Commission Regulation (EEC) No 2568/91Dangos y teitl llawn

5.2. Quantitative analysis U.K.

5.2.1. Determination of the composition U.K.

Apart from exceptional cases, use the internal normalization method, i.e. assume that the whole of the components of the sample are represented on the chromatogram, so that the total of the areas under the peaks represents 100 % of the constituents (total elution).

If the equipment includes an integrator, use the figures obtained therefrom. If not, determine the area under each peak by multiplying the height of the peak by its width at mid-height, and where necessary take into account the various attenuations used during the recording.

5.2.2. Method of calculation U.K.

5.2.2.1. General case U.K.

Calculate the content of a given component i, expressed as a percentage by mass of methyl esters, by determining the percentage represented by the area of the corresponding peak relative to the sum of the areas of all the peaks, using the following formula:

where:

Ai

is the area under the peak corresponding to component i;

A

is the sum of the areas under all the peaks.

Give the result to one decimal place.

Note 7: U.K.

In this general case, the result of the calculation based on relative areas is considered to represent a percentage by mass. For the cases in which this assumption is not allowed, see 5.2.2.2. U.K.

5.2.2.2. Use of correction factors U.K.

In certain cases, for example in the presence of fatty acids with fewer than eight carbon atoms or of acids with secondary groups, when using thermal conductivity detectors or where the highest degree of accuracy is particularly required, correction factors should be used to convert the percentages of peak areas into mass percentages of the components.

Determine the correction factors with the help of a chromatogram derived from the analysis of a reference mixture of methyl esters of known composition, carried out under operating conditions identical with those used for the sample.

For this reference mixture, the percentage by mass of component i is given by the formula:

where:

mi

is the mass of component i in the reference mixture;

m

is the total of the masses of the various components of the reference mixture.

From the chromatogram of the reference mixture (4.4) calculate the percentage (area/area) for component i as follows:

where:

Ai

is the area under the peak corresponding to component i;

A

is the sum of the areas under all the peaks.

The correction factor is then calculated as:

Commonly, the correction factors are expressed relative to K_C16, so that the relative factors become:

For the sample, the content of each component i, expressed as a percentage by mass of methyl esters, is:

Give the results to one decimal place.

5.2.2.3. Use of an internal standard U.K.

In certain analyses (for example where not all of the fatty acids are quantified, such as when acids with four and six carbons are present alongside acids with 16 and 18 carbons, or when it is necessary to determine the absolute amount of a fatty acid in a sample) it is necessary to use an Internal Standard. Fatty acids with five, 15 or 17 carbons are frequently used. The correction factor (if any) for the Internal Standard should be determined.

The percentage by mass of component i, expressed as methyl esters, is then given by the formula:

where:

Ai

is the area under the peak corresponding to component i;

As

is the area under the peak corresponding to the Internal Standard;

K′i

is the correction factor for component i (relative to K_C1);

K′s

is the correction factor for the Internal Standard (relative to K_C16);

m

is the mass, in milligrams, of the test portion;

ms

is the mass, in milligrams, of the Internal Standard.

Give the results to one decimal place.