Mass Spec w/ a twist!

[ilovemedi]

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So according to TBR, if v>E/B, then the cation deflects UP. if = then it deflects straight. if v<E/B, deflects DOWN.

So my question is, what if an ANION was used and not a cation (just for fun). My instructor said something about if the "voltage (from the plate) was reversed, the anion will curve downward in the magnetic field". I thought that the above information (v>E/B) depended on whether it curves up or down, not charge...?????

 

[circulus vitios]

You have two forces/equations at play in a velocity selector: the force from the electric field and the force from the magnetic field.

Fe = qe
Fb = qv X b

If v=E/B the particle will pass through undeflected regardless of the magnetic field's orientation or the charge of the particle (positive or negative.)

If v&#8800;E/B then it will be deflected. Whether its deflected up or down depends on the orientation of the magnetic field and the charge of the particle.

If v>E/B then the particle will be deflected in accordance with Fb = qv X B because this is the only equation at play that involves the particle velocity. Using this equation and the right hand rule will give you the direction of force on a standard test charge, which is always assumed to be positively charged. If you reverse the magnetic field's orientation then the direction will be reversed as given by the right hand rule. If you use a negative test charge then the direction of the force will also reverse, as the equation and the right hand rule always assume a positive charge.

If v<E/B then the particle will be deflected in the opposite direction.

 

[ilovemedi]

Thank circulus vitios!! So basically if 1) the charge changes to - and 2) if the orientation of the magnetic field changes (i.e. from inside to outside), then the direction it gets deflected is reversed?

 

[circulus vitios]

Thank circulus vitios!! So basically if 1) the charge changes to - and 2) if the orientation of the magnetic field changes (i.e. from inside to outside), then the direction it gets deflected is reversed?

Replace "and" with "or" and that will be correct.