How do mass spectrometers work?

A mass spectrometer is a piece of lab equipment used to measure the relative atomic masses of atoms and molecules. It is widely used to identify substances in the lab.

The process is often automated and requires a number of sequential steps that take places in a specially built vacuum chamber. 

1. The sample is injected either as a gas or vapour

2. An electron gun creates a beam of electrons which collide with the sample atoms and, by removing electrons, ionise them. This creates cations which generally have a 1+ charge; about 5% gain a 2+ charge. Molecules often fragment producing a range of molecules with different masses.

3. A series of negatively charged plates with a small gap between accelerate the cations. This creates a beam of cations in which the lighter cations move faster than the heavier ones.

4. A magnetic field applied perpendicular to the direction of travel causes the beam to be deflected. The amount of deflection is dependent on the charge-to-mass ratio (m/z); heavier ions are deflected less than lighter ones. In this way the sample beam is separated by mass. 

5. The magnetic field is gradually adjusted so that ions of increasing mass strike a detector one after another. As the cations strike they remove electrons from the detector, this results in an electric current proportional to the abundance of the specific ion. 

This data is fed into a computer which creates a mass spectrum; a graph of m/z against relative abundance. By comparing the peaks to known sample graphs the identity of the original sample can be ascertained. 

George B. A Level Chemistry tutor, GCSE Science tutor, GCSE Biology t...

2 years ago

Answered by George, an A Level Chemistry tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£24 /hr

Tiarnan B.

Degree: MChem (Masters) - York University

Subjects offered:Chemistry, Maths+ 2 more

-Personal Statements-

“I am a fun, open and qualified tutor. I guarantee by choosing me you will reach where you want to be and beyond, all while enjoying the subject!”

£20 /hr

Raghav A.

Degree: Chemical Engineering (Masters) - Bath University

Subjects offered:Chemistry, Physics+ 2 more

-Personal Statements-

“If you are looking for a tutor who is not only approachable and passionate about his subjects, but willing to go the extra mile, I am here to help”

£24 /hr

Julia M.

Degree: Sustainable Agriculture and Food Security (Masters) - Edinburgh University

Subjects offered:Chemistry, Maths+ 1 more


“Enthusiastic friendly tutor looking for curious and ambitious students for fun and relaxed lessons”

About the author

George B.

Currently unavailable: for regular students

Degree: Chemistry (Masters) - Oxford, Jesus College University

Subjects offered:Chemistry, Science+ 3 more


“About Me: As a second year chemist at Oxford University I naturally have a love for anything and everything science. Science can be challenging at times however, with the right approach, it can be anengaging and exciting subject. I ho...”

You may also like...

Posts by George

How can aldehydes and ketones be distinguished?

How do mass spectrometers work?

What's the difference between speed and velocity?

Other A Level Chemistry questions

Write an expression and units for equilibrium constant for this reaction: 2SO2(g)+O2(g)<-->2SO3(g)

Why is phenylamine a weaker organic base than ethylamine?

How do I calculate the percentage by mass of a metal within an impure substance?

Explain why fluorine is more reactive than chlorine.

View A Level Chemistry tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss