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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 fraction >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. In order to meet this condition, the sample is diluted with ethanol before treatment, since the exact concentration is 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 simulated input signal of the 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 chromatogram. 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. The program temperature increase program (a) is adopted in the accuracy and precision of the determination and evaluation method of the absolute weight correction factor.
3. Parameters of the accumulative instrument 4. Selection of quantitative methods 4.1 Determination of absolute mass correction factor Since the analytically pure cyclohexylamine used by us was used to detect cyclohexylamine and aniline under this test condition, pure aniline was analyzed 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 and split ratio of aniline calibration solution, the correction factor relative to cyclohexylamine is relatively stable and close. After obtaining this conclusion, instead of correcting the peak area with relative correction factors, we used the area fraction directly to obtain the mass fraction, and the area was normalized using 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 acid-base titration, and whether the results obtained by the two methods were significantly different. 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.
Application of Gas Chromatography in Cyclohexylamine
The aniline is vaporized and hydrogenated with preheated hydrogen under the action of a catalyst to produce a crude cyclohexylamine, which is purified by distillation to obtain cyclohexylamine. Cyclohexylamine is mainly used for 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 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.