Progress in analysis of organic trace substances by gas chromatography
The progress in the analysis of organic trace substances by gas chromatography was reviewed with 63 references.
Gas chromatography; Organic trace analysis; Pretreatment; summary
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Trace analysis refers to the determination of substances with low content in samples, usually called trace components. The concept of trace analysis is a dynamic concept, which changes with the development of science and technology. Liang Hanchang [1] believes that modern trace analysis refers to the detection of components with a concentration of10-9-100×10-6 or lower in pure substances or mixtures. Zhu Minghua [2] thinks that the analysis of components with content lower than 100 ppm is called trace analysis.
With the development of national economy and the continuous emergence of high and new technologies, various industries and fields have higher and higher requirements on the purity and quality of substances, and the trace components in the environment and organisms will also have a great impact on nature and organisms, thus promoting and promoting the development of trace analysis technology. Therefore, it is of great practical significance to study and establish a more sensitive and accurate trace analysis method.
Many analytical methods, such as gas chromatography [3], liquid chromatography [4], mass spectrometry, infrared spectroscopy, Raman spectroscopy [5], capillary electrophoresis [6], electrochemistry [7], capillary electrochromatography-electrospray ionization mass spectrometry [8], derivative spectrophotometry [9] and so on, can be used for the analysis of organic trace amounts. Gas chromatography has many outstanding advantages, such as high separation efficiency, good selectivity, high sensitivity, fast analysis speed, less sample consumption by direct injection, and simultaneous analysis of multiple components by one injection, which is especially suitable for the analysis of organic trace substances. However, the analysis of organic trace amounts is a wide-ranging, difficult and demanding task, which includes not only the detection sensitivity and separation problems that the instrument should have solved, but also the extremely critical contents such as sample collection, transportation, storage and preparation.
1. 1 gas chromatography pretreatment of organic trace analysis samples
The analysis of organic pollutants in the environment (including environmental hormones), some ingredients in food and impurities in drugs mostly involves the detection of trace levels, which must adapt to complex factors such as different substrates and a large number of substances, and is a systematic trace analysis work. In the early days, people devoted themselves to developing chromatographic analysis methods with high sensitivity and selectivity. Through twenty years' practice, people realize that sample pretreatment is a link that can't be ignored in the whole analysis method, and it is often the key to the success or failure of analysis. China has made some progress in sample pretreatment technology, but it is not balanced. This paper briefly introduces the progress of sample pretreatment technology at home and abroad in recent years.
1. 1. 1 solvent extraction
Solvent extraction is one of the most commonly used treatment techniques for various samples. Liquid-solid extraction (LSE) and liquid-liquid extraction (LLE) have been the most widely used sample pretreatment methods, such as Soxhlet extraction, which can enrich and eliminate matrix interference. In the past, EPA 500, 600 and 800 series methods in the United States mostly used this scheme, but the disadvantage was that it consumed a lot of organic solvents (10 mL) and was easy to introduce new interferents (in solvents).
Micro-solvent extraction and continuous extraction have been improved in methods and equipment. The former only needs 100- 1000μL solvent for each extraction, and the sensitivity is improved. The detection limit of trace organic compounds in seawater by continuous extraction combined with gas chromatography can reach 10 ppt (octane) [10].
Rapid solvent extraction (ASE) is an extraction technology introduced by Bruce et al. from 1995 [1 1]. The pretreatment process suitable for solid and semi-solid samples is pressurizing (7- 12 MPa, max. 20 MPa) and heating (50-200℃). ASE has been widely used in the monitoring of herbicides, phosphorus-containing pesticides, polychlorinated dibenzofurans and polychlorinated biphenyls in floating dust, sediments, food and fish, and its recovery rate and relative standard deviation are better than those of general extraction methods.
1. 1.2 microwave extraction
Microwave extraction refers to the process of extracting the components to be detected from the sample matrix with organic solvent under the action of microwave energy. In the past, microwave treatment was only used for inorganic analysis, but it was gradually extended to organic analysis since the late 1980s. Microwave extraction has the advantages of fast extraction speed, low solvent consumption and high recovery rate, and can process multiple samples at the same time. Mainly suitable for solid or semi-solid samples. The principle of microwave extraction is that polar molecules absorb microwave energy to heat polar solvents such as methanol, ethanol, acetone and water. Because the extraction process is carried out in a closed tank, the internal pressure can reach above 65438±0 MPa, so the boiling point of the solvent is much higher than that of the solvent under normal pressure. In this way, microwave extraction can reach the extraction temperature that can not be achieved by using the same solvent under normal pressure, and the extraction efficiency is improved. The microwave extraction experiment of organochlorine pesticides showed that the recovery rate was the highest when the extraction temperature was 120℃. Microwave extraction technology has been applied to the extraction and determination of polycyclic aromatic hydrocarbons, pesticide residues, organometallic compounds, heavy metals and toxic elements in soil, sediments, marine organisms, food and vegetables. The recovery rate is generally better than Soxhlet extraction and ultrasonic extraction [13], and it is easy to realize automation [14]. However, microwave leakage may occur in the application of microwave extraction technology. As a new technology, it needs further research.
1. 1.3 liquid phase microextraction
Liquid phase microextraction or solvent microextraction is a new sample pretreatment technology developed by 1996, which was first proposed by Jeannot and Cantwell [15]. In this technology, organic droplets are hung on the needle of a gas chromatography (GC) micro sampler to extract substances. Micro-sampler can be used as both gas chromatography sampler and micro-separatory funnel. There are two types of LPME: dynamic and static. Static LPME, using 10μL micro sampler to extract 1μL solvent, immersing it in water sample, and the organic matter in water sample is distributed into organic solvent through diffusion. After a period of time, the solvent was pumped back to the sampler and analyzed by GC. Different from the static LPME operation, the dynamic LPME uses a micro-sampler to extract 1μL solvent, immerses the micro-sampler into the sample, extracts 3μL sample into the sampler, stays for a certain time, pushes out 3μL sample, and so on, and takes the organic solvent for GC analysis. This technology is developed on the basis of liquid-liquid extraction. Compared with liquid-liquid extraction, LPME can provide considerable sensitivity and even better enrichment effect. At the same time, the technology integrates sampling, extraction and concentration, and has the characteristics of high sensitivity, simple operation, rapidity and cheapness. In addition, it requires very little organic solvent (several to dozens of μL), and it is a new environmental-friendly sample pretreatment technology, especially suitable for the determination of trace and ultra-trace pollutants in environmental samples. In addition, LPME technology only needs a stirrer, an ordinary micro sampler or porous hollow fiber when processing samples. These characteristics make it easy to combine liquid phase microextraction with portable gas chromatograph, and it is expected to analyze environmental pollutants simply and quickly on the spot, so it has a broader application prospect [16].
1. 1.4 micro distillation
Distillation includes simple distillation, fractional distillation, vacuum distillation, steam distillation and so on. Distillation technology is the first choice for refining volatile and semi-volatile organic compounds. However, distillation is usually not the first choice for chromatographic sample preparation. Micro-distillation technology has the advantages of short distillation time, preparation of various samples and distillation of small samples, and can be successfully used for sample refining before chromatographic analysis or pre-separation of mixed samples. Tim Mansfeldt used micro distillation technology to determine cyanide in soil [17], and achieved good results.
1. 1.5 solid phase extraction (SPE)
Solid phase extraction (SPE) is a sample pretreatment technology developed in the early 1970s. SPE is mainly used for the separation and enrichment of trace or trace target compounds in complex samples. For example, solid phase extraction can be used for sample pretreatment such as the analysis of drugs and their metabolites in biological fluids (such as blood and urine), the analysis of effective or harmful components in food, and the analysis of various pollutants in environmental water samples. In this technology, the target compound in the liquid sample is adsorbed by solid adsorbent, separated from the matrix and interfering compounds of the sample, and then eluted with eluent or desorbed by heating to achieve the purpose of separating and enriching the target compound. According to statistics, nearly 50% of environmental samples now use this method. Solid phase extraction is a method combining purification and enrichment, especially suitable for water samples. The sample size is not limited, from a few milliliters to dozens of liters. Technically speaking, SPE is close to ordinary displacement chromatography. The sample passes through the extraction bed by gravity or pressure to remove the matrix, enrich the analyte, and then elute with a small amount (several milliliters) of appropriate solvent to recover the analyte.
The main stationary phases used in SPE are silica gel, reversed phase C 18 stationary phase (RP-C 18), graphitized carbon black, styrene-divinylbenzene series polymers, polydimethylsiloxane (PDMS) and so on. The selectivity of these stationary phases is different from that of different organic compounds. SPE can use the selectivity of stationary phase to extract various organic compounds from samples, thus improving the analytical sensitivity of target substances. There are two kinds of extraction beds for solid phase extraction. One is cylindrical, and the loading capacity of commercial pre-packed column is about 100 ~ 500mg. The other is that fine particles are mixed with PTFE fiber to form a disc, and the loading capacity is about 30mg ~ 10g. Its advantages are that the bed is thin and tight, not easy to leak, and the sample can pass quickly (~ 660). When determining nonpolar pesticide residues such as organochlorine by gas chromatography-electron capture detector (GC-ECD), alumina-silver salt adsorption column is generally used, and the purification and separation effect of silica gel adsorption column is not as good as alumina column.
SPE is mainly used for trace analysis. Its biggest advantage is that it reduces the use of high-purity solvents and is easy to automate. When combined with thermal desorption device, it can avoid using solvents, and reduce the experimental cost and solvent post-treatment cost. Compared with LLE, solid phase extraction avoids the emulsification problem that is easy to occur in LLE. However, for some samples, the blank value of solid phase extraction is higher and the sensitivity is not as good as LLE method. There are also some problems in the extraction of polar compounds. Later, SPE-GC/GC-MS 18] online analysis method was gradually developed. On-line method has the advantages of automatic analysis, less analyte loss, less external pollution and high precision, and is suitable for the analysis of a large number of samples. However, the disadvantages are sequential operation and inflexible procedures, which lead to complex or even impossible optimization of different steps.
1. 1.6 solid phase microextraction
In recent years, the sample pretreatment technology of solid-phase microextraction (SPME) is developed on the basis of SPE, but it does not separate all the substances to be detected, but realizes the separation through the balanced distribution between samples (such as water samples) and extractant (solid phase). The basic technology of this method is to immerse the elastic time line with proper coating (line diameter 100- 150μm) in the sample (immersion mode) or place it in the upper space of the sample (headspace mode). After a period of time (2-30 min), the analyte in the sample is adsorbed on the coating, and the adsorption amount is proportional to the original concentration of the analyte in the sample. SPME retains the advantages of SPE, avoids the shortcoming of high blank of SPME samples, and completely avoids using solvents. This method has achieved good results in the determination of volatile organic compounds in water. With polysiloxane as the coating, it meets the requirements for the determination of volatile organic compounds in drinking water (EPA524.2 method). This method has also been successfully applied to the monitoring of chlorobenzene, polychlorinated biphenyls, PCDD, herbicides, pesticides and phenol. In the discharged water. The data are basically parallel to the liquid-liquid extraction method, and the RSD is slightly lower [19]. It is also satisfactory to pretreat chlorophenol in water with polyacrylic acid coating and SPME method of GC-MS [20].
The coated silk was suspended in the headspace of water sample, and the headspace SPME technology was developed through the balanced distribution of analyte and coating in the gas phase. By properly increasing the equilibrium temperature or reducing the volume of headspace (gas phase), this method can even be applied to the analysis of substances with slightly higher boiling point in water, which shortens the sample extraction time and is easy to determine volatile organic compounds in various media [2 1]. Headspace-solid-phase microextraction (HS-SPME) is the most widely used headspace analysis method at present. Its reproducibility is comparable to that of static headspace method, and its sensitivity is comparable to that of dynamic headspace method.
1. 1.7 headspace sample preparation technology
Headspace gas chromatography is not a new technology, and it has been used since the early days of gas chromatography. Headspace separation technology is widely used to separate volatile substances in liquid or solid samples. The principle is: under the condition of constant temperature, the volatile substances in the sample are distributed between gas-liquid (or gas-solid) phases, and when the equilibrium is reached, the gas phase on the liquid is taken for GC analysis. Therefore, equilibrium temperature and equilibrium time are the main factors affecting the analytical sensitivity. The accuracy of analysis mainly depends on good constant temperature state and analysis environment. In addition, it should be noted that sample bottles and bottle sealing plugs cannot adsorb samples. Headspace separation has the following characteristics: (1) can be used to determine trace volatile components in samples (liquids and solids) that cannot be directly evaporated without special treatment of samples; (2) The chromatographic column will not be polluted by direct injection of water samples or high-boiling substances or nonvolatile components; (3) Because the concentration of volatile components in the gas phase is higher than that of other components, the detection sensitivity of volatile components can be improved. (4) No reagent is used, the operation is simple, and it can be combined with gas chromatography.
1. 1.8 Purge and trap method (dynamic headspace method)
Purge and trap method can be regarded as a continuous headspace technology, which is mainly used for the analysis of volatile substances in samples. Theoretically, this method can determine all volatile organic compounds in water. The principle of purging and trapping is that according to the volatile characteristics of many organic compounds, the volatile substances are purged from the sample by gas, and the purged components are blown away by the adsorbed compounds, which are directly analyzed by chromatograph. This can enrich the trace organic matter in water to a concentration that can be detected by chromatography. This method not only overcomes the problem that the main peak of solvent masks other peaks in chromatographic separation, but also has higher detection sensitivity than static headspace, and is more suitable for trace and ultra-trace analysis. The laboratory of the US Environmental Protection Agency uses purge and capture technology to determine volatile organic compounds in drinking water and various environmental samples. When using purge and trap gas chromatography, it is best to use a large diameter (0.54 mm) capillary column. If packed column is used, the cold column sampling method should be selected to separate the components well. In addition, purge flow rate and purge capture time are the main factors that affect the sensitivity of analysis, and it is best to obtain them by experiments with standard samples under known conditions. Some studies on enrichment of trace organic compounds in water by gas stripping have been carried out in China, but the recovery rate of volatile organic compounds is low and unstable, and its application range is narrow. Xu Lijuan [23] and others improved the stripping device, deeply and systematically studied the influence of stripping experimental conditions on the yield of volatile organic compounds, and determined the best enrichment conditions. Based on the experiment of synthesizing samples, a variety of water samples were qualitatively and quantitatively analyzed by gas chromatography-mass spectrometry with gas stripping enrichment, and satisfactory results were obtained.
1. 1.9 Supercritical fluid extraction (SFE)
Supercritical fluid extraction (SFE) is a special separation technology developed in recent years. SFE mainly uses supercritical CO2 as extractant, which has both gas permeability and liquid distribution. The solubility of supercritical fluid is close to that of liquid, but its viscosity is close to that of gas, and its diffusion coefficient is between that of liquid and gas, that is, it has good solubility and efficient transmission capacity. At present, the most commonly used fluid CO2 has a critical temperature of 365,438 0.3℃ and a critical pressure of 7.38 MPa. CO2 in the effluent volatilizes under normal pressure, and the substance to be detected is dissolved in the solvent for analysis. Compared with the traditional solvent extraction method, SFE has many advantages. First of all, it can avoid using a lot of solvents, improve extraction efficiency, reduce analysis time and reduce the possibility of sample pollution. It is especially suitable for samples with complex and changeable components in environment and biology [24] and can be automated. SFE has only been developed in recent years, and many experimental parameters and conditions need to be further optimized and clarified. The pressure and temperature of the extract can be well controlled, but some other problems, such as the extraction of cell tissue, the speed of the extract passing through cells, the residence time and the interference of sample substances, need further study [25].
1. 1. 10 membrane separation technology
Membrane separation is one of the new technologies developed in recent years, which can be used in analytical chemistry. It can be separated from each other by using the difference of propagation speed between the substance to be detected and the solvent or between the substance to be detected and macromolecular substances (such as protein or other polymers). Membrane extraction is to extract the target analyte (donor) in the sample solution into the extractant (acceptor) with a membrane. If the system is kept for a long time, phase balance can be established. During the sample processing, transfer the target analyte from the donor to the receptor as much as possible. Membrane extraction can be combined with RP-HPLC [26], GC [27,28] and capillary electrophoresis (CE) online. Membrane extraction overcomes the interference of water itself and has high selectivity. However, the low polarity membrane is not suitable for analyzing polar organic pollutants. Many organic pollutants in water samples have been successfully determined by membrane extraction [29], and some membranes have high enrichment times for low-concentration substances in water.
1.1.11ultrasonic suspension technology
Ultrasonic suspension technology is a containerless processing technology which uses acoustic radiation force to suspend an object on the sound pressure node of ultrasonic standing wave field. This technology can handle samples with a volume of several μL or even dozens of pL in a non-contact way, avoiding the loss of analytes caused by uncertain adsorption, memory effect and container wall pollution, and eliminating the interference of cell reaction caused by the interaction between container wall and sample and the optical interference caused by container wall. And there is no special requirement for the physical and chemical properties of suspended objects. It is a powerful tool based on single particle or small droplet research, especially suitable for deep supercooling (far from solidification equilibrium state) research and small volume trace analysis of materials, which can reduce the detection limit by 1-3 orders of magnitude. The application of ultrasonic suspension technology in biological science and biotechnology has attracted more and more attention, showing attractive prospects. Nevertheless, it is still in its infancy, and there is basically a blank in China.
Reviewing the sample pretreatment technology, considerable achievements have been made, but scientists of organic trace analysis are still trying to develop new technologies and methods that are more effective, more reasonable, simpler and more reliable. Because the sources and existing forms of various samples are complex and the substances to be detected are diverse, it is impossible to find a unified or "universal" pretreatment method. According to the test requirements and sample conditions, make a suitable plan according to local conditions. Among all the known methods, solid-phase extraction and solid-phase microextraction will continue to develop, with a wider range of applications and a higher degree of automation. For solid samples, in addition to improved liquid-solid extraction (rapid, microwave-assisted, etc.). ), supercritical fluid extraction will get better selectivity and treatment effect with the deepening understanding of its mechanism. The application of membrane technology, especially microdialysis and supporting liquid membrane, is a noteworthy development trend. The combination of chromatographic techniques such as GC/GC, LC/GC and LC/CE (capillary electrophoresis) will provide a wider application field for sample analysis, especially for organic trace analysis. Headspace method (including purging and trapping) will still be the main pretreatment method for volatile organic compounds in samples. Other sample pretreatment techniques, such as electrochemical enrichment and immunochemical chromatography, are also noteworthy developments. Intelligent sample pretreatment scheme with the help of computer technology will also be a research direction.