Mass Spectroscopy Flashcards
state 3 uses of the mass spectrometer
1) for decuding the Ar
2) for finding the Mr eg. a biological compound
3) for forensics analysis
state the order of mass spectroscopy
1) ionisation
2) acceleration
3) ion drift/ separation
4) detection
state the two types of ionisation and what they are used for
electron impact (used for elements and low Mr compounds)
electrospray ionisation (used for high Mr compounds eg. proteins)
describe electron impact ionisation (2 points)
used for elements and low Mr compounds
- high speed, high energy electrons are fired from an electron gun
- each one knocks off one electron from each atom/molecule, to form a +1 ion
give the general equation for electron impact ionisation
X(g) –> X+(g) + e-
describe electrospray ionisation (4 points)
used for high Mr compounds eg. proteins
- sample is dissolved in a polar solvent
- then pushed into a vacuum through a hypodermic needle at high pressure
- a high voltage is applied to the end of the needle where the spray emerges
- the particles gain a proton, H+, and ionised particles are separated from the solvent, leaving a gas of positive ions which are eventually attracted to the negative plate
give the general equation for elecrospray ionisation
X(g) + H+(g) –> XH+ (g)
describe acceleration (3 points)
- ions are accelerated using an electric field (-vely charged)
- all ions have the SAME KINETIC ENERGY, where Ek = 1/2mv^2
- those with a lower mass to charge ratio experience greater acceleration, whereas heavier ions have higher m/z and so experience less acceleration
describe ion drift/ separation (4 points)
- ions leave the electric field with a constant speed and Ek
- they enter the flight tube with length d
- as there is no more field to influence the ions speed, they drift through at the same speed they left with
- lighter ion have smaller mass to charge ratio, having experienced greater acceleration they drift quicker and take less time to reach the detector than heavier ions with larger m/z
describe detection (4 points)
- detector is a -vely charged plate
- a current is produced as ions hit the plate and the time taken to do so is recorded
- more ions = larger current produced
- the data is used to calculate m/z values needed for mass spec, and a computer converts the number of collisions into relative abundance/intensity
why is ionisation essential?
only charged particles will be accelerated by the field
label the axis on a mass spec graph
x axis = mass to charge ratio, m/z
y axis = relative abundance
define relative atomic mass, Ar
Ar = the average mass of 1 atom of an element / 1/12th mass of 1 atom of 12C
give the equation for finding relative atomic mass, Ar
Ar = (relative abundance1 x mass1) + (relative abunance2 x mass2) / sum of relative abundances etc.
how do you find the relative molecular mass, Mr using a mass spec graph
describe method for each type of ionisation
electron impact:
- the peak with the greatest m/z value is from the molecular ion
- so the m/z gives the Mr
- smaller peaks will appear around it due to isotopes, and lower ones due to fragmentation
electrospray:
- largest m/z peak is for XH+
- so this value -1 will give the Mr
