---
title: "Mass Spectrometry: Ionisation Methods and Mass Analysers"
description: "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 "
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source: https://lightmysky.com/learn/science/mass-spectrometry-ionisation-methods-and-mass-analysers-mt_JTTb7FM1L4.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Chemistry · Ages 20 to 21
Page: https://lightmysky.com/learn/science/mass-spectrometry-ionisation-methods-and-mass-analysers-mt_JTTb7FM1L4

## Ready when they can

- 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

## Lesson: Making ions and sorting them by mass

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.

**Example.** 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.

**Tip.** 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.

**Recap.** Ionise gently to keep the molecule, sort by mass to charge ratio, and read each landing spot as a mass.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Ion Exchange and Electrophoretic Separations](https://lightmysky.com/learn/science/ion-exchange-and-electrophoretic-separations-mt_6xaiivXbAd)
- [Charged Particles Moving in a Magnetic Field](https://lightmysky.com/learn/science/charged-particles-moving-in-a-magnetic-field-mt_evuhZ6cSfI)
- [Mass Spectra and Infrared: Identifying an Unknown](https://lightmysky.com/learn/science/mass-spectra-and-infrared-identifying-an-unknown-mt_yotOEg9OTn)

## Opens up

- [Accurate Mass and Isotope Patterns as Formula Evidence](https://lightmysky.com/learn/science/accurate-mass-and-isotope-patterns-as-formula-evidence-mt_68vjdAPNrc)