BS 17265:2019 pdf download – Foodstuffs – Determination of elements and their chemical species – Determination of aluminium by inductively coupled plasma optical emission spectometry (ICP-OES)
6.2 Digestion vessels, e.g. of polyfluoropolymers or quartz with volumes from 70 ml to 100 ml.
6.3 Test tube shaker, optional.
6.4 Analytical balance, capable to weigh to the nearest milligram.
To ensure complete dissolution of aluminium the following shall be respected:
— ensure that the sample is sufficiently homogeneous;
— in case of incomplete digestion or high aluminium contents, it may be beneficial to use a test portion as small as possible (at least 200 mg for dry samples) in order to digest aluminium
compounds as completely as possible;
— for samples containing silicates, digestion temperatures above 220 °C may be necessary in order to dissolve the aluminium as completely as possible;
— in case of samples with low water content, first add water and mix intensively, before adding concentrated nitric acid;
— do not use hydrogen peroxide for digestion.
Further instructions regarding test portions and digestion are given in B.2 and B.3.
For pressure digestion according to EN 13805, different vessels may be used depending on the instrument type and manufacturer. The maximum test portion and the minimum liquid volume depend on the pressure stability of the respective vessels. The specifications according to 7.1.2 refer to digestion vessels with volumes from 70 ml to 100 ml and a minimum liquid volume of 5 ml. All indications in 7.1.2 shall be adjusted to the digestion instrument used. For safety reasons the manufacturer’s specifications shall be strictly followed.
7.1.2 Digestion procedure Before digestion, different amounts of water, depending on the different types of food , are added in order to obtain comparable acid concentrations in the final digestion solution.
The amount of water to be added depends on the test portion and thus on the content of carbon and water in the food type. Add just as much water to the test portion that is necessary to suspend the food completely. Then complete the test portion with water to reach 3 g. Water is also added to fat-containing foods, even if a suspension is hardly possible. Example for digestion of powdery food: Add 2,5 ml of water to the dry food, e.g. 0,5 g of flour. Mix the contents of the digestion vessel intensively, e.g. on a test-tube shaker. Repeat shaking every 10 min to 15 min until the suspension is homogeneous. No agglutinated particles shall remain, and the sample material shall be welled. Fat-containing samples require more time and repeated shaking. After at least 30 min, add 2,5 ml of nitric acid (5.1) to the sample. Mix thoroughly again to obtain a homogeneous suspension.
After the pre-reaction has subsided, close the digestion vessel, and start the digestion. Example for digestion of a sample with high water content, e.g. lettuce: Directly add 2,5 ml of nitric acid (5.1) to 3 g of lettuce and mix thoroughly. It is not necessary to add water. After the pre-reaction has subsided, close the digestion vessel, and start the digestion. A digestion temperature of at least 200 °C to be kept for at least 20 min is required for a complete digestion in the microwave. The digestion conditions depend on the manufacturer’s specifications, the reactivity of the sample, the maximum pressure stability of the digestion vessel, and the attainable temperature. To monitor the laboratory-specific digestion conditions, it is recommended to carry out measurements with reference materials containing certified contents . If precipitations or turbidities are visible in the digestion solutions, low findings can occur. In these cases, use smaller test portions and higher digestion temperatures up to 300 °C in order to minimize or avoid possible low findings.
NOTE Turbidities or small amounts of precipitates adhering to the wall or bottom of the vessel are only visible in vessels with high transparency (e.g. aluminium-free quartz vessels). Plastic vessels are often not completely transparent, making precipitates difficult to discern. Fill up the digestion solution obtained by pressure digestion to a defined volume, e.g. 20 ml. Add the internal standard (5.4.3) (e.g. 1 000 µl) to an aliquot of the digestion solution, e.g. 5 ml, and fill up with water to 10 ml.
Separate insoluble residues before measurement. If the digestion solution has a different final volume than 20 ml and a different dilution factor than 2 after addition of the internal standard, adapt the acid concentration of the calibration solutions respectively. All test solutions shall have approximately the same acid concentration as the calibration solutions. The digestion solution shall be measured within 2 weeks.BS 17265 pdf download.