Detection of hydroxypropyl starch as a food additive

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A. 5  Determination of Chloropropanol

A.5.1 Reagents and Materials

A.5.1.1  Anhydrous ether

A.5.1.2; Chloropropanol: Contains 75% - chloro-2-propanol and 25% - chloro-1-propanol

A.5.1.3 Silica: 250um~150um

A.5.1.4 Sulfuric acid

A.5.1.5; Sodium hydroxide

A.5.1.6; Anhydrous sodium sulfate

A.5.1.7 Sulfuric acid solution: c (1/2H2SO4)=2mol/L. Measure 60mL of sulfuric acid, slowly inject 1000mL of water, cool, and shake well

A.5.1.8; Sodium hydroxide solution: 25%. Weigh 25g of sodium hydroxide and dissolve it in 75mL of water

A.5.1.9 Waxy corn starch: Unsaturated waxy corn starch

A.5.2 Instruments and Equipment

A.5.2.1 Gas Chromatography: It is recommended to use a dual column chromatograph equipped with a flame ionization detector.

A.5.2.2 Pressure bottle: A 200mL pressure bottle equipped with a neoprene pad and glass stopper

A.5.3 Chromatographic Column and Typical Chromatographic Operating Conditions

A.5.3.1 Chromatographic Column: Capillary Column, 30m × 0.32mm (inner diameter), filled with polyethylene glycol 20M (PEG20M)

A.5.3.2 Column box temperature: Program temperature rise, 50 ℃ insulation for 5 minutes, increase to 220 ℃ at a rate of 10 ℃/min, and insulation for 2 minutes

A.5.3.3 Injection port temperature: 250 ℃

A.5.3.4 Detector temperature: 250 ℃

A.5.3.5 The combustion gas is hydrogen (47mL/min), the auxiliary gas is air (400mL/min), and the compensation gas is nitrogen (30mL/min)

A.5.3.6 The carrier gas is nitrogen (25mL/min) with a split ratio of 2:1

You can also choose a chromatographic column with equivalent separation effect and corresponding chromatographic conditions

A.5.4 Analysis Steps

A.5.4.1 Preparation of Chloropropanol Standard Solution

Use a 10uL syringe to take 5uL of Chloropropanol.

. Accurately weigh the mass of the syringe and inject the sample into a 500mL volumetric flask containing some water. Reweigh the mass of the syringe and record the difference as the mass of chloropropanol added. Dilute with water to the mark and mix evenly. The solution contains approximately 12.5ug/mL of chloropropanol. This solution needs to be prepared and used immediately

A.5.4.2 Sample Preparation

Weigh 10g of the sample to the nearest 0.001g, place it in a pressure bottle, add 25mL of sulfuric acid solution, clamp the top of the bottle, and rotate the sample until it is completely dispersed.

. Heat the bottle in a boiling water bath for 10 minutes, then rotate the bottle to fully mix its contents. Continue heating in the boiling water bath for 15 minutes, cool to room temperature in air, and then mix it with a sodium hydroxide solution to pH=7. Add 7g of anhydrous sodium sulfate and stir for 5-10 minutes with a magnetic stirrer until the sodium sulfate is completely dissolved. Filter with a absorbent cotton and wash the pressure resistant bottle with a small amount of water. Transfer the solution to a 500mL separator equipped with polytetrafluoroethylene, extract 5 times with 50mL anhydrous ether, and maintain each extraction for at least 5 minutes to achieve phase separation. Transfer all ether extracts to a graduated flask, place in a water bath at 50 ℃ to 55 ℃, and concentrate to 4mL

Note: The ether extract of the sample may contain foreign residual impurities that interfere with the resolution of the chromatogram. These residual impurities may be degradation products caused by the hydrolysis process. The problem caused by this can be solved by the following purification treatment: concentrate the ether extract to about 8mL to replace the aforementioned 4mL. Heat 10g of silica soil at 130 ℃ for 16 hours, fill it into a corresponding sized chromatographic column and gently tap it. Add 1g of anhydrous sodium sulfate to the top of the column, wet the column with 25mL of anhydrous ether, and use a small amount of ether to assist in quantitatively passing the concentrated extraction solution through the column. Wash with 25mL anhydrous ether three times, collect the eluent and transfer it to a concentrator for concentration to 4mL

Cool the extract to room temperature, use a small amount of anhydrous ether to assist in quantitative transfer to a 5.0mL volumetric flask, dilute with anhydrous ether to the required volume, and mix evenly

A.5.4.3 Preparation of reference solution

Add 10g of waxy corn starch to the nearest 0001g in 5 pressure bottles, and add 25mL of sulfuric acid solution to the bottles.

. Then add 0.05L, 0.1mL, 0.2mL, 0.4mL, and 1.0mL of chloropropanol standard solution, so that their concentrations are 0mg/kg, 0.1mg/kg, 0.2mg/kg, 0.4mg/kg, and 1.0mg/kg according to starch, respectively. Calculate the exact concentration of chloropropanol in the bottle based on the mass of chloropropanol in the standard solution of chloropropanol. Clamp the top of the bottle and rotate the sample in the bottle until it is completely dispersed. Then start from A.5.4.2 "Putting the bottle into a boiling water bath..."

A.5.4.4 Determination

Inject 1.0uL of the reference solution, and there should be sufficient time between each injection to ensure complete recording of the corresponding signal peaks of the two isomers of chloropropanol, and clean the chromatographic column.

. Record and add the signal peak areas of two chloropropanol isomers with reference to their respective reference samples. Using the same operating conditions, inject 1.0uL of concentrated extract from the sample, record and add the signal peak area of the sample

The calibration curve is obtained by converting the actual mass of chloropropanol isomers used into chloropropanol concentration (mg/kg) based on the peak area of each reference signal. The concentration of mixed chloropropanol in the sample (mg/kg) is determined by using the sum of peak areas corresponding to 1-chloro-2-propanol and 2-chloro-1-propanol in the sample. When mastering the entire operation process proficiently and ensuring that the calibration curve obtained from the reference sample is linear and reproducible, the number of reference samples can be reduced to one, and a mixture containing approximately 5mg/kg of chloropropanol isomers can be obtained

A.5.4.5 Result Calculation

Chloropropanol w1 is expressed in mg/kg, and calculated according to formula (A.2):

In the formula:

C - the concentration of chloropropanol (sum of isomers) in the reference solution, in milligrams per kilogram (mg/kg)

A1- The total area of signal peaks generated by chloropropanol isomers in the sample solution

A2- Sum of signal peak areas generated by chloropropanol isomers in the same solution

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