CN1328582C - Method for enriching and trapping gas trace component for chromatograph capillary tube column analysis - Google Patents

Abstract

The invention relates to gas chromatographic analysis, which is a method for enrichment and capture of trace components in gas using _CO2 zone refrigeration in capillary column analysis; using liquid _CO2 zone refrigeration source, through the nozzle to the pre-column or capillary The head of the column is locally refrigerated to enrich the trace components in the gas sample. The method of the present invention has high quantitative precision for chromatographic analysis of trace components, fast analysis speed, large enrichment ratio, reasonable use of cooling capacity, small equipment investment, convenient operation, can be directly used in combination with capillary columns, and is also effective for gas chromatographs. There is no special requirement, so it is easy to promote and has broad application prospects.

Description

A method for enrichment and capture of gas trace components in chromatographic capillary column analysis

technical field

The invention relates to gas chromatographic analysis, in particular to a method for enrichment and capture of gas trace components using _CO2 cold spray zone refrigeration in capillary column analysis.

Background technique

Gas chromatography plays an important role in the analysis of volatile samples due to its advantages of high sensitivity, high resolution, fast analysis, low instrument price and convenient operation. However, for trace component analysis, due to the detection limit Due to the limitations, pretreatment is often required, among which preconcentration is an important means to solve the analysis of trace components; it is to enrich the components to be measured in the sample by physical and chemical methods before sample analysis, and at the same time partially remove the sample A kind of method of the principal component in; Common method has: solvent absorption method, cryogenic trapping method (document 1.Kaiser R E.Anal Chem, 1973,45:965-967; Document 2.Mcclenny W A, Pleil J D, Holdren M W et al.Anal Chem, 1984,56:2947～2951), normal temperature solid adsorption method (document 3.Frank W, Frak H.Chromatogrphia, 1990,29:571～574; document 4.Brown R H, PumellC JJ Chromatogr , 1997, 178: 79-90). The solvent absorption method not only consumes a large amount of solvent and causes pollution, but also has a large number of solvent peaks, which interferes with the analysis and limits its use. Cryogenic trapping method is also called purge and trap method (document 5.Ceulemans M, Adams F CJ Anal AtSpectrom, 1996, 11:201-206; document 6.Jo-Anne A Jackson, William R Blair, Frederick E Brickman, Warren P Iverson.Environ Sci Technol, 1982, 16(2): 110-119), because a large amount of water is trapped at the same time, which often affects the chromatographic separation, and the cold trap equipment is expensive, complicated to operate, time-consuming and laborious. The limitation of the normal temperature solid adsorption method is that it is not suitable for the analysis of low boiling point components and requires an analytical device. Common commercial purge traps include: VOCARB3000, VOCARB4000, BTEXTRAP, GERSTEL TDS 2 ^PLUS and GERSTEL TDS 2, etc., which have good performance, but are very expensive and highly specific. General-purpose enrichment methods require a large amount of sample (generally hundreds of milliliters to several liters), and the long desorption time often leads to broadening of the chromatographic injection band and an increase in the minimum detection concentration (document 7.Cropper F R, Simon Kaminsky. Anal Chem, 1963, 35: 735～739; literature 8. Panknow J F, Luo W T, Isabelle L M et al. Anal Chem, 1998, 70: 5213～5221); There are strict requirements.

Gas chromatography plays an important role in the analysis of gas trace components, among which pre-concentration is the most important means to solve the analysis of trace components in gas. The currently commonly used analysis methods have certain limitations. When a large amount of sample is enriched, the long desorption time often leads to broadening of the chromatographic injection band and an increase in the minimum detection concentration. When combined with a capillary column The problem is even more serious; although the commercialized equipment has good performance, its structure is complicated, the price is very expensive, and its specificity is very strong; when the refrigerant is liquid nitrogen, the refrigeration temperature can reach below -60°C, but the equipment cost and The operating costs are very high; when CO ₂ is used, the refrigeration temperature can only reach ~-30°C, and many compounds with low boiling points cannot be captured.

Solid-phase microextraction (SPME), which has developed rapidly in recent years, integrates sampling, extraction, concentration, and sample injection. Because it is easy to carry, it truly realizes on-site sampling and enrichment of samples, and is very good in the analysis of liquid samples. Application: There are also reports in the literature on the concentration of gas samples, but there are still certain difficulties in quantification, and the accuracy is difficult to guarantee, and it is not suitable for the analysis of low boiling point components.

Contents of the invention

The purpose of the present invention is to provide a method for enriching and trapping gas trace components in capillary column analysis with high precision, high speed, low investment and simple operation.

To achieve the above object, the technical scheme adopted in the present invention is:

Use the liquid CO ₂ regional cooling source to locally cool the head of the pre-column or capillary column (-60℃～-70℃) through the nozzle, and use the low temperature to cold capture the gas trace components. When a large number of gas samples are injected Based on the enrichment of trace components in the sample. Because the enriched components are directly focused on the column head of the chromatographic column under the double action of low temperature and stationary liquid, when the chromatographic furnace heats up, the enriched components are well separated in the chromatographic column, and finally in the detector was checked out.

The spout is a stainless steel tube with an internal diameter of 0.75mm, and the 1-5mm place from its outlet is processed into a slit to control _the CO at the spout. The flow rate is 1000-1500ml/min; the distance between the spout and the capillary column is preferably 1-3mm ; The gas sample can also be a pre-enriched sample in an adsorption tube.

The present invention has the following advantages:

1. High quantitative precision. The present invention adopts a very small nozzle to directly form a small area refrigeration at the column head of the capillary column, and directly captures each component to be enriched at the column head of the capillary column. The analysis of the captured components is carried out simultaneously with the chromatographic analysis process without auxiliary heating. The sampling method is equivalent to the capillary column cooling column head sampling, so it can obtain good quantitative and separation effects. Experiments have proved that this patent can simultaneously Obtaining satisfactory enrichment and chromatographic separation is self-evident compared with the existing purge and trap devices.

2. Large enrichment ratio, good focusing effect, and small investment in equipment. The present invention utilizes a liquid _CO2 regional refrigeration source to perform local refrigeration on the capillary column head in a small area (refrigerating temperature can reach -60°C to -70°C), utilizes low temperature to realize cold capture of the column head, and conducts cooling on the basis of a large amount of gas sampling. Trace components are enriched. Experiments have proved that the enriched components are directly focused on the column head of the capillary column under the double action of low temperature and fixative; satisfactory focusing effect can also be obtained at a very high enrichment ratio. At the same time of chromatographic analysis, the components enriched at the head of the column are desorbed and then separated on the chromatographic column; since the process is equivalent to that of cold-column head injection, the column efficiency of the capillary column is almost not lost, so during chromatographic analysis, the enriched Satisfactory separation and quantification results are obtained for each component in the chromatographic column, and at the same time, no auxiliary analytical heating equipment is required; the equipment of the present invention has small investment, convenient operation, easy promotion, large enrichment ratio, reasonable cooling capacity, and can be used with capillary columns It can be used directly and has no special requirements for the gas chromatograph. The applicant refitted the domestic chromatograph, and the experiment proved that the component (xylene) with a boiling point above 100°C was well captured, and the recovery rate could reach 96% to ₁₀₅ %. The mixed sample of C ₁₀ and triphenyl/air proves that the capture effect is also very good for benzene with a boiling point of only 80°C; Sampling) after desorption, cold capture and re-enrichment, good experimental results are also obtained, which demonstrates its technical superiority and broad application prospects.

Description of drawings

Fig. 1 is the flow chart of _CO2 pressure purification liquefaction;

Fig. 2 is the temperature distribution diagram of different distances directly in front of the spout;

Fig. 3 is the pressure-enthalpy diagram of _CO2 ;

Fig. 4 is the flow chart of cold capture enrichment analysis;

Fig. 5 is the chromatographic separation spectrum of 500 μ L xylene/air sample gas;

Fig. 6 is the chromatographic separation figure of 10 * 500 μ L xylene/air sample gas;

Fig. 7 repeats the experiment of Fig. 5 the next day;

Fig. 8 is the chromatographic separation diagram of 15 * 500 μ L xylene/air sample gas in the second day; (the components in the figure are the same as Fig. 6)

Fig. 9 is a chromatographic separation diagram of 500 μL triphenyl and C ₇ , C ₈ , C ₁₀ hydrocarbon/air sample gas;

Figure 10 is a chromatographic separation diagram of 40×500 μL triphenyl and C ₇ , C ₈ , C ₁₀ hydrocarbon/air sample gas;

Figure 11 is a flow chart of desorption and enrichment again;

Fig. 12 is the chromatographic analysis spectrogram of gas in medicine cabinet;

Fig. 13 is the chromatographic analysis spectrum of laboratory gas.

Detailed ways

Example

1) CO ₂ cooling source

The _CO2 refrigeration source used in this example is prepared with reference to " _CO2 pressure purification and liquefaction method and device and application in district refrigeration" (patent application No. 02144879.5), using _CO2 in the gas phase pressure purification, ice -Liquefaction under pressure under water conditions, sudden decompression through the resistor, and the use of a large amount of cold energy released when the liquid CO ₂ phase changes into dry ice for refrigeration. The flow process of _CO2 pressure purification and liquefaction is shown in Figure 1. The specific structure is: _CO2 gas source 1, pressure gauge 2 with a range of 10Mpa, and a built-in dehydrogenation agent (such as activated carbon, etc., which is used in this embodiment. Activated carbon) purifier a3, built-in dehydrating agent (for example: silica gel, molecular sieve, etc., what this embodiment uses is silica gel) purifier b4, coil pipe 6 and gas resistance device 7 are formed by connecting in series through pipelines, wherein: Said coiled pipe 6 (copper tube of external diameter 3mm) is placed in ice-water bath (or ethanol-water-dry ice bath) 5, and the outlet of purifier a3 and purifier b4 is all blocked with sintered stainless steel mesh; this example The resistance device 7 that adopts is to use the stainless steel pipe of 1/4 inch inner diameter 0.75mm, process into a wedge-shaped slit at 3mm inward from the outlet as the spout, so the inner diameter of the spout is 0.75mm; the outer diameter is 1 /4 inches, it is advisable to use the resistance of the slit to control the CO ₂ flow rate of the nozzle to be 1000-1500ml/min.

In the process of Figure 1, the nozzle is used as the resistance device, the thermocouple (EU-2) K with an outer diameter of 0.5mm is used as the sensitive element, and the XL43 digital display meter of Shanghai Yinhe Instrument Factory is used to measure the temperature, and the temperature at different distances from the front of the nozzle The distribution is shown in Figure 2 and Table 1.

Table 1 Distribution of temperature in the region near the vent under different CO ₂ pressures

P _co2 ＝4.5Mpa (gas cylinder temperature is 10ε, cold bath temperature is 0ε) P _co2 ＝4.0Mpa (gas cylinder temperature is 5ε, cold bath temperature is)ε) P _co2 ＝3.0Mpa (the temperature of the cold bath is -18ε) Distance from nozzle mm Temperature ε Distance from nozzle mm temperature 8 Distance from nozzle mm temperature ε 3 -60 3 -70 2 -66 5 -50 5 -48 3 -66 10 -46 10 -twenty four 20 -twenty four 20 -10 30 -13 30 -3

It can be seen from Table 1 that the temperature of the spout is not only related to the distance, but also related to the pressure of gas CO ₂ : when the temperature of the gas cylinder is low, the pressure of CO ₂ in the gas cylinder is also low (see Figure 3) , the temperature of the corresponding spout is higher (only 3mm is the exception due to the influence of flow velocity), and the temperature gradient is also large, and vice versa; secondly, the temperature of the spout is also related to the temperature of the cold bath, the lower the temperature of the cold bath, the lower the temperature of the spout .

2) Cold capture for enrichment of xylene/air gas samples

The process of cold capture for capillary chromatographic column head enrichment is shown in Figure 4, wherein: 8 is the vaporization chamber, 9 is the cold nozzle, 10 is the chromatographic column, and 11 is the detector; the distance between the nozzle and the chromatographic column is 3 mm in the experiment .

Chromatographic analysis conditions:

Chromatograph: Shanghai Kechuang Chromatography Instrument Company GC-900

Acquisition and processing system: N-2000 dual-channel workstation of Institute of Intelligent Information Engineering, Zhejiang University

Column: $\mathrm{FFAP}30m\≅0.25\mathrm{mm}\≅0.5\mathrm{\μm}$

Vaporization chamber: 150°C; hydrogen flame detector: 180°C; split ratio 65:1;

Carrier gas: nitrogen. Column temperature: 40°C (1min)→10°C/min→120°C (constant temperature)

Sample: xylene/air;

Cold bath: -18℃, _CO2 pressure is 3.5Mpa

Add cold spray when the furnace temperature is 40°C, slowly inject samples with a syringe through the vaporization chamber, stop the cold spray after sample injection, heat up the chromatographic furnace and record at the same time, and the chromatographic analysis spectra are shown in Figures 5, 6, 7, and 8.

Fig. 5 is the chromatographic separation spectrum of 500 μ L sample gas (the mixed gas of 0.2 μ L xylene diluted in 500 ml air);

Figure 6 is a chromatographic separation diagram of 10 × 500 μL sample gas;

Fig. 7 repeats the experiment of Fig. 5 the next day;

Fig. 8 is the chromatographic separation diagram of repeating the experiment of Fig. 6 on the second day and injecting 15×500 μL of sample gas.

From the comparison of chromatographic separation diagrams 5 to 8, it can be seen that when the sample is injected, the chromatographic column head adopts cold spray, and each chromatographic peak is coldly captured and enriched at the column head of the chromatographic column, and then separated in the column as the temperature of the chromatographic furnace rises, as shown in Fig. The middle number is the peak area. From the comparison of the area of m-xylene, it can be seen that the efficiency of cold capture reaches 96% and 105% respectively, and the previously undetectable ethylbenzene and p-xylene can be detected after being enriched. .

3) Cold capture is used to enrich hydrocarbons and aromatics/air mixtures

Sample: triphenyl and C ₇ , C ₈ , C ₁₀ hydrocarbons/air;

Cold bath; -20℃, _CO2 pressure is 4.4Mpa;

The rest are the same as Example 2.

The chromatographic operation is the same as in Example 2. From the comparison of Figures 9 and 10, it can be seen that when 500 μL of sample gas is injected, almost all components are not detected. Each chromatographic peak is cold-captured and enriched at the head of the chromatographic column, and then separated in the column as the temperature of the chromatographic furnace rises, and is detected in the detector. It can be seen from the chromatogram that both C ₇ with a boiling point of 98°C and benzene at 80°C are well captured and enriched; because the C ₁₀ sample contains impurities, the peak is tailed, and there are small peaks appearing behind it.

4) Cold capture is used to enrich and analyze actual samples

Due to the low concentration of actual gas samples, a large enrichment ratio is often required, and most of them are sampled in different places. We use adsorption tubes to sample and enrich first, and then heat them into the vaporization chamber for cold capture and enrichment; in order to obtain more For a good focusing effect, a 20cm-long empty capillary column with an inner diameter of 0.25mm is connected to the head of the chromatographic column, and cold capture is carried out on the empty capillary column; the desorption and enrichment process is shown in Figure 11, where: 13 is the carrier gas, and 14 is the six-way valve , 15 is a balance tube, 16 is an adsorption tube, 8 is a vaporization chamber, 9 is a cold spout, 10 is a chromatographic column, 11 is a detector, 17 is an empty capillary, and 18 is a joint.

Chromatographic analysis conditions: Shanghai Kechuang desorption device, the adsorption tube is filled with 0.35 grams of 60/80 mesh TENAXTA, the six-way valve is connected to the vaporization chamber with a 0.25mm inner diameter empty capillary, and the empty capillary is directly inserted into the vaporization chamber.

Chromatographic column: SE-30 25m×0.25mm×0.25μm; carrier gas: nitrogen; split flow: 50:1

Cold bath: 0°C; CO ₂ : 4.8Mpa

Column temperature: 30°C (1min)→5°C/min→120°C (constant temperature)

Sample: The adsorption tube takes the air in the medicine cabinet (enriched about 1 liter of air).

The rest are the same as Example 2. After the adsorption tube is connected to the analysis system, the carrier gas is passed, the furnace temperature is controlled at 30°C, and the cold spraying is started; then the adsorption tube is heated up with the heating furnace, and the time is counted at the same time; it rises to 300°C in 8 minutes, and the cold spraying is stopped at 24 minutes after the constant temperature. At the same time, the chromatographic furnace starts to heat up, and the chromatogram is shown in Figure 12; it can be seen from the figure that the peaks that appeared before 27 minutes are all cold-captured and uncaptured breakdown components, and the chromatographic peaks that appear after 30 minutes are all enriched compound.

Figure 13 is a chromatogram of the air in the laboratory after it is enriched with an adsorption tube (about 1 liter of air enriched), and the conditions are the same as in Figure 12. It can be seen from the figure that the chromatographic peaks that appear after 30 minutes are all from the laboratory Compounds that are enriched in the air are much lower in concentration than the air in the medicine cabinet. The drift of the baseline around 36 minutes is caused by the increase of the resistance when the adsorption tube is heated up and the change of the carrier gas flow rate, which also exists in Figure 12.

Claims (2)

1. a method for enrichment and capture of gas trace components in chromatographic capillary column analysis is characterized in that: utilize liquid _CO Regional refrigeration source, carry out partial refrigeration to precolumn or capillary column column head by spout, to realize Enrichment of trace components in gases; The spout is a stainless steel tube with an inner diameter of 0.75mm, and there is a wedge-shaped slit at the inward 1-5mm of the spout outlet, and the slit forms resistance to control the CO at the spout _The flow rate is 1000-1500ml/min; the spout and the capillary column The distance is 1 ~ 3mm. 2. The enrichment and capture method according to claim 1, characterized in that: the gas sample is a gas trace component or a sample pre-enriched in an adsorption tube.

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