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How are ions produced inside a mass spectrometer?
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In a mass spectrometer, the conversion of an analyte into ions or ionic fragments occurs within the ion source. This process is primarily achieved through the following methods:
- Electron Bombardment: This is the most widely used ionization technique. Electrons extracted from a glowing filament are used to bombard the gaseous analyte, typically removing one or more electrons to create positive ions.12
- Chemical Ionization: To reduce fragmentation, a reagent gas (such as methane) is mixed with the sample and subjected to electron bombardment. The resulting reagent ions then ionize the sample through proton or charge transfer reactions.2
- Negative Ion Formation: Negative ions can be produced via methods like arc discharges or electron impact. Specialized techniques include thermal ionization or the use of a cesium sputter source, where atoms or molecules capture an electron from a cesium layer.3
- Collisions: Analyte molecules may also be converted into ions by undergoing collisions with other ions.1
Once formed, these ions are accelerated into the mass analyzer for separation based on their mass-to-charge ratios.1