Organic Analysis Flashcards
(35 cards)
What is mass spectrometry used for?
To find the abundance and mass of each isotope in an element so we can find the relative atomic mass.
To find the relative molecular mass of substances made of molecules
What are the stages of ToF spectrometry?
Ionisation, acceleration, flight tube and detection
What are the two forms of ionisation?
Electron impact and electrospray ionisation
Explain how electron impact ionisation works
Sample is vapourised and high energy electrons are fired at it from an electron gun which knocks off an electron to form a +1 ion
What is an electron gun?
A hot wire filament with a current running through it which emits electrons
Write an equation for electron ionisation of a sample
X + e- —–> X+ +2e-
What kind of samples is electron impact used for?
Those with low formula mass
Explain how electrospray ionisation is used
Sample dissolved in a volatile solvent (water/methanol) and injected through a fine hypodermic needle to give a fine mist. The tip of the needle is connected to the +ve terminal so the sample gains a proton as they leave the needle
Write an equation for electrospray ionisation of a sample
X + H+ —-> XH+
Why is electrospray ionisation known as soft ionisation?
Fragmentation rarely takes place
Why are +ve ions accelerated using an electric field?
To ensure they all have the same kinetic energy
What does the velocity of each particle depend on?
Their mass
What is the trend between mass and velocity of particles?
Lighter particles have a faster velocity and heavier ones have a slower velocity
What happens in the flight tube?
+ve ions travel through a hole in the -vely charged plate into a tube. They set off at the same time but the lighter ions travel faster and start to separate out so they reach the detector first
What happens in detection?
+ve ions hit the -ve plate and they gain an electron to become neutral. movement of electrons creates current which can be measured to calculate how many ions hit the plate.
What are on the axis of the mass spectrum?
x axis - mass over charge
y axis - abundance
When an element goes through the mass spectrum why are there lots of lines?
Due to isotopes of the element
When an organic compound goes through the mass spectrum why are there lots of lines?
Due to fragment ions and the original molecule
What is the molecular ion?
The line with the largest m/z value which has been produced by the molecule which has only lost one electron
What is the cause of more than one molecular ion peak?
When there are isotopes in the molecule that affect the mass
Why is high resolution mass spectrometry used?
To distinguish between compounds which have the same Mr to one dp by using 4dp
Why is the Ar of C12 exactly 12.0000?
Because the relative atomic masses of all other elements are measured relative to the C12 isotope.
What is the natural frequency of vibration of the bond?
The frequency that a covalent bond naturally vibrates at that is within the infrared region of the EM spectrum
How does infrared radiation?
IR radiation shone onto an organic compound. The bond only absorbs radiation that has the same frequency as the natural frequency of the bond
