# Physics question ?

Based on the understanding of mass spectrometers, answer the three following questions.

1. A uniform magnetic field in the detection chamber of a mass spectrometer causes ions to travel in a circular path. Create an equation for the radius of the path.

2.How is the radius of the path an ion makes in a detection chamber affected by the ion’s mass?

3. Explain how a mass spectrometer is used to determine if a professional baseball player used an illegal performance-boosting substance.

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• 1 month ago

Let m = mass of ion, q = charge on ion, B = magnetic field strength, r = radius of curvature of ion path, v0 = speed ion enters the chamber with.

Assume magnetic field is pointing in a perpendicular direction to that of the ion's velocity when it enters the magnetic field..  You know from Lorenz's law that the force on the ion due to the magnetic field is

F = q*v0*B   and that force is balanced by the ion's centripetal force sine the magnetic field does no work on the ion

F = mv0^2/r = q*v0*B  --> solve for r

r = m*v0/(q*B)

Note that r is proportional to m so if m increases, r increases.

You can find an illegal substance in blood or urine by taking a mass spec of a "clean" sample and comparing it to one where PES is suspected.  If teh suspect sample is clean, both mass specs will look identical (mass peaks at the same places).  If it is tainted, then there will be other mass peaks in the spectrum due to the presence of the substance.