2.2.1 Mass Spectrometry: Determining Atomic Masses

Question 1

Mass Spectrometry: Determining Atomic Masses

Answer 1

• Mass spectrometry can be used to determine the molecular mass of a sample.
• Mass spectrometry data shows the number of particles detected versus the mass of the particle, with the highest mass of the partical, with the highest mass being the molecular mass.

Question 2

electrons and kinetic energy

Answer 2

• The behavior of a charged particle in a magnetic or electric field depends on its charge-to-mass ratio. For example, J. J. Thomson was able to calculate the charge-to-mass ratio of electrons by observing their behavior when subjected to magnetic and electric fields.
• Mass spectrometry (MS) uses this concept to determine the molecular mass of a sample. In mass spectrometry, an ionized gaseous sample is accelerated to a known kinetic energy. The particles of the sample are then deflected in a magnetic field.
• Particles with a higher charge-to-mass ratio (in other words, lighter particles) are deflected more easily. Therefore, a lower magnetic field is required to deflect a lighter particle. The magnetic field required to deflect a particle into the detector is proportional to the mass of the particle.

Question 3

note

Answer 3

• Mass spectrometry data shows the number of particles
  detected versus the mass of the particle, with the highest mass being the molecular mass.
• For example, for butane (H3CCH2CH2CH3), the heaviest particle gives a signal corresponding to a mass of 58 g/mol—the molecular mass of butane. However, several different masses will also be detected, each corresponding to a different fragment of butane. Analysis of these different masses can yield information about the connectivity of the molecule. For example, the sequence of a protein can be determined from the fragmentation pattern detected in a mass spectrometry plot

Question 4

Carbon has 6 protons. A sample of carbon was placed in the mass spectrometer. A signal was detected at 12 and at 13. Which of the following statements is best explaination?

Answer 4

The sample contained two isotopes of carbon.

Question 5

What does a mass spectrometer measure?

Answer 5

the mass to charge ratio of an atom

Question 6

Whose experiments and techniques were used in the mass spectrometer experiments?

Answer 6

Thomson

Question 7

Which statement best describes isotopes?

Answer 7

• Isotopes of a particular element have the same number of protons, but have different numbers of neutrons.
• Isotopes of a particular element are chemically identical, but they differ in mass.

Question 8

The number of ______________ in an atom determines the chemical nature of the atom.

Answer 8

protons

Question 9

True or false?

A molecule is a collection of atoms bound together.

Answer 9

true

Question 10

When the magnetic field is applied to particles in the mass spectrometer, which ions are deflected (more on a per charge basis)?

Answer 10

The lighter ions are deflected more

Question 11

What two types of fields are applied in the mass spectrometer?

Answer 11

Electric and magnetic fields

Question 12

A sample of an element is placed in a mass spectrometer. More than one signal is detected. What does this most likely indicate?

Answer 12

The atoms have varying numbers of neutrons

Question 13

An atom of a hypothetical element has 1 proton and can have 0, 1, or 2 neutrons. A sample containing three isotopes of the hypothetical element is placed in a mass spectrometer. Which of the following is true?

Answer 13

There will be three signals, one for each isotope of a different mass.