mass spectroscopy Flashcards
owhy are mass numbers often seen as decimals?
due to isotopes
why do all isotopes of an element react in a similar way?
electron configuration
what does abundance tell us?
abundance tells us how common an isotope is, for example there could be 69% of copper 63 but 31% of copper 65 atoms.
why is mass spectrometry a powerful analytical technique + useful?
it is the most useful instrument (machine) for accurate determination of the relative atomic mass of an element or molecules even, based on the abundance and mass of each of its isotopes
therefore
briefly explain how mass spectrometry works?
we take a sample of the element we are interested in and place it into a sample chamber where it gets ionised and passes through the mass spectrometer through acceleration though
a spectrum is produced of mass / charge ratio against abundance
what is this spectrum useful in doing?
it ca be used to find the relative isotopic abundance and its atomic and molecular mass therefore
what is the peak with the highest mass called
the molecular ion peak (M+)
what is the peak with the largest abundance called
the base peak
briefly explain time of flight mass spectrometry?
common - all particles of the sample to be analysed are ionised and are forming 1+ ions
these 1+ ions are then accelerated to high speeds, deflected through the spectrometer and then arrive at the detector
(as they hit the detector, the mass spectrum graph is produced)
why is the whole of the apparatus kept under a high vaccum?
to prevent any ions produced from colliding with the molecules in the air therefore
what are the key stages in time of flight mass spectroscopy?
- ionisation
- acceleration
- ion drift, detection
explain what is involved in the ionisation process
all of the atoms are converted into positive ions and the two key methods/ways in which the sample could be ionised are through Electron Impact or electrospray ionisation
explain what happens in electron impact ionisation
this process is used for elements and substances which have a lower molecular mass
the sample is vaporised + bombarded with high energy electrons (the electrons are fired from an electron gun)
- the electron gun is a hot wire filament which emits electrons as a current runs through it
as the sample is bombarded by these electrons, an electron is knocked off each particle, forming a 1+ ion
X(g) -> X+(g) + e-
what are the 1+ formed from electron impact ionisation called?
molecular ions (M+ ions)
they are attracted to the negatively charged plate. this accelerates them through the mass spectrometer where they increase the kinetic energy of the ions
- all of the ions with the same charge will have the same kinetic energy and the molecular ion can be broken further and this is known as fragmentation
what happens during the fragmentation process
the fragments are accelerated through the sample and hit the detector, causing different peaks to show on the mass spectrum produced
explain what happens during electrospray ionisation
it is used for substances that have a higher molecular mass
fragmentation is unlikely to happen therefore electrospray ionisation is called a ‘soft ionisation technique’
the solvent is INJECTED in the mass spectrometer using a hypodermic needle and produces a fine mist/aerosol
the needle is attached to a high voltage power supply, so as the sample is injected, the particles are ionised by gaining a proton from the solvent
X(g) + H+ -> XH+(g)
the solvent EVAPORATES and the XH+ ions are attracted towards a negatively charged plate which accelerates the ions through the mass spectrometer
explain what happens during the acceleration process
the 1+ ions formed from ionisation are accelerated using an electric field.
this electric field increases the kinetic energy of the ions; all ions which have the same charge will have the same kinetic energy
however since 1+ ions got the same kinetic energy, their velocity will depend on their mass. lighter ions will move faster and heavier ions will move slower
therefore
explain what happens during the acceleration process
the 1+ ions formed from ionisation are accelerated using an electric field.
this electric field increases the kinetic energy of the ions; all ions which have the same charge will have the same kinetic energy
however since 1+ ions got the same kinetic energy, their velocity will depend on their mass. lighter ions will move faster and heavier ions will move slower
therefore
explain what happens during the ion drift process
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once the 1+ ions go through the negatively charged plate they begin to stop accelerating and drift down the chamber into a flight tube
(this is where the term ‘time of flight’ derives from - the time of flight of each 1+ ion in this tube depends on their velocity with the lighter ions moving faster than the heavier ions
explain what happens during the detection process
once they pass through the mass spectrometer, the 1+ ions will hit a negatively charged detector plate and gain an electron following this.
the gaining of an electron discharges the ion and causes a current to be produced.
if we have different isotopes moving down the drift chamber then ions of the lighter isotope will have greater velocity and will reach the detector first
the time taken to move down the drift chamber is used by the machine to determine the mass of an isotope
the size of the current produced when each isotope hits the detector is used to determine the abundance of each isotope; a more abundant isotope will produce a greater current than a less abundant isotope
state the relationship between the size of the current and abundance
the size of the current is proportional to the abundance of the ions hitting the plate and gaining an electron as a result.
what is the detector plate connected to
a computer which produces the mass spectrum
state the equation for calculating the kinetic energy
KE = 1/2 mv^2
KE = kinetic energy in J
m = mass in kg
v = velocity in ms-1
to calculate the velocity, state the equation
v = √2KE/m
to calculate the time, state the equation
t = d√2KE/m
because the mass of the ion is in g, how do you calculate its mass in kg?
mass of ion x 10^-3/6.02 x 10^23
when interpreting a time of flight graph, what do the peaks represent?
the amount of possible isotopes an element has
what does the y axis of the graph show us?
the relative abundance for the two isotopes which is given as a percentage of the totals and often shown at the top of each peaks.
what does the x axis show us?
we have the m/z ratio which is the ratio of the mass of each ion to its charge
almost all of the ions have a single positive charge so we can think of the m/z ratio as simply the relative mass of the ion.
