Abstract: Abstract: This testing method originated from the notice on the issuance of guidelines and plans for the national rectification and random sampling inspection work against illegal addition of non-edible substances and abuse of food additives.

1 Principle After the liquid milk sample precipitates protein and removes fat, it is analyzed by ion chromatography, detected by a conductivity detector, and quantified by an external standard method.

2 Experimental part 2.1 Reagents and materials 2.1.1 The test water is ultrapure water 2.1.2 Acetonitrile (chromatographically pure)

2.1.3 Small column for solid phase extraction: OnGuard RP column (2.5cc), or equivalent (such as C18), activated with 5 ml of methanol and 10 ml of water before use.

2.1.4 Standard thiocyanate: Beijing Chemical Plant 2.1.5 Standard thiocyanate stock solution Dry the standard thiocyanate in an 80 degree oven for 2 hours. Accurately weigh 1.6732g of dried potassium thiocyanate in a 1000ml volumetric flask, make a volume and mix well. A standard stock solution of 1000 ppm thiocyanate is obtained.

2.1.6 The standard intermediate solution of thiocyanic acid takes 1 mL of the standard stock solution of thiocyanic acid, put it in a 100 mL volumetric flask, and add water to the mark. This solution contains 10 mg / L thiocyanic acid.

2.1.7 Thiocyanic acid standard use solution Pipette 0.1, 0.2, 0.5, 1.0, 2.0 mL of thiocyanic acid standard intermediate solution, dilute with water to a 10 mL volumetric flask, the concentration is 0.1, 0.2, 0.5, 1.0, 2.0 mg / L.

2.2 Instruments 2.2.1 Ion chromatograph: equipped with eluent generator and conductivity detector; 2.2.2 centrifuge: refrigerated centrifuge.

2.3 Sample processing Take 4mL of liquid milk sample, add 5mL of acetonitrile to precipitate protein, take supernatant and dilute 10 times, pass RP column (or freeze centrifuge) to remove fat and then go to the machine.

2.4 Reference conditions for ion chromatography Column: strong hydrophilic anion exchange column. IonPac AS16, 4.0 × 250mm analytical column; IonPac AG16, 4.0 × 50mm guard column; or other equivalent.

Mobile phase: KOH solution, gradient elution. The eluent is generated online by the eluent online generator.

Flow rate: 1.0 mL / min; suppressor: ASRS-300 type suppressor, 4 mm; suppressor suppression mode: external water mode, suppression current 175 mA; column temperature: 30 ° C; injection volume: 100 μL.

3 Result calculation X = c * 9 * 10/4 ………………………… (1)

In the formula: X——the content of thiocyanic acid in liquid milk, the unit is microgram per milliliter (μg / mL); c——the concentration of thiocyanic acid in the sample solution obtained from the standard curve, the unit is microgram per milliliter (μg / mL); 9—the sum of the volume of liquid milk and the volume of acetonitrile, in milliliters (mL); 4—the volume of liquid milk, in milliliters (mL); 10—dilution multiple; the calculation result retains two digits effective number.

4 Precision Under repeatable conditions, the absolute difference between two independent test results of thiocyanic acid obtained is not greater than 5% of its arithmetic mean.

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