Take gas chromatograph used in laboratory as an example, there are more than 1 kinds of domestic types and models, and the technical performance, functional characteristics, price and operating characteristics of different products are quite different. In addition, the samples analyzed are various, and the analysis purposes and requirements are different. For those chromatographic users who have not worked for a long time and have little experience, it is really not easy to choose an instrument with appropriate performance/price ratio according to their own needs. In order to help you buy a chromatograph quickly, well and economically, several considerations on how to buy a gas chromatograph are classified and analyzed for your reference.
1. Analysis of the sample:
⑴ The composition and state of the sample itself, whether gaseous, liquid, solid or mixed, can be directly analyzed by gas chromatograph?
⑵ Whether the tested component is thermally unstable, easy to decompose or easy to catalyze the reaction. Whether the changes of time, temperature and pressure will cause the changes of the tested components;
(3) Whether there are smoke, suspended solids, high-Buddha-point components and corrosive components in the sample. In order to consider how to collect samples and how to pretreat them;
(4) Is the sample source easy? Allow the consumption of samples, which is conducive to the selection of sampling methods;
(5) components that do not need to be analyzed and approximate concentration range;
[6] The number of times samples need to be analyzed every day and the interval between two analyses;
2. what is the purpose of the analysis?
(1) qualitative analysis: the analyzed components are known or unknown, and are there any standards?
(2) quantitative analysis: in that range-constant (1-1`~1-3); Semi-trace (1-3~1-5); Trace (1-5~1-7); Trace (1-6~1-9) or ultra-trace (≤ 1-9)
(3) Quantitative accuracy and analytical accuracy are much simpler if semi-quantitative requirements are met.
3. Positioning of purchasing units:
(1) Scientific research institutes-demanding;
(2) monitoring and analysis center-accurate and reliable;
(3) On-site analysis of the first line-repeated reproduction;
4. Theoretically, the same sample may be analyzed by various instruments. By comparing the performance/price ratio, operating characteristics and maintenance services of the instruments, the reasons for choosing gas chromatograph for analysis are listed.
5. consult and find out whether there are national standards, trade standards, enterprise standards or relevant foreign reference materials for the analyzed samples. If there are, the functions and technical requirements of the instruments to be used in general occasions will be given in the standards.
6. Whether there are analysts who do similar samples around this work unit will be of direct help to the selection and the establishment of chromatographic analysis methods in the future;
7 collect samples and data of various types of gas chromatographs (including accessories) to make basic preparations for the final selection;
8 is the task long-term? Or short term? Decide how much to invest and what kind of instruments to choose because of different tasks. Do you want to make long-term plans?
9 what are the existing conditions? For a new analysis task, there are many units, and the existing instruments are properly reformed and the analysis method is re-established, which is completely competent for the work; If the conditions are met, there is no reason to invest heavily in buying new instruments and equipment;
1 considering the working efficiency, operating cost, own manpower (technical level) and financial conditions of the selected instruments and equipment, it is not easy to choose those so-called high, refined and sophisticated products. In short, it is mainly practical economy.
11. Chromatographic data processing device is the necessary equipment to finally give the analysis results. It should be selected according to the specific requirements of the types and formats of the information required for the analysis results, and the chromatographic workstations with excessive functions and prices must not be selected without considering the financial resources. Some samples have few components and are well separated. Why spend tens of thousands of dollars to buy a computer with low efficiency and time-consuming operation to process the data?
12. Usage and installation location of the instrument:
Although the installation location of the gas chromatograph is not strict compared with that of the optical instrument, the following conditions should be paid attention to when operating some detectors and working with high sensitivity:
(1) Usage: temperature, humidity, atmospheric pressure, vibration, electromagnetic interference, presence or absence of corrosive gas, ventilation, stray light, water source, dust, etc. may work on the instrument.
(2) The installation base of the instrument is stable and earthquake-resistant, and its area, location and maintenance are convenient;
(3) whether the supply method, installation operation and purity of gas source can meet the requirements.
Notes on purchasing gas chromatograph-(2): Scope of application and use of gas chromatograph
In principle, any substance with small molecular weight, certain volatility, which will not decompose under vaporization or column temperature, or with large molecular weight, can be derived into volatile compounds through various treatments, and can also be analyzed by gas chromatography. Specific to the current commercial gas chromatograph, the general GC is suitable for the analysis group with the Buddha point below 35℃, and the high temperature GC can analyze the components with the Buddha point below 5℃. Among instrumental analysis methods, chromatographic analysis has unique advantages compared with other instrumental analysis methods because it can be separated and detected at the same time. Because of its high separation efficiency, sample analysis of isomers, optical isomers and trace components is almost indispensable for complex mixtures of multiple components. At present, with the continuous development of detection technology, sample processing technology and microelectronics technology, the detection limit of GC has expanded from the initial 1-2 to 1-13, and even some analyses can reach 1-15 orders of magnitude. However, at present, the commodity conventional detectors matched with gas chromatography can not give characteristic signals according to the composition of the analyzed components, and the qualitative analysis by conventional GC is still limited. For this kind of problems, it is necessary to use a variety of instruments to analyze and cooperate with each other. At present, the instrument analysis methods used for on-line coupling mainly include: GC/MS, GC/FIR, LC/MS, LC/NMR and so on.
Notes on purchasing gas chromatograph-(3): Rough classification of gas chromatograph users
Different types of GC users have great differences in analysis purposes and requirements. Therefore, the function, performance and operating characteristics of the same type of instruments are also very different. As a new customer, we should first position ourselves in that level category. It is also one of the main factors for choosing a good instrument. We can roughly divide GC users into three categories:
1. Research institutes and research departments in colleges and universities set up by the state: mainly used as analysis and testing means for life science, environmental science, new material science, forensic science, military science, aerospace science, archaeological research, agriculture and forestry, geological ocean, drug dynamic function and toxicology research.
2. research institutes, analysis centers, testing centers and monitoring centers set up by national ministries, bureaus, corporations, key industries in the national economy and large enterprises. For example, typical industries include: chemical industry, petroleum industry, metallurgical industry, energy (nuclear energy, coal) industry, semiconductor industry, machinery industry, pharmaceutical industry, light industry (food, daily chemical industry) and commerce.
3. customers used for front-line analysis, such as: process control and quality assurance in factory production process, quality supervision of import and export commodities, disease diagnosis, sanitation and epidemic prevention, industrial hygiene investigation and evaluation, public security investigation and evidence collection, monitoring of military equipment (air quality monitoring in military environment, such as missile launching site and submarine), biological products analysis, daily analysis of environmental monitoring, and on-site analysis of resource development (calculation of calorific value of oil field and natural gas), Quality monitoring of daily commodities (cosmetics, perfume composition, glass, ceramics, paper industry, etc.), quality monitoring of decoration materials, teaching laboratories, etc.
Instructions for purchasing gas chromatograph-(4): Classification of gas chromatograph
Gas chromatograph can be based on (1) separation principle; ⑵ Type of chromatographic column; (3) the application scope and other methods of classification. However, in terms of the source of samples (different fields) and the purpose of analysis, it is more suitable to know the classification method of instrument application scope for choosing what kind of gas chromatograph. Below we introduce this classification method in detail. This paper lists the classification of different gas chromatographs according to their application purposes. It should be pointed out that some kinds of instruments can have more than two types of application scope and purposes, such as: GC for analysis can also be equipped with some small pure substance preparation accessories for preparing GC.
category 1: analytical uses include online analysis (industrial chromatography) and offline analysis. Off-line analysis includes two types: laboratory use and field use (portable): laboratory use is divided into multi-detector and multi-function, single detector (simple) and special use.
category 2: preparation of pure substances: it is divided into small preparation accessories for qualitative purposes and large devices (instruments) for industrial preparation;
class 3: determination of physical and chemical constants of substances, such as single constant (specific surface area) and multiple constants (specific surface area and pore size classification).
1. Off-line gas chromatograph:
Off-line gas chromatograph generally refers to gas chromatograph used for analysis in laboratory or field. At present, these instruments account for more than 9% of the tens of thousands of gas chromatographs in operation in China. Due to different objects to be analyzed, different purposes of analysis and different occasions of use, there are great differences in the functions, performance indicators, structural characteristics and prices of the instruments. In order to meet the needs of different analysis, hundreds of models of gas chromatograph with different types and uses have been developed and produced. For the convenience of purchase, they are divided into: ① multi-detector multi-function gas chromatograph; ② General gas chromatograph with single detector or double detection; Special gas chromatograph for the analysis of products; ④ Portable gas chromatograph and other four categories are introduced:
1. Multi-detector multi-function gas chromatograph:
At present, multi-detector multi-function gas chromatograph is still the mainstream in the production and use of gas chromatograph, and the characteristics of such instruments are as follows: ① there are many detectors; ② Attached (auxiliary) pieces are complete; ③ thoughtful design; ④ great flexibility; ⑤ It has excellent performance, and can be combined into instruments with different analysis processes according to the needs of the analyzed samples, which can basically meet the needs of different analysis objects, especially suitable for the needs of users of the first class or part of the second class. From another point of view, the shortcomings of this kind of instrument mainly include: ① the structure of the instrument is complex; ② Require high level of operators; ③ It is difficult for users to maintain themselves; (4) transport to form a high cost; ⑤ General use efficiency is low; 6. The cost is expensive. In addition, users need to set up their own chromatographic analysis methods according to the analysis objects. When choosing this kind of instrument, we should pay attention to the following points:
(1) Are the detectors we need now or in the future? ⑵ How many detectors can be installed in the instrument at ordinary times? Is it convenient to change the gas circuit related to the detectors? (3) What kinds of accessories (accessories) are there? Do you need to buy them all? Can users install these accessories themselves? (4) Can the gas path system meet the needs of different analysis processes? Is it really a fully vitrified system? In addition, is it convenient and reasonable to install all kinds of valves, especially when multi-channel analysis is realized? 5] What are the advantages and disadvantages of the type and structure characteristics of the sampler compared with similar instruments? If the selection is not appropriate, it will have adverse effects on the future sample analysis; [6] The volume of chromatographic column box should be as large as possible; In addition, it is convenient to replace and install the chromatographic column; Whether the sealing method is advanced; (7) The degree of automation in the analysis process should be high enough (for example, the automatic procedure of capillary can be realized and the aging process can be repeated); (8) The comprehensive technical indexes of the instrument can not only meet the requirements of micro-analysis but also meet the requirements of trace analysis or ultra-trace analysis; Levies the operation interface should be convenient and friendly! So as to facilitate the setting of various parameters, reduce misoperation and improve work efficiency.
1. Single-detector (double-detector) general gas chromatograph:
Compared with multi-function multi-detector gas chromatograph, this kind of gas chromatograph is only equipped with 1~2 commonly used detectors, and the injection system is only equipped with 1~2 kinds, and the maximum temperature is generally not higher than 35℃. In order to expand the functions of such instruments, it is often necessary to add some accessories or accessories and make appropriate modifications. However, because the design and manufacture are aimed at a certain detector and special function, it is easy to make it reasonable. Therefore, in some special functions, the technical performance (such as stability, signal/noise ratio) and operating characteristics are no less than those of multi-detector gas chromatograph. In addition, the instrument has relatively simple structure, convenient operation, self-care in maintenance, low price and high working efficiency, which is beneficial to popularization. Especially suitable for the second (third) users, the analysis work is single and the technical force is weak.
3. Special gas chromatograph for an analysis
This kind of instrument is essentially equipped with a qualified chromatographic column on the general gas chromatograph with single (double) detectors, and the manufacturer helps users to establish a set of methods for an analysis by experiments before leaving the factory. Often called turnkey project. It can be done that the actual sample can be analyzed as long as there is power supply and the instrument arrives and is installed. Of course, in addition to the instrument fee, the manufacturer should also appropriately charge the software fee for establishing chromatographic analysis methods. However, because the manufacturer specializes in producing such instruments, it is very cost-effective for customers in terms of cost, time and technical performance. For example, special gas chromatographs that can be supplied in batches in China are: (1) natural gas; (2) liquefied petroleum gas; (3) gas; (4) refinery gas; 5] transformer oil gas; [6] Air quality in coal mines; (7) Composition analysis of SF6 for air; (8) Total hydrocarbons in the atmosphere (non-methane) (9) Trace water; ⑽ Trace gas in metal (alloy) glass; (11) Standard methods for hygienic inspection of benzene, toluene and xylene in the air of residential areas; (12) Determination of total volatile organic compounds (TVOC) in indoor air. It can be said that as long as there are national standards, business standards and enterprise standards or specific analysis requirements can be put forward, they can be customized into special gas chromatographs through consultation with domestic manufacturers.
4 Portable gas chromatograph for field use:
Portable gas chromatograph is generally a special type, which is a portable instrument designed and produced for different analysis tasks, and can obtain analysis results immediately when used in the field. On-site chemical analysis provides useful information, which is conducive to making decisions quickly. Some analysis tasks can't be completed in the laboratory. In addition, because of the flexibility of field sampling, the need for multiple sampling is reduced, and a lot of expenses can be saved. At present, foreign portable gas chromatograph is mainly used in the field of environmental protection, court investigation, industrial hygiene and so on, and there are also many application examples in the field analysis of chemical weapons application. The main difference between portable gas chromatograph and conventional instruments is that it needs independent power supply and portable gas source. Because of field use, there are high requirements on instrument structure and reliability. Therefore, the price of such instruments abroad is not low. For the same analysis purpose, it is still difficult to develop and produce such instruments in China, and it is even more difficult to popularize them only by importing them. What is gratifying is that Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has developed and identified a "micro-gas chromatograph" that is competent for some work of on-site analysis of foreign portable instruments.
3. On-line gas chromatograph:
On-line gas chromatograph is also called process gas chromatograph or industrial gas chromatograph. Compared with other gas chromatographs, it requires automation and higher degree of computerization, and the service life of chromatographic column should be long enough to ensure that the instrument can run continuously for a long time (the maintenance period is more than 6 months), and the analysis data should be accurate, reliable and stable.