UKC

When I learned about the basics of how electric and magnetic fields are used in mass spectrometers for my A level many moons ago I recall it thusly for a bainbridge mass spectrometer.

Charged particles are accelerated using an electric field perpendicular to their motion. All given same kinetic energy = Vq as it is assumed that they all have same charge. Therefore larger mass particles have lower velocities etc.

Particles then at range of speeds enter into velocity selector using electric and magnetic fields to exert forces opposite to each others. The only particles that get through are those with velocity equal to E/B.

These particles then enter a magnetic field and follow a circular path whose radius (and hence the detector they hit) is proportional to their mass. Fine.

Except for the fact that now it comes to teaching it on a new spec it occurs to me that if.....
1) all particles are acclerated by initial electric field and given same Ek = Vq and
2) the velocity is proportional to square root of 1/mass
then by using the the velocity selector to allow only particles with velocity E/B through then it only allows particles with the same mass through as well.

Clearly it is more complicated than this and whilst the students do not need any more than this basic level understanding (thank christ as I clearly do not have the answer) it may well be that one of them spots this. How can I answer it with the required level of understanding? All the texts I find of an A level standard make no mention of this.

Many thanks

In reply to twm.bwen:

I'm not that familiar with Mass Spec, but I have used them from time to time. I don't know the answer to your question, but this is what I would expect from the design of the Bainbridge spectrometer and its uses:

The ionization and acceleration process does not always give each ion exactly the same kinetic energy. So the ions entering the velocity selector have a range of different kinetic energies (how wide a range depends on the ion source used), and there will be some of both types of ions getting through. For this to work, the ions have to be fairly similar in mass. This makes the Bainbridge design good for distinguishing between ions with similar mass, but less good for looking at ions with very different masses. So the Bainbridge is common used for looking at isotope abundance in elements, and other designs are better for other uses.

In reply to twm.bwen:

Is this at A-Level? Which spec are you teaching that needs to know about this?

In reply to twm.bwen:

it only allows particles with the same mass through as well.

Isn't that what you want?

In reply to Jamie Wakeham:
OCR A 5.1.2
"Describe /explain the deflection of particles in mass spectrometers" owtte

P

In reply to lowersharpnose:

Exactly. So why have the velocity selector other than to say, if they have known charge from uonisation process (as assumed) and velocity E/B and are deflected by field stren2 then thr radius is used to measure the mas of the ion?gth B.
It still doesnt answer how an electric field accelerates different mass particles to the same speed.

Sorry about typos. whenever the keyboard pops up on my phone the test panel dissappears so I'm doing it blind and its impossibke to go back and correct.

In reply to twm.bwen:

electric field accelerates different mass particles to the same speed.

I don't think it does.

In reply to twm.bwen:

The part of the description that is not clear to me is how the ioniser/linear accelerator can produce particles with equal charges, differing masses but equal velocities. I guess perhaps the initial accelerating field varies cyclically or is swept but if that were the case you'd think you could infer mass from the output of the velocity selector and the initial field strength alone. Perhaps there's more to it than that and there's something I don't know about the ionisation process, some randomness in initial velocity maybe. Perhaps the subsequent beam bender and detector is there to handle particles with more than one electron missing or simply improve resolution and or produce a photographic record that can be interpreted at leisure.

jk

In reply to twm.bwen:

For OCR A, I think you're overthinking this one. I don't think you need to include the velocity selector at all - just teach how the accelerating voltage is used to inject the ions and then solve Bqv = mv^2/r to find a unique value for q/m. Something like FPF Jan '13 q3 is pretty typical of how they set the questions.

Admittedly FPF Jun '10 q3 does have a simple velocity selector in it, in that it asks candidates to deduce that you could have an acceleration in one direction due to an E field and balance it in the other direction with a B field. But the way it's set is that it leads you by the nose to this conclusion, implying that the examiner did not expect candidates to know about this idea beforehand. This doesn't mean that teaching it isn't a good call, of coure, but I'd use this question as a guide for how difficult it could get in an exam (in fact if I were you I'd use this question specifically to teach how a velocity selector could be made).

For what it's worth, of all the A levels I tutor in, I regard OCR A as one of the more predictable ones - they don't have a habit of setting way-outside-the-box stuff all that often.