Determination of Methanol and Ethanol in Cosmetics by Agilent Gas Chromatograph

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Cosmetics are a mixture of various raw materials that have been properly formulated. Cosmetics come in a wide variety of raw materials and properties. Cosmetics are chemically mixed substances that are processed by natural, synthetic or extracted various substances as raw materials and processed by heating, stirring and emulsification. Ethanol is a kind of solvent that cosmetics often use in the production process, such as makeup remover, lotion, toilet water, perfume, hair gel, mousse, etc. If ethanol is present in a large amount in cosmetics, it will cause itching, allergies and other symptoms. Methanol is an intermediate product in the preparation of ethanol. It is highly toxic and can cause headache, dizziness, fatigue, dizziness, drunkenness, convulsions, paralysis, and even coma. Optic nerve and retinopathy can be blurred, double vision, etc. The heavy one is blind. Methanol enters the body through the respiratory tract and is metabolized to formic acid and formaldehyde in the body, both of which are much more toxic than methanol. The hygienic standard for cosmetics strictly stipulates that the methanol content should not exceed 0.2%. Increasing efforts to control the content of methanol and ethanol in cosmetics is of great significance to safeguarding the health and rights of consumers.

At present, there are reports on the detection methods of methanol and ethanol in cosmetics, mainly by colorimetry, spectrophotometry and gas chromatography. The gas chromatographic method of Agilent gas chromatograph is more effective in the determination of methanol and ethanol in cosmetics. Accurate, fast and sensitive. Based on the advantages and disadvantages of the above methods, a method for the determination of methanol and ethanol in cosmetics by gas chromatography internal standard method with quantitative accuracy, wide linear range, good reproducibility and good recovery rate was established.

1 Experimental part

1.1 Instruments and reagents

Agilent Gas Chromatograph 7890B (Agilent, USA) [equipped with FID detector and ChemStation]; DB-FFAP capillary column (60m × 0.32mm, 1.0μm, Agilent, USA); DB-1 capillary column (60m × 0.32mm, 3.0 μm, Agilent, USA; DB-624 capillary column (60 m × 0.25 mm, 1.4 μm, Agilent, USA); 0.1 mg electronic balance; vortex mixer; centrifuge; Sartorius-arium ultrapure water; Turbo Matrix 40 trap automatic headspace sampler (Perkin Elmer, USA), 22mL headspace bottle and matching aluminum cap and rubber gasket (Perkin Elmer, USA), headspace bottle capper (Perkin Elmer, USA); 0.45 Mm filter; 2 ml syringe.

Ethanol, methanol (chromatographic purity, Ministry of Agriculture Environmental Protection Research Institute); isopropanol (chromatographically pure, American TEDIA Tiandi Reagent Co., Ltd.); N, N-dimethylformamide, ethanol for liquid chromatography (Tianjin Kemi) Eurochemical Reagent Co., Ltd.); deionized water after purification by Sartorius-arium ultrapure water unit.

1.2 Chromatographic conditions

The carrier gas is high purity nitrogen, the flow rate is 1.0mL/min, constant current mode; the inlet temperature is 240 °C, the split injection is 10:1; the oven temperature is programmed: the initial temperature is 80 °C (maintained 1 min), ramped to 120 ° C at 5 ° C / min (for 2 min), then increased to 240 ° C at a rate of 35 ° C / min (held for 10 min); detector temperature: 240 ° C; injection volume of 1 μL.

1.3 Preparation of standard solutions

Accurately weigh 1.0 g of each of the methanol and ethanol standards in a volumetric flask, and determine the volume to 10 mL with N,N-dimethylformamide to prepare a standard solution stock solution of methanol and ethanol (concentration: 100 mg/mL). 1 mL of the mixed standard stock solution was taken in a 10 mL volumetric flask, and the volume was determined by N,N-dimethylformamide to prepare a standard mixed solution of methanol and ethanol (concentration: 10 mg/mL).

Take 2, 5, 20, 100, 500 μL of the standard mixture of methanol and ethanol in a 25 mL volumetric flask, accurately add 10 μL of isopropanol as an internal standard, and determine the volume of 25 mL with N,N-dimethylformamide. Then the standard working solution concentration is 0.8, 2, 8, 40, 200 mg / L, respectively, through the membrane, to be used on the machine.

1.4 Sample Processing

Weigh 5.00g of the sample into a 50mL volumetric flask, accurately add 20μL of isopropanol as the internal standard, determine the volume with N,N-dimethylformamide, and after vortexing, take a small amount of solution and transfer it to the centrifuge tube. After centrifugation at 5000 rpm for 3 min, the membrane was passed through and the machine was used.

2 Agilent gas chromatograph to determine the content and analysis of methanol and ethanol in cosmetics

2.1 Sample solvent selection

Most cosmetics are water-soluble substances, but there are also a small number of cosmetics with large oil content. In this experiment, the sample was diluted with water and N,N-dimethylformamide as a solvent. Figure 1 shows the gas chromatographic analysis of the same positive sample after dilution with the above two solvents. As can be seen from the figure, with water as the solvent, the baseline and noise are higher. In addition, the water phase will cause some damage to the column when entering the capillary column, and the repeatability is poor. N,N-dimethylformamide as a solvent has good repeatability and reproducibility, and can dissolve various types of cosmetics. Therefore, this experiment uses N,N-dimethylformamide organic phase as a sample. Solvent.


2.2 Selection of internal standard substances

Methanol and ethanol have low boiling points and are extremely volatile substances. If the external standard method is used for quantification, methanol and ethanol will be slightly volatilized during the preparation of the standard working solution, resulting in inaccurate sample quantification. An internal standard substance close to the boiling point of both should be selected to eliminate the quantitative deviation. Therefore, in this paper, isopropanol is used as an internal standard substance to simultaneously detect the methanol and ethanol content in cosmetics.

2.3 Selection of chromatographic conditions

In this experiment, the samples were separated by non-polar DB-1 capillary column, medium polarity DB-624 capillary column and strong polarity DB-FFAP capillary column. Figure 2 shows the separation spectrum of a standard working solution of 20 mg/L on the above three capillary columns. It can be seen from the figure that the peak shape of the DB-624 capillary column is poor, the tailing is serious, and the resolution is not good; the peak shape of the DB-1 capillary column is better, but also slightly tailed. The DB-FFAP capillary column has a symmetrical peak shape, and the isopropanol retention time is between methanol and ethanol, which has a good separation between the two, and the retention time difference between isopropanol and methanol and ethanol is less than 2.8 min. More favorable to quantitative accuracy. Therefore, this experiment selected DB-FFAP strong polarity capillary column for sample analysis.




2.4 Standard curve and detection limit

In this experiment, the internal standard method was used for quantification. The standard solution was diluted with N, N-dimethylformamide into standard working solutions of different concentrations, and 1 μL was injected under the determined chromatographic conditions to obtain the peak area (Y). The concentration (X, mg/L) was linearly regressed and estimated by 3 times the signal-to-noise ratio. The low detection concentration of Zui in this method was 0.2 mg/L, and the detection limit was 2 μg/g.

Table 1 shows the linear equations, correlation coefficients, linear ranges, relative errors, and measurement times of the internal standard method. It can be seen from the table that the internal standard method for determining the content of methanol and ethanol in cosmetics has a wide linear range, relatively low relative error, and saves a large amount of detection time.

Table 1 Results of linear regression test of internal standard method and headspace method

method   name   Linear equation    Correlation coefficient    Linear range    Relative error   Measurement timeInternal standard method

Methanol   Y=0.682499X+0.20493    R2=0.9998      0.8~500mg/L       <2.1%       0.5h

Ethanol      Y=1.840692X+0.62135    R2=0.9997      0.8~500mg/L        <2.4%      0.5h


2.5 Method recovery and precision

Weigh 5.0g of blank samples of different viscosities separately, add appropriate amount of standard mixed solution, and make methanol and ethanol concentrations equivalent to 20, 60, 150 and 300μg/g. Add and recover these four addition levels according to 1.4 operation steps. Each level was repeated 6 times, and four levels of addition recovery were obtained. Table 2 gives the results of the blank sample recovery experiment of the internal standard method. From the table, the recovery rate of the sample obtained by gas chromatography internal standard method can reach 92.7~104.7%.


Recovery test results (n=6)

Add level     name      Frost A recovery rate (%)      Milk B recovery rate (%)      Water C recovery rate (%)



                    Methanol                 94.4                     100.1                         103.2

20                Ethanol                    92.5                      100.9                         103

                   Methanol                 93.8                       99.8                          103.1

60               Ethanol                    93.3                       98.7                           103.4

                   Methanol                94.6                        100.6                         104

150             Ethanol                    94                           99.9                          103.5

                   Methanol                92.7                        98.3                          104.7

 300            Ethanol                     93.1                       99.5                         104.5


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