Fundamental Particles, Mass number and Isotopes

Relative mass of electron

1/1840

Electron location in atom

In the shells orbiting the nucleus

Atomic number definition

The number of protons in the nucleus of an atom

Mass number definition

The number of protons and neutrons in the nucleus of an atom

Isotopes definition

Atoms of the same element with a different number of neutrons

Why do isotopes have the same chemical properties?

Isotopes have the same number of electrons in the outer shell

What can TOF mass spectrometry be used to measure?

• Relative atomic mass of an element

• Relative molecular mass of a compound

What are the 4 key stages of TOF mass spectrometry?

• Ionisation

• Acceleration

• Separation of ions

• Detection

What are the two different methods of ionisation in mass spectrometry?

• Electron impact

• Electrospray ionisation

What are the steps for electron impact ionisation? Give the general equation.

1. Sample is injected into spectrometer and vaporised

2. High energy electrons from electron gun are fired at the sample

3. The high energy electrons knock off an electron from each particle to form positive ions

• General equation: M_(g) → M⁺_(g)+ e^-

What are the steps for electrospray ionisation? Give the general equation.

1. Sample is dissolved in polar solvent

2. Sample is then injected through a fine hypodermic needle to give a fine mist

3. The tip of the needle is attached to the positive terminal of a high voltage supply

4. Particles gain a proton (H⁺ ion) from the solvent to form positive ions

• General equation: M + H⁺ → MH⁺

What kind of solvent is the sample dissolved in for electrospray ionisation and why is this important?

• Volatile solvent

• To ensure it evaporates to leave positive ions (MH⁺)

How are the ions accelerated and what does this give them?

• An electric field is applied to accelerate the positive ions

• Gives all ions with the same charge the same kinetic energy

What is the equation for kinetic energy? Give the units of mass and velocity.

• K.E= 1/2mv²

• mass → kg

• velocity → ms^-1

What mass of ions travel through the mass spectrometer to reach the detector first?

Lighter ions

How are the ions detected?

• At the detector each positive ion gains an electron

• This generates a current

• The greater the abundance of the ion, the higher the current

• General equation: M⁺+ e^- → M

Why is the whole spectrometer kept under a vacuum?

To prevent ions from colliding with molecules in the air

What is the equation containing velocity, distance and time? Give the units of all 3.

• velocity = distance/time

• velocity → ms^-1

• distance → metres

• time → seconds

What is the equation to calculate the mass of one ion (kg)

• mass of one ion (kg) = (relative isotopic mass x 10^-3)/ 6.022×10²³

• 6.022 × 10²³ = Avogadro constant

Define isotopic mass

The mass of an isotope relative to 1/12^th the mass of an atom of carbon-12

Define relative atomic mass

The weighted average mass of all the isotopes relative to 1/12^th the mass of an atom of carbon-12

What is the formula for relative atomic mass

∑(mass x abundance of each isotope)/ total abundance