Mass Spectrometry: Ionisation Methods and Mass Analysers
A mass spectrometer has to make an ion, sort it by mass to charge ratio and count it. The choice of ionisation decides whether the molecule survives, and the choice of analyser decides how finely the sorting is done.
What a learner can do afterwards
- Compares a hard and a soft ionisation method by what appears in the spectrum
- Explains how a quadrupole, a time of flight and a magnetic sector each separate ions
- Defines resolving power and says what it lets an analyst distinguish
- Explains why multiply charged ions extend the mass range of an instrument
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A mass spectrometer must do three jobs: make an ion, sort it by mass to charge ratio, and count it. The ionisation choice decides whether the molecule survives. Hard ionisation shatters the molecule, so the spectrum fills with fragments and often shows no molecular ion. Soft ionisation keeps the molecule intact, so the molecular mass appears clearly.
Sorting works by bending flight paths with fields. In a magnetic sector analyser, ions fly at known speeds across a magnetic field that curves each path into a circle of radius r equal to mv over qB. Heavier ions bend less, lighter ions bend more, and more highly charged ions bend more strongly, so each mass to charge ratio lands at its own spot on the detector.
The other analysers use the same ratio with different tricks. A quadrupole filters ions by their stability in an oscillating field, letting one ratio through at a time. A time of flight tube gives every ion the same push and times the race: lighter ions arrive first. Resolving power then says how finely the sorting is done: higher resolving power separates masses that sit closer together.
Soft methods can also stack several charges onto one big molecule, and those multiply charged ions extend the mass range of the instrument. A heavy molecule carrying several charges behaves like a lighter singly charged one, so it lands inside the working window. If a spectrum shows no molecular ion, suspect hard ionisation and switch to a softer method to reveal the molecular mass.
Ionise gently to keep the molecule, sort by mass to charge ratio, and read each landing spot as a mass.
More to read
11.7 Applications of Magnetic Forces and Fields
How does a mass spectrometer sort ions?
A mass spectrometer is a machine that separates charged particles by their charge-to-mass ratio. Being able to push charged particles around and sort them lets scientists work out what matter is made of.
When a charged particle moves across a magnetic field, the field pushes it into a curved path. If the particle's velocity is at right angles to the field, the path is a circle. The radius of that circle is r = mv divided by qB, where m is the particle's mass, v is its speed, q is its charge, and B is the strength of the magnetic field.
That one formula is the heart of the magnetic-sector analyser. Heavier ions bend less, lighter ions bend more, and ions with bigger charge bend more strongly. So if you send a beam of ions through the magnet, each mass-to-charge ratio lands at its own spot on the detector.
The same physics also powers the cyclotron, a machine that speeds charges up to very high energies.
22.11 More Applications of Magnetism
Mass spectrometry: bending ions to measure them
The curved paths that charged particles follow in magnetic fields can be put to good use. A charged particle moving at right angles to a magnetic field travels in a circle. The radius of the circle is r = mv over qB.
This is exactly how a mass spectrometer works. The sample is turned into ions, the ions are given a push so they all move at known speeds, and then they pass through a magnetic field. Because the radius depends on mass divided by charge, ions with different mass-to-charge ratios curve by different amounts and arrive at different places on the detector.
Think of it like a race track where heavier runners are harder to turn. The field is the corner, and each ion comes out of the corner at its own angle. By measuring where the ions land, you can work out their masses and figure out what the sample contains. This is the classic magnetic-sector design, one of the oldest and still one of the most precise ways to sort ions.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.