Rapid determination of impurities in carbon dioxide for food by gas chromatography

This article describes the experimental principle and method of rapid determination of carbon monoxide, methane, propane, total hydrocarbon, methanol, ethylene oxide, acetaldehyde, vinyl chloride, benzene, and other common impurities in carbon dioxide by gas chromatograph. By using food additive carbon dioxide as a sample, the rapid analysis conditions, such as chromatographic column, valve switching time, and column box temperature control, were optimized. The retention time of impurity components in food additive carbon dioxide was determined using the external standard method. The relative standard uncertainty of CH4, C3H8, CO, CH3OH, CH2CH2O, CH3CHO, CH2CHCl, and C6H6 analysis is within 3% by using the established analytical method proposed in this article, which is equivalent to the national standard GB 1886.228-2016. The linear correlation coefficient r is greater than 0.9995, and the linear error is not greater than 2.0, indicating excellent linearity. In the meantime, in the case of 6 consecutive sample injections, the detection time is shortened by nearly three times. The experimental results show that the analytical method established in this article is suitable for the rapid determination of impurities in carbon dioxide for food, which can improve the accuracy of experimental results and work efficiency.


Introduction
Carbon dioxide is an important substance widely used in carbonated beverages, beer, and food preservation, which is closely related to daily life.The food industry of China has a huge demand for nearly 2 million tons of carbon dioxide every year, which is significantly increasing year by year.Therefore, it is necessary to detect harmful impurities in carbon dioxide used in food.However, at present, the relative testing method and traceability system have not been established in China.Large companies such as Coca-Cola require carbon dioxide purchased in China to be sent to foreign institutions for testing, indicating that there is still space for improvement in this field in China.Currently, food-grade carbon dioxide in China is mainly a by-product of chemical products, which is the carbon dioxide gas source.National food safety standard GB 1886.228-2016Food additive -Carbon dioxide was officially implemented in 2017, which specifies detection indicators and methods.However, some detection methods proposed in the standard are relatively cumbersome, require complex types of equipment, lack traceability systems, and have high analytical uncertainty.Therefore, this article established a rapid detection method for carbon dioxide in food additives, which can quickly and accurately determine the concentration of carbon dioxide in food, solving the problems present in the national standard.
The carbon dioxide used in food contains impurities such as carbon monoxide, methane, propane, total hydrocarbons, methanol, ethylene oxide, acetaldehyde, vinyl chloride, and benzene, all of which but carbon monoxide are hydrocarbons with a large number of hydrogen atoms, and carbon monoxide can be converted into methane through catalytic hydrogenation.Therefore, GC -FID (Flame Ionization Detector) is used for the detection of impurities in this article [1][2] .Due to the differences in the use of separation columns for each component in GC (Gas Chromatography), in order to achieve the goal of simultaneous detection of target impurity components in a single injection, separation channels are added in the original single flow chromatography system.Based on the difference in peak time of each channel, separation components in different channels respond separately on the FID detector.By adding devices such as automatic switching pneumatic valves and resistance columns, unnecessary and longer retained components in each separation channel are cut and blown back without affecting the peak detection of the target component, saving chromatographic separation and detection time [3] .In order to further improve sensitivity and repeatability, the chromatographic conditions were optimized by increasing quantitative ring volume [4] .

Experimental principle of GC -FID
When organic compounds burn in a hydrogen flame, chemical ionization reactions occur, producing positive and negative ions.There is a potential difference between the emitter and collector of the detector, and ion pairs form an ion flow between the two electrodes.The electron flow collected by the collector generates a voltage signal through the high resistance load, which is amplified by a light microcurrent amplifier and transported to the recorder to obtain the corresponding chromatographic peaks of the substance [5][6] .

Experimental conditions
In order to achieve the optimal experimental conditions for analyzing impurities in carbon dioxide used as a food additive, the initial temperature of the columns is adjusted to 80℃, the temperature is kept for 5 minutes, and then the column temperature is elevated to 120℃ at the rate of 10℃/min.The temperature is kept for 5 minutes, and the size of the sample ring is set at 1 mL.The detailed specific experimental conditions are shown in Table 1.

Experimental process
The pneumatic valve V2 is switched.The sample gas is set to respectively enter the quantitative Loop 1, Loop 2, and Loop 3 through the 3-2 and 5-4 channels of V1, as well as the 10-9, 6-5, and 2-1 channels of V2, to complete the injection.Carrier gas 2 (SFV2) brings the sample from Loop 1 into the TDX-01 column.CO and CH4 components are first separated by the chromatographic column, and CO is converted into CH4 [7][8] through a methanator, then CH4 is detected by the FID-A detector, and V1 is activated to return to the initial position.The remaining components are purged and emptied by carrier gas 1 (SFV1), preventing the components with a long retention time in the TDX-01 column from remaining on the column and a large amount of CO2 entering methanation.At the same time, V2 is started, and carrier gas 4 (SFV4) will bring the sample from Loop 3 into the KB-PLOT Q column through the injection port.CH4, C3H8, CH2CH2O, CH3OH, CH3CHO, and CH2CHCl [9][10][11] will be preseparated by the KB-PLOT Q column before entering the FTD-B detector for detection.Carrier gas 3 (SFV3) will bring the sample from Loop 2 into the SCBWQ column for separation, and total volatile hydrocarbons (THC) are detected by the FID-A detector.CO, CH4, and THC are displayed in the FID-A chromatogram, while CH4, C3H8, CH2CH2O, CH3OH, CH3CHO, CH2CHCl, and C6H6 are sequentially displayed in the FID-B chromatogram.CH4 can be quantified in both the FID-A and FID-B chromatograms, which can be used as a tool for validation and comparison.

Separation order of impurity components in food additive carbon dioxide
The chromatographic columns used in the experiment are all non-polar or low-polar, and the components of smaller relative molecular weight and lower boiling point flow out earlier.If the boiling points of the components are the same or similar, saturated alkanes flow out first.Components with complex isomerism flow out earlier when saturated hydrocarbons are with the same carbon number.Unsaturated hydrocarbons with double bonds in the middle peak earlier than those without.In order to fully determine the peak time of each component, the external standard method was used to determine the retention time of impurity components in food additive carbon dioxide, and the gas chromatography analysis spectrums are shown in Figures 2 and 3. Figure 2 shows the channel FID-A detection results, including volatile THC (0.503 min), CO (1.105 min), and CH4 (1.817 min).

Linearity and detection limit
Under the optimal analysis conditions elaborated in Section 2.3, CH4, CO, and C3H8 standard gases with concentrations of 5.00 μmol/mol, 25.0 μmol/mol, and 50.0 μmol/mol, as well as CH2CH2O, CH3OH, CH3CHO, CH2CHCl, and C6H6 standard gases with concentration of 1.00 μmol/mol, 2.00 μmol/mol, and 5.00 μmol/mol, are injected into the modified SC-3000 GC to perform gas analysis.The concentration of standard gas is made as the x-axis and the chromatographic peak area is made as the y-axis for linear regression.The detection limit is 3 times the baseline noise of the mass of the corresponding impurity component.The experimental results of linearity and detection limit are shown in Table 2.

Conclusion
This article modified an SC-3000 GC to determine the impurities in food additive carbon dioxide.The modified GC is equipped with 2 valves, 4 columns, 3 quantitative tubes, and 2 FID detectors that can automatically merge to achieve rapid and accurate analysis of various impurities in food additive carbon dioxide.The retention time of impurity components in food additive carbon dioxide was determined using the external standard method, which effectively achieved qualitative analysis.The established analytical method and optimal experimental conditions can quickly and accurately analyze the impurity content of THC, CO, CH4, C3H8, CH2CH2O, CH3OH, CH3CHO, CH2CHCl, and C6H6 through only one injection.The analytical method has good linearity and high precision, which is consistent with the national standard GB1886.228-2016Food additive -Carbon dioxide, but with more than 3 times efficiency.The experimental results show that the method presented in this article is suitable for the determination of impurities in food additive carbon dioxide, providing a new and rapid analysis method for impurity analysis in food additive carbon dioxide.

Figure 1 .
Figure 1.The internal configuration diagram of GC after renovation.

Figure 2 .
Figure 2. The gas chromatography analysis spectrum of channel FID-A.

Figure 3 .
Figure 3.The gas chromatography analysis spectrum of channel FID-B.

Table 1 .
Detailed specific conditions.

Table 2 .
The experimental results of linearity and detection limit.

Table 3 .
The experimental results of repeatability.