5.3. Preparation of the ‘addition’ curve
NB:
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 (yi) and the corresponding value of the responses calculated from the regression line (zi) divided by the mean value ( 
5.3.1. Preparation of sample
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.
5.3.2. Procedure
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.