Atomic Structure Flashcards
What is an isotope
Atoms with the same number of protons, but different numbers of neutrons
What’s the atomic number
The number of protons in the nucleus
What’s the mass number
The total number of protons and neutrons in the atom
Properties of isotopes
Similar chemical properties because same electronic structure. They may have slightly varying physical properties because they have different masses.
What does a mass spectrometer do
The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify the elements
What are the steps of time of flight mass spectrometry
Ionisation of species
Ions are accelerated
Ions drift area
Detection area
Why does this need to occur in a vacuum
Otherwise air particles would ionise and register on the detector
Explain the two methods of ionisation
Electron impact- vaporised sample injected at low pressure, electron gun fires high energy électrons at the sample, knocks out an outer electron, forming positive ions with different charges
Electro spray ionisation- the sample is dissolved in a volatile, polar solvent which is injected through a fine hypodermic needle giving a fine mist or aerosol, the tip of the needle has high voltage. Tip of the needle the sample molecule (M) gains a proton (H+) from the solvent forming MH+, the solvent evaporates away while the MH+ ions move towards a negative plate
When is electron impact used
When it’s for elements and substances with low formula mass. Electron impact can cause larger molecules to fragment.
When is electrospray ionisation used
For larger organic molecules. The softer conditions of this technique mean fragmentation doesn’t occur
What are the ions accelerated by
An electric field to a constant kinetic energy
Why do the ions need to be the same kinetic energy
Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity
Look at the flight tube aspect in the text book
Page number unknown
What is the M/Z ratio
Mass divided by charge value
What happens at detection
The ions reach the detector and generate a small current, which is fed to a computer for analysis. The current is produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the abundance of the species.
