Determination of ethanol content

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Section 11: Determination of Ethanol Content 1. Method Summary: Gas chromatography is used, under selected operating conditions, to separate impurities such as ethanol from methanol, with detection by a flame ionization detector. The quantitative correction factor was determined, and the mass fraction of ethanol was calculated using the internal standard method. II Reagents and Materials (1) Anhydrous ethanol ； (2) Isopropanol [(CH3)2CHOH] ； Chromatographically pure, internal standard ； (3) Sorbitol (C6H14O6): Chromatographic stationary phase ； (4) Pickled 6201 type carrier: 0.18mm~0.25mm ； (5) Methanol ； The mass fraction of ethanol shall not exceed 0.001%; if the ethanol content is higher than this value, the background value shall be deducted ； (6) Hydrogen: volume fraction of not less than 99%, dried and purified using silica gel and molecular sieves ； (7) Nitrogen: volume fraction of not less than 99.95%, dried and purified using silica gel and molecular sieves ； (8) Air: Dried and purified using silica gel and molecular sieves. 3. Analytical instruments: (1) Gas chromatograph: Any model of gas chromatograph equipped with a flame ionization detector, whose sensitivity and stability meet the requirements specified in GB/T 9722－1988. (2) Chromatography column: stationary phase preparation ； Weigh 30 g of sorbitol and place it in a 600 ml beaker; add 300 ml of methanol to dissolve it. Place the beaker in a water bath and warm it slightly; once sorbitol has dissolved, add 70 g of the acid-washed 6201 type support. Stir gently, and after the methanol solvent has evaporated, place the mixture in an oven for drying before sieving it for later use. The packing and aging of the chromatography column shall be carried out in accordance with the provisions of 7.1.3 and 7.1.4 in GB/T 9722-1988. (3) Recorder: Chromatography data processor or recorder ； (4) Micropipettes: 10μL, 100μL. (5) Typical operating conditions: The chromatography columns recommended by this standard and their typical operating conditions are shown in Table 2. Other chromatography columns and operating conditions that can achieve the same level of separation may also be used. The typical chromatogram is shown in Figure 1, and the typical retention times are listed in Table 3. Chromatography column and typical operating conditions: Column length/column inner diameter: 5 m–6 m/3 mm–4 mm; Column oven temperature: 100°C; Vaporization chamber temperature: 150°C; Carrier gas (nitrogen) flow rate: 30 ml/min–40 ml/min; Hydrogen flow rate: 40 ml/min; Air flow rate: 500 ml/min–600 ml/min; Stationary phase: Sorbitol: acid-washed 6201 = 30:70; Injection volume: 2 μL–10 μL. Analysis steps: (1) Preparation of isopropyl alcohol internal standard solution: Take 0.5 ml of isopropyl alcohol and place it in a 100 ml volumetric flask, then dilute to the mark with methanol and mix well. (2) Determination of the correction factor: Add 0.5 ml of ethanol to a 100 ml volumetric flask that contains a known mass of dry substance; weigh it – the mass of ethanol is m1. Then dilute with methanol to the mark and weigh again; the mass of the solution is m2. This solution constitutes the ethanol standard solution. Take 6 volumes of dry 25 ml methanol, and using a micropipette (or microliter pipette), add 100 μl of the isopropanol internal standard solution to each, as well as 0 ml, 0.05 ml, 0.10 ml, 0.20 ml, 0.50 ml, and 1.00 ml of the ethanol standard solutions respectively. Dilute to the mark with methanol and mix well; this solution serves as the calibration standard solution (the one containing 0 ml of ethanol standard solution is the blank solution). The peak areas of ethanol and isopropyl alcohol are measured separately; then, the area of the ethanol peak in the blank sample is subtracted to obtain the corrected peak area. Subsequently, the quantitative correction factor (f’) is calculated using equation (3): file:///C:\Users\lenovo\AppData\Local\Temp\ksohtml\wpsEF0E.tmp.png file:///C:\Users\lenovo\AppData\Local\Temp\ksohtml\wpsEF2E.tmp.png f’= ———————— Where: m1 ---------- the mass of ethanol in the standard ethanol solution, in grams (g) ； m2---------The value of the mass of the ethanol standard solution, in grams (g) ； V-----------The value of the volume of the ethanol standard solution, in milliliters (ml) ； Ai---------Adjusted peak area of ethanol ； As--------isopropanol peak area ； V1-------The value of the volume of the calibration standard solution, in milliliters (ml) (V1=25). The average quantitative correction factor (f’) is calculated from the individual quantitative correction factors. (3) Sample determination: Take a dry 25 ml volumetric flask, inject 100 μl of isopropanol internal standard solution using a micropipette, dilute to the mark with the sample, and mix well. Measure the peak areas of ethanol and isopropanol. V. Calculation of results: The mass fraction w3 of ethanol, expressed as a percentage, is calculated using equation (4): w3 = f’ × Ai/As. Where: f’------------- Average quantitative correction factor ； Ai--------------Ethanol peak area ； As--------------is the peak area of isopropanol. 6. The allowable value is taken as the arithmetic mean of the results of two parallel measurements. When the mass fraction of ethanol is less than or equal to 0.01%, the relative deviation between the results of two parallel measurements is not greater than 30% ； When the mass fraction of ethanol is greater than 0.01%, the relative deviation between the results of two parallel measurements is not more than 10%.