Because in foods (246). A highly sensitive method

Because of
some drawbacks of TLC, a novel SPE method has been developed to separate
4-desmethyl-, 4-monomethyl-, and 4,4-dimethyl sterols in vegetable oils, which
is shown in Figure 13 (170). After separation of different sterol
fractions by TLC or SPE, they are analysed by GC to obtain their composition
and also quantity. Figure 14 shows separation of trimethylsilyl ether
derivatives of the purified 4,4?-dimethyl sterols of hazelnut oil and virgin
olive oil by GC.To
determine the free and esterified
sterol concentrations of different edible oils, free and esterified sterols
seperated by SPE, and then saponified, and quantified as trimethylsilyl ether
derivatives using GC. Determination of individual values for free and
esterified ?-sitosterol,
campesterol, stigmasterol, brassicasterol, ?5-avenasterol,
sitostanol, campestanol, and cholesterol was possible (38).

GC methods are recommended for total plant sterols, where several distinct
components can be determined (e.g. campesterol, stigmasterol, sitosterol, ?5-avenasterol)
(250). Steryl classes (free
forms and conjugates) and structural groups are separated by adsorption
chromatography on the basis of their polarity (256).
 GC-FID chromatogram of sterols in refined
rapeseed oil is shown in Figure 15. HPLC techniques are less often used to
separate free PS than GC, but some applications exist. One of the reasons for
limited use of HPLC may be that there is not such a general and sensitive HPLC
detector for PS as the flame ionization detector in GC and also its poor separation
quality. However, with new separation techniques and detection possibilities,
attempts have been made to apply HPLC to the analysis of free phytosterols in
foods (246).
A highly sensitive method for quantification of PS
based on HPLC has been developed by derivatization with the benzoyl
chromophore. Introduction of the chromophore, benzoyl group, to PS via
derivatization greatly improved the UV response at 254 nm. Quantification of PS
was effectively performed by HPLC analysis with methyl benzoate as the internal
standard after derivatization (258).

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