Page 168 - Analytical method for food addtives

E249–50: Nitrites 113

the sample with the aid of the suspension apparatus. Immediately cool the
glass beaker with the test portion. Fill a cup of the sample exchanger with the
test portion, in such a way that the separated fat remains in the glass beaker.
• Dried milk products: Weigh, to the nearest 0.005 g, 2.500 g of the test sample
in a glass beaker. Add 50 mL of ammonium and sodium solution III and
suspend the test sample with the aid of the suspension apparatus. Fill a cup of
the sample exchanger with the test portion.
• Liquid milk products: Weigh, to the nearest 0.01 g, 10.00 g of the test sample
in a 50 mL volumetric flask. Make up to the 50 mL mark with ammonium and
sodium solution III and mix. Fill a cup of the sample exchanger with the test
portion.

Determination
Start the pump, the spectrometer, the recorder and the data processing equipment
of the analyser. Run the reagents through the analyser according to the diagram
(Fig. 2, BS EN ISO 14673-2:2001) for 15 min. Fill the sample exchanger of the
analyser in the following order: start with the 5 tubes filled with sodium nitrate
standard solutions placed in order of increasing nitrate content followed by the
tubes with the test portion. Start the sample exchanger of the analyser. Activate the
data processing equipment according to the directions for use. When the
determination is finished flush the system with an alkaline detergent for at least 15
min and subsequently with water for at least 15 min.

Dilutions
If the nitrite content of the test portion is higher than the nitrite content of the
highest sodium nitrate standard solution, repeat the analysis in duplicate by again
preparing two new test portions according to ‘Preparation of test portion’. Dilute
these new test portions in such a way that the expected results given by the recorder
will be lying between the result of the lowest but one sodium nitrate standard
solution and the highest standard solution. Then transfer the thus diluted test
portion into a cup.

Reference sample
Examine the reference test sample at least in duplicate, according to ‘Determination’.

Calibration curve
Calculate the most suitable calibration curve and the correlation coefficient for
both the sodium nitrate and the sodium nitrite standard solutions. The obtained
correlation coefficient should be >0.9985.

Checking the drift of standard solutions
Check regularly the drift using the appropriate standard solution. The drift path
between two successive (drift) standards may be max 10 %. The drift path between
the highest and lowest results obtained on one day may be max 20 %. When this
value is exceeded check the cause and repeat the examinations.