Mass Spectroscopy Flashcards
What is the Mass Spectroscopy, and how does it work?
Atoms are too small to be weighed. The relative masses of atoms can be obtained by using a mass spectrometer.
In a mass spectrometer, particles are turned into positive ions and accelerated. The resulting detection of the ions depends on their ‘mass to charge’ ratio (m/z).
The results produce a mass spectrum which portrays the different ions in order of their m/z value.
Mass spectrometry was initially used to show the identity of isotopes.
It is now used to calculate molecular masses and characterise new compounds.
Describe Stage1 Ionistation, ELECTRON BOMBARDMENT
The high energy electrons come from an ‘electron gun’ which is a hot wire filament with a current running through it that emits electrons. This usually knocks off one electron from each particle forming a 1+ ion.
Describo Stage 1 Ionisation, ELECTROSPRAY.
Sample is dissolved in a polar, volatile solvent to ensure evaporation and is then is pumped into a capillary tube. The particles are ionised by gaining a proton (ie an H+ ion which is simply one proton) from the solvent as they leave the needle producing XH+ ions (ions with a single positive charge and a mass of Mr + 1).
What are the equations for stage 1 ionisation, ELECTRON BOMBARDMENT?
X(g) + e– –> X+(g) + 2e–
OR
X(g) —> X+(g) + e–
What is the equation for stage 1 ionisation, ELECTROSPRAY?
X(g) + H+ —> XH+(g)
Describe Stage 2: Acceleration.
An electric field is applied to give all ions of the same charge the same amount of kinetic energy (KE).
KE = ½ mv2 .
The ions then enter the drift tube.
Describe Stage 3: Ion Drift.
If ions have the same KE, their velocity will depend on their mass.
The drift tube is field free, so ions are separated based on their different velocities.
Describe Stage 4: Detection
The time of flight is recorded as a current is produced by the positive ions hitting the detector.
