Page 159 - Analytical method for food addtives

104 Analytical methods for food additives

Calculation of the nitrate content
Calculate the mass fraction of nitrate, W NO , expressed as nitrite ion, in milligrams
3
per kilogram, with equation [9.3]:
[9.3]
W NO 3 = W NO 2 +NO 3 – W NO 2
The conversion factor from nitrite to nitrate is 1.35.

Regression graph
Alternatively, the calculation may also be carried out mathematically using a
regression graph. (For further information on this method see DD ENV 12014-
3:1998.)

Ion-exchange chromatographic (IC) method for the determination of
nitrate and nitrite content of meat products 15
Sample preparation
Homogenise the laboratory sample with the appropriate equipment. Take care that
the temperature of the sample material does not rise above 25 ºC. If a mincer is
used, pass the sample at least twice through the equipment. Weigh, to the nearest
10 mg, 10 g of the homogenised sample into e.g. a 150 mL wide neck conical flask
(test portion).

Extraction and clarification
The analysis should be performed in a single working day.
Add 50 mL of water at a temperature of 50 ºC to 60 ºC to the test portion in the
wide neck flask. Mix thoroughly with the homogeniser. Rinse the homogeniser
with water and add the washings to the flask. Quantitatively transfer this slurry into
a 200 mL volumetric flask by rinsing the wide neck flask with water. Add 50 mL
of acetonitrile. Mix gently. Allow to cool to room temperature. Dilute to the mark
with water.
Filter first through the fluted filter paper and then through a membrane filter of
pore size of approximately 0.45 µm. If this solution is clear, it may be injected. If
the solution is still not clear, filter through a membrane filter of pore size of
approximately 0.22 µm (sample test solution). Prepare a blank replacing the test
portion by 10 mL of water.

Preparation of the calibration graph
To plot a calibration graph, inject equal volumes of the standard solutions first and
then the blank solution under the conditions as described above.
Check the linearity of the calibration graph.

IC operating conditions
The column used must be found satisfactory to adopt the following parameters: