Time of Flight (TOF) Mass spectrometer Flashcards
what are the 4 stages in a TOF mass spectrometer
- ionisation
- acceleration
- ion drift
- detection
what are the 2 types of ionisation
electrospray ionisation
electron impact ionisation
describe electronspray ionisation
- The sample is dissolved in a volatile solvent and injected through a fine needle giving a fine mist
- A high voltage is applied to the tip of the needle
- This causes each particles to gain a proton as it leaves the needle
- The solvent evaporates
describe electron impact ionisation
the sample is vaporised
an electron gun is used to fire high energy electrons at the sample
this knocks an outer electron off each particle so they become +1 positive ions
describe what happens in acceleration
an electric field is applied to give all the ions with the same charge a constant kinetic energy
this electric field accelerates the ions
the velocity of each particle depends on its mass as they all have the same kinetic energy
how does the mass of the particles affect its velocity
the lighter the particle, the faster it will move
what 2 properties of the ion determines its path through the magnetic field
the mass of the ion
the charge of the ion
kinetic energy =
1/2 x mass x velocity^2
describe ion drift
-the ions enter a region with no electric field( the flight tube)
-the ions are now separated based on their different velocities
-smaller ions arrive at the detector first as they have more kinetic energy
describe detection
when ions reach the detector, their flight times are recorded
the ions generate a small current which is fed to a computer for analysis
the current is produced by electrons transferring from the detector to positive ions
what does a TOF mass spectrometer do
identifies different isotopes and determines the mass of separate atoms or molecules
what are the uses of mass spectrometry
drug testing in sport
quality control in the pharmacuetical industry
suggest a reason why all the peaks in a mass sprectrum are not all the same height
the height of the peak is directly proportional to the abundance of the isotope. One isotope may be more abundant than the other
what does the x axis in a mass spectrum show
the mass to charge ratio (m/z)
what does the y axis in a mass spectrum show
relative intensity/abundance
what does a peak in the spectrum represent
a positively charged ion
how can you tell which peak gives you the molecular mass of the compound
it is the last peak, NOT THE HIGHEST
What does the tallest peak show you
the base peak represents the most stable fragment
why is the mr of ethanol 46
the peak with the highest m/z is the molecular ion
how do you calculate the molecular formula from the empirical formula
n x empirical formula = molecular mass (given)
n x (Mr of empirical formula) = molecular mass
xn = molecular mass
n = molecular mass / x
why might there be 3 peaks in a mass spectrum
the element has 3 different isotopes
why might the height of 2 peaks be the same
there are equal amounts of each isotope. They are equally abundant
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.
(Ions will interact with and) be accelerated (by an electric field)
Ions create a current when hitting the detector OR ions create a current in the detector. Allow (ions) can be detected
Describe how the molecules are ionised using electrospray ionisation.
(Sample is) dissolved (in a volatile solvent) Allow named solvent (eg water/methanol)
(Injected through) needle at high voltage
Each molecule/particle gains a proton
Describe how ions are formed in a time of flight (TOF) mass spectrometer. (for electrospray ionisation)
A high voltage is applied to a sample in a polar solvent
the sample molecule, M, gains a proton forming MH+
Describe how ions are formed in a time of flight (TOF) mass spectrometer. (for electron impact ionisation)
the sample is bombarded by high energy electrons
the sample molecule loses an electron forming M+
A TOF mass spectrometer can be used to determine the relative molecular mass of molecular substances.
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer.
Ions, not molecules, will interact with and be accelerated by an
electric field
Only ions will create a current when hitting the detector
Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.
Positive ions are accelerated by an electric field
To a constant kinetic energy
The positive ions with m / z of 104 have the same kinetic energy as
those with m / z of 118 and move faster
Therefore, ions with m / z of 104 arrive at the detector first
what is electrospray ionisation used for
high Mr compounds such as proteins
what is electron impact ionisation used for
elements and ions
Give 2 reasons why you need to ionise the isotopes of an element before they can be analysed in a TOF mass spectrometer (2)
Ions will interact with and be accelerated by an electric field
Ions create a current when hitting the detector and so can be detected
Describe how the molecules are ionised using electrospray ionisation (3)
Sample is dissolved in a volatile solvent
Injected through a needle at high voltage
Each molecule/particle gains a proton
