Mass Spectrometer

What is a mass spectrometer used for

• mass spectrometer is the most useful instrument for the accurate determination of relative atomic masses (Ar)

• determines amss of seperate atoms or molecules

• relative atomic mass are measured on a scale on which mass of one atom of 12C is defined as exactly 12

• no other isotpoe has RAM that is exactly a whole number bc proton and neutron both have mass of exactly 1

RAM formula/defintion

ar= average mass of 1 atom/ 1/12 mass of 1 atom of 12C

Mr defintion/formula

average mass of molecule / 1/12 mass of 1 atom of 12C

What is a TOF and what does it do

Are there different types of mass spectroemetr

several types of mass spectrum

What principle do all mass spectrum function from

• all work on principle of formign ions from sample and then seperating ions according to ratio of charge to their mass- so TOF works on this principle

Outline basic steps of TOF

• ionisation- substance(s) are converted to positive ions

• accelerated- ions accelerated to high speeds (

• drift

• detection-arrive at detector:

What is the TOF apparatus kept in

• whole apparatus is kept under a high vaccum to prevent ions that are produced colliding with molecules from the air

What are the 2 types of ionisation

• electron impact ionisation

• electrospray ionisation

What happens in electron impact ionisation(heavier elements)

• smaple is vaporised

• high energy electrons from an electron gun is fired at sample

• knocks off an electron from each particle forming a +1 ion

formula: X(g) + e- —→ x+(attracted to neg plate) + 2e-

What happens in electrospray ionisation(lighter elements)

• sample dissolved in a volatile solvent

• and injected into a fine hollow needle → a fine mist is formed

• tip of needle is attached to a positive terminal of a high power voltage supply

• produces tiny psotively charged droplets that have gained a proton from the solvent

• particles are then able to be ionised from solvent solution as it leaves the needle- solvent evaporates from the droplets into vaccum and droplets get smaller until they may contain no more than a single positively charged ion

X + H+(g) —→ XH+(g) ( positive ion which will be attracted towards the negative plate)

What species are electrospray ionisation used for

• used for speces w high molecular mass ( i/e proteins) as soft ionisation techniques prevent fragmentation

• peak that corresponds to the ion = mr of the sample +1

What happens in acceleration

• positive ions are attracted towards a negatively charged plates and accelerate towards it

• positive ions that are accelerated by an electric field

• this rnables ions with the same charge to have the same KE(kinetic energy)

KE=1/2 mv2

-velocity depends on mass(lighter particles , faster velocity)

-amount of acceleration and hence final velocity dpend son mass:charge ratio

• lighter ions and more highly charged ions acheive a higher speed

What happens in ion drift

• ions drift through a hole through negatively charged plates

• no electric feild

• ions form a beam and travel along tube called flight tube to a detector

• KE of ions remain constant ( vaccum) → hence velocity remains constant

• velocity depends on mass : charge ratio

• TOF of each particle through flight tube depends on velocity , therefore mass

time is directly proprtional to mass sqaure rooted

What happens in detection

• positive ions hit a negatively charged plate- arrive at the detector

• lighter ones are first because they have higher velocities, flight times are recorded

• positive ions gain an electron from the detector forming particles ( atom, fragment, molecule in a solvent)

• generates a movement of electron and hence a current flows

size of current is directly proprtional to abundance of ion

What happens in data anylisis

• signal from detector is passed to a computer which generates a mass spectrum

What else can the mass spectrometer be used for

• can be used to identify different isotopes that make up an element

• detects individual ions so diff isotopes are detected seperately b/c they have diff masses

What happens in low resolution mass spectrometry

What are the 2 type of mass spectromety

• high resolution mass spectrometry

• low resolution mass spectrometry

What is high resolution mass spectrometry

• mass spectrometer measures relative atomic mass to 5 decimal places of an atomic mass unit

What is low resolution mass spectrometry

• measuring relatomic amss to 1 decimal place of a unit

calculate atomic mass from abundance- isotopes

(mass number x abundance) + (mass number x abundance)/100