Your forensic chemistry group, working closely with local law
enforcement agencies, has acquired a mass spectrometer similar to
that discussed in the text. It employs a uniform magnetic field
that has a magnitude of 0.68 T. To calibrate the mass spectrometer,
you decide to measure the masses of various carbon isotopes by
measuring the position of impact of the various singly ionized
carbon ions that have entered the spectrometer with a kinetic
energy of 23 keV. A wire chamber with position sensitivity of 0.46
mm is part of the apparatus. What will be the limit on its mass
resolution (in kg) for ions in this mass range, that is, those
whose mass is on the order of that of a carbon atom?
Answer in kg
given uniform magnetic field B = 0.68 T
kE = 23*10^3 eV
d = 0.46 mm
mass resolution limit = dm
hence
from kE we get
kE = 0.5mv^2
23*10^3*1.6*10^-19 = 0.5*m*v^2
mv^2 = 7360*10^-18
now, for length l of spectrometer
the deflection d is given by
d = R(1 - cos(wt))
now, Rsin(wt) = l
and
for speed v of charged particel of charge q and mass m
qvB = mv^2/R
qB = mv/R
amd v = w*R
hence
d = (mv/qB)(1 - cos(t*qB/m)) = (mv/qB)(1 - sqroot(1 - l^2*q^2B^2/m^2v^2))
hecne mass resolutionis given by
dm = d*qB = 0.46*10^-3*0.68*1.6*10^-19 = 50048*10^-27 kg
mass of one carbon atom = 12*1.6*10^-27 kg
hence
dm = 2606.66 atoms of carbon atom
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