Time of Flight Mass Spectrometry

Question 1

what is a mass spectrometer?

Answer 1

an instrument which is used to measure the relative mass of isotopes

Question 2

once the sample is in the mass spectrometer (in a vacuum) it undergoes 5 seperate stages

what are these stages?

Answer 2

1) ionisation
2) acceleration
3) ion drift
4) ion detection
5) data analysis

Question 3

why is a vacuum required in a mass spectrometer?

Answer 3

to prevent the ions colliding with molecules in the air

Question 4

what are the 2 types of ionisation called?

Answer 4

Electrospray Ionisation (high voltage)

Electron Impact

Question 5

What are the stages of electrospray ionisation?

Answer 5

1) dissolve the sample in a volatile, polar solvent

2) inject the sample through a needle at a high voltage

3) each particle in the sample gains a proton (H+) to form +1 ions

Question 6

what are the stages of electron impact ionisation ?

Answer 6

1) high energy electrons are fired from an electron gun

2) these cause an electron to be knocked of from each particle

3) to form 1+ ions

Question 7

what is stage 2 of mass spectrometry and describe it

Answer 7

it is acceleration

the positive ions are then accelerated by an electric field at a constant kinetic energy

the velocity of the ion depends on the mass of the ion

Question 8

what is stage 3 of mass spectrometry and describe it

Answer 8

it is ion drift

the ions are allowed to drift into a flight tube of a known distance and the time it takes for the ions to hit the detector is recorded

the time of flight depends on the velocity which depends on the mass of the ion

Question 9

what is stage 4 of mass spectrometry ?

Answer 9

it is ion detection

when positive ions hit the detector (a negatively charged plate) they pick up an electron which causes a current to flow

the greater the abundance of positive ions hitting the detecter , the bigger the current produced

abundance is directly proportional to current

Question 10

what is stage 5 of mass spectrometry

Answer 10

it is data analysis

the detector is linked to a amplifier which is linked to a recorder which converts the current a peak that is shown on a mass spectrum

the height of the peak is proportional to the current which is therefore proportional to the abundance of that ion

the peaks are displayed on a plot of ABUNDANCE v MASS/CHARGE (m/s)

as the ions produced are usually +1 the mass is the same as the m/z

Question 11

peaks in a mass spectrum tells us what?

Answer 11

how many isotopes an element has

Question 12

what is the x and y axis in a mass spectrometer

Answer 12

x axis is the m/z ratio (mass of each ion to its charge)

y axis is the relative abundance %

Question 13

what is relative isotopic mass

and give 3 points

Answer 13

the mass of an atom of an isotope compared with 1/12th of the mass of carbon-12

1) there is one isotopic mass for each isotope of an element

2) relative isotopic mass is ALWAYS a whole number

3) relative isotopic mass has no units

Question 14

what is electron impact ionisation (fragmentation)

Answer 14

covalently bonded molecules can have their bonds broken during electron impact ionisation and break into fragments (fragmentation)

Question 15

does fragmentation occur to all molecules (electron impact ionisation)

Answer 15

no some molecules stay intact and gain a positive charge

Question 16

what do the unfragmented molecules look like on a mass spectrum?

and what does the peak tell us

Answer 16

the unfragmented and positively charged molecules will have the peak with the highest m/z value which is called the MOLECULAR ION PEAK

the peak tells us the Mr of the molecule/sample

Question 17

explain electrospray ionisation (fragmentation)

Answer 17

very little fragmentation occurs with electrospray ionisation, so there are much less peaks in a mass spectrum

Question 18

how do we find the Mr of the sample using electrospray ionisation mass spectrum

Answer 18

a proton has a mass of 1 (H+).

we look at the major peak (biggest m/z value) and take away 1

Question 19

how to we find the mass of each 1+ particle in kg?

Answer 19

mass number / avogandros constant

➗

1000

Question 20

relative atomic mass calculation?
Ar

Answer 20

average mass of one atom of an element
➗
1/12th of the mass of one atom of carbon-12

Question 21

with both methods of fragmentation what will be there 1 greater the molecular ion peak?

Answer 21

a small peak after the molecular ion peak due to H1 or C13

Question 22

what does volatile mean

Answer 22

easily evaporates

Question 23

what’s the equation for electrospray ionisation

Answer 23

X(g) + H+ ——> XH+(g)

each particle gains a proton (H+) and atomic mass increases by 1

Question 24

what happens to the solvent whilst the XH+ ions are attracted towards a negative plate and are accelerated

Answer 24

the solvent evaporates away since its volatile

Question 25

what is equation for electron impact ionisation

Answer 25

X(g) —-> X+(g) + e-

Question 26

what the mass of an electron

Answer 26

1/1860