Atomic Structure- Mass Spectrometry Flashcards
what is mass spectrometry used for
-finding abundance and mass of isotope
-finding relative molecular mass
what happens in time of flight/ acceleration
ions are accelerated by electric field so they all have the same kinetic energy
what are the 2 types of ionisation
electron impact and electrospray ionisation
what happens in electron impact
-sample is vaporised
-electron gun fires high energy electrons at it
-electron gun is hot wire filament w/ current
-this knocks off 1 electron forming a 1+ion
-the 1+ ion are attracted towards negative electric plate
equation for electron impact
X(g)—> X+(g) + e-
what happens electrospray ionisation?
-sample is dissolved in volatile solvent
-injected through fine hypodermic needle to give fine mist (aerosol)
-tip of needle has high-voltage power supply
-particles are ionised by gaining a proton (H+ ion) from solvent and produce XH+ ions
-solvent evaporates while XH+ ions attracted to negative charge plate
equation for electrospray ionisation
X(g)+ H+ —> XH+ (g)
how is a h+ ion formed in electrospray ionisation
H has 1 electron and 1 proton. Ionisation looses an electron, leaving a positive charged proton
when is electrospray ionisation used
-for substances with high molecular mass including biological molecules eg. proteins
-fragmentation rarely takes place
when is electron impact used
-for elements and substances with low formula mass eg organic/inorganic molecules
when molecules are ionised in electron impact, the 1+ion formed is known as
a molecular ion
equation for kinetic energy
KE= 1/2 x m x v²
equation for velocity
v= √ (2KE / m)
lighter particles will have ____ velocity
heavier particles will have ____velocity
faster
slower
what happens in stage 2 acceleration
positive ions are attracted to negative charged plate
what happens in stage 3 flight tube
-positive ions travel thru a hole in negatively charged plate
-ToF depend on velocity and mass
equation for time of flight
t= d/v
rearrange time of flight equation with velocity
t= d √(m/2KE)
what happens in stage 4 detection
-positive ions hit negatively charged plate
-positive ions get discharged by gaining electrons from plate
-this generates movement, causing an electric current to be measured
-size of current gives measure of no. of ions hitting plate
what does the mass spectrum show
-mass to charge ratio (mass) (m/z)
-abundance of ion
equation for relative atomic mass *******
(abundance 1x mass 1) + (abundance 2x mass 2) / mass 1+ mass2
what does high m/z value in electron impact graph mean
means its from the molecular ion and gives relative molecular mass
why are there small peaks in m/z graphs
molecular ions contain different isotopes
why is a negative charged plate used
for positive ions to attract to negative plate
