Commission Regulation (EC) No 152/2009Show full title

C. DETERMINATION OF THE TRACE ELEMENTS IRON, COPPER, MANGANESE AND ZINC U.K.

1. Purpose and scope U.K.

The method makes it possible to determine the trace elements iron, copper, manganese and zinc in feed. The limits of quantification are:

• iron (Fe): 20 mg/kg

• copper (Cu): 10 mg/kg

• manganese (Mn): 20 mg/kg

• zinc (Zn): 20 mg/kg.

2. Principle U.K.

The sample is brought into solution in hydrochloric acid after destruction of organic matter, if any. The elements iron, copper, manganese and zinc are determined, after appropriate dilution, by atomic absorption spectrometry.

3. Reagents U.K.

Introductory comments U.K.

For preparation of the reagents and analytical solutions use water free from the cations to be determined, obtained either by double distilling water in a borosilicate glass or quartz still or by double treatment on ion exchange resin.

The reagents must be of at least analytical grade. Freedom from the element to be determined must be checked in a blank experiment. If necessary, the reagents must be further purified.

In place of the standard solutions described below, commercial standard solutions may be used provided that they are guaranteed and have been checked before use.

3.1. Hydrochloric acid (d:1,19 g/ml). U.K.

3.2. Hydrochloric acid (6 mol/litre). U.K.

3.3. Hydrochloric acid (0,5 mol/litre). U.K.

3.4. Hydrofluoric acid 38 % to 40 % (v/v) having an iron (Fe) content of less than 1 mg/litre and a residue after evaporation of less than 10 mg (as sulphate)/litre. U.K.

3.5. Sulphuric acid (d: 1,84 g/ml). U.K.

3.6. Hydrogen peroxide (approximately 100 volumes of oxygen (30 % by weight)). U.K.

3.7. Standard iron solution (1 000 μg Fe/ml) prepared as follows or equivalent commercially available solution: dissolve 1 g of iron wire in 200 ml of 6 mol/litre hydrochloric acid (3.2), add 16 ml of hydrogen peroxide (3.6) and make up to one litre with water. U.K.

3.7.1. Working standard iron solution (100 μg Fe/ml) prepared by diluting one part of the standard solution (3.7) with 9 parts of water. U.K.

3.8. Standard copper solution (1 000 μg Cu/ml) prepared as follows or equivalent commercially available solution: U.K.

• dissolve 1 g of copper in powder form in 25 ml of 6 mol/litre hydrochloric acid (3.2), add 5 ml of hydrogen peroxide (3.6) and make up to one litre with water.

3.8.1. Working standard copper solution (10 μg Cu/ml) prepared by diluting 1 part of the standard solution (3.8) with 9 parts of water and then diluting 1 part of the resulting solution with 9 parts of water. U.K.

3.9. Standard manganese solution (1 000 μg Mn/ml) prepared as follows or equivalent commercially available solution: U.K.

• dissolve 1 g of manganese in powder form in 25 ml of 6 mol/litre hydrochloric acid (3.2) and make up to one litre with water.

3.9.1. Working standard manganese solution (10 μg Mn/ml) prepared by diluting 1 part of the standard solution (3.9) with 9 parts of water and then diluting 1 part of the resulting solution with 9 parts of water. U.K.

3.10. Standard zinc solution (1 000 μg Zn/ml) prepared as follows or equivalent commercially available solution: U.K.

• dissolve 1 g of zinc in strip or leaf form in 25 ml of 6 mol/litre hydrochloric acid (3.2) and make up to one litre with water.

3.10.1. Working standard zinc solution (10 μg Zn/ml) prepared by diluting 1 part of the standard solution (3.10) with 9 parts of water and then diluting 1 part of the resulting solution with 9 parts of water. U.K.

3.11. Lanthanum chloride solution: dissolve 12 g of lanthanum oxide in 150 ml of water, add 100 ml of 6 mol/litre hydrochloric acid (3.2) and make up to one litre with water. U.K.

4. Apparatus U.K.

4.1. Muffle furnace with temperature regulation and preferably recorder. U.K.

4.2. Glassware must be of resistant borosilicate type and it is recommended to use apparatus which is reserved exclusively for trace element determinations. U.K.

4.3. Atomic absorption spectrophotometer meeting the requirements of the method with regard to sensitivity and precision in the required range. U.K.

5. Procedure(1) U.K.

5.1. Samples containing organic matter U.K.

5.1.1. Ashing and preparation of the solution for analysis(2) U.K.

5.1.1.1. Place 5 to 10 g of sample weighed to the nearest 0,2 mg in a quartz or platinum crucible (see Note (b)), dry in an oven at 105 ^oC and introduce the crucible into the cold muffle furnace (4.1). Close the furnace (see Note (c)) and gradually raise the temperature to 450 to 475 ^oC over about 90 minutes. Maintain this temperature for 4 to 16 hours (e.g. overnight) to remove carbonaceous material and then open the furnace and allow to cool (see Note (d)). U.K.

Moisten the ashes with water and transfer these in a beaker of 250 ml. Wash the crucible out with a total of about 5 ml of hydrochloric acid (3.1) and add the latter slowly and carefully to the beaker (there may be a vigorous reaction due to CO₂ formation). Add hydrochloric acid (3.1) dropwise with agitation until all effervescence has stopped. Evaporate to dryness, occasionally stirring with a glass rod.

Next add 15 ml of 6 mol/litre hydrochloric acid (3.2) to the residue followed by about 120 ml of water. Stir with the glass rod, which shall be left in the beaker, and cover the beaker with a watch-glass. Bring gently to the boil and maintain at boiling point until no more ash can be seen to dissolve. Filter on ash-free filter paper and collect the filtrate in a 250 ml volumetric flask. Wash the beaker and filter with 5 ml of hot 6 mol/litre hydrochloric acid (3.2) and twice with boiling water. Fill the volumetric flask up to the mark with water (HCl concentration about 0,5 mol/litre).

5.1.1.2. If the residue in the filter appears black (carbon), put it back in the furnace and ash again at 450 to 475 ^oC. This ashing, which only requires a few hours (about three to five hours), is complete when the ash appears white or nearly white. Dissolve the residue with about 2 ml of hydrochloric acid (3.1), evaporate to dryness and add 5 ml of 6 mol/litre hydrochloric acid (3.2). Heat, filter the solution into the volumetric flask and make up to the mark with water (HCl concentration about 0,5 mol/litre. U.K.

Notes: U.K.

(a)

In determining trace elements it is important to be alert to the risks of contamination, particularly by zinc, copper and iron. For this reason, the equipment used in preparing the samples must be free of these metals.

To reduce the general risk of contamination, work in a dust-free atmosphere with scrupulously clean equipment and carefully washed glassware. The determination of zinc is particularly sensitive to many types of contamination, e.g. from glassware, reagents, dust, etc.

(b)

The weight of sample to be ashed is calculated from the approximate trace element content of the feed in relation to the sensitivity of the spectrophotometer used. For certain feed low in trace elements it may be necessary to start with a 10 to 20 g sample and make up the final solution to only 100 ml.

(c)

Ashing must be carried out in a closed furnace without injection of air or oxygen.

(d)

The temperature indicated by the pyrometer must not exceed 475 ^oC.

5.1.2. Spectrophotometric determination U.K.

5.1.2.1. Preparation of calibration solutions U.K.

For each of the elements to be determined, prepare from the working standard solutions given in points 3.7.1, 3.8.1, 3.9.1 and 3.10.1 a range of calibration solutions, each calibration solution having an HCl concentration of about 0,5 mol/litre (and (in the cases of iron, manganese and zinc) a lanthanum chloride concentration equivalent to 0,1 % La (w/v).

The trace element concentrations selected must lie within the range of sensitivity of the spectrophotometer used. The tables below show, by way of example, the compositions of typical ranges of calibration solutions; depending, however, on the type and sensitivity of spectrophotometer used it may be necessary to select other concentrations.

Iron U.K.

μg Fe/ml	0	0,5	1	2	3	4	5
ml working standard solution (3.7.1) (1 ml = 100 μg Fe)	0	0,5	1	2	3	4	5
ml HCl (3.2)	7	7	7	7	7	7	7
+ 10 ml of lanthanum chloride solution (3.11) and make up to 100 ml with water

Copper U.K.

μg Cu/ml	0	0,1	0,2	0,4	0,6	0,8	1,0
ml working standard solution (3.8.1) (1 ml = 10 μg Cu)	0	1	2	4	6	8	10
ml HCl (3.2)	8	8	8	8	8	8	8

Manganese U.K.

μg Mn/ml	0	0,1	0,2	0,4	0,6	0,8	1,0
ml working standard solution (3.9.1) (1 ml = 10 μg Mn)	0	1	2	4	6	8	10
ml HCl (3.2)	7	7	7	7	7	7	7
+ 10 ml of lanthanum chloride solution (3.11) and make up to 100 ml with water

Zinc U.K.

μg Zn/ml	0	0,05	0,1	0,2	0,4	0,6	0,8
ml working standard solution (3.10.1) (1 ml = 10 μg Zn)	0	0,5	1	2	4	6	8
ml HCl (3.2)	7	7	7	7	7	7	7
+ 10 ml of lanthanum chloride solution (3.11) and make up to 100 ml with water

5.1.2.2. Preparation of solution for analysis U.K.

For the determination of copper, the solution prepared from point 5.1.1 can normally be used directly. If necessary to bring its concentration within the range of the calibration solutions, an aliquot portion may be pipetted into a 100 ml volumetric flask and made up to the mark with 0,5 mol/litre hydrochloric acid (3.3).

For the determination of iron, manganese and zinc, pipette an aliquot portion of the solution prepared from point 5.1.1 into a 100 ml volumetric flask, add 10 ml of lanthanum chloride solution (3.11) and make up to the mark with 0,5 mol/litre hydrochloric acid (3.3) (see also point 8 ‘Observation’).

5.1.2.3. Blank experiment U.K.

The blank experiment must include all the prescribed steps of the procedure except that the sample material is omitted. The calibration solution ‘0’ must not be used as the blank.

5.1.2.4. Measurement of the atomic absorption U.K.

Measure the atomic absorption of the calibration solutions and of the solution to be analysed using an oxidising air-acetylene flame at the following wavelengths:

• Fe: 248,3 nm

• Cu: 324,8 nm

• Mn: 279,5 nm

• Zn: 213,8 nm

Carry out each measurement four times.

5.2. Mineral feed U.K.

If the sample contains no organic matter, prior ashing is unnecessary. Proceed as described in point 5.1.1.1 starting from the second paragraph. Evaporation with hydrofluoric acid may be omitted.

6. Calculation of results U.K.

Using a calibration curve, calculate the trace element concentration in the solution to be analysed and express the result in milligrams of trace element per kilogram of sample (ppm).

7. Repeatability U.K.

The difference between the results of two parallel determinations carried out on the same sample by the same analyst shall not exceed:

• 5 mg/kg, in absolute value, for contents of the trace element concerned up to 50 mg/kg,

• 10 % of the higher result for contents of the trace element concerned from 50 and up to 100 mg/kg,

• 10 mg/kg, in absolute value, for contents of the trace element concerned from 100 and up to 200 mg/kg,

• 5 % of the higher result for contents of the trace element concerned above 200 mg/kg.

8. Observation U.K.

The presence of large quantities of phosphates may interfere with the determination of iron, manganese and zinc. Such interference must be corrected by addition of lanthanum chloride solution (3.11). If, however, in the sample the weight ratio Ca + Mg/P is > 2, addition of lanthanum chloride solution (3.11) to the solution for analysis and to the calibration solutions may be omitted.

(1)

Other methods of digestion may be used provided they have been demonstrated to have similar results (such as microwave pressure digestion).

(2)

Green fodder (fresh or dried) is liable to contain large amounts of vegetable silica, which may retain trace elements and must be removed. For samples of these feed, therefore, the following modified procedure must be followed. Carry out operation 5.1.1.1. as far as the filtration. Wash the filter paper containing the insoluble residue twice with boiling water and place it in a quartz or platinum crucible. Ignite in the muffle furnace (4.1) at a temperature below 550 ^oC until all carbonaceous material has completely disappeared. Allow to cool, add a few drops of water followed by 10 to 15 ml of hydrofluoric acid (3.4) and evaporate to dryness at about 150 ^oC. If any silica remains in the residue, redissolve it in a few millilitres of hydrofluoric acid (3.4) and evaporate to dryness. Add five drops of sulphuric acid (3.5) and heat until no more white fumes are given off. After the addition of 5 ml of 6 mol/litre hydrochloric acid (3.2) and about 30 ml of water, heat, filter the solution into the 250 ml volumetric flask and make up to the mark with water (HCl concentration about 0,5 mol/l). Proceed then with the determination from point 5.1.2.