1.4 - THE MASS SPECTROMETER Flashcards
What is the mass spectrometer the most useful instrument for?
for the accurate determination of relative atomic masses
Formula for relative atomic mass?
average mass of 1 atom ---------------------------------- 1 mass of 1 atom of 12C --- 12
Formula for relative molecular mass?
average mass of molecule -------------------------------------- 1 mass of 1 atom of 12C --- 12
What does the mass spectrometer determine?
mass of separate atoms
Example of use of mass spectrometers?
used by forensic scientists to help identify substances such as illegal drugs
On what principles do mass spectrometers work?
work on the principle of forming ions from the sample and then separating the ions according to the ratio of their charges to their mass
What type of instrument is a mass spectrometer described to be?
time of flight (TOF) instrument
What happens in a time of flight mass spectrometer? (3)
substance(s) in sample are converted to positive ions
accelerated to high speeds (which depend on their mass to charge ratio)
arrive at detector
Where is the apparatus held under?
under a high vacuum
Why is the apparatus held under a high vacuum?
to prevent ions that are produced colliding with molecules from the air
What are the two types of ionisation used in this process?
electrospray ionisation
electron impact
What happens in electrospray ionisation?
sample is dissolved in a volatile solvent and forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply
What does the electrospray ionisation produce?
tiny positively charged droplets that have gained a proton from the solvent
What happens to the solvent? (electrospray ionisation)
evaporates from the droplets into the vacuum and the droplets get smaller and smaller until they may contain no more than a single positively charged ion
What happens in electron impact?
sample is vaporised and high energy electrons are fired at it from an electron gun