Ionizace záchytem elektronu: Porovnání verzí
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==Electron capture detector== |
==Electron capture detector== |
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[[File:Gas_chromatography_electron_capture_detector.gif|thumb|350 px|Diagram of a gas chromatograph coupled to an electron capture detector <ref>[http://www.esrl.noaa.gov/gmd/hats/insitu/insitu.html ESRL Global Monitoring Division - Halocarbons and other Atmospheric Trace Species |
[[File:Gas_chromatography_electron_capture_detector.gif|thumb|350 px|Diagram of a gas chromatograph coupled to an electron capture detector <ref>[http://www.esrl.noaa.gov/gmd/hats/insitu/insitu.html ESRL Global Monitoring Division - Halocarbons and other Atmospheric Trace Species]</ref>]] |
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An [[electron capture detector]] most often uses a [[radioactive]] source to generate electrons used for ionization. Some examples of radioactive [[isotopes]] used are <sup>3</sup>H, <sup>63</sup>Ni, <sup>85</sup>Kr, and <sup>90</sup>Sr. The gas in the detector chamber is ionized by the radiation particles. Nitrogen, argon and helium are common carrier gases used in the ECD. Argon and helium need to be combined with another gas, such as methane, in order to prevent immediate conversion into [[metastable]] ions. The combination will extend the lifetime of the metastable ions (10<sup>−6</sup> seconds). The methane will cool the electrons during the collisions.<ref>{{Cite journal | doi = 10.1016/S0021-9673(00)92077-6| title = Electron capture detection in gas chromatography| journal = Journal of Chromatography A| volume = 98| issue = 2| page = 323| year = 1974| last1 = Pellizzari | first1 = E. D. }}</ref> The addition of methane will enhance the ability to form negative ions under high pressure because it will adjust the thermal energy to be similar to the energy distribution of the ions. Methane is the most common gas used because it can produce many positive ions when it collides with electrons. These positive ions will then form low energy electrons used for ionization: |
An [[electron capture detector]] most often uses a [[radioactive]] source to generate electrons used for ionization. Some examples of radioactive [[isotopes]] used are <sup>3</sup>H, <sup>63</sup>Ni, <sup>85</sup>Kr, and <sup>90</sup>Sr. The gas in the detector chamber is ionized by the radiation particles. Nitrogen, argon and helium are common carrier gases used in the ECD. Argon and helium need to be combined with another gas, such as methane, in order to prevent immediate conversion into [[metastable]] ions. The combination will extend the lifetime of the metastable ions (10<sup>−6</sup> seconds). The methane will cool the electrons during the collisions.<ref>{{Cite journal | doi = 10.1016/S0021-9673(00)92077-6| title = Electron capture detection in gas chromatography| journal = Journal of Chromatography A| volume = 98| issue = 2| page = 323| year = 1974| last1 = Pellizzari | first1 = E. D. }}</ref> The addition of methane will enhance the ability to form negative ions under high pressure because it will adjust the thermal energy to be similar to the energy distribution of the ions. Methane is the most common gas used because it can produce many positive ions when it collides with electrons. These positive ions will then form low energy electrons used for ionization: |
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:<chem>2CH4+ + 2e^- -> CH4+ + CH3+ + H + \underbrace{2e^-}_{(secondary)} + \underbrace{2e^-}_{(primary)}</chem><ref name="OngHites1994"/> |
:<chem>2CH4+ + 2e^- -> CH4+ + CH3+ + H + \underbrace{2e^-}_{(secondary)} + \underbrace{2e^-}_{(primary)}</chem><ref name="OngHites1994"/> |
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Verze z 19. 3. 2021, 12:30
Ionizace záchytem elektronu je ionizace atomu nebo molekuly v plynné fázi nastávající v důsledku zachycení elektronu v atomovém obalu za vzniku iontu s obecným vzorcem A-. Reakce probíhá podle tohoto schématu:
- A + e- -> [M]A-
kde M nad šipkou označuje, že k zachování energie a hybnosti je třeba třetí částice (molekularita reakce je rovna třem).