Chemical ionization is a soft ionization technique used in mass spectrometry. All mass spectrometry devices require their samples to first be ionized before the mass-to-charge ratio of the different components in the sample can be measured and displayed on a mass spectrum. Mass spectrometry is something I have covered extensively and I will link a playlist containing all my relevant videos on the topic by the end of this one.
The principle of chemical ionization is to use a reagent ion which we will call X+ to react with the analyte molecule A to achieve ionization of the analyte A. Let us look at an example:
In this equation we have hydronium plus our analyte A. When these two substances react, the resulting products are our analyte, containing an extra positively charged hydrogen and water. In other words our analyte acquires an extra proton from the hydronium!
However, the reagent ion also has to be generated with the help of an ion source. Commonly used ion sources mainly include radiation source, hollow cathode discharge power source, and ordinary glow discharge power source. Polonium-210 and Americium-241 are the most common sources of radiation. The ionization process begins with the alpha particles emitted by Polonium-210 and Americium-241. It has high energy and can collide with the reagent gas to generate reagent ions and electrons. If the generated electrons are high enough, they too may collide with the reagent gas to form new reagent ions and electrons. When using H3O+ as a reagent ion, a hollow cathode discharge power source is commonly used, which can produce 99.5% of H3O+ ions. Then these H3O+ ions can chemically react with our target analyte and produced an ionized sample which travels into the mass analyzer. The mass analyzer can then measure the mass-to-charge ratio of the ion and display it in a mass spectrum.
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