Application of Gas Chromatography in Analysis of Cyclohexylamine

Abstract : The combination of better chromatographic conditions was determined by experiments. The established method has good accuracy and repeatability.

The aniline is vaporized and hydrogenated with the preheated hydrogen under the action of a catalyst to produce crude cyclohexylamine, which is purified by distillation to obtain cyclohexylamine. Cyclohexylamine is mainly used in the production of food additives cyclamate, synthetic rubber accelerators, desulfurizers, corrosion inhibitors, water treatment, synthetic pesticides, petroleum product additives, gel coagulants and high-grade dyes. Since cyclohexylamine is strongly alkaline and can react with acids to form salts quantitatively, it can be analyzed for its purity by acid-base titration. However, the results of this method have no obvious effect on the content of cyclohexylamine impurities, while the cyclohexylamine industry standard HG. /T2816-1999 There are strict control indicators for aniline. We use gas chromatography. Due to the use of HP5 silica capillary columns and the introduction of program temperature, some operating conditions (such as carrier gas flow rate, column temperature, gasification temperature, and injection volume) are based on specific instrument performance during operation. Adjust appropriately to find better combinations. For this reason, we conducted experiments.

l, the main instruments and reagents 1.1 Instruments (a) HP4890D gas chromatograph;

(b) HP5 capillary column (non-polar);

(c) Detector: Flame Ionization Detector (FID);

(d) Logger: HP3955 integrator;

(e) Sample injector: 10 μL microsyringe.

1.2 Reagents (a) Cyclohexane;

(b) cyclohexylamine;

(c) Aniline;

(a)-cyclohexylamine;

(e) Anhydrous ethanol. The above reagents use analytical grade.

1.3 Air source (a) High-purity nitrogen gas number > 99.999%;

(b) High purity hydrogen volume fraction >99.999%;

(c) Dry compressed air.

2, chromatographic instrument operating conditions (1) detection temperature 280 °C;

(2) gasification temperature 250 °C;

(3) column temperature 50 °C;

(4) Initial time 1min;

(5) Heating rate 10 °C/min;

(6) End temperature 100 °C;

(7) End time 3min;

(8) carrier gas (nitrogen) flow rate 35mL/min;

(9) Combustion gas (hydrogen) flow rate 43mL/min;

(10) Combustion gas (compressed air) flow rate 400mL/min;

(11) The split ratio is 12:1. In the determination of the absolute weight correction factor, in order to better emphasize the peak area, we use the “climbing method” conditional test for the split ratio parameter from 10:1 to 11:1, 12:1, and 15:1. 20:

1, 25:1, 30:1, 35:1, and 40:1 were tested separately to compare the reproducibility of peak areas. Progressively find a better split ratio of 12:1.

(12) Injection volume: When using the normalization method, the peak height of the subject (or after attenuation) should be more than 70% of the full scale on the recorder. To meet this condition, the sample was diluted with ethanol before treatment, since the exact concentration was not required by the normalization method. Our approach is to use drops of 2 drops of cyclohexylamine product, diluted with absolute ethanol to about 2mL, injection volume of 1.0mL, so that the analog input signal of the measured peaks of each component is less than 1000mV, in order to achieve quantitative .

(13) Column temperature: Capillary columns are generally programmed to heat up. Based on the retention time of the two impurity components of aniline and dicyclohexylamine monitored by the cyclohexylamine industry standard HG/T2816-1996 relative to the main cyclohexylamine and the boiling point and polarity of each component, an empirical test was conducted. . For example, the most polar bicyclohexylamine can peak before 8.5 min under various test conditions. The retention time is not very close, so we decided to use a first-order procedure to increase the temperature, and the initial temperature, initial time, heating rate, and end The temperature and the end time are determined by the physicochemical properties of the solvent and the components, and empirical tests are also performed to evaluate the chromatograms. The two program heating programs (a) and (b) are used to obtain the main body of the area normalization method. There is no significant difference in quality scores. From the chromatogram, the separation effect between the components was good. The total peak time of the sample was about 8.5 min, which is suitable for normal sample analysis. It can be seen that the feasible program for temperature increase is not the only one, and the program temperature increase program (a) is adopted in the accuracy and precision of the absolute weight correction factor determination and evaluation method.

3. Parameters of the accumulative instrument 4. Selection of quantitative methods 4.1 Determination of absolute mass correction factor As we used the analytically pure cyclohexylamine in this test condition to detect cyclohexylamine and aniline, and analyze pure aniline in this test. Cyclohexylamine, cyclohexane, and dicyclohexylamine were not detected under the conditions, so when preparing a single calibration solution of cyclohexylamine, cyclohexane, and dicyclohexylamine, aniline was used as a solvent. Analytically pure cyclohexylamine solvent was used as the single standard solution for aniline and the area of ​​the aniline peak carried by the cyclohexylamine solvent was subtracted accordingly. To avoid overlapping contamination, no calibration mixture was prepared. Using a weighing method (approximated to 0.0002g), three standards each having a similar index to the calibration component were prepared and measured according to the measurement conditions of the sample.

4.2 Whether to use the relative quality correction factor The aniline content in cyclohexylamine is relatively strict in the control project of the industry standard. When analyzing the purity of the main body, the content of the impurity aniline must be accurately analyzed. Through experiments, changing the operating conditions such as the concentration of aniline calibration solution, the ratio of splitting, etc., the correction factor relative to cyclohexylamine is relatively stable and close. After obtaining this conclusion, instead of correcting the area of ​​the peak with relative correction factors, we used the area fraction directly to obtain the mass fraction, and the area was normalized by the integrator. The measured aniline content and the aniline content obtained by the single standard method were obtained. For comparison, the measured mass fraction of cyclohexylamine was compared with the mass fraction of cyclohexylamine measured by the acid-base titration method, and whether there was a significant difference in the results obtained by the two methods was judged. In the table, the mass fraction of cyclohexylamine measured by the area normalization method has deducted moisture. Under the same conditions, different samples were analyzed in parallel, and then the results of each pair of parallel analysis were examined by T test. After comparison, there was no significant difference in the measurement results. For several other impurity components, quality correction factors have not been used because they are not required by industry standards and are small in content. Therefore, the quantitative method uses an integrator to calculate the area normalization method and avoids the preparation of standard samples.

5. Method error In order to verify the accuracy of the method, recovery tests are usually performed. However, since the time-programming and [INTG] functions provided in the integrator are used in the area normalization method, solvent peaks are not integrated. Therefore, the concentration of cyclohexanamine products does not affect the groups calculated by the area normalization method. The fractional mass fraction is not applicable to the recovery test of this method. We used this method to repeat the analysis of known pure cyclohexylamine (mass fraction greater than 99.8%) for 12 times. The analytical results (measured values) were in good agreement with the standard values. Analyze the precision of the results. The data can be considered as high precision and repeatability. In addition, through analytical tests, it is proved that this method is also applicable to the analysis of cyclohexylamine hydrogenation products, pre-fractions, and raffinate, and it is effective in tracking analysis of cyclohexylamine production.

6, the application of a variety of operating conditions fixed parameters, through the integrator function and command to establish a method stored in the integrator memory. Before the analysis, the method is first loaded into the integrator's active workspace, and analysis is automatically performed according to the method, which makes the operation easier and faster.