Polar organic matter online analysis method and system based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer

Abstract

The invention belongs to the technical field of aerosol thermal desorption analysis, and discloses a polar organic matter online analysis method and system based on comprehensive two-dimensional gas chromatography and an aerosol thermal desorption analyzer. Comprising four stages of sampling and GC * GC-MS analysis, standard liquid loading and injection, thermal desorption and derivatization and back flushing in the whole working period of a TAG-GC * GC-MS instrument. The TAG-GC * GC-MS system provided by the invention not only overcomes various limitations of a traditional analysis method, but also provides a more accurate and rapid POCs analysis means through an efficient and automatic working process, and provides important technical support for air quality monitoring and pollution source tracking.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerosol thermal desorption analysis, and particularly relates to an on-line analysis method and system for polar organic compounds based on comprehensive two-dimensional gas chromatography and an aerosol thermal desorption analyzer. Background Art

[0002] Due to the fact that the pollution process of atmospheric fine particulate matter (PM 2.5 ) has a wide range, a long duration, and a high degree of pollution, it has a significant impact on atmospheric visibility, air quality, human health, etc., and has become one of the prominent atmospheric pollution problems at present. As an important component of PM 2.5 , studying its chemical composition not only helps to evaluate the impact of secondary organic aerosol (SOA) on air quality and human health, reveal its role in climate change, but also enables in-depth understanding of the atmospheric chemical reaction process, helps to identify pollution sources and optimize pollution control strategies, and at the same time provides a basis for the source analysis of atmospheric pollutants. The hundreds of polar organic compounds (POCs) identified in PM 2.5 are the main components of OA, including aliphatics, aromatics, mono / dicarboxylic acids, aromatic acids, nitro compounds, etc. Such compounds can account for 30-45% of OC and are important components of OA.

[0003] The detection methods of POCs are generally divided into two ways: offline and online. Offline detection includes three steps: sample collection, pretreatment, and sample analysis. Usually, it is collected by a high-volume sampler using a quartz filter membrane. To meet the detection limit requirements of the analytical instrument, offline sampling usually needs to increase the sampling duration to improve the capture amount. The sampling time is usually in units of hours or days, and the time resolution is low; in addition, the long sampling process cannot ensure the consistency of sampling conditions, destroys the gas-particle equilibrium state of polar components, and causes calculation errors; moreover, POCs have low volatility and high polarity, resulting in difficult effective separation and detection in gas chromatography. Therefore, before offline analysis, derivatization treatment needs to be carried out to turn them into compounds that are more volatile, more stable, and have less interaction with the chromatographic column. However, the treatment steps are complex and time-consuming, and the operation process further increases the uncertainty of the experimental results.

[0004] In recent years, the aerosol thermal desorption analyzer (TAG) technology developed has shown the advantage of high-time-resolution real-time monitoring in OA monitoring, significantly improving the monitoring level of PM 2.5 . Through the PM with an hourly time resolution 2.5Quantitative analysis is used to obtain the dynamic changes of atmospheric pollutants in real time, providing more accurate data support to cope with sudden pollution events. In addition, the TAG combined with on-line derivatization technology can effectively detect trace POCs in complex mixtures. However, when analyzing complex samples such as environmental aerosols or biological samples, it is difficult for the gas chromatography-mass spectrometry (GC-MS) equipped in the TAG system to avoid the problem of poor separation efficiency. When isomers or compounds with similar structures in the sample pass through the chromatographic column at the same time, due to their similar physical and chemical properties, the retention times overlap and they cannot be effectively separated, resulting in unresolved complex mixtures (UCM). To overcome this problem, an effective method is to introduce two-dimensional gas chromatography (GC×GC) technology. This technology first performs preliminary separation on the first-dimensional chromatographic column, and then quickly transfers the components to the second-dimensional chromatographic column for further separation. With this dual separation technology, the separation efficiency is significantly improved. However, the current comprehensive two-dimensional technology is still in the off-line analysis stage, and the processing and analysis of each sample usually need to be operated separately, resulting in a long analysis cycle. This means that it cannot respond in real time to the rapid changes in air quality or pollution events, restricting its application in emergency pollution monitoring and rapid decision-making.

[0005] Based on the TAG coupled with comprehensive two-dimensional gas chromatography and on-line derivatization system, the present invention constructs a comprehensive two-dimensional thermal desorption aerosol analyzer (TAG-GC×GC-MS) to achieve in-situ on-line quantitative analysis of POCs substances including organic acids, fatty acids, sugars, etc. at the molecular level with high time resolution and high species identification rate. This technology will provide scientific support for the refined management and effective treatment of future PM 2.5 in the future. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the present invention provides an on-line analysis method and system for polar organic compounds based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer.

[0007] The present invention is realized as follows. An on-line analysis method for polar organic compounds based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer, the method comprising the following steps:

[0008] The entire working cycle of the TAG-GC×GC-MS instrument includes four stages: sampling and GC×GC / MS analysis, loading and injecting standard solution, thermal desorption and derivatization, and backflushing;

[0009] During sampling, through a PM 2.5The cyclone separator of the cutting head sucks ambient air at a flow rate of 10 L / min for 30 min, and then removes the gaseous organic compounds therein through a multi-channel carbon remover, and collects the particulate matter into the thermal desorption unit CTD. Before thermal desorption, 5 μL of an internal standard mixed solution containing deuterated compounds is added to the CTD unit, and the sample is desorbed from the CTD by a two-step thermal desorption method. At the same time, the gasified POCs are derivatized in situ with a saturated derivatization reagent and a helium (He) carrier gas stream, and the derivatization reagent used is N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA), which can derivatize POCs containing -OH and -COOH groups into non-polar or weakly polar compounds. The in-situ derivatization is divided into three stages: 1. Derivatization with a mixture of 10 sccm He and 40 sccm MSTFA for 6 min; 2. Derivatization with a mixture of 16 sccm He and 64 sccm MSTFA for 6 min; 3. Derivatization with 100 sccm pure He for 8 min. The analyte after thermal desorption is reconcentrated into a focused trap (FT) at 45 °C. Then, the FT is heated to 315 °C, and at the same time, backflushing is carried out with helium at a flow rate of 4 ml / min, and the analyte to be measured is introduced into a one-dimensional GC column (DB-5MS, specification: 30 m × 0.25 mm × 0.25 μm) through a valve-less injection system. This step takes 12 min; subsequently, the separation and analysis of GC×GC-MS are started, and the analysis duration is 61 min. The TAG system collects the next ambient sample while the GC×GC-MS system is running, and so on.

[0010] Furthermore, the heating program of the CTD is that the initial temperature is set at 45 °C and maintained for 2 min, then it is heated to 310 °C at a rate of 50 °C / min and maintained at 310 °C for 12 min, and the sample is desorbed from the CTD, and at the same time, in-situ derivatization is carried out in the He stream of the saturated derivatization reagent.

[0011] Furthermore, the heating program of the =GC×GC is as follows: First, it is maintained at 45 °C for 5 min, then heated to 195 °C at a rate of 6 °C / min and maintained at this temperature for 2 min, then the temperature is raised to 300 °C at a rate of 7 °C / min and maintained at this temperature for 12 min, and finally heated to 310 °C at a rate of 10 °C / min and maintained for 1 min.

[0012] Furthermore, the operation mode of two-dimensional GC is to connect the second-dimensional GC column through a solid-state thermal modulator on the basis of the original one-dimensional GC column, and finally analyze through a detector. In this operation mode, the flow rate of the first-dimensional column is the same as that of the second-dimensional column.

[0013] Furthermore, the modulator is a key part of two-dimensional gas chromatography. The current modulation method is thermal modulation, using the solid-state thermal modulator SSM1810 / 1820 of Snow View Technology, which is mainly composed of a cold zone in the middle and two hot zones on both sides, respectively called the left inlet hot zone and the right outlet hot zone; inside the cold zone, a metal block is close to the semiconductor refrigeration element to achieve cooling. The two hot zones are heated by a heating plate; a copper needle is installed between the two hot zones and the cold zone, one end of which is locked on the wall of the hot zone to effectively conduct the temperature of the hot zone to the entire length of the copper needle; the other end extends into the cold zone, keeping a certain distance from the metal block in the cold zone without contact; such a design not only ensures extremely fast temperature switching, but also limits the thermal crosstalk between the cold zone and the hot zone, and realizes independent control of the three temperature zones. The copper needles in the two hot zones and the central hole of the metal block in the cold zone are precisely aligned to ensure that a fused silica capillary modulation column can pass through smoothly; there is a rotating solenoid valve behind the right hot zone, and its extended shaft extends into the hot zone and connects to a metal swing arm; the modulation column is fixed to the end gripper of the metal swing arm, so as to convert the circular rotation of the solenoid valve into horizontal movement of the modulation column in the three temperature zones; the modulation column is bent into a column ring in each of the two hot zones, and extends into the column temperature box through the copper tube transmission line fixed at the bottom of the hot zone, and is connected to the inlet end of the one-dimensional column and the outlet end of the two-dimensional column.

[0014] Furthermore, the modulation process is achieved by the back-and-forth movement of the modulation column between different temperature zones; when the modulation column moves to the left, a certain section thereof moves from the cold zone to the inlet hot zone, and the compounds previously condensed or retained in the cold zone are reheated and released; at the same time, another section moves from the outlet hot zone to the cold zone, so that the compounds released from the red part of the first stage are recondensed or retained in the black part of the second stage; after a period of time, when the modulation column moves to the right, the first stage re-enters the cold zone, and the compounds distilled from the first-dimensional column are cold-condensed to prevent them from entering the two-dimensional column. The second stage moves to the outlet hot zone, and the compounds previously cold-condensed in the second stage are quickly released to the second-dimensional column for two-dimensional analysis; the modulation column continuously shuttles between the three temperature zones, forming a classic two-stage thermal modulation process;

[0015] The separated compounds enter the mass spectrometer and first undergo electron bombardment (EI) in the ion source. During this process, the sample molecules are bombarded by high-energy electrons, causing the molecules to break and the compounds to be ionized to form charged ions. The generated ions are accelerated and focused and enter the quadrupole mass spectrometer. The accelerating electric field enables the ions to gain kinetic energy and form a high-energy ion beam. The ions are separated in the mass spectrometer according to their mass-to-charge ratio (m / z). Ions with different mass-to-charge ratios move in different ways in the analyzer, thereby achieving separation. The separated ions are captured by the detector to generate a mass spectrum showing the relationship between the relative abundance of the ions and the mass-to-charge ratio. By comparing with the NIST library, the compounds in the sample can be identified and qualitative and quantitative analysis can be performed.

[0016] The present invention also provides an on-line analysis system for polar organic compounds based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer, and the system includes:

[0017] A sampling unit, including a cyclone separator with a PM 2.5 cutting head and a multi-channel carbon remover, which collects ambient air at a flow rate of 10 L / min and removes gaseous organic compounds, so that particulate matters are collected into the thermal desorption unit (CTD);

[0018] A thermal desorption and derivatization unit (CTD), which is used to gasify the trapped organic compounds by programmed temperature rise and introduce them into a focused trap (FT), and simultaneously complete in-situ derivatization of POCs in a helium gas stream;

[0019] A gas chromatography unit (GC×GC), including a one-dimensional gas chromatography column (DB-5MS, 30 m×0.25 mm×0.25 μm) and a two-dimensional gas chromatography column, and an analyte is introduced by using a valve-less injection system and a back-flushing technique;

[0020] A modulation unit, including a solid-state thermal modulator, the modulator has a cold zone and two hot zones (a left inlet hot zone and a right outlet hot zone), and heat is conducted to the modulation column through a copper needle to achieve two-stage thermal modulation of compounds;

[0021] A detection unit, including an electron impact ion source (EI) and a quadrupole mass analyzer, which is used for ionization, mass separation and detection of compounds, and generates mass spectrometry data;

[0022] A data processing unit, which is used to receive the mass spectrometry data of the detection unit, and perform qualitative and quantitative analysis of compounds by comparing with a database.

[0023] Furthermore, the temperature rise program of the thermal desorption unit (CTD) includes:

[0024] The initial temperature is set at 45 °C and lasts for 2 min;

[0025] It is heated to 310 °C at a rate of 50 °C / min and maintained for 12 min to desorb the sample and perform derivatization in a helium gas stream of the derivatization reagent.

[0026] Furthermore, the operating parameters of the gas chromatography unit (GC×GC) include:

[0027] One-dimensional chromatographic column temperature rise program: maintain at 45 °C for 5 min, heat to 195 °C at a rate of 6 °C / min, maintain for 2 min, then heat to 300 °C at a rate of 7 °C / min and maintain for 12 min; finally, heat to 310 °C at a rate of 10 °C / min and maintain for 1 min.

[0028] Flow control of two-dimensional gas chromatography: The flow rate of the first-dimensional column is the same as that of the second-dimensional column, and a solid-state thermal modulator is used to achieve sample enrichment and release.

[0029] Furthermore, the modulation unit uses the reciprocating movement of the modulation column between the cold zone and the hot zone for sample modulation. When the modulation column moves to the left, the condensed compounds are reheated and released, while at the same time, the other end enters the cold zone for compound cold aggregation; when the modulation column moves to the right, the condensed compounds are released to the second-dimensional column for separation and analysis, and finally detected by a mass spectrometry system.

[0030] Combined with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by the present invention are as follows:

[0031] First, when analyzing potential polar organic pollutants, traditional techniques usually rely on membrane sampling and laboratory off-line derivatization analysis methods, facing problems such as low time resolution, cumbersome operation steps, and possible loss of samples during long-term sampling. In addition, although two-dimensional gas chromatography (GC×GC) has been applied in laboratory off-line analysis, it still relies on a complex pretreatment process and is difficult to meet the requirements for rapid and efficient analysis of POCs.

[0032] To overcome the above deficiencies, the in-situ on-line derivatization TAG-GC×GC-MS technology proposed by the present invention uses a TAG system for air sampling at the front end, which can quickly capture POCs within 30 minutes, significantly reducing the loss of samples during long-term sampling. The GC×GC-MS analysis at the back end can greatly improve the separation efficiency compared with traditional gas chromatography methods. Especially when dealing with complex samples, it can separate compounds with similar structures, thereby improving the sensitivity and selectivity, reducing the co-elution phenomenon, and ensuring that the compounds in the chromatogram are arranged more regularly.

[0033] By combining TAG with GC×GC-MS, this system can achieve automated on-line derivatization and analysis of POCs. The collected samples do not need to go through traditional storage, transportation, and pretreatment steps and can be directly introduced into GC×GC-MS for real-time analysis. This automated operation not only shortens the overall analysis time (the collection and analysis can be completed within two hours), but also avoids sample contamination and loss caused by multiple processing links in traditional off-line methods. In addition, the hourly data resolution is of great significance for timely capturing the changes in POCs during pollution events, and then for pollutant source tracing and assessment.

[0034] While ensuring high resolution and high sensitivity, the TAG-GC×GC-MS system significantly reduces the input of human and material resources and the cost of sample pretreatment. Its rapid sampling, automated operation, and efficient separation and analysis capabilities provide important technical support for ambient air quality monitoring and pollution source tracking. Compared with traditional methods, this system can not only obtain high-quality data in a short time, but also better meet the research needs of the dynamic changes of POCs, laying a solid technical foundation for the subsequent in-depth analysis of pollution characteristics and source apportionment.

[0035] Second, as the creative auxiliary evidence of the claims of the present invention, it is also reflected in the following important aspects:

[0036] (1) The expected benefits and commercial value after the transformation of the technical solution of the present invention are as follows: This technical solution has great industrialization potential, especially in the field of ambient air monitoring. It is expected that after the integrated TAG-GC×GC-MS system with on-line derivatization enters the market, it will gradually replace traditional off-line detection equipment, significantly improving the detection efficiency and data quality of POCs. From the market perspective, this technology has a wide range of potential users and good market expectations.

[0037] (2) The technical solution of the present invention fills the technical gaps in the domestic and international industries: The TAG-GC×GC-MS system formed by this technical solution fills the gap in the lack of on-line detection technology for POCs at home and abroad.

[0038] (3) The technical solution of the present invention solves the technical problems that people have been eager to solve but have never succeeded in: a. The problem of on-line analysis of polar organic compounds (POCs) with high time resolution. Traditional POCs analysis methods mainly rely on off-line sampling and laboratory analysis, and the sampling time is usually several hours or even several days, with low time resolution. The long sampling process cannot guarantee the consistency of sampling conditions, and the sample may volatilize, degrade and other changes during the sampling process, resulting in inaccurate analysis results. The present invention realizes the on-line analysis of POCs with high time resolution through the TAG-GC×GC-MS system. The sampling time is only 30 minutes, and the derivatization, separation and detection of the sample can be completed in a short time, significantly improving the time resolution and solving the problem that traditional methods cannot capture the dynamic changes of POCs in real time. b. The problem of efficient separation and detection of POCs in complex samples. Traditional gas chromatography-mass spectrometry (GC-MS) technology often faces problems such as low separation efficiency and serious co-elution phenomenon when analyzing complex samples, especially for structural similar isomers or the coexistence of multiple compounds, it is difficult to achieve effective separation. The present invention realizes the efficient separation of POCs in complex samples by introducing comprehensive two-dimensional gas chromatography (GC×GC) technology and combining with a solid-state thermal modulator, significantly improving the separation efficiency and detection sensitivity, and solving the problem that traditional GC-MS technology is difficult to cope with complex sample analysis.

[0039] (4) The technical solution of the present invention overcomes technical biases: a. The bias of traditional off-line analysis methods. The analysis of POCs mainly relies on off-line sampling and laboratory analysis, believing that on-line analysis is difficult to achieve high time resolution and high sensitivity detection. The present invention successfully realizes the on-line analysis of POCs through the TAG-GC×GC-MS system, overcomes the bias of traditional off-line analysis methods, and proves the feasibility and superiority of on-line analysis technology in POCs detection. b. The bias that GC-MS technology cannot cope with complex samples. Traditional GC-MS technology is often considered difficult to achieve efficient separation when analyzing complex samples, especially for compounds with similar structures. The present invention significantly improves the separation efficiency by introducing comprehensive two-dimensional gas chromatography (GC×GC) technology and combining with a solid-state thermal modulator, overcomes the limitations of traditional GC-MS technology in dealing with complex samples, and proves the advantages of GC×GC technology in complex sample analysis. Description of the Drawings

[0040] Figure 1 It is a schematic diagram of the TAG-GC×GC-MS system provided by the embodiment of the present invention;

[0041] Figure 2 It is the GC×GC operation mode provided by the embodiment of the present invention;

[0042] Figure 3It is the structure diagram of the solid-state thermal modulator provided by the embodiment of the present invention;

[0043] Figure 4 It is the modulation process of the solid-state thermal modulator provided by the embodiment of the present invention.

[0044] Figure 5 It is the establishment of the TAG method provided by the embodiment of the present invention;

[0045] Figure 6 It is the TAG work log provided by the embodiment of the present invention;

[0046] Figure 7 It is the two-dimensional spectrum of POCs saccharides (taking levoglucosan calibration as an example) provided by the embodiment of the present invention;

[0047] Figure 8 It is the extraction mass spectrum provided by the embodiment of the present invention;

[0048] Figure 9 It is the detailed peak result information (taking saccharides as an example) provided by the embodiment of the present invention;

[0049] Figure 10 It is the front view and partial view of TAG-GC×GC-MS provided by the embodiment of the present invention;

[0050] Figure 11 It is the comprehensive two-dimensional solid-state thermal modulator provided by the embodiment of the present invention. Detailed implementation manners

[0051] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0052] The schematic diagram of this technical instrument is as Figure 1 shown. The entire working cycle of the comprehensive two-dimensional thermal desorption aerosol analyzer (TAG-GC×GC-MS) instrument includes four stages: sampling and comprehensive two-dimensional gas chromatography mass spectrometry (GC×GC / MS) analysis, loading and injecting standard solution, thermal desorption and derivatization, and backflushing. During sampling, through the PM 2.5The cyclone separator of the cutting head (10 L / min, BGI Inc., Waltham, MA) aspirates ambient air, and then removes gaseous organic compounds through a multi-channel carbon adsorber, concentrating particulate matter into the CTD unit. The sampling time is 30 min. Subsequently, the CTD is heated by programmed temperature rise, vaporizing the trapped organic compounds, which are then introduced into the subsequent FT. The temperature rise program of the CTD is as follows: the initial temperature is set at 45 °C and maintained for 2 min, then it is heated to 310 °C at a rate of 50 °C / min and held at 310 °C for 12 min. The sample is desorbed from the CTD and undergoes in-situ derivatization simultaneously in a He stream carrying a saturated derivatization reagent. Among them, N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA, Sigma-Aldrich) is used as the derivatization reagent for organic compounds containing hydroxyl (-OH) and carboxyl (-COOH) groups. The in-situ derivatization is divided into three stages, namely, derivatization with a mixture of 10 sccm He and 40 sccm MSTFA for 6 min, derivatization with a mixture of 16 sccm He and 64 sccm MSTFA for 6 min, and derivatization with 100 sccm pure He for 8 min. Before thermal desorption, 5 μL of the internal standard mixed solution (containing deuterated compounds) is added to the CTD unit. The analytes after thermal desorption are reconcentrated into a focusing trap at 45 °C, and then the FT is heated to 315 °C while backflushing with 4 mL / min of He, and the analytes to be measured are introduced into a one-dimensional chromatographic column (DB-5MS, specifications: 30 m × 0.25 mm × 0.25 μm) through a valve-less injection system. This step takes 12 min. Subsequently, the separation and analysis of GC×GC-MS are initiated. Among them, the temperature rise program of GC×GC is as follows: maintain at 45 °C for 5 min, heat to 195 °C at a rate of 6 °C / min, hold at this temperature for 2 min, heat to 300 °C at a rate of 7 °C / min, hold at this temperature for 12 min, and finally heat to 310 °C at a rate of 10 °C / min and hold for 1 min. The analysis time for each sample is 61 min. The TAG system collects the next ambient sample simultaneously while the GC×GC-MS system is running, and so on.

[0053] As Figure 2 shown, the operation mode of two-dimensional GC is to connect the second-dimensional GC column through a solid-state thermal modulator based on the original one-dimensional GC column and finally analyze through a detector. In this operation mode, the flow rate of the first-dimensional column is the same as that of the second-dimensional column.

[0054] The modulator is a key part of two-dimensional gas chromatography. Currently, the modulation methods are mainly divided into gas flow modulation and thermal modulation. This technology uses the solid-state thermal modulator SSM1810 / 1820 of Xuejing Technology. Its main structure is as Figure 3As shown. It mainly consists of a cold zone in the middle and two hot zones on both sides (referred to as the left inlet hot zone and the right outlet hot zone respectively). Inside the cold zone, a metal block is close to the semiconductor refrigeration element to achieve cooling. The two hot zones are heated by heating plates. A copper needle is installed between the two hot zones and the cold zone, one end of which is locked on the wall of the hot zone to effectively conduct the temperature of the hot zone to the entire length of the copper needle; the other end extends into the cold zone and keeps a certain distance from the metal block in the cold zone without contact. This design not only ensures extremely fast temperature switching, but also limits the thermal crosstalk between the cold zone and the hot zone, and realizes independent control of the three temperature zones. The copper needles in the two hot zones and the center hole that penetrates the metal block in the cold zone are precisely aligned to ensure that a fused silica capillary modulation column passes smoothly. There is a rotating solenoid valve behind the right hot zone, and its extended shaft extends into the hot zone to connect a metal swing arm. The modulation column is fixed to the end gripper of the metal swing arm, so as to convert the arc rotation of the solenoid valve into the horizontal movement of the modulation column in the three temperature intervals. The modulation column is bent into a column ring in each of the two hot zones, and extends into the column oven through a copper tube transmission line fixed at the lower part of the hot zone to connect with the one-dimensional column (inlet end) and the two-dimensional column (outlet end).

[0055] The modulation process is achieved by moving the modulation column back and forth between different temperature zones ( Figure 4 ). When the modulation column moves to the left, one section (indicated by red in the figure, the first level) moves from the cold zone to the inlet hot zone, and the compounds that were previously condensed or retained in the cold zone are reheated and released. At the same time, another section (indicated by black in the figure, the second level) moves from the outlet hot zone to the cold zone, so that the compounds released from the red part of the first level are recondensed or retained in the black part of the second level. After a period of time, when the modulation column moves to the right, the first level re-enters the cold zone and cold-condenses the compounds distilled from the first-dimensional column to prevent them from entering the two-dimensional column. The second level moves to the outlet hot zone and quickly releases the compounds that were previously cold-condensed in the second level to the second-dimensional column for two-dimensional analysis. The modulation column continuously shuttles between the three temperature zones, forming a classic two-stage thermal modulation process.

[0056] The separated compounds enter the mass spectrometer and first undergo electron bombardment (EI) in the ion source. During this process, the sample molecules are bombarded by high-energy electrons, causing the molecules to break and the compounds to be ionized to form charged ions. The generated ions are accelerated and focused and enter the quadrupole mass spectrometer. The accelerating electric field causes the ions to gain kinetic energy and form a high-energy ion beam. The ions are separated in the mass spectrometer according to their mass-to-charge ratio (m / z). Ions with different mass-to-charge ratios move in different ways in the analyzer, thereby achieving separation. The separated ions are captured by the detector and a mass spectrum is generated, showing the relationship between the relative abundance of the ions and the mass-to-charge ratio. By comparing with the standard mass spectrometer library (NIST), the compounds in the sample can be identified and qualitative and quantitative analysis can be performed.

[0057] Taking a single run of a calibration sample as a specific implementation example, this document details how to qualitatively and quantitatively analyze POCs using this technology.

[0058] First, set up the method on the TAG system operation software. The calibration sample system automatically prepares external standards, solvents, and internal standards. Using a two-step thermal desorption method, the sample is desorbed from the CTD and simultaneously in-situ derivatized, then transferred to a one-dimensional GC column. Subsequently, it is analyzed by GC×GC through a two-dimensional column connected by a modulator. The specific method settings are as Figure 5 shown.

[0059] After the method settings are completed, next, establish the sequence. After uploading the sequence to the TAG system, save the work log. In the work log, the temperature changes in the heating part and the flow rate information of each part during the sequence run are recorded. After the instrument runs, the real-time sequence running status of the instrument can also be queried in the status information of the software, which provides convenience for subsequent troubleshooting of instrument problems.

[0060] After the complete sequence run is finished, open the two-dimensional chromatogram analysis software (Canvas). First, obtain the structured two-dimensional chromatogram of POCs. POCs elute in order of volatility, appearing as strips on the chromatogram, and the color intensity represents the concentration of the substance.

[0061] In Canvas, load the NIST mass spectrometry library for qualitative analysis. First, perform automatic integration through the peak detection function of the two-dimensional software. After turning on the peak selection switch, randomly select a peak on the two-dimensional chromatogram. The mass spectrum of the selected compound is displayed in the mass spectrometer (blue), and the highest matching compound of this compound is also displayed on the chromatogram simultaneously (red). The detailed information of this compound is shown in the matching compound information table below the chromatogram. From Figure 8 it can be seen that the main ion fragments of this compound are at m / z = 206, 220, and 192, which are the characteristic ion peaks of levoglucosan. First, it is determined as a carbohydrate. In the matching compound information, the substance matched in the NIST library is a levoglucose derivative, and the reverse matching degree is 758. Then, based on the elution time of levoglucosan, it is finally determined that this substance is levoglucosan, thus completing the qualitative analysis of one substance.

[0062] Repeat the above method to identify and qualitatively analyze all polar substances (such as sugars, acids, alcohols, etc.) in the sample, and export the peak areas of all polar substances for quantitative analysis. In the Canvas software, after peak integration, click on the peak results to view the detailed information of all integrations, such as retention time, peak area, etc. By exporting the peak areas of the corresponding POCs, quantitative analysis is performed using the internal standard method.

[0063] After importing the peak areas into Excel, the quantitative part will take constructing a standard curve as an example again. The stock solutions of polar organic compounds (sugar alcohols, fatty acids, straight-chain dicarboxylic acids, oxygen-containing carboxylic acids, aromatic acids, nitro-hydrocarbon compounds, secondary organic aerosol tracers) are placed in 10 mL reagent bottles and diluted several times with a mixed solution of organic solvents such as methanol, acetonitrile, and dichloromethane to prepare working standard solutions. Finally, the working solution concentrations of different POCs are 0.30 - 0.45 ng / μL. Table 1 shows the corresponding internal standards (IS) and other detailed information of the external standards (ES), and Table 2 lists the detailed information on the preparation of the IS standards. After injecting different volumes (ranging from 5 to 30 μL) of the working standard solutions and a fixed volume (5 μL) of the IS into the CTD, a standard curve is constructed by fitting the normalized peak areas of the ES to the corresponding IS. As can be seen from Table 2, the R 2 values of the standard curves are all above 0.90, indicating good linear responses.

[0064] Table 1. Quantitative ions and corresponding internal standards of polar organic compounds

[0065]

[0066]

[0067] Table 2. R information list of the standard curve 2 Information list

[0068]

[0069]

[0070] Figure 10 Fig. 1 shows the front view and partial view of the TAG-GC×GC-MS, where part labeled 1 is the above-mentioned CTD, part labeled 2 is the standard sample injection system, part labeled 3 is the focusing trap, part labeled 4 is the comprehensive two-dimensional solid-state thermal modulator, and parts labeled 5a and 5b are the on-line derivatization systems.

[0071] Figure 11 Fig. 2 shows the detailed drawings of the components of the comprehensive two-dimensional solid-state thermal modulator. Part labeled 1 is the inlet hot zone heat insulation pad, part labeled 2 is the outlet hot zone heat insulation board, part labeled 3 is the guide rod, part labeled 4 is the gripper, part labeled 5 is the modulation column, part labeled 6 is the millet grain, which is used to connect the modulation columns at both ends of the inlet hot zone and the outlet hot zone. Part labeled 7 is the semiconductor refrigeration zone.

[0072] As described above, the above are only the specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. An on-line analysis method for polar organic compounds based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer, characterized in that, The method comprises the following steps: Through the cyclone separator of the PM 2.5 The cyclone separator of the cutting head sucks ambient air at a flow rate of 10 L / min, and uses a multi-channel carbon remover to remove gaseous organic compounds, so that particulate matter is collected in the thermal desorption unit (CTD); The temperature of the CTD is programmed to vaporize the trapped organic matter and introduce it into the focused trap (FT); In situ derivatization was performed by adding N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) as a derivatization agent to CTD; The FT was heated to 315 °C and backflushed with 4 mL / min of helium, and the analytes were introduced into a one-dimensional gas chromatography column (DB-5MS, 30 m × 0.25 mm × 0.25 μm); Separation and detection were performed by comprehensive two-dimensional gas chromatography (GC×GC) and mass spectrometry detection system (MS), and the entire analysis cycle was 61 minutes.

2. The on-line analysis method for polar organic compounds according to claim 1, characterized in that The temperature raising program of the CTD includes: the initial temperature is set to 45°C, and after holding for 2 minutes, the temperature is raised to 310°C at a rate of 50°C / min and maintained for 12 minutes, while derivatization is performed in a helium flow of a saturated derivatization reagent.

3. The on-line analysis method of polar organic compounds according to claim 1, characterized in that, The temperature rise program of the GC×GC is: Keep at 45℃ for 5min; Raise the temperature to 195°C at a rate of 6°C / min and hold for 2 min; The temperature was raised to 300°C at a rate of 7°C / min and maintained for 12 min.

4. The on-line analysis method for polar organic compounds according to claim 1, characterized in that, The two-dimensional gas chromatography system adopts a solid-state thermal modulator, which includes a cold zone and two hot zones, namely a left inlet hot zone and a right outlet hot zone. The cold zone is cooled by a metal block close to a semiconductor refrigeration element, and the two hot zones are heated by a heating plate, and the heat is transferred to the modulation column through a copper needle.

5. The on-line analysis method for polar organic compounds according to claim 4, characterized in that, The modulation process is achieved by moving the modulation column back and forth between the cold zone and the hot zone. When the modulation column moves to the left, one section of it moves from the cold zone to the inlet hot zone, so that the condensed compounds are released again, and at the same time, the other section enters the cold zone from the outlet hot zone, so that the compounds distilled from the first-dimensional column are cooled and aggregated; when the modulation column moves to the right, the condensed compounds are quickly released to the second-dimensional column for analysis.

6. The on-line analysis method for polar organic compounds according to claim 1, wherein The separated compounds enter the mass spectrometer, are ionized by electron impact (EI) in the ion source, and enter the quadrupole mass spectrometer for mass separation. Ions with different mass-to-charge ratios move along different trajectories in the analyzer and are captured by the detector to generate a mass spectrum for qualitative and quantitative analysis of the compounds.

7. An on-line analysis system for polar organic compounds based on comprehensive two-dimensional gas chromatography and aerosol thermal desorption analyzer, characterized in that The system includes: Sampling unit, including PM 2.5 The cyclone separator and multi-channel carbon remover of the cutting head collect ambient air at a flow rate of 10 L / min and remove gaseous organic compounds, so that particulate matter is collected in the thermal desorption unit (CTD); Thermal desorption unit (CTD), used to vaporize the trapped organic matter by programmed temperature and introduce it into the focused trap (FT), while completing in-situ derivatization in the helium flow; Gas chromatography unit (GC×GC), including a one-dimensional gas chromatography column (DB-5MS, 30m×0.25mm×0.25μm) and a two-dimensional gas chromatography column, and using a valveless injection system and backflush technology for analyte introduction; The modulation unit includes a solid-state thermal modulator, the modulator has a cold zone and two hot zones (a left inlet hot zone and a right outlet hot zone), and heat is conducted to the modulation column through a copper needle to achieve two-stage thermal modulation of the compound; A detection unit, including an electron impact ion source (EI) and a quadrupole mass spectrometer, is used for ionization, mass separation and detection of compounds and to generate mass spectral data; The data processing unit is used to receive the mass spectrometry data from the detection unit and perform qualitative and quantitative analysis of the compounds by comparing the database.

8. The on-line analysis system for polar organic compounds according to claim 7, wherein The temperature-raising program of the thermal desorption unit (CTD) includes: The initial temperature is set at 45°C and maintained for 2 min; It is heated to 310°C at a rate of 50°C / min and held for 12 min to desorb the sample and perform derivatization in the helium gas stream of the derivatization reagent.

9. The on-line analysis system for polar organic compounds according to claim 7, wherein The operating parameters of the gas chromatography unit (GC×GC) include: The temperature-raising program of the one-dimensional chromatographic column: maintained at 45°C for 5 min, heated to 195°C at a rate of 6°C / min, held for 2 min, and then heated to 300°C at a rate of 7°C / min and held for 12 min; The flow control of two-dimensional gas chromatography: the flow rate of the first-dimensional column is the same as that of the second-dimensional column, and a solid-state thermal modulator is used to achieve sample enrichment and release.

10. The on-line analysis system for polar organic compounds according to claim 7, characterized in that, The modulation unit uses the reciprocating movement of the modulation column between the cold zone and the hot zone to modulate the sample. When the modulation column moves to the left, the condensed compound is reheated and released, and at the same time, the other end enters the cold zone for compound cold aggregation; when the modulation column moves to the right, the condensed compound is released to the second-dimensional column for separation and analysis, and finally detected by the mass spectrometry system.

Images