Application Progress of Headspace Gas Chromatography in Analytical Chemistry

Fengfeng ZHANG1, Yu ZHANG1, Xiaoxiang QIU

1.College of Chemistry and Material, Weinan Normal University, Weinan 714099, China; 2.Key Laboratory of Ecology and Environment of River Wetland in Shaanxi Province, Weinan 714099, China

Abstract This paper briefly expounds the basic principle and classification of headspace gas chromatography, summarizes its application in food analysis, environmental analysis and medical analysis, and forecasts the application prospect of headspace gas chromatography in analytical chemistry in the future.

Key words Headspace gas chromatography, Food analysis, Environmental analysis, Medical analysis, Application progress

1 Introduction

Gas chromatography (GC) is a major technique for the separation and analysis of multicomponent mixtures in samples, which has a history of more than 50 years. It is a mature method for separation and analysis of complex mixtures. In 1901, the Russian botanist Tswett discovered the phenomenon related to chromatography[1]. GC method uses gas as mobile phase, which can also be called carrier gas. Helium, nitrogen and hydrogen are the most commonly used carrier gases. Solid or liquid as stationary phases can be used to determine the components with low boiling point in various samples. However, there are some shortcomings in gas chromatography alone, and the volatile components in solid samples cannot be determined directly. It must be treated as a liquid by a complex pretreatment method and then injected into the chromatographic column, which is overcome by headspace chromatography. Headspace chromatography refers to the indirect determination of volatile components in solids and liquids by ordinary gas chromatography. Headspace gas chromatography is more widely used, the injection is more accurate, the detection limit is lower, and the reproducibility is better. In view of the unique advantages of headspace chromatography, headspace chromatography has become a widely used GC technology[2]. It is used in food odor, volatile organic compounds in industrial wastewater, environmental pollution and so on. This paper mainly describes the application of headspace chromatography in food, environment, medicine and so on, and forecasts the application prospect of headspace chromatography in analytical chemistry in the future.

2 Classification of analytical methods of headspace gas chromatography

Headspace gas chromatography[3]consists of three processes, one is sampling, the second is injection, and the third is gas chromatography analysis. According to the different methods of sampling and injection, headspace analysis is divided into static headspace analysis method and dynamic headspace analysis method.

2.1 Static headspace chromatographyStatic headspace chromatography refers to the determination of the components in the vapor phase when the liquid or solid is placed in a constant temperature sealed environment and the gas chromatography is used to analyze the components in the vapor phase when the gas-liquid or gas-solid equilibrium is reached. Its sample injection mode includes manual sample injection device and automatic sample injection device.

2.1.1Manual injection device. The manual injection device is equipped with an air-tight syringe. In general, this method of injection is used as long as there is a constant temperature tank (water bath or oil bath) with accurate temperature control outside the airtight syringe. Static headspace is relatively simple in the application of device, but it also has some shortcomings. One is that when the sample enters the syringe from the headspace device, the pressure change will affect the actual injection volume to a certain extent. In order to control the change of the actual injection volume, this problem can be overcome by using an air-tight syringe with pressure locking. The other is that when the temperature of the syringe is low, some samples with higher boiling point will condense, resulting in sample loss. The syringe can be combined with the six-way valve, and the temperature of the valve body can be used to control the temperature of the sample. This method is used to eliminate the influence of syringe temperature and only qualitative analysis is made. The manual injection is an economical method, but if you want to carry out accurate quantitative analysis, the analysis results of automatic headspace injection device will be more accurate.

2.1.2Automatic sample injection device. According to its principle, it can be divided into three kinds: syringe injection system, pressure balance headspace injection system and pressure control quantitative tube injection system.

The pressure balance headspace injection system consists of a pressure control valve and a gas injection syringe. When the pressure of the sample flask is equal to the pre-pressure of the GC column, the mixed gas of the carrier gas and the sample gas enters the GC column through the heated delivery pipe. This collection mode controls the injection volume by controlling the time of the process. When using this device, the balance should be controlled, and the pressure in front of the GC column should be higher than that in the sample flask, so as not to cause inaccurate analysis results. In practice, efforts should be made to meet the pressure balance.

As for the pressure control quantitative tube injection system, the main purpose of the system is to apply a certain pressure to the sample flask so that the pressurized gas in the sample flask enters the quantitative tube through the probe. After that, all the carrier gas brings all samples into GC for analysis through the quantitative tube.

2.2 Dynamic headspace chromatographyDynamic headspace chromatography is to "purge" the volatile components of the sample with flowing gas, then use a trap to adsorb the swept materials, and then send the sample to GC for analysis by pyrolysis absorption. Therefore, it is also known as purging-trapping injection technology, the injection device is the purging-trapping injection device. In this device, liquid (water) is placed in the sample tube, and the gas passes through the liquid storage tank and blows out the volatile components into the trap. After purging, the carrier gas enters the GC through the trap. When purging, there will be a large amount of water vapor in the trap, which will affect the quantitative accuracy and separation results. An effective method should be used to remove water.

3 Application of headspace chromatography in analytical chemistry

3.1 Application in food analysisLi Wenqingetal.[4]used headspace gas chromatography to determine the content of main sulfur compounds in garlic after cooking. During cooking, a series of sulfur-containing compounds were produced, such as diallyl disulfide, and diallyl trisulfide. The separation was carried out by DM2 WAX quartz elastic capillary column with equilibrium temperature of 403 K and equilibrium time of 20 min, and stirring speed of 500 r/min. Within the range of concentration investigated, the linear relationship of the method was good. The detection limit was 0.001-0.025 mol/L, the linear correlation coefficient was more than 0.999, and the average recovery of the three sulfur compounds was 97.1%-103.0%. The method has the advantages of small sample size, high sensitivity and simple operation. There is no need for organic solvent extraction and filtration and enrichment. It has been applied to the determination of main sulfur content in garlic at different cooking temperatures with satisfactory results. Ma Yusongetal.[5]used headspace gas chromatography-mass spectrometry in the process of identifying the flavor components of sesame oil. 71 components were identified by DB-624 capillary column at injection port temperature of 250℃. The elements making up the compounds were effectively detected, which laid a solid foundation for the research of scientists in this field. Du Lijunetal.[6]used headspace gas chromatography in the determination of cyanide in wine. The gas chromatographic column for separation was WAX. The detection limit was 0.02 mg/L, the quantitative limit was 0.05 mg/L, the injection time was 0.03 min, and the equilibrium temperature was 50℃. This method is fast, simple, reduces the detection time, and is an environmentally friendly and economical detection method. In the detection of large-scale sample volume, this method has certain pertinence, and the results of the analysis are more accurate.

3.2 Application in environmental analysisXie Lietal.[7]used headspace gas chromatography-mass spectrometry in the analysis of seven kinds of volatile organic compounds in instant noodle printing and packaging materials. The capillary column used in the detection process was VOC. The equilibrium time was 40 min, the equilibrium temperature was 80℃, and the detection limit was 0.004-0.007 mg/m3. The correlation coefficients were all greater than 0.999 1, and the recovery was 93%-95%. The results showed that the method was stable and accurate. Lu Qingetal.[8]determined five volatile substances in coatings, 1,1-trichloroethane, p-chloro-trichlorotoluene, acetone, dichloromethane and methyl acetate by headspace gas chromatography-mass spectrometry. The chromatographic column used in the detection process was DB-VRX, which was accurate and convenient. Li Runyan[9]used headspace gas chromatography-mass spectrometry in the determination of volatile organic compounds in cosmetics. The residues of 14 kinds of organic compounds were determined, and the HP-5MS quartz capillary column was used as the separation column. The mobile phase was carrier gas, and the equilibrium temperature was 85℃ when the equilibrium time was 30 min. The relative standard deviation of the measured value was less than 5.0%, and the accuracy of this method was high. Zhou Liangchunetal.[10]used headspace gas chromatography to determine the content of benzene volatiles in automobile interior decoration materials. In the optimization of the chromatographic column, the CP-WAX57C chromatographic column was used. The equilibrium temperature was 100℃, the equilibrium time was 30 min, and the correlation coefficient was more than 0.999. The linear relationship was good and the measurement accuracy was high, which was especially suitable for the determination of plastics and other materials in automobile. Fan Zhanchunetal.[11]used headspace gas chromatography-mass spectrometry to determine volatile halogenated hydrogen in soil with 70℃ as its equilibrium temperature and 40 min as its equilibrium time.

The detection limit was relatively low, and the accuracy was good when the detection range is 2.2-5.9 μg/kg. Pu Xuewei[12]used dynamic gas chromatography and HP-VOC column in the determination of volatile organic compounds in water. The mobile phase was helium, the purging temperature was 40℃, the purging time was 15 min, and the purging flow rate was 40 mL/min. The desorption temperature was 200℃ and the desorption time was 2 min. The detection deviation was 0.3%-12.0%, and the recovery was 79.8%-114.0%. This method was used to accurately detect volatile organic compounds in water. This method has the advantages of simple operation, high recovery rate and accurate results. Huang Changchun[13]used headspace gas chromatography-mass spectrometry in the process of determining the content of trace trimethylamine in air. The column was CP-VoIamine, the equilibrium temperature was 80℃, the equilibrium time was 40 min, and the injection time was 0.2 min. The scanning mode was SIM mode, the recovery was 94%-102% in the range of 0.05-2.00 μg, and the detection limit was 0.3 μg/m3when the sampling volume was 30 L. The method was convenient to operate and the result was accurate.

3.3 Application in medical analysisFang Qiuxue[14]used headspace gas chromatography and hydrogen flame ion detector to detect the components in the original drug of retegabine. The analytical column was DB624, the equilibrium time was 15 min, and the equilibrium temperature was 110℃. After determination, the detection limit of triethylamine was 0.01%, the detection limit of ethanol was 0.007 5%, the detection limit of tetrahydrofuran was 0.002 4%, the detection limit of n-hexane was 0.000 48%, and the recovery was 100%-102%. The linear coefficients were more than 0.995. The method has high sensitivity, high accuracy and good linear relationship. Yang Junhui[15]used headspace gas chromatography-mass spectrometry in the analysis of volatile components in perilla leaves in different harvest periods. The chromatographic column was HP-5 MS, the injection port temperature was 150℃, and the mobile phase was helium. 52 components were detected using this method, and 20 of them were contained in all components. The volatile components in perilla leaves showed different rules because of their different growth periods. According to the results, it can be known that this method is more suitable for the determination of volatility of perilla leaves in different periods. Zhang Linetal.[16]used ionic liquid-static headspace gas chromatography in the measurement of camphor and eucalyptus oil in "10 drops". The capillary column used for chromatography was TM-5, the equilibrium temperature was 140℃ and the equilibrium time was 30 min. The detection limits of camphor and eucalyptus oil were 0.160 and 0.026 5 mg/L, respectively, showing a good linear relationship. Camphor and eucalyptus oil essence had good linearity in their respective ranges. This method has the advantages of high accuracy, simplicity, no pollution to the environment and satisfactory results.

Luo Zhonghuaetal.[17]used headspace gas chromatography in the determination of organic residual solvents in clopidogrel hydrosulfate. The chromatographic conditions were as follows: DB-624 capillary column, nitrogen as carrier gas, injection volume of 1 mL. The external standard method was used in the determination of organic residual solvents in clopidogrel sulfate, and the accuracy was 90.0%-107.0%. The method is simple, accurate and satisfactory. Zheng Chaoetal.[18]used static headspace gas chromatography in the determination of dimethyl sulfoxide in organic residual solvent. The diluent for the preparation of the sample was liquid paraffin, which successfully avoided the interference caused by the matrix. The equilibrium temperature was 160℃, the equilibrium time was 10 min, and the detection limit was 1.0 μg/g. The method is simple, with low detection limit and high sensitivity. Ma Bokaietal.[19]used headspace chromatography in the determination of six residual solvents in topiramat. Among them, the residual solvents had varying degrees of toxicity, which was very harmful to human body. Headspace gas chromatography was used to determine six residual solvents in topiramat. They had a good linear relationship in their own linear range. The quantitative limits were: methanol 0.006%, ethanol 0.005%, dichloromethane 0.012%, ethyl acetate 0.002 5%, 1, 4-dichlorohexane 0.007 6%, pyridine 0.004%. Under the premise of three additive agents, the recovery rate was between 92.3% and 100.3%. This method is simple and rapid.

4 Prospect

Headspace analysis is an ideal method for sample pretreatment. Compared with SPE and traditional liquid-liquid extraction, headspace successfully avoids some coextracts’ interference in analysis, and reduces the interference of sample matrix in the analysis. Furthermore, headspace chromatography is the simplest method in gas chromatography. Because of the good quantitative effect of GC, the headspace chromatography also has certain authority in quantitative analysis. In recent years, headspace analysis technology has been more and more widely used in analytical chemistry, and has been concentrated in the fields of food volatile component analysis, environmental analysis, drug and food residual solvent analysis, criminal investigation analysis and so on. There are also some drawbacks to the headspace GC. For example, its linear range is narrow, below 0.1%-1.0%. In recent years, headspace solid phase microextraction technology has also developed rapidly. As a solvent-free extraction technology, it has the advantages of environmental friendliness, simple operation and high sensitivity, and it is easy to be combined with chromatographic technology for automatic analysis. It is favored by more and more analytical chemists. The method of headspace solid microextraction combined with gas chromatography was used in the determination of chlorothalonil in water. This method has the advantages of simple operation, low sample usage, high accuracy and no use of any organic solvents. It can be believed that the technology will play a greater role in scientific research and practical application in the future because of its uniqueness and further development. With the development of science and technology, the chemical mechanism and nano-scale problems of life phenomena will appear one after another. Separation must be addressed to solve these problems, and chromatographic analysis will play an irreplaceable role.