Unit 1: Section 1 - Atomic Structure Flashcards
Relative atomic mass definition
The average mass of an atom of an element on a scale where an atom of carbon-12 is 12 (an average, so not usually a whole number)
What are the relative charges and masses of the subatomic particles?
Protons - +1 1
Neutrons - 0 1
Electrons - -1 1/2000
What letters represent mass number and atomic number?
Mass number - A
Atomic number - Z
Relative isotopic mass
The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12 (usually a whole number)
Relative molecular mass
The average mass of a molecule on a scale where an atom of carbon-12 is 12
What can a mass spectrometer be used to find?
Relative atomic mass, relative molecular mass, relative isotopic abundance and to identify elements
What is the process when a sample is squirted into a TOF mass spectrometer?
- Ionisation
- Acceleration
- Ion drift
- Detection
What are the 2 ways of ionising the sample in TOF mass spectrometry?
- Electrospray ionisation
- Electron impact ionisation
What is electrospray ionisation
- The sample is dissolved and pushed through a small nozzle at high pressure
- A high voltage is applied to it, causing each particle to gain an H+ ion
- The sample is turned into a gas made up of positive ions
What is electron impact ionisation?
- The sample is vaporised and an ‘electron gun’ is used to fire high energy electrons at it
- This knocks one electron off each particle, so they become +1 ions
What is acceleration in TOF spectrometry?
The positively charged ions are accelerated by an electric field so that they all have the same kinetic energy (this means that the lighter ions will end up moving faster than the heavier ions)
What is ion drift in TOF spectrometry
The ions enter a region with no electric field, so they just drift through it (lighter ions will drift through faster than heavier ions)
What is ion detection in TOF spectrometry?
- Because lighter ions travel at higher speeds in the drift region, they reach the detector in less time than heavier ions.
- The detector detects charged particles, an electrical current is produced in the detector when a charged particle hits it
- A mass spectrum is produced
What is a mass spectrum?
- mass/charge against abundance (often as a percentage)
- If the sample is an element, each line will represent a different isotope of that element
- If the spectrum was produced using electrospray ionisation, then you have to take away one from the m/z ratio to get the relative mass of each isotope
How to work out relative atomic mass from a mass spectrum
- For each peak read the % relative isotopic abundance from the y- axis and the relative isotopic mass from the x-axis, and multiply them together to get the total mass for each isotope
- Add up these totals
- Divide by 100 (percentages were used)
- If the relative abundance is not given as a percentage, divide by the sum of the relative abundances instead of 100