GB/T 21126-2026Determination of sodium formaldehyde sulfoxylate in wheat flour, rice flour and their products (English PDF)

5.1 Reagents

5.1.1 n-Hexane (C6H14). chromatographic grade.

5.1.2 Acetonitrile (C2H3N). chromatographic grade.

5.1.3 Phosphoric acid (H3PO4).

5.1.4 Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O). 5.1.5 2,4-Dinitrophenylhydrazine (C6H6N4O4).

5.1.6 Sodium chloride (NaCl).

5.1.7 Hydrochloric acid (HCl).

5.1.8 Anhydrous ethanol (C2H6O).

5.2 Reagent Preparation

5.2.1 Hydrochloric acid-sodium chloride solution [6% (volume fraction) hydrochloric acid; 20 g/L sodium chloride]. Weigh 20 g of sodium chloride into a 1000 mL volumetric flask. Dissolve in a small amount of water, add 60 mL of hydrochloric acid, and then add water to the mark.

5.2.2 Disodium hydrogen phosphate solution (0.5mol/L). Weigh 18g of disodium hydrogen phosphate dodecahydrate, dissolve in water and bring the volume to 100mL.

5.2.3 Purification of 2,4-dinitrophenylhydrazine. Weigh approximately 20 g of 2,4-dinitrophenylhydrazine into a beaker, add 167 mL of acetonitrile and 500 mL of water, and mix thoroughly. Stir to dissolve, filter out insoluble impurities using qualitative filter paper, and let the filtrate stand overnight. Filter the crystals through qualitative filter paper, and react with water and anhydrous ethanol respectively. After washing 5 to 6 times, place in a dryer for later use.

5.2.4 Derivatization solution (2 g/L). Weigh.200 mg of 2,4-dinitrophenylhydrazine prepared in 5.2.3, dissolve it in acetonitrile and bring the volume to 100 mL. Incubate at room temperature. Store away from light.

5.3 Standard Products Formaldehyde (CH2O, CAS No.. 50-00-0) standard solution. 1000 mg/L, or other nationally certified and certified standard samples. Standard reference materials.

Note. Formaldehyde standard solution can also be prepared and calibrated using 36%~38% formaldehyde solution. For specific procedures, please refer to Appendix A of GB 5009.307-2025.

5.4 Preparation of Standard Solutions

5.4.1 Formaldehyde standard intermediate solution (100 µg/mL). Accurately pipette 1 mL of 1000 mg/L formaldehyde standard solution, dilute with water and bring to a final volume. Prepare a 10 mL solution of formaldehyde standard intermediate with a mass concentration of 100 µg/mL. Store at 2°C~8°C away from light. Shelf life is 3 months.

5.4.2 Formaldehyde Standard Working Solution. Accurately measure a certain amount of formaldehyde standard intermediate solution and dilute it with water to prepare a 2µg/mL formaldehyde standard working solution. Prepare as needed.

5.5 Materials

5.5.1 Centrifuge tubes. 20mL, 50mL.

5.5.2 Organic filter head. 0.45µm pore size.

7.1 Sample Preparation

7.1.1 Preparation of powdered samples. For wheat flour and rice flour samples, take no less than 500g of representative sample from all samples and directly pack it into a clean container. Store in a clean, sealed container for later use.

7.1.2 Preparation of dried samples. For dried noodles and dried rice noodles, take a representative sample of no less than 500g from all samples, and cut or chop it. Use a knife grinder (6.5) to grind the particles to a size of less than 1 mm, mix them well, put them into a clean container, seal and store for later use.

7.1.3 Preparation of wet samples. For steamed buns, fresh noodles or rice noodles, wet rice noodles, and fresh rice sheets, take no less than 500g from the total sample. Representative samples were chopped into particles no larger than 5 mm, mixed thoroughly, and placed in a clean container, sealed, and refrigerated for later use.

7.2 Sample Pretreatment

7.2.1 Formaldehyde Extraction For dried or powdered samples, weigh 5g (accurate to 0.01g) and place it in a 150mL Erlenmeyer flask with a stopper (6.1), then add 50mL of hydrochloric acid-chlorine mixture. Sodium solution (5.2.1) was placed on a reciprocating shaker (6.6) and shaken for 40 min to extract. For wet sample preparation, weigh 20g (accurate to 0.01g) and place it in a tissue homogenizer (6.7), add.200mL of hydrochloric acid-sodium chloride solution (5.2.1). After crushing for 5 minutes, transfer the entire contents to a 250 mL Erlenmeyer flask with a stopper (6.1), place it on a reciprocating shaker (6.6), and shake to extract for 40 minutes. Take about 20 mL of the extract into a centrifuge tube (5.5.1) and centrifuge at 10000 r/min for 15 min in a high-speed centrifuge (6.9). Collect the supernatant. spare.

7.2.2 Derivatization Measure

2.00 mL of formaldehyde extract (7.2.1) into a 50 mL centrifuge tube (5.5.1), and add 1 mL of disodium hydrogen phosphate solution (5.2.2). Add

0.5 mL of derivatization solution (5.2.4) to a final volume of 10 mL with water and mix well. Place the centrifuge tube in a constant temperature water bath (6.10) and heat at 50°C for 40 minutes. Take the centrifuge tube... Cool to room temperature with running water.

7.2.3 Derivative Extraction Accurately add

5.00 mL of n-hexane (5.1.1) to the solution in the centrifuge tubes, and place the centrifuge tubes on a vortex mixer (6.8) for vortexing. Swirl for 1 min, then centrifuge at 10000 r/min for 5 min in a high-speed centrifuge (6.9). Take the supernatant organic solution and pass it through an organic filter (5.5.2). Filter and store in a

1.5 mL sample vial for later use.

7.3 Instrument Reference Conditions

7.3.1 Chromatographic column. C18 column (4.6mm×250mm, 5µm), or a chromatographic column with equivalent performance.

7.3.2 Column temperature. 30°C.

7.3.3 Mobile phase. Acetonitrile-water (volume ratio 70.30).

7.3.4 Flow rate.

0.8 mL/min.

7.3.5 Detection wavelength. 350nm.

7.3.6 Injection volume. 10 µL.

7.4 Creation of Standard Working Curves Measure

0.00 mL,

0.10 mL,

0.25 mL,

0.50 mL,

4.00 mL of formaldehyde standard working solution (5.4.2) respectively into... Add 2 mL of hydrochloric acid-sodium chloride solution (5.2.1) to a 50 mL centrifuge tube (5.5.1), and follow the derivatization and derivative extraction steps in

7.2 to obtain... Prepare a series of formaldehyde standard working solutions. Inject the standard working solutions sequentially into the high-performance liquid chromatograph according to the formaldehyde mass (0.0 µg, 0.2 µg, 0.5 µg, 1.0 µg, 2.0 µg, 4.0 µg, 8.0 µg), from low to high, and determine the corresponding peak areas (see Appendix A, A.1). A standard working curve was plotted with formaldehyde mass as the x-axis and the peak area of formaldehyde derivative phenylhydrazone as the y-axis.

7.5 Measurement The sample solution was injected into a high-performance liquid chromatograph (6.11) for qualitative analysis based on retention time, and the concentration of methyl methacrylate (M) in the sample solution was determined based on the corresponding peak area. Aldehyde mass (see A.2, A.3). If the response value of formaldehyde derivatives in the sample solution exceeds the range of the standard curve, the determination of formaldehyde mass should be based on the measured formaldehyde content. The sample was diluted appropriately before analysis.

8.Results Presentation The formaldehyde sodium bisulfite content (calculated as formaldehyde) in the sample is calculated according to formula (1).