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
What is an electron gun?
hot wire filament with a current running through it that emits beam of high energy electrons
What does the electron gun usually do?
knocks off one electron from each particle forming a 1+ ion:
X(g) + e^- arrow X^+(g) + 2e^-
What is the acceleration process?
positive ions are attracted towards a negatively charged plate and accelerate towards it
What ions achieve a higher speed?
lighter ions and more highly charged ions
What is the ion drift?
ions pass through a hole in the negatively charged plate, forming a beam and travel along a tube (flight tube), to a detector
What happens during detection?
ions with the same charge arrive at the detector, lighter ones are first as they have higher velocities. flight times are recorded. positive ions pick up an electron from the detector, which causes a current to flow
What happens during data analysis?
signal from the detector is passed to a computer which generates a mass spectrum
What can the mass spectrometer be used to do?
identify the different isotopes that make up an element
How are different isotopes detected by the mass spectrometer?
it detects individual ions, so different isotopes are detected separately as they have different masses
What does the peak height give?
the relative abundance of each isotope
To what decimal place can mass spectrometers measure relative atomic masses?
to five decimal places of an atomic mass unit
What is the name of the mass spectrometer that can measure relative atomic masses to five decimal places
high resolution mass spectrometry
To what decimal point is most work done to?
one decimal point
What is work done to one decimal place called?
low resolution mass spectrometry
How may isotopes does chlorine have?
two
What are the isotopes of chlorine?
35Cl and 37Cl
What ratio do the isotopes of chlorine occur in?
3:1
35Cl 35Cl 35Cl 37Cl
What is the percentage of the chlorine isotopes in naturally occurring chlorine gas?
75% 35Cl and 25% 37Cl
Example of space probe that carries mass spectrometers
Mars Rover Curiosity
What are the mass spectrometers in space probes used for?
used to identify the elements in rock samples
What did the mass spectrometer that landed on Titan used for?
identify and measure the amounts of the gases in Titan’s atmosphere
after landing it also analysed vaporised samples of the surface
