mass spectroscopy

Question 1

owhy are mass numbers often seen as decimals?

Answer 1

due to isotopes

Question 2

why do all isotopes of an element react in a similar way?

Answer 2

electron configuration

Question 3

what does abundance tell us?

Answer 3

abundance tells us how common an isotope is, for example there could be 69% of copper 63 but 31% of copper 65 atoms.

Question 4

why is mass spectrometry a powerful analytical technique + useful?

Answer 4

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

Question 5

briefly explain how mass spectrometry works?

Answer 5

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

Question 6

what is this spectrum useful in doing?

Answer 6

it ca be used to find the relative isotopic abundance and its atomic and molecular mass therefore

Question 7

what is the peak with the highest mass called

Answer 7

the molecular ion peak (M+)

Question 8

what is the peak with the largest abundance called

Answer 8

the base peak

Question 9

briefly explain time of flight mass spectrometry?

Answer 9

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)

Question 10

why is the whole of the apparatus kept under a high vaccum?

Answer 10

to prevent any ions produced from colliding with the molecules in the air therefore

Question 11

what are the key stages in time of flight mass spectroscopy?

Answer 11

• ionisation
• acceleration
• ion drift, detection

Question 12

explain what is involved in the ionisation process

Answer 12

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

Question 13

explain what happens in electron impact ionisation

Answer 13

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-

Question 14

what are the 1+ formed from electron impact ionisation called?

Answer 14

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

Question 15

what happens during the fragmentation process

Answer 15

the fragments are accelerated through the sample and hit the detector, causing different peaks to show on the mass spectrum produced

Question 16

explain what happens during electrospray ionisation

Answer 16

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

Question 17

explain what happens during the acceleration process

Answer 17

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

Question 18

explain what happens during the acceleration process

Answer 18

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

Question 19

explain what happens during the ion drift process

Answer 19

nh^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^&&&&7 (my dog did this btw)
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

Question 20

explain what happens during the detection process

Answer 20

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

Question 21

state the relationship between the size of the current and abundance

Answer 21

the size of the current is proportional to the abundance of the ions hitting the plate and gaining an electron as a result.

Question 22

what is the detector plate connected to

Answer 22

a computer which produces the mass spectrum

Question 23

state the equation for calculating the kinetic energy

Answer 23

KE = 1/2 mv^2
KE = kinetic energy in J
m = mass in kg
v = velocity in ms-1

Question 24

to calculate the velocity, state the equation

Answer 24

v = √2KE/m

Question 25

to calculate the time, state the equation

Answer 25

t = d√2KE/m

Question 26

because the mass of the ion is in g, how do you calculate its mass in kg?

Answer 26

mass of ion x 10^-3/6.02 x 10^23

Question 27

when interpreting a time of flight graph, what do the peaks represent?

Answer 27

the amount of possible isotopes an element has

Question 28

what does the y axis of the graph show us?

Answer 28

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.

Question 29

what does the x axis show us?

Answer 29

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.