Electron capture detector Table of contents [hide] Overview Development process Simple working mechanism of ECD Classification of ECD

Electron capture detector (electron capture detector), referred to as ECD.

The electron capture detector is also a kind of ionization detector. It is a selective and highly sensitive detector. It only detects electronegative substances, such as halogen, sulfur, phosphorus, etc. Nitrogen substances have signals. The stronger the electronegativity of the substance, that is, the greater the electron absorption coefficient, the higher the sensitivity of the detector. However, there is no signal for electrically neutral (no electronegativity) substances, such as alkanes. [Edit this paragraph] Overview 1. ECD came out in 1961. It, FID and chromatographic temperature-programmed analysis are called the three major breakthroughs in the development of chromatographs;

2. It is a highly sensitive and highly selective detector, which is particularly sensitive to electronegative substances;

1. The minimum detection amount can reach 10-13 grams (γ-666), and the sensitivity ratio to carbon tetrachloride and n-hexane is 4×108 times;

4. It is mainly used to analyze and determine the electronegativity of halides, phosphorus (sulfur) compounds, peroxides, nitro compounds, metal organic compounds, metal chelates, steroid compounds, polycyclic aromatic hydrocarbons and ***-conjugated hydroxyl compounds. substance. In addition, it can also analyze 1PPM oxygen;

5. Use chemical conversion methods to make derivatives with strong electronegative properties to expand the scope of use of electron capture detectors;

6. ECD has become one of the most widely used detectors in the fields of food inspection, pesticide residues in animals (plants) and environmental testing (water, soil, air pollution, etc.). [Edit this paragraph] Development process Since the advent of ECD, people have continuously improved and perfected it to make its structure and performance more ideal. In the past few decades, the two most practical developments have been the use of 63Ni radioactive source instead of 3H radioactive source and the use of fixed base current pulse modulated voltage power supply instead of other power supply methods. The main advantage of using the 63Ni source is that the detector temperature can be operated at 350~400°C, thereby reducing contamination problems during operation and improving the detection limit. The fixed base current pulse modulation voltage is used for power supply, which extends the linear range to 104 and the dynamic range to 105, and increases the stability of the detector. [Edit this paragraph] The concise working mechanism of ECD ECD is a type of radioactive ionization detector. It uses radioactive isotopes to emit beta-particles with a certain energy during the decay process as the ionization source. When only pure molecules carrying it are When passing through the ion source, they are ionized into positive ions and free electrons under the bombardment of β-particles. Under the action of the applied electric field, both ions and electrons will move in a direction. Because electrons move much faster than positive ions, positive ions The probability of recombination of ions and electrons is very small. As long as the conditions are certain, a certain ion flow (base flow) will be formed. When the carrier gas enters the ion chamber with a trace amount of electronegative components, the electrophilic components will capture a large number of electrons. Negative ions or negatively charged molecules are formed. Because the moving speed of negative ions (molecules) is similar to that of positive ions, the recombination probability of positive and negative ions is 105 to 108 times higher than the recombination probability of positive ions and electrons, so the base flow drops significantly, so the instrument outputs a negative polarity electrical signal. Therefore, contrary to FID, the measured component output through ECD produces a negative peak in data processing.

There are more than four types of electronegative substances that capture electrons and are dissociated in the ion chamber. But practice shows that the main ionization forms are dissociation and non-dissociation.

In the dissociation reaction, when a polyatomic molecule AB enters the ion chamber, the sample molecule AB reacts with an electron and dissociates into a free radical and a negative ion. For example, CL, Br, and I compounds of aliphatic hydrocarbons are dissociated types; In the non-dissociative reaction, sample AB reacts with an electron to generate a negatively charged molecule, such as derivatives of hydroxyl, F, CH3, ON, OCH3, etc. of aromatic hydrocarbons and polyaromatic hydrocarbons, which belong to the non-dissociative type; the dissociative type is in In most cases, a certain amount of energy must be absorbed, and the electron absorption cross section will increase with temperature. Therefore, the dissociation type is beneficial to improving sensitivity when the temperature is higher. The non-dissociative type releases energy, and the electron absorption cross section will decrease as the temperature of the detector increases. Therefore lower temperature is beneficial to improve sensitivity. In addition, theoretically speaking, oxygen has a strong ability to capture electrons. The presence of oxygen will interfere with the work of the ECD. However, some people have found that the carrier gas contaminated by oxygen can improve the sensitivity of the ECD to halogenated hydrocarbons; in the carrier gas N2 Similar results will be obtained by adding N2O. If several millionths of N2O is mixed into N2, ECD will also have a greater impact on methane, ethane, benzene, ethanol and CO2. The working mechanism of ECD is very complicated. This is because during the ECD analysis process:

1. There are too many forms of impurities with different contents, and they change under various circumstances. The proportion of these impurities in the ECD information is still unclear;

2. The rate at which positive ions are lost due to space charge diffusion and the proportion of these positive ions in the ECD current are also not very clear;

3. The impact of a specific pool structure on various pool reaction phenomena and the degree of additional changes caused by changing the pool structure have yet to be summarized in practice.

In view of the above reasons, sometimes the analysis results of the same instrument are often different, so people often say that ECD is the most likely to cause misunderstandings. Practice has proved: before operating the ECD, be familiar with its basic working principles and some issues that should be paid attention to during operation. Once you master its regularity, routine operations may be simpler than TCD or FID. [Edit this paragraph] Classification of ECD There are many classification methods for ECD. If you are familiar with these classification methods, you can better understand their operating characteristics, so that you can choose them reasonably when different analysis needs are needed.

1. Classification by ion source used

Ionization sources used for ECD include radioactive isotope sources and non-radioactive sources. Although non-radioactive ECDs are commercially available and have the advantage of being non-radioactive, they require the use of high-purity He and the addition of certain rare gases as carrier gases during operation. The ECD structure and electronic equipment are also relatively complex, and there are still some operational characteristics. There are some shortcomings, so it is currently in the stage of improvement, promotion and use.

2. Classified by type of radioactive source: it can be divided into two types: 63Ni and 3H.

⑴ ECD requirements for radioactive sources

① Safety in use

Radioactive isotopes may produce α, β, and γ rays during the decay process. α is a high-speed helium nucleus that is positively charged;

β rays are a high-speed electron that is negatively charged; γ rays are electromagnetic waves with extremely short wavelengths. These three types of rays all have a certain amount of energy and can ionize gases and other substances. Among them, alpha rays have the strongest ionization ability. Alpha rays can produce 105 ion pairs per centimeter of travel, and beta rays can produce 102~103 ion pairs per centimeter. Gamma is weaker, producing only one pair of ions per centimeter. Although α-ray ionization efficiency is high, it is too noisy. Gamma rays require sufficient ion current and a large dose of radioactive materials. Gamma rays have strong penetrating power and are very harmful to the human body.

The β source has moderate ionization and penetration strengths, so it is most suitable as an ionizing radiation source;

② The radiation energy of the source must be large enough to provide the necessary ion flow;

③ Ray The range must be short enough, which is conducive to structural design and safety. Although this is contradictory to the requirements for radiation energy, so both must be taken into consideration during use;

④ The half-life must be long enough;

⑤ The operating temperature should be high;

⑵ The two ideal radioactive sources that have been used in ECD are 63Ni and 3H. The following table shows their performance comparison.