Mass Spectrometry Flashcards
What is mass spectrometry
Analytical technique used to identify different isotopes and their abundance
How does time of flight method work
Records time taken for ions of each isotope to reach detector, those with higher m/z are slower than those with lower
What happens during ionisation
Sample of element vapourised and injected into mass spectrometer in ionisation chamber where high speed electrons are fired at the sample causing electrons to be removed from the atoms forming 1+ ions
What happens during acceleration
Positively charged ions are accelerated towards a negatively charged detection plate by electric field
What happens during ion drift
Ions deflected by magnetic field (produced by electromagnet) into curved path depending on m/z
What happens during detection
When the positive ions hit the negatively charged detection plate they gain an electron and produce a flow of charge, the greater the current produced the greater the abundance of that ion
What is produced by mass spectrometry
A spectra with relative abundance of each isotope on y axis and m/z on x axis
Order of stability of ions
Tertiary>secondary>primary
What is a molecular/parent ion
The positive ion formed when a molecule loses one electron, species is radical as one electron has been lost from a pair in the molecule leaving one unpaired electron
What peak is the mass of the molecular ion
The peak with the largest m/z value
Advantages of instrumental analysis over chemical analysis
Quicker and more accurate, does not destroy the sample by reacting it and can use very small sample so very sensitive
Advantages of chemical analysis over instrumental analysis
Cheaper to purchase instruments and instrumental analysis graphs sometimes need expert to interpret
Why is the inside of a mass spectrometer a vacuum
To prevent atoms in air from interfering with the ionised particles
What is fragmentation
Process in which a molecular ion breaks into smaller ions/radicals/neutral molecules
Where do molecules break up more readily
At weak bonds or at bonds which give rise to more stable fragments
Example of fragmentation of ethanol
CH3CH2OH ——> [CH2OH•] + [CH3]+
Or [CH2OH]+ + [CH3•]
What is base peak in mass spectrum
The tallest peak in a mass spectrum due to the ion with the greatest relative abundance
What is M+1 ion
Ion with mass to charge ratio one higher than parent ion, due to 13C isotope present in organic compounds
Typical fragments and their formulae
-15=CH3+
-29=CH3CH2+
-43=CH3CH2CH2+
-57=CH3CH2CH2CH2+
Major peak you would find in mass spectrum for propanal but not propanone
29 (CH3CH2)+ as carboxyl group at end of chain in propanal
