High Purity Gas Analysis Gas Chromatograph

Analytical testing of ultrapure gas and high purity gas is a branch of the trace analysis discipline. It is a professional subject with narrow scope but practical significance to study gas purity analysis and trace impurity determination. With the rapid development of China's economy, new requirements have been put forward for high-purity gas not only in terms of quantity, quality, and type, but also for research, development and production of corresponding national standards, detection theories, methods and detection instruments. Put forward higher requirements. Advanced testing equipment can not only guide the control and reform of the production process, but also ensure product quality and avoid disputes between manufacturers and users.

The term "ultra-pure gas" was defined at the National Ultrapure Gas Test Annual Meeting in 1964, where the gas purity was 5 "9" (99.999%) or more and the total impurity was 10x10-6V/V (ie, 10ppm). The following gases are in the category of ultrapure gas. However, the development in 50 years has changed this definition; five "9" gases have been referred to as high purity gas, and six "9" or more purity gases have been identified as Ultrapure gas: The purity analysis of gases with purity greater than 99% is calculated by subtracting the impurities, and the gas purity analysis is actually the detection of trace or trace impurity gases in the gas. Only a brief summary of the current status of gas chromatographic detection of impurity gases, and proposed some ideas on the concept of understanding.Water is also an impurity gas, and is a very special, ubiquitous gas impurities, due to detection methods Special, this article does not discuss.

I. Status Quo of Gas Chromatographic Analysis of Pure Gases Gas chromatography impurities in pure gas are irreplaceable due to their various advantages. If many components can be detected at the same time, the analysis time is short, the operation is simple, the analysis technology is flexible, the price is low, and the advantages of automatic detection and computer control can be achieved. Therefore, the products are welcomed by the users. National standards for pure gas analysis have already been used for gas chromatography. However, the status quo still has a large gap with the international community.

1. Technical research and innovation From the published papers, there have been domestic boom times in the 1980s and 1990s to study and analyze trace impurity gases. However, in recent 10 years, new detection methods, technologies, instruments, and detectors have made slow progress, lack of innovation, and the number of papers published has been reduced. There is no long-term unified planning and stable investment, and the professional research and analysis team has grown, and at the same time, the quality has yet to be improved. So far, there is no monograph of "high-purity gas analysis technology" published.

2. The “National Standard” reaction technology is generally behind the “national standards” related to high-purity gas. Among them, the analytical method is significantly behind the international level, and the level of instrumentation is low. Some of them can be similar to the others. Some "standards" still use colorimetry as the main method, and detection methods cannot be instrumentalized. For example, the problem reflected in the "Medicine Oxygen Standard" (GB8982-1998) is the most concentrated. In the standard "technical specifications", except for the oxygen content index (≥ 99.5%), the impurity content has no data and is "tested according to the prescribed method." All of the prescribed methods are chemical absorption or colorimetry. Only "qualified" and "unqualified" results of the analysis, no data records. This is unfavorable for guiding manufacturers. To date, most manufacturers do not have the technology and conditions for comprehensive sampling according to the “Regulations”. The use of units (hospitals and related research units) is more difficult to invest in the formation of analysts and conditions. Some manufacturers only submit inspections to scientific research units, and inspections are not allowed after the inspection.

Second, the detector and detection technology 1, the detector is currently used for pure gas impurity gas analysis of gas chromatography detector are as follows.

(1) Thermal Conductivity Detector (TCD)

The best indicator of the detector can be detected in ppm. With the variable temperature concentration method can be detected ppb level, such as high-purity hydrogen, ultra-pure hydrogen detection can be measured 0.1ppb (0.1x10-9V/V) level. [4, 6, 12, 20]

(2) Gas detector This detector can detect 0.1 ppm hydrogen impurity in the national standard for testing high purity nitrogen (GB/T8980-1996).

(3) Hydrogen flame ionization detector (FID)

The national standard (GB/T 8984.1—3—1997) for the determination of carbon monoxide, carbon dioxide, and hydrocarbons in gases is determined directly after conversion using flame ionization, and the minimum concentration can be detected at 0.1 ppm. With a variable temperature concentration can be measured to 1ppb.

(4) Modified ion detector (M-ArID) [10,14]

The argon ionization detector can be modified to detect hydrogen, oxygen, nitrogen, methane, carbon monoxide, and carbon dioxide impurity gases in high purity argon, with a minimum detection concentration of 0.1 ppm.

(5) Helium ionization detector (HeID) [3,16]

Most of the detectors use the detection of impurity gases in high purity helium, and direct detection can reach 1ppb [3,16]. It also cooperates with switching technology to detect other impurity gases in high purity gas [15].

(6) Electron Capture Detector (ECD)

The detector can detect trace oxygen (ppb) in high purity nitrogen, argon, carbon monoxide and other gases.

There are also zirconium oxide detectors [17] and ion mobility detectors.

2, detection technology Gas chromatography analysis of gas impurities using the detection technology variable temperature concentration method, column conversion method, column switching method and process change method.

(1) Variable-Temperature Concentration Method Variable-Temperature Adsorption Concentration method is a method in which a certain amount of impurity gas in a sample gas is adsorbed at a low temperature on a sorbent in a sample tube and the sample is thawed and heated for injection. Therefore, the actual injection volume is much larger than the sample volume (102~104 times), and the impurity gas is changed from ppb level to ppm level [1,4]. This method requires that the bottom gas is not frozen, adsorbed or has a boiling point higher than the impurity gas, such as impurities in concentrated hydrogen, hydrocarbons in oxygen, and the like. In addition to temperature swing adsorption and enrichment methods, there are chemical reaction concentration methods and special concentration methods, which are used in the detection of specialty gases.

(2) Conversion method in column The conversion method in the column is a catalyst or chemical reaction tube (which can control a certain temperature) after the sample is injected, before or after the chromatography column. One of the impurity gases participates in the reaction and becomes another gas that is detected. For example, carbon monoxide and carbon dioxide did not respond to the flame ionization detector, but they turned into methane after the nickel catalyst (having hydrogen participation) responded and could detect 0.1 ppm. In combination with concentration, ppb can be detected. This method is also used when analyzing trace amounts of water by gas chromatography. Traces of water react with calcium carbide (Ca2C) in the column to generate acetylene, and flame ionization can detect moisture up to less than 1 ppm.

(3) Column switching method [2,7]

This method is also called multidimensional chromatography. It is a valve or "valve-free" switch after the majority of the main component (gas) is cut off, and the remaining impurity gas is detected after secondary separation [2]. For example, high-sensitivity helium ionization detectors are used to detect impurities in oxygen, hydrogen, and impurity gases in helium [15].

(4) The process change method uses the column and parallel connection of the chromatographic column to separate the impurity gas, and can also be used in combination with the above three methods to achieve the purpose of separating and detecting multiple impurity gases. Detectors can also be used in series and in parallel, but detectors that are required to satisfy the concatenation must use the same carrier gas.

Most of the above technologies are for the detection of conventional gases, and special techniques for more special high purity gas should be used [21-28].

The “background”, “minimum detection concentration”, and “quantity and error” appearing in the high-purity gas analysis proposed by gas chromatographic analysis of high-purity gas are theoretical problems in trace analysis. Practical problems. We do not describe the derivation of the theory here, but use the theory to solve the problem. We mainly focus on the actual understanding of the problem and welcome participation in the discussion.

1. The background “background” refers to the measured impurity gas contained in the chromatographic carrier gas and the measured impurity gas contained in the bottom gas of the standard gas; the test system (carrier gas system and sample gas system) brings The impurity gas (mainly oxygen and nitrogen) is leaked. Most concentration detectors require the background value to be much smaller than its minimum detection concentration. When the background value in the carrier gas is greater than the concentration of a certain impurity in the sample gas, the impurity gas will have a "reverse peak". The gas requirements for preparing standard gas are more stringent, otherwise the background value directly affects the quantification. However, the exception is the mass detector (mainly flame ionization detector). When we detected hydrocarbon impurities in oxygen, we found that with pure nitrogen (4 "9") as the carrier gas (hydrogen and air are the same), we can also detect 0.1ppm of hydrocarbons (generally domestic chromatograph) or even 20-50ppm. Hydrocarbon impurity gas. This is because the impurity in the carrier gas (oxygen, hydrogen, carbon monoxide, and carbon dioxide on the FID does not respond, and the methane content is ≤5 ppm), because the concentration does not change, can only make the detector (FID) produce a corresponding stable "base flow" as long as Base balance, low noise, 0.1ppm of hydrocarbon impurities can still produce a peak in the baseline. Gas chromatography quantitatively determines the minimum detection peak: the minimum peak height is twice the noise value. In the past, domestic instruments were only able to detect 0.1ppm of hydrocarbons because the total noise value of the instrument was high. As long as the total noise value is reduced by an order of magnitude, the minimum detection peak will increase by an order of magnitude. The FID sensitivity index of our company is still Mt ≤ 1×10-11g/s (n-hexadecane), which is the same as other manufacturers' indexes, but the total noise value is low. Therefore, "the hydrogen flame gas chromatograph can only detect similar values ​​to the background current" is unfounded. We then use the concentration method to make acetylene detection in hydrocarbons detect ppb levels.

The most significant influence of the background value is the analysis of nitrogen and oxygen in the high-purity gas (the main component in the air). The purified carrier gas still has a background effect, which is mainly caused by the leakage of gas. If there are joints, valves, watches, etc. in the pipeline, it will inevitably leak air. We can only control a certain amount of leakage so that it does not affect the analysis. It also means that "leakage" is absolute, and "leakage" is only relative. For example, when using a high-purity argon analyzer, the purity of the pure gas must be better than six "9" in order to ensure analysis. We purify the carrier gas to 7 "9" or more, leaving a certain "space" so that the "leakage" still meets the analysis requirements. It is not allowed to use organic materials for sealing on high-purity gas analyzers (because nitrogen and oxygen in the air can penetrate into the system through organic materials), including various rubbers, PTFE, etc. [27]; Use as little as possible. The sample gas (gas) flow path also has the problem of air leakage. We have found that changing the analysis operation can determine if there is a leak method and can roughly determine the leak size. There is also the saying that "in the detection of <20ppb ultra-low trace gas composition, can not use copper and general stainless steel materials," the argument is also based on. Both helium ionization detectors and electron capture detectors can directly detect ppb-level detectors. On the instrument, we still use common copper and stainless steel tubes as the connecting tube and column, just to clean and replace the tubes. Meet the requirements [27]. In the calibration method, some people say that "the standard content of commercially available bottled gas is within 1 to 20 ppm. Use this concentration of standard gas to check the content of 1 to 20 ppb concentration. The two data sizes are related to 1000 times. This is completely Unreasonable." This is an outsider's criticism of the expert's words! It seems that critics can produce standard gas with a content of 1-20ppb. Because it is clear that no one can currently obtain the purity of 9 "9" with the standard gas. Based on the fact that only standard ppm gas can be prepared at the present time, the purity of the first bottom gas is guaranteed; the second problem is the gas distribution error. As long as we check the primary [GBW] and secondary [GBW(E)] gas reference materials (more than 300 kinds) [11] approved by the National Bureau of Quality and Technical Supervision, we can know that the minimum formulation concentration of the gas in the cylinder can only reach 1ppm. The third consideration is that the concentration method can convert ppb into ppm and the detector can guarantee a linear range. Therefore, it is not surprising to check the ppb level with ppm standard gas. Theoretical and technical researches have, in many aspects, made the "impossible" and "unreasonable" things, making it possible under certain conditions.

2, the minimum detection concentration The minimum detection concentration of the chromatographic detector depends on, first, the sensitivity of the detector. The higher the sensitivity, the lower the impurity concentration detected by the detector; second, the smaller the peak width of the chromatogram peak (the higher the column efficiency), the lower the concentration of the detected impurity gas. Specifically for the minimum peak height must be twice the noise signal. The minimum detection value for any chromatographic detector is the minimum detection concentration. Instrument operators should be aware that values ​​below this concentration (without peaks) should not be "0", but should be less than the minimum detected concentration value. In theory, it is also considered that it can only be close to "0" but not equal to "0". The "0" displayed for a continuously monitored digital display should also be seen because the display cannot show a value less than its minimum detection concentration. Therefore, when all the instruments are detecting impurities, if the content cannot be measured, then in the report, the component content should be written less than the minimum detected concentration value, and cannot write “0”.

3, the quantitative and error Each gas chromatograph analysis of high-purity gas must have a corresponding test impurity gas sensitivity and better repeatability. We believe that the quantitative error of the instrument depends on the accuracy (reliability) of the standard gas under the condition of ensuring the above conditions and meeting the requirements of the background. The relative error of repeated measurements for trace components is often much greater than the constant analysis. For the detection repeatability (relative error) of 0.1-10 ppm concentration, the relative error of 10-20% is also good; the impurity gas of PPT level can be allowed to have a difference of one order of magnitude. In the quantitative method, the "external standard method" is used, that is, calibration using standard gas. In the laboratory, it is possible to use a variety of standard gas preparation methods to correct and obtain higher reliability. However, it is very difficult to use in actual use. Therefore, the country's requirements for the preparation of standard gas are very strict.