[plasos]

Hi.

I’ve read through the mass spectrometry process many times but it says in my A level book that “the magnetic field strength is slowly increased.  As this happens different ions (ones with lower mass/charge ratio) can reach the detector”

Surely as you increase the magnetic field strength it would allow molecules with a LARGER mass/charge ratio to reach the detector since they have a higher momentum and require a higher force for deflection.

I’m fairly new to chemistry but feel convinced that the textbook is mistaken 

[Aldebaran]

You should find this link useful: https://www.chemguide.co.uk/analysis/masspec/howitworks.html#top

Also if you are studying UK A-level chemistry I would encourage you to explore the chemguide site. It is pitched at UK A-level and is very extensive in its subject matter. It is also reliable in its information.

[Borek]

Surely as you increase the magnetic field strength it would allow molecules with a LARGER mass/charge ratio to reach the detector since they have a higher momentum and require a higher force for deflection.

Unless I am missing something - you are right.

[Perci]

Hi! if your asking why the lighter ions travel faster towards the detector then: (if not just ignore what I'm saying)

In mass spec all the ions are accelerated to the same kinetic energy using an electric field, before they enter the flight tube.

K_e = ¹/₂ x mass x (velocity)²

so, for the sake of example,
K_e = 5
mass for first ion = 2
mass for second ion = 4

so, for the first ion:

• K_e = ¹/₂ x mass x (velocity)²
• 5 = ¹/₂ x 2 x (velocity)²
• 10 = 2 x (velocity)²
• 5 = (velocity)²

and the second ion:

• K_e = ¹/₂ x mass x (velocity)²
• 5 = ¹/₂ x 4 x (velocity)²
• 10 = 4 x (velocity)²
• 2.5 = (velocity)²

Hopefully this explains why they travel at different speeds depending on their mass...