[F1ANNEX V U.K. DETERMINATION OF THE COMPOSITION AND CONTENT OF STEROLS AND TRITERPENES DIALCOHOLS BY CAPILLARY-COLUMN GAS CHROMATOGRAPHY
Textual Amendments
1. SCOPE U.K.
The method describes a procedure for determining the individual and total sterols and triterpene dialcohols content of olive oils and olive-pomace oils.
2. PRINCIPLE U.K.
The oil, with added α-cholestanol as an internal standard, is saponified with potassium hydroxide in ethanolic solution and the unsaponifiable matter is then extracted with ethyl ether.
The sterols and triterpene dialcohols fraction is separated from the unsaponifiable matter by thin-layer chromatography on a basic silica gel plate. The fractions recovered from the silica gel are transformed into trimethylsilyl ethers and then analysed by capillary column gas chromatography.
3. APPARATUS U.K.
The usual laboratory equipment and in particular the following:
250 ml flask fitted with a reflux condenser with ground-glass joints.
500 ml separating funnel.
250 ml flasks.
Complete apparatus for analysis by thin-layer chromatography using 20 x 20 cm glass plates.
Ultraviolet lamp with a wavelength of 254 or 366 nm.
100 μl and 500 μl microsyringes.
Cylindrical filter funnel with a G3 porous septum (porosity 15-40 μm) of diameter approximately 2 cm and a depth of 5 cm, suitable for filtration under vacuum with male ground-glass joint.
50 ml vacuum conical flask with ground-glass female joint, which can be fitted to the filter funnel (point 3.7).
10 ml test tube with a tapering bottom and a sealing glass stopper.
Gas chromatograph suitable for use with a capillary column with split injection system, consisting of:
A thermostatic chamber for columns capable of maintaining the desired temperature with an accuracy of ± 1°C;
A temperature-adjustable injection unit with a persilanised glass vaporising element and split system;
A flame ionisation detector (FID);
Data acquisition system suitable for use with the FID detector (point 3.10.3.), capable of manual integration.
Fused-silica capillary column of length 20 to 30 m, internal diameter 0,25 to 0,32 mm, coated with 5 % diphenyl - 95 % dimethylpolysiloxane (SE-52 or SE-54 stationary phase or equivalent), to a uniform thickness between 0,10 and 0,30 μm.
Microsyringe, of 10 ml capacity, for gas chromatography, with cemented needle suitable for split injection.
Calcium dichloride desiccator
4. REAGENTS U.K.
4.1. Potassium hydroxide minimum titre 85 %. U.K.
4.2. Potassium hydroxide ethanolic solution, approximately 2 N. U.K.
Dissolve 130 g of potassium hydroxide (point 4.1) with cooling in 200 ml of distilled water and then make up to one litre with ethanol (point 4.10). Keep the solution in well-stoppered dark glass bottles and stored maximum two days.
4.3. Ethyl ether, for analysis quality. U.K.
4.4. Potassium hydroxide ethanolic solution, approximately 0,2 N. U.K.
Dissolve 13 g of potassium hydroxide (point 4.1) in 20 ml of distilled water and make up to one litre with ethanol (point 4.10).
4.5. Anhydrous sodium sulphate, for analysis quality. U.K.
4.6. Glass plates (20x20 cm) coated with silica gel, without fluorescence indicator, thickness 0,25 mm (commercially available ready for use). U.K.
4.7. Toluene, for chromatography quality. U.K.
4.8. Acetone, for chromatography quality. U.K.
4.9. n-Hexane, for chromatography quality. U.K.
4.10. Ethyl ether, for chromatography quality. U.K.
4.11. Ethanol of analytical quality. U.K.
4.12. Ethyl acetate of analytical quality. U.K.
4.13. Reference solution for thin-layer chromatography: cholesterol or phytosterols, and erythrodiol 5 % solution in ethyl acetate (point 4.11). U.K.
4.14. 2,7-dichlorofluorescein, 0,2 % in ethanolic solution. Make slightly basic by adding a few drops of 2 N alcoholic potassium hydroxide solution (point 4.2). U.K.
4.15. Anhydrous pyridine, for chromatography quality (see Note 5). U.K.
4.16. Hexamethyl disilazane of analytical quality. U.K.
4.17. Trimethylchlorosilane of analytical quality. U.K.
4.18. Sample solutions of sterol trimethylsilyl ethers. U.K.
To be prepared at the time of use from sterols and erythrodiol obtained from oils containing them.
4.19. α-cholestanol, purity more than 99 % (purity must be checked by GC analysis). U.K.
4.20. α-cholestanol internal standard solution, 0,2 % solution (m/V) in ethyl acetate (point 4.11). U.K.
4.21. Phenolphthalein solution, 10 g/L in ethanol (point 4.10). U.K.
4.22. Carrier gas: hydrogen or helium, gas-chromatographic purity. U.K.
4.23. Auxiliary gases: hydrogen, helium, nitrogen and air, of gas-chromatographic purity. U.K.
4.24. n-Hexane (point 4.9)/ethyl ether (point 4.10) mixture 65:35 (V/V). U.K.
4.25. Silylation reagent, consisting of a 9:3:1 (V/V/V) mixture of pyridine/hexamethyl disilazane/trimethylchlorosilane. U.K.
5. PROCEDURE U.K.
5.1. Preparation of the unsaponifiable matter. U.K.
5.1.1. Using a 500 μl microsyringe (point 3.6) introduce into the 250 ml flask (point 3.1) a volume of the α-cholestanol internal standard solution (point 4.20) containing an amount of cholestanol corresponding to approximately 10 % of the sterol content of the sample. For example, for 5 g of olive oil sample add 500 μl of the α-cholestanol solution (point 4.20) and 1 500 μl for olive-pomace oil. Evaporate until dryness with a gentle current of nitrogen in a warm water bath, after cooling the flask, weigh 5 ± 0,01 g of the dry filtered sample into the same flask. U.K.
Note 1 : Animal or vegetable oils and fats containing appreciable quantities of cholesterol may show a peak having a retention time close to cholestanol. If this occurs, the sterol fraction will have to be analysed in duplicate with and without internal standard. U.K.
5.1.2. Add 50 ml of 2 N ethanolic potassium hydroxide solution (point 4.2) and some pumice, fit the reflux condenser and heat to gentle boiling until saponification takes place (the solution becomes clear). Continue heating for a further 20 minutes, then add 50 ml of distilled water from the top of the condenser, detach the condenser and cool the flask to approximately 30 °C. U.K.
5.1.3. Transfer the contents of the flask quantitatively into a 500 ml separating funnel (point 3.2) using several portions of distilled water (50 ml). Add approximately 80 ml of ethyl ether (point 4.10), shake vigorously for approximately 60 seconds, periodically releasing the pressure by inverting the separating funnel and opening the stopcock. Allow standing until there is complete separation of the two phases (Note 2). U.K.
Then draw off the soap solution as completely as possible into a second separating funnel. Perform two further extractions on the water-alcohol phase in the same way using 60 to 70 ml of ethyl ether (point 4.10).
Note 2 : Any emulsion can be destroyed by adding small quantities of ethanol (point 4.11). U.K.
5.1.4. Combine the three ether extracts in one separating funnel containing 50 ml of water. Continue to wash with water (50 ml) until the wash water no longer gives a pink colour on the addition of a drop of phenolphthalein solution (point 4.21). U.K.
When the wash water has been removed, filter on anhydrous sodium sulphate (point 4.5) into a previously weighed 250 ml flask, washing the funnel and filter with small quantities of ethyl ether (point 4.10).
5.1.5. Evaporate the solvent by distillation on a rotary evaporator at 30 °C under vacuum. Add 5 ml of acetone and remove the volatile solvent completely in a gentle current of air. Dry the residue in the oven at 103±2 °C for 15 min. Cool in desiccators and weigh to the nearest 0,1 mg. U.K.
5.2. Separation of the sterol and triterpene dialcohols fraction (erythrodiol + uvaol) U.K.
5.2.1. Preparation of the basic thin layer chromatography plates. Immerse the silica gel plates (point 4.6) about 4 cm in the 0,2 N ethanolic potassium hydroxide solution (point 4.5) for 10 seconds, then allow to dry in a fume cupboard for two hours and finally place in an oven at 100 °C for one hour. U.K.
Remove from the oven and keep in a calcium chloride desiccator (point 3.13) until required for use (plates treated in this way must be used within 15 days).
Note 3 : When basic silica gel plates are used to separate the sterol fraction there is no need to treat the unsaponifiable fraction with alumina. In this way all compounds of an acid nature (fatty acids and others) are retained on the spotting line and the sterols band is clearly separated from the aliphatic and triterpene alcohols band. U.K.
5.2.2. Place hexane/ethyl ether mixture (point 4.24) (Note 4) into the development chamber, to a depth of approximately 1 cm. Close the chamber with the appropriate cover and leave thus for at least half an hour, in a cool place, so that liquid-vapour equilibrium is established strips of filter paper dipping into the eluent may be placed on the internal surfaces of the chamber. This reduces developing time by approximately one-third and brings about more uniform and regular elution of the components. U.K.
Note 4 : The developing mixture needs to be replaced for every test, in order to achieve perfectly reproducible elution conditions, alternative solvent 50:50 (V/V) n-hexane/ethyl ether may be used. U.K.
5.2.3. Prepare an approximately 5 % solution of the unsaponifiable (point 5.1.5) in ethyl acetate (point 4.12) and, using the 100 μl microsyringe, depose 0,3 ml of the solution on a narrow and uniform streak on the lower end (2 cm) of the chromatographic plate (point 5.2.1). In line with the streak, place 2 to 3 μl of the material reference solution (point 4.13), so that the sterol and triterpene dialcohols band can be identified after developing. U.K.
5.2.4. Place the plate in the developing chamber prepared as specified in point 5.2.2. The ambient temperature should be maintained between 15 and 20 °C (Note 5). Immediately close the chamber with the cover and allow eluting until the solvent front reaches approximately 1 cm from the upper edge of the plate. Remove the plate from the developing chamber and evaporate the solvent in a flow of hot air or by leaving the plate for a short while, under a hood. U.K.
Note 5 : Higher temperature could worsen the separation. U.K.
5.2.5. Spray the plate lightly and uniformly with the 2,7-dichlorofluorescein solution (point 4.14) and then leave to dry. When the plate is observed under ultraviolet light, the sterols and triterpene dialcohols bands can be identified through being aligned with the spots obtained from the reference solution (point 4.13). Mark the limits of the bands along the edges of the fluorescence with a black pencil (see TLC plate figure 3). U.K.
5.2.6. By using a metal spatula, scrape off the silica gel of the marked area. Place the finely comminuted material removed into the filter funnel (point 3.7). Add 10 ml of hot ethyl acetate (point 4.12), mix carefully with the metal spatula and filter under vacuum, collecting the filtrate in the conical flask (point 3.8.) attached to the filter funnel. U.K.
Wash the residue in the flask three times with ethyl ether (point 4.3) (approximately 10 ml each time), collecting the filtrate in the same flask attached to the funnel, evaporate the filtrate to a volume of 4 to 5 ml, transfer the residual solution to the previously weighed 10 ml test tube (point 3.9), evaporate to dryness by mild heating, in a gentle flow of nitrogen, make up again using a few drops of acetone (point 4.8), evaporate again to dryness,
The residue contained in the test tube must consist of the sterol and triterpene dialchols fractions.
5.3. Preparation of the trimethylsilyl ethers. U.K.
5.3.1. Add the silylation reagent (point 4.25) (Note 6), in the ratio of 50 μl for every milligram of sterols and triterpene dialcohols, in the test tube containing the sterol and triterpene fraction, avoiding any uptake of moisture (Note 7). U.K.
Note 6 : Ready for use solutions are available commercially. Other silylation reagents, such as, for example, bistrimethylsilyl trifluor acetamide + 1 % trimethylchlorosilane, which has to be diluted with an equal volume of anhydrous pyridine, are also available. U.K.
Pyridine can be replaced by the same amount of acetonitrile.
5.3.2. Stopper the test tube, shake carefully (without overturning) until the compounds are completely dissolved. Leave to stand for at least 15 minutes at ambient temperature and then centrifuge for a few minutes. The clear solution is ready for gas chromatographic analysis. U.K.
Note 7 : The slight opalescence, which may form, is normal and does not cause any anomaly. The formation of a white flock or the appearance of a pink colour is indicative of the presence of moisture or deterioration of the reagent. If this occurs, the test must be repeated (only if hexamethyldisilazane/trimethylchlorosilane is used). U.K.
5.4. Gas chromatographic analysis. U.K.
5.4.1. Preliminary operations, capillary column conditioning. U.K.
5.4.1.1. Fit the column (point 3.11) in the gas chromatograph, by attaching the inlet end to the split injector and the outlet end to the detector. U.K.
Carry out general checks on the gas chromatograph unit (leaks from the gas circuits, detector efficiency, efficiency of the splitting system and recording system, etc.).
5.4.1.2. If the column is being used for the first time, it is recommended that it be subjected to conditioning: passing a gentle flow of gas through the column itself, then switch on the gas chromatography unit and begin a gradual heating, up to a temperature of at least 20 °C above the operating temperature (Note 8). Hold this temperature for at least two hours, then place the entire unit in operating mode (adjustment of gas flows and splitting, ignition of the flame, connection with the computing system, adjustment of the column, detector and injector temperature, etc.) and then record the signal with a sensitivity at least two times greater than that one intended for the analysis. The course of the base line must be linear, without peaks of any kind, and must not show drift. U.K.
A negative straight-line drift indicates leakage from the column connections; a positive drift indicates inadequate conditioning of the column.
Note 8 : The conditioning temperature must always be at least 20 °C less than the maximum temperature specified for the stationary phase used. U.K.
5.4.2. Choice of operating conditions. U.K.
5.4.2.1. The operating conditions are as follows: U.K.
Column temperature: 260 ± 5 °C;
Injector temperature: 280-300 °C;
Detector temperature: 280-300 °C;
Linear velocity of the carrier gas: helium 20 to 35 cm/s; hydrogen 30 to 50 cm/s;
Splitting ratio: from 1:50 to 1:100;
Instrument sensitivity: from 4 to 16 times the minimum attenuation;
Recording sensitivity: 1 to 2 mV full scale;
Amount of substance injected: 0,5 to 1 μl of TMSE solution.
These conditions may be changed according to the characteristics of the column and gas chromatograph, so as to obtain chromatograms, which meet the following requirements:
The retention time for the ß-sitosterol peak should be at 20 ± 5 min;
The campesterol peak should be: for olive oil (mean content 3 %) 20 ± 5 % of full scale; for soybean oil (average content 20 %) 80 ± 10 % of full scale;
All the present sterols must be separated. In addition to being separated the peaks, they must also be completely resolved, i.e. the peak trace should return to the base line before leaving for the next peak. Incomplete resolution is, however, tolerated, provided that the peak at RRT 1,02 (Sitostanol) can be quantified using the perpendicular.
5.4.3. Analytical procedure U.K.
5.4.3.1. By using the 10 μl microsyringe, take 1 μl of hexane, draw in 0,5 μl of air and then 0,5 to 1 μl of the sample solution. Raise the plunger of the syringe further, so the needle is emptied. Push the needle through the membrane of the injector and after one to two seconds, inject rapidly, and then slowly remove the needle after around five seconds. U.K.
An automatic injector can be used as well.
5.4.3.2. Carry out the recording until the TMSE of the present triterpene dialcohols are completely eluted. The base line must continue to meet the requirements (point 5.4.1.2). U.K.
5.4.4. Peak identification U.K.
Identify individual peaks on the basis of retention times and by comparison with the mixture of sterol and triterpene dialcohols TMSE, analysed under the same conditions (see Appendix).
The sterols and triterpene dialcohols are eluted in the following order: cholesterol, brassicasterol, ergosterol, 24-methylen-cholesterol, campesterol, campestanol, stigmasterol, Δ7-campesterol, Δ5,23-stigmastadienol, clerosterol, ß-sistosterol, sitostanol, Δ5-avenasterol, Δ5,24-stigmastadienol, Δ7-stigmastenol, Δ7-avenasterol, erythrodiol and uvaol.
The retention times for ß-sitosterol, for SE-52 and SE-54 columns, are shown in Table 1.
Figures 1 and 2 show typical chromatograms for some oils.
5.4.5. Quantitative evaluation. U.K.
5.4.5.1. Calculate the areas of the α-cholestanol and the sterol and triterpene dialcohols peaks by using the computing system. Ignore peaks for any compound which are not included (ergosterol must not be calculated) among those listed in Table 1. The response factor for α-cholestanol should be considered equal to 1. U.K.
5.4.5.2. Calculate the concentration of each individual sterol, in mg/kg of fatty material, as follows: U.K.
where:
=
peak area for sterol x, in computing system counts;
=
area of the α-cholestanol peak, in computing system counts;
=
mass of added α-cholestanol, in milligrams;
=
mass of the sample used for determination, in grams.
6. EXPRESSION OF THE RESULTS U.K.
6.1. Report individual sterol concentrations as mg/kg of fatty material and their sum as ‘total sterols’. U.K.
The composition of each of the individual sterols and of the erythrodiol and uvaol should be expressed to one decimal point.
Total sterol composition must be expressed without any decimal point.
[F26.2. Calculate the percentage of each individual sterol from the ratio of the relevant peak area to the total peak area for sterols: U.K.
where:
=
peak area for x;
=
total peak area for sterols.]
Textual Amendments
6.3. Apparent β-sitosterol: Δ5-23-stigmastadienol + clerosterol + β-sitosterol + sitostanol + Δ5-avenasterol + Δ5-24-stigmastadienol. U.K.
6.4. Calculate the percentage of erythrodiol and uvaol: U.K.
where
=
sum area for sterol in computing system counts;
=
area of Erythrodiol in computing system counts;
=
area of Uvaol in computing system counts;
Appendix
Determination of the linear speed of the gas U.K.
With the gas chromatograph set to normal operating conditions, inject 1 to 3 μl of methane (or propane) and measure the time taken by the gas to pass through the column, from the time of injection to the time at which the peak appears (tM).
The linear speed in cm/s is given by L/tM, where L is the length of the column in centimetres and tM is the measured time, in seconds.
Table 1
Relative retention times for sterols
| Peak | Identification | Relative retention time | ||
|---|---|---|---|---|
| SE 54 column | SE 52 column | |||
| 1 | Cholesterol | Δ-5-cholesten-3 ß -ol | 0,67 | 0,63 |
| 2 | Cholestanol | 5 α -cholestan-3 ß -ol | 0,68 | 0,64 |
| 3 | Brassicasterol | [24S]-24-methyl-Δ-5,22-cholestadien-3 ß -ol | 0,73 | 0,71 |
| * | Ergosterol | [24S] 24 methy Δ5-7-22 cholestatrien 3ß-ol | 0,78 | 0,76 |
| 4 | 24-methylene-cholesterol | 24-methylene-Δ-5,24-cholestadien-3 ß -o1 | 0,82 | 0,8 |
| 5 | Campesterol | (24R)-24-methyl-Δ-5-cholesten-3 ß -ol | 0,83 | 0,81 |
| 6 | Campestanol | (24R)-24-methyl-cholestan-3 ß -ol | 0,85 | 0,82 |
| 7 | Stigmasterol | (24S)-24-ethyl-Δ-5,22-cholestadien-3 ß -ol | 0,88 | 0,87 |
| 8 | Δ-7-campesterol | (24R)-24-methyl-Δ-7-cholesten-3 ß -ol | 0,93 | 0,92 |
| 9 | Δ-5,23-stigmastadienol | (24R,S)-24-ethyl-Δ-5,23-choIestadien-3 ß -ol | 0,95 | 0,95 |
| 10 | Clerosterol | (24S)-24-ethyl-Δ-5,25-cholestadien-3 ß -ol | 0,96 | 0,96 |
| 11 | ß -sitosterol | (24R)-24-ethyl-Δ-5-cholesten-3 ß -ol | 1,0 | 1,0 |
| 12 | Sitostanol | 24-ethyl-cholestan-3 ß -ol | 1,02 | 1,02 |
| 13 | Δ-5-avenasterol | (24Z)-24-ethylidene-Δ-cholesten-3 ß -ol | 1,03 | 1,03 |
| 14 | Δ-5-24-stigmastadienol | (24R,S)-24-ethyl-Δ-5,24-cholestadien-3 ß -ol | 1,08 | 1,08 |
| 15 | Δ-7-stigmastenol | (24R,S)-24-ethyl-Δ-7-cholesten-3 ß -ol | 1,12 | 1,12 |
| 16 | Δ-7-avenasterol | (24Z)-24-ethylidene-Δ-7-cholesten-3 ß -ol | 1,16 | 1,16 |
| 17 | Erythrodiol | 5α olean-12en-3ß28 diol | 1,41 | 1,41 |
| 18 | Uvaol | Δ12-ursen-3ß28 diol | 1,52 | 1,52 |
Figure 1
Gas chromatogram of the sterol and triterpene dialchols fraction of a lampante olive oil (spiked with internal standard)
