mass number and isotopes

mass number

number of protons and neutrons

atomic number

number of protons

how to calculate number of neutrons

mass number - atomic number

what is an isotope

an atom of the same element with the same number of protons but a different number of neutrons

why do isotopes have similar chemical properties

they have the same electronic structure

two uses of the time of flight mass spectrometer

find the abundance and mass of each isotope in an element

find the relative molecular mass of substances made of molecules

what is stage 1 of TOF

ionisation

what are the two techniques for ionisation

electron impact

electrospray

what happens in electron impact

the sample being analysed is vaporised and then high energy electrons are fired at it from an electron gun

this knocked off one electron from each particle forming a +1 ion

the +1 ions are then attracted to a negative plate where they are accelerated

what is an electron gun

a hot wire filament with a current running through it that emits electrons

equation for electron impact

X(g) + e^- → X⁺(g) + 2e^-

what elements and substances is electron impact used for

those with low formula mass

what happens in electrospray ionisation

the sample is dissolved in a volatile solvent and injected through a fine hypodermic needle to give a find mist

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

the particles are ionised by gaining a proton from the solvent when they leave the needle

this produces XH+ ions

the solvent evaporates away and the XH+ ions are attracted to a negative plate where they are accelerated

what substances is electrospray ionisation used for

those with higher molecular mass

equation for electrospray ionisation

X(g) + H⁺ → XH⁺

what is stage 2 of TOF

acceleration

what happens during acceleration

the positive ions are accelerated using an electric field so that they all have the same kinetic energy

what is the formula for kinetic energy

KE = ½ x mv²

units for kinetic energy of the particle

joules

units for mass of particle

kg

units for velocity of the particle

m/s

formula for velocity

v = square root (2KE/m)

relationship between velocity and mass

heavier = slower

lighter = faster

what is stage 3 of TOF

flight tube

what happens in the flight tube

the positive ions travel through a hole in the negatively charged plate into a tube

the time of flight of the particle through the tube depends on the velocity which depends on the mass

formula for the relationship of time , distance and velocity

t = d/v

formula for time combined with formula for velocity

t = d x square root (m/2KE)

relationship between time of flight and mass of ion

time of flight is proportional to the square root of the mass of the ions

what is stage 4 of TOF

detection

what happens in detection

the positive ions hit a negatively charged electric plate

when they hit the plae the positive ions are discharged by gaining electrons from the plate

this generates a movement of electrons hence an electric current that is measured

the size of the current gives a measure of the number of ions hitting the plate

relationship between current and abundance

the greater the current the greater the abundancy of that ion

what does the mass spectrum show

m/z ratio and abundance of each ion that reaches the detector

what is the m/z ratio equivalent to

the mass of the ion

how can you calculate relative atomic mass (Ar)

m/z x abundance of each isotope / combined abundance of all isotopes

how can you know the Mr of a molecule that underwent electron impact

the peak with the largest m/z

how can you know the Mr of a molecule that underwent electrospray

the peak with the largest m/z - 1