Physics

Lab 4-2 Mass Spectrometer Lab

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Mass Spectrometer

A mass spectrometer is a device determines what elements and isotopes are present in a sample.
Element
refers to the type of atom, such as hydrogen, carbon, iron, etc.
Isotopes
are atoms of the same element but with different numbers of neutrons. Mass spectrometers are sometimes used in forensic science when detectives find an unknown substance at a crime scene and need to figure out what it might be.

Left: Photo of a mass spectrometer. Right: Diagram of how a mass spectrometer works.

Elements to be identified are processed so that an electron is removed from each atom, making the atom have a net positive charge equal to the charge on a single proton. The elements are accelerated to high speeds and passed through a velocity selector, which uses a combination of electric fields and magnetic fields to only allow atoms through with a particular velocity. See

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/maspec.html

#c3

for information on how a velocity selector works.

Atoms entering the velocity selector have a positive charge equal to

1

proton. Therefore, when they enter the electric field that is pointing “up” (see diagram above), they feel

a) a force pointing up because positive charges feel a force in the same direction as the electric field.

b) a force pointing down because positive charges feel a force in the opposite direction as the electric field.

c) no force, because the direction of motion is perpendicular to the electric field.

d) a force coming out of the page perpendicular to both the electric field and the direction of motion.

e) a force going into the page perpendicular to both the electric field and the direction of motion.

Select the letter of your answer here:___ Choose an item.___

These same positively charged atoms are also entering a magnetic field coming out of the page. When they enter this magnetic field they feel

a) a force coming out of the page, in the same direction as the magnetic field.

b) a force going into the page, in the opposite direction as the magnetic field.

c) no force, because they are going perpendicular to the magnetic field

d) a force going down, perpendicular to both the magnetic field and the velocity, using the right-hand rule.

e) a force going up, perpendicular to both the magnetic field and the velocity, using the left-hand rule.

Select the letter of your answer here:___ Choose an item.___

Of the two forces acting on the moving atom, which is or are proportional to the speed of the atom?

a) The electric field

b) The magnetic field

c) Both the electric and magnetic fields

Select the letter of your answer here:___ Choose an item.___

Then the atoms move into the area with a magnetic field only. The magnetic force on moving charged particles causes each atom to move in a circular path.

The atoms move in a circular path because

a) the force on the atom is in the opposite direction as the velocity.

b) the force on the atom is perpendicular to the velocity.

c) the force on the atom is in the same direction as the velocity.

Select the letter of your answer here:___ Choose an item.___

The atoms collide with a detector which marks where each atom strikes. The further from the opening the atom gets, the more mass it has. The mass helps researchers identify the type of atom or isotope.

To explore how the mass spectrometer works, open the following simulation

https://iwant2study.org/lookangejss/00workshop/2017FelixPaco/day4/ejss_model_massspectrometerv10/massspectrometerv10_Simulation.xhtml

Let’s say we have the following values:

v = 3000

, speed; B = 5.000 mT, Magnetic field strength in milliTesla.

The Tesla is a unit of magnetic field strength that will cause a particle with 1 C of charge moving (perpendicular to the field) at 1 to feel a force of 1 Newton.

When a charged particle is moving perpendicular to a magnetic field, the force on the particle is

, where
q
= charge,
v
= speed, and
B
= magnetic field strength.

Standard units for the equation are Coulombs (charge), (speed), and Tesla (magnetic field). Results should come out in standard units of force: Newtons.

For more detail on this derivation and on the mass spectrometer, see this presentation on the Internet: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/maspec.html

Observe the paths of two isotopes of Carbon.

Each isotope has a positive charge of one proton.

Use the following values for q, v, and B to calculate
the force on the isotopes:

Note: for calculation purposes, assume all readings on the simulation are good to 4 significant figures.

q = 1.600 x 10–19 C

v = 3000

B = 0.005 T

Note: to enter 1.60 x 10–19 in a calculator press
1.60 EE (–) 19 keys.

The display will show
1.6e-19 for this number.

When calculating values using number in scientific notation, it helps a great deal to write the formula and then write the values that will be entered into the calculator as a way of focusing on the proper way to enter such numbers into the calculator without losing track of what you have done and what you have yet to do.

Write each substitution value below before doing your calculation. Then write your answer.

F = q v B = ()(Click or tap here to enter text.)(Click or tap here to enter text.) =
Be sure to enter units with all numbers.

Note: The proper way to write this is using the power of ten, not the way the calculator presents the number. (6.4 x 10-14 not
6.4 e-14
)

Since the force is applied to the particle perpendicular to the velocity, the particle moves in a circle. We will call the radius of this circle, R. The mechanics of circular motion can show that the force, mass, speed, and radius are related in the following equation:

Combining this with the formula for the magnetic force, we have:

Solving for R, we get

In any given trial, the values for v, q, and B are the same, so the only thing that affects the radius of the path is the mass of the particle. Notice in your simulation the mass determines the radius of the curve. Carbon-12 has a mass in atomic mass unit (amu) of 12 u. Each amu is 1.66053906660(50)×10−27 kg.

Which isotope has the path with the bigger radius?

a) Carbon-12

b) Carbon-14

c) Both have the same radius

Select the letter of your answer here:___ Choose an item.___

Therefore, which isotope has the larger mass?

a) Carbon-12
b) Carbon-14

c) Both have the same radius hence the same mass.

Select the letter of your answer here:___ Choose an item.___

Click the reset button on the simulation and change the isotope pair to
Neon-20 and Neon-22 ( )

The number is the number of protons and neutrons in the isotope, so we expect these to have more mass than the carbon.

What is different about these paths compared to the Carbon isotope paths?

a) Neon isotope paths have a smaller radius than the Carbon paths.

b) Neon isotope paths have the same radius as the Carbon paths.

c) Neon isotope paths have a greater radius than the Carbon paths.

Select the letter of your answer here:___ Choose an item.___

The reason for this must be that

a) Neon isotopes have less mass than Carbon isotopes.

b) Neon isotopes have the same mass as Carbon isotopes.

c) Neon isotopes have more mass than Carbon isotopes.

Select the letter of your answer here:___ Choose an item.___

Neon-20 radius = Click or tap here to enter text. cm = Click or tap here to enter text. m

Neon-22 radius = Click or tap here to enter text. cm = Click or tap here to enter text. m

If we rearrange the equation on page 4 for R, we can get an equation that calculates the mass of each particle. The radius for Neon-20 and Neon-22 are 12.4 cm and 13.45 cm respectively.

Calculate the mass of Neon-20 and Neon 22 using the values above for R, q, B, and v.

Ask for help if you have trouble with this calculation. Don’t be intimidated by scientific notation. For all the exponents and complexity, you are still just multiplying three numbers together and dividing the result by a 4th number. Your result should be in kilograms and have an exponent that is around –26. If you get an exponent different from this, double check your numbers and your method of entering the numbers into the calculator. Get help if needed.

Start each calculation by writing the values substituted for each variable.

Mass of Neon-20= (red beam) = =

Mass of Neon-22 (blue beam) = =

Check your result as follows:

Neon-20 has 10 protons and 10 neutrons. Neon-22 has 10 protons and 12 neutrons.

The mass of a proton is 1.6726×10–27 kg. The mass of a neutron is 1.6749×10–27 kg.

Calculate the mass of Neon-20 and Neon-22 using this information:

Mass of Neon-20 = (10)x(1.6726×10–27 kg)+(10)x(1.6749×10–27 kg) = kg

Mass of Neon-22 = (10)x(1.6726×10–27 kg)+(12)x(1.6749×10–27 kg) = kg

Your results will not match exactly the results from the mass spectrometer. This is because in the world of the nucleus, mass can turn into energy that binds the nucleus together.

Extra Credit

Reset the simulation and select the isotope U-235 and U 238.

These two isotopes of uranium are very important in generating nuclear energy for electricity and in nuclear warheads.

Use the simulation to measure the mass of each isotope. Follow the same general procedure as on page 7-9 and express your answer amu (atomic mass units).

Record your settings and then use them to calculate the mass.

R1= Click or tap here to enter text.

R2= Click or tap here to enter text.

velocity = v = Click or tap here to enter text.

Magnetic field = B = Click or tap here to enter text.

charge = q =

1.660 x 10–27 kg = 1 amu

Give answers amu units.

Mass 1 = Click or tap here to enter text. (should be mass of U-235)

Mass 2 = Click or tap here to enter text. (should be mass of U-238)

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