6.4
Mass Spectrometers
This lesson details the instrumentation of a mass spectrometer—a physical instrument to perform mass spectrometry on analyte molecules and record the…
The mass spectrometer ionizes molecules and produces a mass spectrum from the fragment distribution.
It performs three functions: ionization of molecules or atoms, separation of the ion fragments, and detection of ions.
In electron ionization, a common ionization method, a vaporized sample in the ionization chamber bombarded with a high-energy electron beam produces radical cations. The radical cation quickly fragments into neutral molecules, radicals, and cations.
The cations are then accelerated into an analyzing chamber by a series of negatively charged accelerator plates.
Many common analyzers apply a magnetic or electrical field to the analyzing chamber, which bends the trajectories of ions based on their mass and charge.
Only ions of particular m/z values have the exact trajectory to pass through the narrow slit placed in front of the ion detector for a given magnetic or electrical field.
By altering the field, the mass analyzer scans through a range of ion masses, with their abundances recorded in the mass spectrum.
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Q1: What are the three main functions of a mass spectrometer?
A mass spectrometer performs three essential functions: ionization of molecules or atoms to create charged species, separation of ion fragments based on their mass-to-charge ratio using magnetic or electrical fields, and detection of ions to record their relative abundance. Together, these functions produce a mass spectrum showing the fragment distribution of the analyzed sample.
Q2: How does electron ionization create ions in a mass spectrometer?
In electron ionization, a vaporized sample in the ionization chamber is bombarded with a high-energy electron beam, typically around 70 eV. These electrons strip electrons from analyte molecules, producing radical cations. The radical cation quickly fragments into neutral molecules, radicals, and cations, which are then directed toward the analyzing chamber for separation and detection.
Q3: Why do mass spectrometers only detect charged ions and not neutral molecules?
Mass spectrometers record only charged species because the electric or magnetic fields in the analyzing chamber control and manipulate ions based on their charge and mass. Neutral molecules lack charge and cannot be deflected by these fields, making them impossible to separate or detect. Only charged fragments reach the detector and contribute to the mass spectrum.
Q4: How does a magnetic field separate ions by mass in a mass spectrometer?
A magnetic field applied in the analyzing chamber bends the trajectories of accelerated ions based on their mass-to-charge ratio. At a constant magnetic field strength, ions with different masses follow different curved paths with varying radii of curvature. A narrow slit positioned before the detector allows only ions of a specific mass to pass through, enabling selective detection of individual ion masses.
Q5: What role do accelerator plates play in a mass spectrometer?
Accelerator plates, positioned after the ionization chamber, accelerate the generated ions toward the analyzing chamber using a series of negatively charged plates. This acceleration gives the ions kinetic energy, which is essential for their subsequent separation by magnetic or electrical fields. The accelerated ions then enter the mass analyzer where they are separated based on mass-to-charge ratio.
Q6: How does scanning the magnetic field produce a complete mass spectrum?
By varying the magnetic field strength, the mass spectrometer can selectively allow ions of different masses to pass through the detector slit one at a time. As the field is scanned through a range of values, each mass-to-charge ratio reaches the detector sequentially. The relative abundance of each ion type is recorded, creating a complete mass spectrum showing all charged species present in the sample.
Q7: What happens to a molecular ion after it forms in the ionization chamber?
After formation, the molecular ion is unstable and quickly undergoes mass spectrometry molecular fragmentation, breaking into smaller charged fragments, neutral molecules, and radicals. Only the charged fragments are accelerated by the accelerator plates and separated in the analyzing chamber. This fragmentation pattern is characteristic of each molecule and provides structural information in the resulting mass spectrum.