3.1.1.2 Mass Number and Isotopes Flashcards
Define isotope
Atoms of the same element that contain the same number of protons but a different number of neutrons, relative to the carbon-12 isotope.
What is the relationship between properties of isotopes?
They have the same chemical properties as they have the same number of electrons, so react in the same way. However, they have different physical properties due to their different masses.
What is the relative atomic mass (Ar)?
Weighted average of the mass number of all of the isotopes of an element, taking into account both their masses and their abundances. It is relative to the mass of an atom of carbon 12
What is mass spectrometry and what is it used for?
A powerful method of instrumental analysis that can be used to…
-find the mass and abundance of each isotope in an element, and therefore its relative atomic mass (Ar).
-identify molecules by finding their relative molecular mass (Mr).
What does a mass spectrometer produce?
A spectrum of mass/charge ratios (m/z) against percentage abundance.
Give a brief overview of how a mass spectrometer produces mass spectra
An ionised sample is accelerated through the mass spectrometer, and detected to determine its m/z ratio, based on its time of flight.
What are the 4 stages in TOF mass spectrometry?
-ionisation
-acceleration
-ion drift
-detection
What does ionisation mean?
To turn atoms or molecules into ions- in this case, 1+ ions.
Describe how electron impact ionisation takes place
-the sample is vaporised so it is a gas
-high energy electrons are fired at the sample from an electron ‘gun’
-one electron per particle is removed from the outer shell on collision
-this forms M+ (molecular ions)
What is the equation for electron impact ionisation?
M(g) -> M+(g) + e-
When is electron impact ionisation used?
For elements and substances that have a low molecular mass
-this is because they are less likely to fragment when hit by the electrons
Why is the mass spectrometer kept under a vacuum?
To prevent ions from colliding with air molecules which would slow them down.
Describe the process of electrospray ionisation
-the sample is dissolved in a volatile solvent
-it is injected into the spectrometer through a thin hypodermic needle, which produces a fine mist
-the needle tip is attached to the positive terminal of a high voltage power supply
-this causes the particles to be ionised by gaining a proton (H+) from the solvent.
When is electrospray ionisation used?
-substances with a large molecular mass-usually biological and organic molecules
-this is because it is ‘soft ionisation’
-these substances would be more likely to fragment on electron impact, so the Mr of the whole molecule would not be measured
Equation for electrospray ionisation
M(g) + H+ -> MH+(g)
What does it mean if a particle has a lower m/z ratio?
The particle will accelerate more
Explain the mass/charge ratio
-mass./charge ratio (m/z) is ratio of a particles mass compared to its charge
-can also be written as m ÷ z
-therefore if the particle has a 1+ charge, then the m/z ratio = Mr or Ar
What is the equation for Kinetic energy and what does it show?
KE = 1/2mv^2
-the velocity of the particle depends on its mass, as all particles have the same kinetic energy
Describe the process of acceleration
-the positive ions are attracted to a negatively charged plate
-this causes the ions to accelerate towards it
-all particles are given the same kinetic energy by the electric field
-this means lighter ions will accelerate faster than heavier ions
Describe the process of detection
-the positive ions strike a negatively charged plate after drifting through the tube
-the ions gain an electron at the plate, which generates a current
-this current is amplified to produce a signal on a computer
What does the velocity of a particle depend on?
It’s mass
-the electric field gives all of the particles the same kinetic energy
Describe the process of ion drift
-the ions leave the electric field at a constant speed
-they enter the flight tube and drift through at the same speed as they left the electric field
-the ‘time of flight’ for each particle is measured, which will depend on its velocity, and therefore its mass.
What does the size of the current tell us?
The size of the current is proportional to the abundance of a particular ion hitting the plate and gaining an electron. (Abundance is proportional to the current for a given isotope).
-It tells us the number of ions with a particular m/z ratio that have struck the plate
What does the abundance on a mass spectra tell us?
The quantity (%) of ions detected for each m/z value. The higher the abundance, the more of that particular ion was detected.
What does the m/z value on a mass spectra tell us?
The ratio of mass to charge, which is usually the Mr/Ar if the charge is +1.
Why do we get an M+1 peak on a mass spectra?
Due to the presence of carbon-13 isotope.
What does the molecular ion peak (M+ peak) tell us, when electron impact ionisation was used?
-it is the furthest peak to the right, with the heaviest m/z
-it tells us the Mr of the molecule
Why might there be multiple peaks on a mass spectra?
From other positive ions that have broken into fragments through fragmentation (shows that the ions were produced by electron impact ionisation).
Why would there only be one peak on a mass spectra?
The ions were produced b y electrospray ionisation, so no fragmentation occurred.
What does the molecular ion peak (M+ peak) tell us when electrospray ionisation was used?
It is representative of the ion MH+, as a proton was added at the start
-this means to find the Mr of the ion, we need to subtract 1, for the H+ proton added (atomic mass of a proton is 1)
-this gives us the Mr of the original molecule
What type of ionisation is used for elements?
Electron impact ionisation because they are unlikely to fragment
How can mass spectra be used to identify isotopes in an element?
Different isotopes have different masses so will show up as different peaks. The ratio of abundances of each isotope can be used to calculate the relative atomic mass of an element.
Why do diatomic molecules have more molecular ion peaks?
Because there are multiple combinations of isotopes that could be bonded together, so these molecules would therefore have different masses due to the presence of these different isotopes.
