mass spectrometer

what is a mass spectrometer

it is an instrument for the accurate determination of relative atomic masses and the abundance and masses of each isotope in an element

what are the different stages in a time of flight mass spectrometer

vacuum, ionisation, acceleration, ion drift, detection, data analysis

what happens in a time of flight mass spectrometer

the substance(s) in the sample are converted to positive ions, accelerated to high speeds and arrive at a detector

what happens in the vacuum stage

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

what are the two ionisation stages

electron impact and electrospray ionisation

what happens in electron impact ionisation

the sample is vapourised and then high energy electrons are fired at it from an electron gun. this knocks off one electron from each particle forming a 1+ ion

what is the electron gun which is used in electron impact ionisation

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

what is electron impact ionisation used for

it is used for substances with a low Mr

what is the electrospray ionisation stage

the sample is dissolved in a volatile solvent and it is injected through a fine hollow needle, the tip of the needle is attached to the positive terminal of a high-voltage power supply

what is produced in electrospray ionisation

tiny positively charged droplets that have gained a proton from the solvent and become ions as a fine mist and the solvent evaporates leaving 1+ ions.

what is electrospray ionisation used for

used for elements and compounds with a high Mr

what happens to the molecular ions in electron impact ionisation

the molecular ion breaks down into smaller fragments and some of them are detected in the mass spectrum

what happens in the Accleration stage

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

in the acceleration stage, which particles are faster

lighter particles have a faster velocity and heavier particles have a slower velocity

what happens in the ion drift stage

the ions leave the electric field through a hole, forming a beam and travel along the tube to the detector

what happens in the detection stage

the detector is a negatively charged plate, when the positive ions hit the detector plate

how can the mass be calculated from the detection stage

when the detector records how long it took to pass through the spectrometer

what does the size of the current give

it gives a measure of the number of ions hitting the plate

advantage of mass spectrometry

highly accurate, minimal sample destruction, relatively quick results, can analyse small samples

disadvantages of mass spectrometry

expensive, specialist equipment required