-dimensional GC with flame ionization detection (GC6GC-FID) off-line combined
with NPLC-GC (Biederman M et al., Eur J Lipid Sci Technol 2008, 110, 1084).
GC6GC-FID enables to group the various classes of wax esters, in particular the
phytol esters, geranylgeraniol esters and the straight-chain esters of palmitic
acids and the unsaturated C18 acids.
Wax esters of various natural sources have been extensively reviewed:
Nicolaides N et al., Lipids 1970, 5, 299
Kolattukudy PE (Ed.) Chemistry and biochemistry of natural waxes, Elsevier, 1976.
Lee RF et al., Alcohols and waxes, in Marine biogenic lipids, fats and oils. (Ackman RG
ed), vol.I, 73-102, CRC Press Inc 1989.
