Selection of online gas analyzers

1. Mass spectrometers and chromatographs are also used for multi-component measurement, but they are quite expensive; other devices that can simultaneously detect multiple components simply do not exist. 2. Analytical cabins are used, and integrated systems are required; well-known domestic companies such as Hangzhou Jucang and Xiedilong also offer integration services. In addition, I think companies like Beijing Geweinik and Beijing Jiuxinglong are also good choices. 3. For laser analyzers, ABB, Siemens, and Shinfuku Mei are suitable options; in China, there are only Focuslight, Snowden, and Yousheng. 4. For laboratory analyzers, Agilent should be considered first.

Beijing York Instrument Technology Development Co., Ltd. acts as the agent for Northern Laser of Canada’s gas analyzers. We can discuss this further

This post was last edited by ljtjbx on 2015-6-19 at 14:43. Basic knowledge of online chromatographic analyzers. Chromatography, also known as chromatoid method or layer chromatography, is a physicochemical analysis technique that takes advantage of the differences in the forces acting between different solutes (samples) and the stationary phase as well as the mobile phase (such as distribution, adsorption, ion exchange, etc.). As these two phases move relative to each other, the various solutes undergo multiple equilibrations between the two phases, resulting in their separation from one another. Its English name is “chromatography”. This word originates from the Greek words “chroma” and “graphein”; when translated literally into English, they become “color” and “writing”. Literally translated into Chinese, it becomes “chromatography”. However, some people render it as chromatography or chromatographic method. In 1906, the Russian scientist Tswett studied the separation of plant pigments and introduced the concept of chromatography ； While studying the pigment components in plant leaves, he poured the extracts of these leaves into vertical glass tubes filled with calcium carbonate, then added petroleum ether to allow it to flow freely; as a result, the various components of the pigments separated from one another, forming bands of different colors. According to the nomenclature of spectra, this method is thus named chromatography. Later, this method was gradually applied to the separation of colorless substances. Although the term \"chromatography\" has lost its original meaning, it is still in use to this day. Stationary phase – In chromatography, the stationary phase is the phase that remains fixed (solid or liquid) ； Mobile phase – The moving phase (usually a gas or liquid) is called the mobile phase. Based on the geometric form of the stationary phase, chromatographic methods can be classified as follows: Column chromatography. In column chromatography, the stationary phase is placed inside a metal or glass column, or it is attached to the inner wall of a capillary tube to form a chromatographic column. The sample moves in one direction from the top to the bottom of the column, thereby undergoing separation. Currently, column chromatography is used in online chromatographs. 　　Paper chromatography utilizes filter paper as a carrier for the stationary phase. The sample is spotted onto the paper, which is then developed using a solvent. The various components appear as spots at different locations on the paper; qualitative and quantitative analyses are performed based on the positions and sizes of these spots. Simply put, a chromatograph is a device that uses the principles of chromatography to separate a mixture through a chromatographic column, and then uses a detector to analyze each component. Unlike the several gas composition analyzers mentioned earlier, chromatographic analyzers can provide a comprehensive analysis of the sample being tested; they can not only identify the various components in a mixture but also determine the concentration of each component. Therefore, chromatographic analyzers are being used more and more widely in scientific experiments and industrial production. Basic principles of chromatographic separation: As can be seen from the above methods, there are two phases in chromatography; one phase remains stationary, and we call it the stationary phase ； The other phase continuously flows through the stationary phase; we call it the mobile phase. The separation principle of chromatography is to utilize the differences in affinities such as distribution coefficients and adsorption capacities of the various substances to be separated within two phases. An external force is used to force the mobile phase containing the sample (gas or liquid) to pass through a stationary phase surface that is fixed in a column or on a plate and is immiscible with the mobile phase. When the mixture carried in the mobile phase flows through the stationary phase, the various components of the mixture interact with the stationary phase. Due to the differences in properties and structure among the components in the mixture, the forces exerted between them and the stationary phase vary in magnitude. As the mobile phase moves, the mixture undergoes repeated distribution equilibria between the two phases, resulting in different retention times for each component in the stationary phase; consequently, they emerge from the stationary phase in a specific order. A detector is installed at the outlet of the chromatography column. When a component flows from the chromatography column into the detector, the detector generates an electrical signal proportional to the concentration of that component. By recording these signals corresponding to each component using a recorder, a chromatogram is formed, as shown in the figure below. The composition of the mixture and the concentration of each component can be determined based on the time at which they appear in the chromatogram and the size of their peaks. Classification of chromatographs: There are many types of chromatographic analysis methods, and different classification approaches can be used based on various criteria. Based on the phase status: In chromatography, the mobile phase can be either a gas or a liquid; accordingly, it can be divided into gas chromatography (GC) and liquid chromatography (LC). The online chromatographs commonly used in industry are generally gas chromatographs. The stationary phase can be either a solid or a liquid coated on a solid; accordingly, gas chromatography and liquid chromatography can be further classified into gas-liquid chromatography, gas-solid chromatography, liquid-solid chromatography, and liquid-liquid chromatography. Components of a gas chromatograph: ① Control and measurement of the gas source and carrier gas. The gas source typically consists of high-pressure cylinders (containing hydrogen, nitrogen, argon, etc.) that serve as storage containers for high-purity gases. These cylinders are equipped with pressure reducers to lower the high-pressure gas to a low pressure level (0.1–0.5 MPa) for use. The gas supplied by the cylinder is called the mobile phase, also known as the carrier gas. The main function of the carrier gas is to transport the sample to the chromatography column and the detector. ②Flow control valves can regulate the flow rate of the carrier gas; commonly used types include pressure regulators and needle valves. An anemometer is used to measure the flow rate of the carrier gas. Commonly used ones include rotameters and soap film flow meters, etc. ③Chromatography column and thermostat. The function of the chromatography column is to separate a mixture into individual components. It is generally composed of stainless steel or copper tubes filled with a stationary phase, and the tubes are U-shaped or screw-shaped. The inner diameter of ordinary column tubes is 2–8 mm; there are also those with even smaller inner diameters, known as capillary chromatographic columns. The length of these column tubes is generally 1–4 meters or more. Thermostat: To maintain a constant temperature inside the chromatography column or detector, these components are often placed within a thermostat. Air constant temperature is commonly used. The component that introduces the sample into the chromatography column is called the injector. For online gas chromatographs, common injection tools include flow path switching valves, column valves, and quantitation tubes. The detector, also known as the analyzer, is used to detect the components that emerge from the column, displaying them as voltage or current signals. Commonly used detectors include those of the thermal conductivity type ； Hydrogen flame ionization type ； There are several types of electron capture and flame photometric detectors. ⑤An automatic recorder serves to record the signals output by the detector, which are used as a basis for qualitative and quantitative analysis. However, modern online chromatographs are not equipped with automatic recorders; instead, they have large LCD displays or a host computer, but interfaces for automatic recorders are still provided. Classification of gas chromatography columns: A chromatography column consists of a column tube and a stationary phase. Based on the diameter of the column tube and the way the stationary phase is packed, they are divided into packed columns and capillary columns. Among the various factors that affect the separation performance of a packed column gas chromatography column, selecting an appropriate chromatographic stationary phase is crucial. It is necessary for the various components to be tested to have different adsorption or distribution properties on the selected stationary phase in order to achieve separation. ①The stationary phase in gas-liquid chromatography (partition chromatography) consists of a high-boiling-point liquid as the stationary liquid and an inert support. Support (or carrier) – a chemically inert, porous solid particle that supports the stationary phase; it has a large surface area and good stability (chemical and thermal). Its particle size and pore size distribution are uniform ； It has a certain degree of mechanical strength and is not easy to break. Types and properties of carriers: Diatomite type: Red diatomite carriers – have good strength, but contain active sites on their surface, which can cause chromatographic peaks to tail when separating polar substances ； It is commonly used to separate non-polar and weakly polar substances. White diatomaceous earth carriers—have low surface adsorption but poor strength, and are commonly used for separating polar substances. Non-diatomaceous earth carriers: fluorine-containing carriers, suitable for the analysis of highly polar and corrosive gases ； Glass microspheres, suitable for the analysis of high-boiling-point substances ； Polymer porous microspheres can be used as adsorbents in gas-solid chromatography as well as supports in gas-liquid chromatography. Pre-treatment of the support: Removing the active centers on its surface to deactivate it. Pickling method (removal of basic active groups) ； Alkali washing method (removal of acidically active groups) ； Silylation (elimination of hydrogen bond formation) ； Vitrification treatment (vitrifying the surface and sealing micropores), etc. ②Stationary liquid – the main component applied to the support to serve as the stationary phase. Requirements for the stationary liquid: good chemical stability: it should not react with the support, the carrier gas, or the components to be analyzed ； Good thermal stability: remains in a liquid state at operating temperatures, has a low vapor pressure, and is not prone to loss ； High selectivity: large difference in distribution coefficients K ； Good solubility: The fixing solution should have a certain solubility for the component being analyzed. Interactions between components and stationary phase molecules: The interaction forces between components and stationary phase molecules generally include electrostatic forces, induced forces, dispersion forces, and hydrogen bond forces. In gas-liquid chromatography, a component can be distributed in the stationary phase only when the force between the component and the molecules of the stationary phase is greater than the force between the molecules of the component itself. Selecting an appropriate fixing agent that creates differences in the forces between the various components to be separated and the fixing agent is necessary to achieve their separation. Classification of fixing fluids: There are over 400 types of fixing fluids, and they are commonly classified based on relative polarity. Selection of stationary phase: Generally, it is determined based on the properties of the sample (polarity and functional groups), with an appropriate stationary phase being chosen in accordance with the principle of \"like dissolves like.\" Specifically, it can be considered from the following aspects: a. To separate non-polar mixtures, non-polar stationary phases are generally used, and the force acting between stationary phase molecules is mainly dispersion force. In the sample, the components elute in order of increasing boiling point. Commonly used ones include: squalane (isotriacetylin), hexadecane, silicone oil, etc ； b. Medium-polarity stationary phases are generally used to separate medium-polarity mixtures. The forces between the components and the molecules of the stationary phase are mainly dispersion forces and induced forces. In the sample, the components elute in order of increasing boiling point. c. When separating polar components, the interaction between polar stationary liquid components and the stationary liquid molecules is mainly orientational force. The components in the sample to be tested appear in peaks in order of increasing polarity. For example, when analyzing acetaldehyde and acrolein using the polar stationary phase polyethylene glycol-20M, acetaldehyde, which has lower polarity, elutes first. d. To separate mixtures of non-polar and polar (easy-to-polarize) components, a polar stationary phase is used: the non-polar components elute first, while the polar (or easily polarizable) components elute later. For example, by using the moderately polar dinonyl phthalate as a stationary phase, benzene (with a boiling point of 80.1°C) and cycloethane (with a boiling point of 80.8°C), which have very similar boiling points, can be separated quantitatively; cyclohexane elutes first. If a non-polar stationary phase is used, it is difficult to separate these two compounds. e. For components that can form hydrogen bonds, strongly polar or hydrogen-bonding stationary phases should be used. For example, in the separation of polyols, nitrile ethers, phenols, amines, etc., the components that are less likely to form hydrogen bonds elute first. Gas-solid (adsorption) chromatographic stationary phase – solid adsorbents ① Activated carbon: a non-polar adsorbent used for the analysis of low-carbon hydrocarbons, gases, and short-chain polar compounds. ②Alumina: a weak (moderately) polar adsorbent, mainly used for the analysis of C1–C4 hydrocarbons and their isomers. ③Silica gel: A strongly polar adsorbent, commonly used for the analysis of sulfides: COS, H2S, SO2, etc. ④Molecular sieves (synthetically produced silicates): highly polar adsorbents used to achieve good separation of H2, O2, N2, CH4, and CO at room temperature. ⑤Polymer porous microspheres: polar and non-polar adsorbents, capable of analyzing polar compounds such as polyols, fatty acids, nitriles, amines, or non-polar compounds such as hydrocarbons, ethers, etc ； It is particularly suitable for analyzing trace amounts of water in organic compounds. Carrier gas for gas chromatographs: The gas used as a carrier gas in gas chromatography is required to have good chemical stability ； High purity ； It’s cheap and easy to obtain ； It can be suitable for the detector being used. Common carrier gases include hydrogen, nitrogen, argon, helium, carbon dioxide, and so on. Among them, hydrogen and nitrogen are inexpensive and have good properties, making them excellent gases for use as carrier gases. Due to its low molecular weight, large molecular radius, high thermal conductivity, and low viscosity, hydrogen is often used as a carrier gas when using TCD. In FID, it is a necessary fuel gas. Currently, in addition to high-pressure hydrogen cylinders, hydrogen generators that produce hydrogen through water electrolysis can also be used as sources of hydrogen. Hydrogen is flammable and explosive, so special attention must be paid to safety when using it. Due to its low diffusion coefficient and high column efficiency, nitrogen is commonly used as a carrier gas in detectors other than TCD. It is used less in TCD mainly because the nitrogen thermal conductivity system has a small size and low sensitivity; however, when analyzing H2, N2 must be used as the carrier gas, otherwise TCD cannot be employed to address the analysis of H2. In terms of the properties as a chromatography carrier gas, helium is similar to hydrogen and boasts the excellent advantage of high safety. However, it is used less due to its high price. Principle for selecting the carrier gas type: When choosing a gas as a carrier gas, the type of detector to be used must be considered first. When using a thermal conductivity cell detector (TCD), employing hydrogen or helium as the carrier gas can improve sensitivity; moreover, hydrogen as a carrier gas can extend the lifespan of the tungsten wire in the thermosensitive element ； A hydrogen flame detector (FID) should use nitrogen as the carrier gas, but hydrogen can also be used ； The electron capture detector (ECD) typically uses nitrogen with a purity greater than ； Nitrogen and hydrogen are commonly used in flame photometric detectors (PFDs). The diffusion coefficient is related to the properties of the carrier gas; it is inversely proportional to the square root of the carrier gas’s molar mass. Therefore, using a carrier gas with a higher molar mass reduces the molecular diffusion coefficient, thereby improving column efficiency. On the other hand, using a carrier gas with a lower molar mass increases the molecular diffusion coefficient, which reduces the resistance to mass transfer in the gas phase and thus improves column efficiency as well. Therefore, when using a carrier gas at low linear velocity, one should choose one with a high molar mass, while at high linear velocity, it is preferable to use one with a low molar mass. Selection of carrier gas purity: In principle, when choosing the purity of a gas, it mainly depends on: ① The analyte ； ② Filler in the chromatography column ； ③ Detector. It is recommended to use gases with higher purity as much as possible, provided that the analysis requirements are met. This not only improves (maintains) the high sensitivity of the instrument but also extends the lifespan of the chromatography column as well as that of the entire instrument (gas path control components, gas filters). Practice has shown that, as mid-to-high-end instruments used for trace analysis, when they are operated for extended periods with gas sources of lower purity, it can be very difficult to restore the instrument’s high sensitivity once analyzing samples with low concentrations is required. For low-end instruments used for constant or semi-micro analysis, the use of high-purity gases not only increases operating costs; moreover, gas purifiers are required to purify the gases, which adds complexity to the gas supply system and makes it more likely to occur leaks or other problems that can affect the proper operation of the instrument. Therefore, it is not recommended to purify such chromatography carrier gas. Furthermore, for certain specific analytical purposes, it is necessary to intentionally add certain \"impurities\" to the carrier gas; for example, an appropriate amount of water vapor is added when analyzing polar compounds, and a trace amount of sulfur is added when using a flame photometric detector (FPD) in order to improve the sensitivity for analyzing sulfides. To operate a helium ionization detector, the neon content must be between 5 and 25 ppm; otherwise, negative peaks or \"W\"-shaped peaks may appear when analyzing hydrogen, nitrogen, and argon.

I have been busy these past few days. Thank you to all the seniors for their guidance. I have another question: what are the advantages and disadvantages of mass spectrometry and chromatography analyzers compared to laser analyzers?

Thank you! I have another question: what is the approximate price difference between mass spectrometers and chromatographs compared to lasers? What are its advantages and disadvantages compared to laser analyzers?

Mass spectrometers and chromatographs are much more expensive than lasers; it’s hard to say exactly how much more, as it depends on the brand you choose. When it comes to mass spectrometers and chromatographs, should one choose domestic or foreign brands? Well-known American brands such as Thermo Fisher and AAI offer mass spectrometers that cost at least 2 to 5 million yuan; for chromatographs, ABB’s models also cost hundreds of thousands to millions of yuan. Compared to lasers, mass spectrometers and chromatographs provide higher measurement accuracy and better stability.

Mass spectrometers and chromatographs are not as expensive anymore