Why don't we just plot abundance against mass if m/e basically equals mass anyway?
Why does the axis have to be the ratio of m/e in the first place? Thanks.
Why does the axis have to be the ratio of m/e in the first place? Thanks.
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Quick answer, e doesn't equal +1 in all cases. The molecule will disintegrate into varying sized particles which will also can also have a varying size of charge. m/e doesn't basically equal mass, there is still a dependence - due to the variation in e - on e.
The long answer: A simple mass spectrometer works by taking a sample and bombarding it with electrons to break it up. The particles then reach a velocity governed by a few equations:
E=QV
E= 0.5mv^2
=> v=sqrt(2QV/m)
After the particle leaves the potential field, it enters perpendicular magnetic fields. Moving charges in a magnetic field experience an acceleration. This acceleration is, again, dictated by a simple equation:
F=ma
F=qvb
v=sqrt(2QV/m)
=> a =m/(sqrt(2QV/m)QB
Rearranging that last equation we can get an equation for m/Q
m/q = (2va^2B^2)^(1/4) (to the power of a fourth is equal to the fourth root)
Looking at this we already know the value of B from the machines specifications. We need to only calculate the velocity of the particle and the acceleration. Watch out, these two are in different directions.
Calulating these two requires the measurment of two things. A) how far the particle travelled and b) the time of its flight. These are easy to measure, and it's then just a case of using simple kinematic equations which can be derived from Newton's laws.
If you do this, you'll see that it's impossible to work out what the charge was or the velocity. Only the ratio between them. For example, if a particle is lighter it will accelerate more quickly through the electric field. It will also, however, accelerate more quickly in the magnetic fields. The higher acceleration in the magnetic field cuts its time of flight so that - even though travelling faster - it travels the same distance as a particle which is twice as massive but has half the charge.
The long answer: A simple mass spectrometer works by taking a sample and bombarding it with electrons to break it up. The particles then reach a velocity governed by a few equations:
E=QV
E= 0.5mv^2
=> v=sqrt(2QV/m)
After the particle leaves the potential field, it enters perpendicular magnetic fields. Moving charges in a magnetic field experience an acceleration. This acceleration is, again, dictated by a simple equation:
F=ma
F=qvb
v=sqrt(2QV/m)
=> a =m/(sqrt(2QV/m)QB
Rearranging that last equation we can get an equation for m/Q
m/q = (2va^2B^2)^(1/4) (to the power of a fourth is equal to the fourth root)
Looking at this we already know the value of B from the machines specifications. We need to only calculate the velocity of the particle and the acceleration. Watch out, these two are in different directions.
Calulating these two requires the measurment of two things. A) how far the particle travelled and b) the time of its flight. These are easy to measure, and it's then just a case of using simple kinematic equations which can be derived from Newton's laws.
If you do this, you'll see that it's impossible to work out what the charge was or the velocity. Only the ratio between them. For example, if a particle is lighter it will accelerate more quickly through the electric field. It will also, however, accelerate more quickly in the magnetic fields. The higher acceleration in the magnetic field cuts its time of flight so that - even though travelling faster - it travels the same distance as a particle which is twice as massive but has half the charge.
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keywords: Mass,of,spectrometry,just,instead,is,mass,Why,measured,as,because,Mass spectrometry - Why is m/e measured, instead of just mass? (as m/e=mass because e=+1)