Mass spectrometry Flashcards
What is a mass spectrometer?
A machine that can be used to analyse elements or compounds
What information does a mass spectrometer give you?
Relative atomic mass, relative molecular mass and the relative abundance of isotopes?
What are the 4 steps involved in the time of flight mass spectrometry?
ionisation
acceleration
ion drift
detection
What are the two types of ionisation?
Electrospray ionisation
Electron impact
What are the stages in electrospray ionisation?
A sample is dissolved in a solvent and injected as a mist
The particles are ionised by gaining a proton from the solvent as they leave the needle, producing XH+ ions
What is the formula for electrospray ionisation?
X(g) + H+ -> XH+(g)
What are the stages in electron impact?
The sample is vapourised and high energy electrons are fired at it
The electrons come from a filament gun
This causes each particle in the sample to lose an electron
What are the formulas for electron impact ionisation?
X(g) + e- -> X+(g) + 2e-
X(g) -> X+(g) + e-
Why do samples have to be ionised?
so the ions are attracted to the negative plate and they can be accelerated as they are positive
What is the second stage in time of flight?
Acceleration
What is acceleration?
The positive ions in the sample are accelerated by an electric field
What does the electric field give to all of the ions during acceleration?
the same amount of kinetic energy
What type of ions have the greater acceleration?
The ions with the lower mass to charge ratio (lighter)
What is the third stage in time of flight mass spectrometry?
Ion drift
What do the ions leave the electric field with?
A constant speed and constant kinetic energy
Which ions will drift at higher speeds through the region with no electric field (ion drift)? Why?
Ions with the lower mass to charge ratio
They drift at the same speed they left the electric field
What is the last stage in time of flight?
Detection
Which ions reach the detection plate first?
The ions with the lower mass to charge ratio (lighter)
What happens at the detector?
The current that is created when ions hit it is detected
It is recorded how long they took to pass through the spectrometer
What is the recording at the detector plate used to calculate?
relative abundance
What is the definition of relative atomic mass?
The average mass of an atom of an element on a scale where an atom of carbon-12 is exactly 12
How do you calculate relative atomic mass with percentages?
1) multiply the percentage relative isotopic abundance and the relative isotopic mass together
2) add up all of the totals
3) divide by 100
How do you calculate the relative atomic mass when it is not given as a percentage?
1) multiply the relative isotopic abundance and the relative isotopic mass together for each isotope
2) add up all of the totals
3) divide by the total relative abundance
