- Latest available (Revised)
- Original (As adopted by EU)

- Latest available (Revised)
- Original (As adopted by EU)

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This is the original version (as it was originally adopted).

CAUTION: VC is a hazardous substance and a gas at ambient temperature therefore, the preparation of solutions should be carried out in a well-ventilated fume cupboard.

Take all the necessary precautions to ensure that no VC or DMA is lost.

When employing manual sampling techniques, an internal standard (3.3) should be used.

When using an internal standard, the same solution must be utilized throughout the procedure.

Accurately weigh to the nearest 0.1 mg a suitable glass vessel and place in it a quantity (e.g. 50 ml) of DMA (3.2). Re-weigh. Add to the DMA a quantity (e.g. 0.1 g) of VC (3.1) in liquid or gas form, injecting it slowly onto the DMA. The VC may also be added by bubbling it into the DMA, provided that a device is used which will prevent loss of DMA. Reweigh to the nearest 0.1 mg. Wait two hours to allow equilibrium to be attained. If an internal standard is to be employed, add internal standard so that the concentration of the internal standard in the concentrated standard VC solution is the same as in the internal standard solution prepared under point 3.3. Keep the standard solution in a refrigerator.

Take a weighed amount of concentrated standard solution of VC (5.1.1) and dilute, to a known volume or a known weight, with DMA (3.2) or with internal standard solution (3.3). The concentration of the resultant dilute standard solution (solution A) is expressed as mg/litre or mg/kg respectively.

The curve must comprise at least seven pairs of points.

The repeatability of the responses(1) must be lower than 0.002 mg VC/litre or kg of DMA.

The curve must be calculated from these points by the least squares technique, i.e., the regression line must be calculated using the following equation:

y = a_{1}x + a_{o}

where:

and:

where:

y

=

the height or area of peaks in any single determination;

x

=

the corresponding concentration on the regression line;

n

=

number of determinations carried out (n ≥ 14).

The curve must be linear, i.e., the standard deviation (s) of the differences between the measured responses (y_{i}) and the corresponding value of the responses calculated from the regression line (z_{i}) divided by the mean value (y) of all the measured responses shall not exceed 0·07.

This shall be calculated from:

where:

and:

where:

y_{i}

=

each individual measured response;

z_{i}

=

the corresponding value of the response (y_{i}) on the calculated regression line;

n

=

≥ 14.

Prepare two series of at least seven phials (4.4). Add to each phial volumes of dilute standard VC solution (5.1.2) and DMA (3.2) or internal standard solution in DMA (3.3) such that the final VC concentration of the duplicate solutions will be approximately equal to 0, 0·005, 0·010, 0·020, 0·030, 0·040, 0·050, etc., mg/litre or mg/kg of DMA and that each phial contains the same total volume of solution. The quantity of dilute standard VC solution (5.1.2) must be such that the ratio between the total volume (μl) of added VC solution and quantity (g or ml) of DMA, or internal standard solution (3.3) does not exceed five. Seal the phials and proceed as described under points 5.4.2, 5.4.3 and 5.4.5. Construct a graph in which the ordinate values show the areas (or heights) of the VC peaks of the duplicate solutions, or the ratio of these areas (or heights) to those of the relevant internal standard peaks, and the abscissa values show the VC concentrations of the duplicate solutions.

Repeat the procedure described under points 5.1.1 and 5.1.2 to obtain a second dilute standard solution with, in this case, a concentration approximately equal to 0·02 mg VC: 1, or 0·02 mg VC/kg of DMA or internal standard solution. Add this solution to two phials (4.4). Seal the phials and proceed as described under points 5.4.2, 5.4.3 and 5.4.5.

If the average of two gas-chromatographic determinations relating to the solution B (5.2.1) does not differ by more than 5% from the corresponding point of the response curve obtained in point 5.1.3, the solution A is validated. If the difference is greater than 5%, reject all the solutions obtained in points 5.1 and 5.2 and repeat the procedure from the beginning.

The curve must comprise at least seven pairs of points.

The curve must be calculated from these points by the least squares technique (5.1.3, third indent).

The curve must be linear, i.e., the standard deviation(s) of the differences between the measured responses (y_{i}) and the corresponding value of the responses calculated from the regression line (z_{i}) divided by the mean value (

The sample of foodstuff to be analyzed must be representative of the foodstuff presented for analysis. The foodstuff must, therefore, be mixed or reduced to small pieces and mixed before the sample is taken.

Prepare two series of at least seven phials (4.4). Add to each phial a quantity, not less than 5 g, of sample obtained from the foodstuff under investigation (5.3.1). Ensure that an equal quantity is added to each phial. Close the phial immediately. Add to each phial for each gram of sample 1 ml, preferably of distilled water, or demineralized water of at least equivalent purity, or an appropriate solvent if necessary. (Note: for homogeneous foodstuffs, addition of distilled of demineralized water is not necessary.) Add to each phial volumes of dilute standard VC solution (5.1.2), containing the internal standard (3.3), if considered useful, such that concentrations of VC added to the phials equal to 0, 0·005, 0·010, 0·020, 0·030, 0·040 and 0·050, etc., mg/kg of foodstuffs. Ensure that the total volume of DMA or DMA containing internal standard (3.3) in each phial is the same. The quantity of dilute standard VC solution (5.1.2) and additional DMA where used, must be such that the ratio between the total volume (μl) of these solutions and the quantity (g) of foodstuff contained in the phial is as low as possible but not more than five and is the same in all phials. Seal the phials and proceed as described under point 5.4.

(1)

See recommendation ISO DIS 5725: 1977.

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