1.1 atomic structure- mass spec Flashcards
what is the order of the 6 stages for electron impact
1.vaporisation
2.ionisation
3.acceleration
4.ion drift
5.detection
6.data analysis
what is vaporisation stage
sample is turned into a gas
what is ionisation stage
high energy electrons from an electron gun knock off an electron from the atoms
X(g)–>X⁺(g) + e⁻
what is acceleration stage (2)
-a negative plate attracts positive ions to give all molecules the same kinetic energy
-speed is dependent on mass of molecule- lighter ions move faster
what is ion drift stage (2)
ions travel along flight tube
-larger m/z ions take longer to reach detector
what is detection stage (3 + equation)
positive ion hits negative plate
causes a current to flow
gives abundance of molecule
X⁺ + e⁻–> X
what is data analysis stage (2)
-size of current is proportional to abundance
-time of flight is converted to m/z
why is the interior of the spectrometer a vacuum
prevents ions colliding with air molecules
what is mass spectrometry and what is it used to find (5)
its an analytical technique
-find relative isotopic mass
-relative isotopic abundance
-identifying elements
-relative molecular mass
why is the relative atomic mass of (copper) not a whole number
a sample of copper is a mixture of 2 isotopes with different abundances
the average mass of these isotopes isn’t a whole number
why do samples need to be positively charged
so they can be accelerated and detected
explain how TOF spectrometry distinguishes between ions with different masses
positive ions are accelerated by an electric field to a constant kinetic energy. Faster ions arrive at the detector first
relative isotopic mass definition
mass of an atom of an isotope of an element/ a twelfth the mass of an atom of carbon-12
rearrange kinetic energy equation to get velocity=
v=sqrt(2KE/m)
rearrange kinetic energy and velocity equation to get time=
t=d x sqrt(m/2ke)
how does ionisation occur in electrospray
sample is dissolved in a volatile solvent and injected through a fine needle at high voltage
each molecule gains a proton
equation for electrospray ionisation
X(g) + H+ -> XH+(g)
what type of substances is electrospray used for (3)
molecules with higher Mr
biological molecules
it is a soft ionisation technique
what type of substances is electron impact used for and why (2)
elements or molecules with low Mr
fragmentation of molecules can occur
relative atomic mass definition
average mass of every atom in every element/mass of one C-12 atom
how is current generated
electron transferred from detector to +1 ion
