[ANNEX IX U.K. SPECTROPHOTOMETRIC INVESTIGATION IN THE ULTRAVIOLET
5. PROCEDURE U.K.
5.1. The sample must be perfectly homogeneous and without suspended impurities. If not, it must be filtered through paper at a temperature of approximately 30 °C. U.K.
5.2. Weigh accurately approximately 0,25 g (to the nearest 1 mg) of the sample so prepared into a 25 ml graduated flask, make up to the mark with the specified solvent and homogenise. The resulting solution must be perfectly clear. If opalescence or turbidity is present, filter quickly through paper. U.K.
NOTE: Generally, a mass of 0,25 to 0,30 g is sufficient for absorbance measurements of virgin and extra virgin olive oils at 268 nm and 270 nm. For measurements at 232 nm, 0,05 g of sample are usually required, so two distinct solutions are usually prepared. For absorbance measurements of olive pomace oils, refined olive oils and adulterated olive oils, a smaller portion of sample, e.g. 0,1 g is usually needed due to their higher absorbance. U.K.
5.3. If necessary, correct the baseline (220-290 nm) with solvent in both quartz cells (sample and reference), then fill the sample quartz cell with the test solution and measure the extinctions at 232, 268 or 270 nm against the solvent used as a reference. U.K.
The extinction values recorded must lie within the range 0,1 to 0,8 or within the range of linearity of the spectrophotometer which should be verified. If not, the measurements must be repeated using more concentrated or more dilute solutions as appropriate.
5.4. After measuring the absorbance at 268 or 270 nm, measure the absorbance at λmax, λmax + 4 and λmax – 4. These absorbance values are used to determine the variation in the specific extinction (ΔΚ). U.K.
NOTE: λmax is considered to be 268 nm for isooctane used as solvent and 270 nm for cyclohexane.] U.K.