Page 170 - Analytical method for food addtives

E249–50: Nitrites 115

Repeatability
The absolute difference between two single test results, carried out simultaneously
or in rapid succession by the same operator under the same conditions on identical
test material, will be greater than 0.5 mg/kg in not more than 95 % cases.

Reproducibility
The absolute difference between two single and independent results obtained by
two operators working in different laboratories on identical test material, will be
greater than 1.0 mg/kg in not more than 95 % of cases.
(For further information on this method see BS EN ISO 14673-2:2001.)

Method using flow injection analyses with in-line dialysis 19

Preparation of test sample: cheese
Prior to analysis, remove the rind or mouldy surface layer of the cheese, in such a
way as to provide a sample representative of the cheese as it is usually consumed.
Grind the test sample by means of an appropriate device; mix the ground mass
quickly, and if possible grind a second time and again mix thoroughly. Transfer the
test sample to an air-tight container to await analysis, which should be carried out
as soon as possible after grinding. If a delay is unavoidable, take all precautions to
ensure proper preservation of the sample and to prevent condensation of moisture
on the inside surface of the container. Ground cheese showing unwanted mould
growth or beginning to deteriorate should not be analysed. Clean the device after
grinding each sample.
Checking the reducing capacity of the cadmium column
Carry out this check at least twice a day at the beginning and at the end of a series
of determinations. Prepare the FIA system for operation as described above
(‘Determination’). Fill a cup with nitrite reference solution. Fill a cup with the
nitrate calibration standard solution. Analyse the nitrate calibration standard
solution and the nitrite reference solution. Divide the peak height found for nitrate
by the peak height for nitrite and multiply by 100 to obtain the percentage reducing
capacity of the column. If the reducing capacity is less than 95 %, the column
should be regenerated (see ‘Regeneration of the cadmium column’).

Regeneration of the cadmium column
Regenerate the column at the end of each day after use, or more frequently if the
check indicates a loss of efficiency. Disable pump flows for carrier solution CS and
reagents R1 and R2. Unscrew the tubing at the inlet of the dialysis module
connecting the injector to the dialysis module. Run water through the injector used
for carrier solution C1 until the system is filled. Connect the cadmium column to
the outlet of the injector. Start the pump again and make 3 to 5 injections of reagent
solution R2 followed by nitrate standard solution. Wash the column by passage of
carrier solution.