SOP: Ultraviolet Spectrum of Citrus Oils
GIFSQ — Global Institute for Food Safety & Quality
Principle
The ultraviolet absorption spectrum of the oil in alcohol is recorded from 260 to 400 nm; a base line is drawn tangent to the curve at the absorption minimum and at the inflection point near 365–370 nm, and the CD value (the maximum absorbance above the base line) is measured and corrected to the standard sample weight — added distilled oil, terpenes or washed oils in cold-pressed oils change the CD value and the wavelength maximum.
Apparatus required
- Spectrophotometer recording in the U.V. region (or manual instrument)
- 1 cm cell (0.1 cm cell for grapefruit and lime oil where the concentration is too great to obtain a satisfactory curve)
Reagents
a. Alcohol (solvent and blank).
Procedure
Accurately weigh 1 g ± 1.0 mg of sample in a glass-stoppered weighing bottle
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Dissolve in alcohol, transfer to a 100 ml volumetric flask, and make to volume with alcohol
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Dilute 25 ml of this solution to 100 ml with alcohol
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Determine the absorbance of this solution in the U.V. region from 260 to 400 nm against alcohol as blank — if a manual instrument is used, note the absorbances at 5 nm intervals from 260 to 315 nm, at 3 nm intervals from 315 to 336 nm, and at 10 nm intervals from 340 to 400 nm; at points of maximum or minimum absorbance, read the absorbance at 1 nm intervals
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Plot the absorbance against wavelength and draw a smooth curve through the points, or use the automatically recorded spectrum
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Draw a straight line (base line) AB, tangent to the curve at the point of minimum absorbance (usually between 285 and 295 nm) and at the inflection point where the curve levels off near 365–370 nm
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Draw the vertical line CD, perpendicular to the abscissa, from the absorbance maximum to intercept the base line
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Subtract the absorbance corresponding to the point of intercept at D from the absorbance at the maximum at C to determine the CD value
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Correct the CD value found for the actual weight of oil taken, calculating the difference on the basis of a 250 mg sample
EOA procedure: the procedure is essentially the same, except that (i) a solution of the oil in alcohol is prepared in the concentration specified in the individual monograph ± 10% (see the table below); (ii) the CD value found is adjusted to a cell thickness of 1.00 cm and the sample concentration as specified — this value should be equal to or greater than the value reported in the monograph, and the wavelength maximum should be within the specified range; the CD value varies with oils but is specific for each of them.
| Oil | Concentration of sample solution (mg/100 ml) | CD value | λ max (nm) |
|---|---|---|---|
| Bitter orange oil | 50 | Minimum 0.32 | 320 ± 3 |
| Bergamot oil, cold pressed | 50 | Minimum 0.32 | 315 ± 3 |
| Grapefruit oil | 50 | Minimum 0.42 | 319 ± 3 |
| Lime oil, cold pressed | 20 | Minimum 0.45 | 319 ± 3 |
| Lemon oil, Arizona, cold pressed | 250 | Minimum 0.20 | not legible in the available copy |
| Lemon oil, distilled | 250 | Maximum 0.01 | 315 ± 3 |
| Mandarin oil, cold pressed | 150 | Minimum 0.50 | 323–343 |
| Orange oil, distilled | 250 | Maximum 0.01 | 350 ± 25 |
| Tangerine oil, cold pressed | 150 | Minimum 0.28 | 325 ± 3 |
| Orange oil, sweet U.S.P., cold pressed | 250 | Minimum 0.24 | 325–330 |
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
- Sale, J.W., J. Assoc. Offic. Anal. Chem., 36, 112 (1953).
- Official Methods of Analysis, Association of Official Analytical Chemists, 12th edn., 1975, p. 340.
- Specifications and Standards, Essential Oil Association of USA, Inc.