Download Mass Spectrometer Practice Problems

Mass Spectrometer Practice Problems
AP Physics
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A mass spectrometer consists of three major parts: an acceleration chamber, a velocity selector and the mass spectrometer
chamber. In the acceleration chamber, atoms are ionized by having electrons stripped away and the ions are then accelerated
by an electric potential. Once they leave the acceleration chamber, they pass through a velocity selector where an electric
field and a magnetic field work together to allow only ions with the exact right speed to pass through; any ion going too fast
or too slow is steered off course and never makes it to the mass spectrometer. In the mass spectrometer, a magnetic field
steers the moving ion in a circular path – with the radius of the circle being proportional to the ion’s mass.
1. An ionized deuteron (a hydrogen atom with a neutron in the nucleus) passes to the east into a velocity
selector built with an electric field of 8.0 kV/m upward and a magnetic field of 40 mT to the north. How
fast is the ion moving if it goes straight through the velocity selector?
2. A pure sample of hydrogen is sent through a mass spectrometer.
a) After having their single electrons removed the hydrogen ions are sent through a velocity selector.
The velocity selector is created with a magnetic field of 0.75 T pointing downward and an electric field
of 1500 N/C pointing west. What is the velocity of the ions that make it through the velocity selector?
b) The singly ionized atoms are sent into a mass spectrometer with a magnetic field of 1.5 mT
downward, where they travel in semicircular arcs before landing on the detector at three different points:
2.78 cm from the entrance, 5.57 cm from the entrance and 8.35 cm from the entrance. What is the mass
for the isotope of hydrogen that lands at each point?
3. A velocity selector built with a 0.5 T magnetic field and a perpendicular 1750 N/C electric field feeds
into a mass spectrometer with a 0.8 T magnetic field. A sample of air is ionized and fed into the mass
spectrometer.
a) What velocity will the particles that pass through the velocity selector and into the mass spectrometer
be traveling?
b) If the particles passing into the mass spectrometer are singly ionized oxygen (2.65601774 × 10-26 kg),
singly ionized nitrogen (2.32526486 × 10-26 kg) and double ionized nitrogen (2.32526486 × 10-26 kg),
how far away from the entrance will each different kind of particle land? (Assume semicircular arcs)
4. You are trying to design a cheap mass spectrometer that will only have a single detector element,
which is positioned 50 cm away from the entrance to the mass spectrometer chamber. Your plan is to
change the magnetic field strength for different substances to get them all to hit the single detector.
a) You use a velocity selector constructed around a 2000 N/C electric field and a 4 T magnetic field.
How fast will the particles that make it into the velocity selector be going?
b) How strong will the magnetic field in the mass spectrometer need to be for each of the following
particles to get it to hit the detector?
i) triply ionized uranium (3.95292579 × 10-25 kg)
ii) singly ionized iron (9.28821405 × 10-26 kg)
iii) doubly ionized helium (6.64647825 × 10-27 kg)
5. In a mass spectrometer, a singly charged ion pass through a velocity selector built with an electric
field of 1.0 x 103 V/m to the east and a magnetic field of 1.0 T upward. It then passes into the deflector
chamber, which uses the same magnetic field as the velocity selector, and moves in a circular path with
a radius of 1.2 cm. What is the mass of the ion?
6. In a mass spectrometer, a doubly charged ion having a particular velocity is selected by using a
magnetic filed of 100 mT perpendicular to an electric field of 1.0 kV/m. The same magnetic field is
used to deflect the ion in a circular path with a radius of 15 mm.
a) What is the mass of the ion?
b) What is the kinetic energy of the ion?
c) Does the kinetic energy of the ion increase or decrease in the deflector chamber? Explain.