Determination of total arsenic in feeds
1 Scope
This document describes the silver diethyl-dithiocarbamate method, hydride generation - atomic fluorescence spectrometry and inductively coupled plasma mass spectrometry for the determination of total arsenic in feeds.
This document is applicable to the determination of total arsenic in formula feed, concentrated feed, concentrate supplement, additive premix, feedstuffs and feed additives.
If the sampling amount is 5g and the constant volume is 50mL, the limit of detection of silver diethyl-dithiocarbamate method is 0.25mg/kg, the limit of quantitation is 0.50mg/kg. If the sampling amount is 5g, the constant volume is 50mL and the dilution ratio is 20, the limit of detection and the limit of quantitation of the hydride generation - atomic fluorescence spectrometry and the inductively coupled plasma mass spectrometry are 0.02mg/kg and 0.05mg/kg respectively.
2 Normative references
The following documents contain requirements which, through reference in this text, constitute provisions of this document. For dated references, only the edition cited applies. For undated references, the latest edition (including any amendments) applies.
GB/T 6682 Water for analytical laboratory use - Specification and test methods
GB/T 20195 Animal feeding stuffs－Preparation of test samples
3 Terms and definitions
No terms and definitions apply for the purposes of this document.
4 Silver diethyl-dithiocarbamate method (arbitration method)
4.1 Principle
After the test sample is treated by acid digestion, direct acid dissolution or dry ashing, the high-valent arsenic is reduced to trivalent arsenic with potassium iodide and stannous chloride, and then hydrogen arsenide is generated from the reaction with fresh hydrogen generated by adding zinc particles and acid. The hydrogen arsenide will be absorbed by silver diethyldithiocarbamate (Ag-DDTC) - triethylamine-chloroform solution to form a red gel-like matter. The color shade of the gel-like matter is directly proportional to the arsenic content. The absorbance is determined with a spectrophotometer and is quantified by comparing with the standard series solution.
4.2 Reagents or materials
Warning: All kinds of strong acids shall be carefully handled, and shall be diluted and taken in a a fume cupboard. When using perchloric acid, care shall be taken to avoid dryout and explosion.
Unless otherwise specified, only analytical reagents are used.
4.2.1 Water: Grade II water as specified in GB/T 6682.
4.2.2 Nitric acid.
4.2.3 Hydrochloric acid.
4.2.4 Chloroform.
4.2.5 Magnesium oxide.
4.2.6 Magnesium nitrate.
4.2.7 L-ascorbic acid.
4.2.8 Arsenic-free zinc granule: with grain size of 3.0mm±0.2mm.
4.2.9 Mixed acid solution: nitric acid+perchloric acid+sulfuric acid = 230+50+30, mix well and store in a brown reagent bottle.
4.2.10 Hydrochloric acid solution (6mol/L): take 250mL of hydrochloric acid (4.2.3) and add 250mL of water, then mix them well.
4.2.11 Magnesium nitrate solution: weigh 180g of magnesium nitrate [Mg(NO3)2·6H2O], dissolve it with water, dilute the solution to 1,000mL, and mix well.
4.2.12 Potassium iodide solution (150g/L): weigh 75g of potassium iodide, and dissolve it with water, dilute the solution to 500mL, and mix well, then store the obtained solution in a brown bottle.
4.2.13 Acidic stannous chloride solution: weigh 40g of stannous chloride (SnCl2·2H2O), dissolve it with 100mL of hydrochloric acid (4.2.3), and then add several tin granules, store the obtained solution in a brown reagent bottle. The solution is available within one week.
4.2.14 Silver diethyldithiocarbamate (Ag-DDTC) - triethylamine-chloroform solution (2.5g/L): put 2.5g (to the nearest 0.0001g) of Ag-DDTC in a dry beaker and add a proper amount of chloroform, add 20mL of triethylamine after complete dissolution, then transfer the solution to a 1,000mL volumetric flask with chloroform, dilute to the scale and mix well, store the solution at 2℃~8℃ in a brown reagent bottle and keep away from light. In case of any precipitate, the solution shall be used after filtration.
4.2.15 Lead acetate solution (200g/L): weigh 40g of lead acetate, dissolve it with water to 200mL and mix well.
4.2.16 Lead acetate cotton: soak the medical absorbent cotton in the lead acetate solution (4.2.15) for 1h, extrude to remove excess lead acetate solution, then keep the medical absorbent cotton loose and air-dry it, or bake it at 90℃~100℃, and store the obtained solution in a closed bottle.
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Silver diethyl-dithiocarbamate method (arbitration method)
5 Hydride generation - atomic fluorescence spectrometry
6 Inductively coupled plasma mass spectrometry
Annex A (Informative) Microwave digestion reference conditions
