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The Preservatives in Food (Scotland) Regulations 1989

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Method of analysis

5.  The analysis shall be carried out as follows—

(a)Preparation and extraction: All the fruit in the sample for analysis is cut in half. Half of each piece of fruit is kept for quantitative determination of the residue of any biphenyl or 2-hydroxybiphenyl present. Pieces of peel are taken from the other halves to give a sample of about 80 g. These pieces are chopped, crushed in the mixer and placed in the 250 ml flask; to this is added 1 ml of 25 per centum hydrochloric acid and 100 ml dichloromethane. The mixture is heated under reflux for 10 minutes. After cooling and rinsing of the condenser with about 5 ml of dichloromethane, the mixture is filtered through a fluted filter. The solution is transferred to the evaporator and some anti-bumping granules are added. The solution is concentrated at reduced pressure at a temperature of 60C to a final volume of about 10 ml. If a rotary evaporator is used, the flask should be kept in a fixed position to avoid loss of biphenyl through the formation of a film of the product on the upper wall of the flask.

(b)Chromatography: 30 g of silica gel and 60 ml of water are placed in a mixer and mixed for one minute. The mixture is then spread on to 5 chromatographic plates to form a layer approximately 0.25 mm thick. The plates covered with this layer are subjected to a stream of hot air for 15 minutes and then placed in an oven where they are kept for 30 minutes at a temperature of 110C.

After cooling, the surface layer of each plate is divided into lanes, 2 cm wide, by parallel lines penetrating the silica gel down to the surface of the glass plate. 50 μl of the extract to be analysed are applied to each lane as a narrow band of contiguous spots approximately 1.5 cm from the lower edge of the plate. At least one lane is kept for the controls consisting of a spot of 1 ul (that is, 10 μg) of the standard solutions of biphenyl and 2-hydroxybiphenyl, one standard per lane. The chromatographic plates are developed in a mixture of cyclohexane and dichloromethane (25:95) in tanks previously lined with filter paper.

(c)Detection and identification: The presence of biphenyl and 2-hydroxybiphenyl is shown by the appearance of spots in ultra-violet light (254 nm). The sodium biphenyl-2-yl oxide will have been converted to 2-hydroxybiphenyl during the extraction in an acid medium, and its presence cannot therefore be distinguished from that of 2-hydroxybiphenyl. The products are identified in the following manner—

(i)biphenyl gives a yellow spot in daylight when sprayed with the TNF solution;

(ii)2-hydroxybiphenyl gives a blue spot when sprayed with the solution of 2,6-dibromo-p-benzoquinonechlorimine, followed by rapid passage through a stream of hot air and exposure to an ammonia-saturated atmosphere.

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