Determination of sorbic acid and benzoic acid content in beverages (2)

4.
Safety reminder

Ethyl ether is narcotic, petroleum ether is extremely flammable, can cause peripheral neuritis, and has a strong irritation to the skin; pre-treatment operations should be carried out in a fume hood, open flames, inhalation, and skin contact should be avoided
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5.
Operation steps

(1) Sample extraction Weigh 2.
50g pre-mixed sample, put it in a 25mL graduated cylinder with stopper, add 0.
5mL hydrochloric acid (1+1) to acidify, extract twice with 15,10mL ether , shake for 1min each time , The upper ether extract is sucked into another 25mL graduated cylinder with a stopper, and the ether extract is combined
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(2) Chromatographic reference conditions

Chromatography column: glass column, inner diameter of 3mm, length of 2m, 60-80 mesh Chromosorb WAW coated with 5% DEGS+1% phosphoric acid fixative
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Air flow rate: the carrier gas is nitrogen, 50mL/min (the ratio of nitrogen to air and hydrogen is selected according to the different instrument models to select the respective optimal ratio conditions)
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Temperature: inlet 230℃; detector 230℃; column temperature 170℃
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(3) Chromatogram The gas chromatograms of sorbic acid and benzoic acid are shown in Figure 5-1
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Figure 5-1 Gas chromatogram of sorbic acid and benzoic acid

The retention time of sorbic acid is 2min 53s, and the retention time of benzoic acid is 6min 8s
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(4) Determine the concentration of each concentration standard in the 2uL standard series of sample injection in a gas chromatograph.

At the same time, 2uL sample solution was injected, and the measured peak height was compared and quantitative with the standard curve

6.

Where x-the content of benzoic acid or sorbic acid in the sample, mg/kg

m ₁ ——The quality of benzoic acid or sorbic acid in the sample solution for determination, ug

V ₁ - was added petroleum ether - diethyl ether (3 + 1) mixed solvent volume, mL

V ₂ ——Injection volume during measurement, uL

m ₂ ——the mass of the sample, g

5—— The volume of the ether extract in the measurement , mL

25——Total volume of sample ether extract, mL

The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of the arithmetic mean

7.

(1) The ether extract should be fully dehydrated with anhydrous sodium sulfate, and water in the sample solution will affect the measurement results

(2) Problems that should be paid attention to when sampling:

Do not hold the needle of the syringe and the sample area; do not have air bubbles in the syringe (when aspirating the sample, it should be slow, fast discharge and then slowly aspirate.

Newbies doing chromatographic analysis often bend the syringe needle and syringe stem because the injection port is screwed too tightly (for example, if the screw is too tight at room temperature, the silicone gasket will expand when the temperature of the vaporization chamber rises.

(3) The main methods to improve the resolution are:

① Increase column length;

②Reduce the amount of sample injection (increase the amount of solvent for solid samples);

③Improve the sampling technology to prevent two injections;

④Appropriately reduce the carrier gas flow rate and chromatographic column temperature (too much decrease will make the chromatographic bee wider);

⑤Increase the temperature of the vaporization chamber;

⑥ The capillary chromatographic column should be split, and an appropriate split ratio should be selected;

⑦Reduce the dead volume of the system.

In short, it must be explored in practice according to specific conditions