Chemistry - Unit 1 - Topic 1 - Time of Flight Mass Spectrometry

what is mass spectrometry used for? (elements)

to calculate the relative atomic mass of element

what is mass spectrometry used for? (compounds)

to identify unknown purified compounds by comparing the mass spectrum produced to another

what are the 6 stages of TOF mass spectrometry?

• vaporisation

• ionisation

• electric field

• ion drift

• detection

• data analysis and mass spectra production

describe the stage of vapourisation

samples of unknown substances are injected into a vacuum and vaporised 

what are the two types of ionisation?

• electrospray ionisation

• electron impact ionisation 

describe electrospray ionisation

• the sample is dissolved in a polar solvent, which is pushed through the sample at high pressure

• a high voltage is applied causing each sample to gain a H+ ion

• the solvent is removed

• this means that the Ars produced in the mass spectra will be 1 more than the true values 

describe electron impact ionisation

• electrons are fired at the sample

• this causes one electron to be knocked off per atom, causing ions to be formed with +1 ions

give an equation for electron impact ionisation including state symbols

A (g) + e- → A+ (g) + 2e-

when would electron impact ionisation be used?

for compounds that have a low risk of fragmentation - low formula mass

when would electrospray ionisation be used?

for compounds with a high risk of fragmentation - high formula mass - as it rarely causes fragmentation

describe the stage of acceleration

• an electric field is applied to the ions giving them all the same kinetic energy

• this causes them to accelerate

• heavier particles move more slowly than lighter particles

describe the stage of ion drift

• ions enter a region with no electric field called the flight tube

• ions are separated based on their different velocities (and thus masses)

• smaller, fast ions travel through the flight tube more rapidly and arrive at the detector first

describe the stage of ion detection

• ions collide with a negatively charged detection plate 

• they are given one electron by the plate, which causes a current to form

• theoretically all ions of the same atomic mass will collide with the plate at the same time and therefore cause a large spike in current 

describe the stage of data analysis

• the flight time at which peaks form corresponds to the Ar 

• a mass spectrum is produced, which is a plot of relative abundance against mass to charge ratio (m/z)

• current is proportional to relative abundance 

describe how to interpret a mass spectrum 

• number of peaks = number of isotopes or different particles (depending on what you are working out)

• the relative atomic mass is the mean mass of the isotopes (combined mass of all isotopes(x abundance)/combined abundance)

• m/z ratio is the same as Ar or Ir

what must you do with Mrs, Ars and Irs in mass spectra from electrospray ionisation?

subtract 1 as a proton is added to each ion

how would a mass spectrum for electron impact ionisation or a compound differ from a mass spectrum for electrospray ionisation?

• the compound may fragment, causing many different peaks and groups of peaks

• the largest peak is the base peak - most stable ion formed

• the last major peak (here 46) is the the Mr of the compound 

• no fragmentation occurs with electrospray

describe how to interpret the mass spectra of molecules, using chlorine as an example

• small molecules can form single ions with a positive charge, or can be broken up into atoms

• so for chlorine, there are 5 possible atoms which should be detected - 35Cl+; 37Cl+; (35Cl-35Cl)+; (35Cl-37Cl)+; (37C-37C)+

• so there will be peaks at 35 and 37 (ratio 3:1) as well as 70,72,74 (ratio 9:6:1 - multiplication of probability)