Define Atomic number
The number of protons in the nucleus
Define Mass number
The total number of protons and neutrons in the nucleus of an atom
Define isotope
Atoms of the same element with the same number of protons but different number of neutrons
What does a TOF stand for and what is it
Time of flight mass spectrometer
A method of mass spectrometry where an ions mass-to-change ratio is determined by time of flight measurement.
Define orbital
The region of sub-shells where the probability of finding electrons is maximum
for example: S sub shell has 1 orbital which can contain 2 electrons.
P sub shell has 3 orbitals which can contain 6 electrons
d sub chill has 5 orbitals which can contain 10 electrons
What is the ionisation stage in TOF spectrometry
First stage of TOF spectrometry. The sample can be ionised by electron spray ionisation or electron impact ionisation
Describe Electron impact ionisation
The sample is vaporised and an electron gun fires high energy electrons at it, causing the outer electron to be knocked off each particle to produce 1+ ions
What is the ion equation for electron impact ionisation?
Ti (g) —–> Ti+ (g) + e–
Disadvantage of electron impact
Electron impact is used for elements
and substances with low formula mass.
Electron impact can cause larger organic
molecules to fragment.
What is the ion equation for electrospray ionisation?
M (g) + H+ ——> MH+ (g)
Describe Electrospray ionisation
The sample is dissolved in a volatile, polar solvent and then injected through a fine hypodermic needle (giving a fine mist or aerosol) A high voltage is applied to the tip of the needle causing the particle to gain a proton, H+ from the solvent forming an ion.
The solvent evaporates away while the MH+ ions move towards a negative plate
What is the advantage of electro spray ionisation?
Electro spray ionisation is used
preferably for larger organic
molecules. The ‘softer’ conditions
of this technique mean
fragmentation does not occur.
What is the second stage in TOF spectrometry and describe it
Acceleration, Positive ions are accelerated by an electric field to constant kinetic energy. Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass.
Formula for K.E ?
K.E =1/2 m V^2
Units for kinetic energy formula
K.E is measured in Joules
mass is measured in kg
Velocity is measured in ms^ -1
What is the third stage in TOF spectrometry and describe it
Ion drift, The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move
faster.
The heavier particles take longer to move through the drift area.
The ions are distinguished by different flight times
Formula used in the ion drift and units?
t= d/V
time in secs
d in m
v in ms^ -1
What is the fourth stage in TOF spectrometry and describe it
Detection, The ions reach the negatively charged detector plate and generate a small current, which is fed to a computer for analysis.
The current is produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the
abundance of the species
For each isotope the mass spectrometer can
measure a m/z (mass/charge ratio) and an
abundance
What is the condition of TOF mass spectrometry
It needs to be under a vacuum
otherwise air particles would
ionise and register on the
detector
