The first component the chemicals encounter in the mass spectrometer is called the ion source, where neutral molecules that elute from the GC column are ionized. A common ion source is an electron ionization (EI) source that usually contains a metal filament, similar to the filament in a light bulb. When an electrical charge is applied to the filament, it emits a stream of electrons at the incoming compounds, breaking them into fragments, and many of them with a positive charge. The pattern of resulting fragments acts as a highly specific “fingerprint” that can be used to identify the chemical.
Within the ion source, a series of electrodes known as lenses direct the charged molecules away from the source and into a quadrupole mass analyzer (or mass filter). A quadrupole consists of four rods to which a direct current voltage and radio frequency are applied. Various combinations of these forces ensure that only fragments of a specific mass (called a mass-to-charge ratio or m/z) will travel down the electric field of the quadrupole toward the detector at a given time.
The mass spectrometer rapidly cycles through different voltages, measuring many m/z ratios. The ions that pass through the instrument are measured by a detector called an electron multiplier, which provides a signal intensity for each ion species present. The recorded data at each point in time of an experiment is called a mass spectrum. The pattern of this mass spectrum can be used for identification purposes, much like a fingerprint. The response recorded for the different ion species can be calibrated for quantitative purposes.
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