Time of Flight (TOF) Mass spectrometer

Question 1

what are the 4 stages in a TOF mass spectrometer

Answer 1

1. ionisation
2. acceleration
3. ion drift
4. detection

Question 2

what are the 2 types of ionisation

Answer 2

electrospray ionisation
electron impact ionisation

Question 3

describe electronspray ionisation

Answer 3

1. The sample is dissolved in a volatile solvent and injected through a fine needle giving a fine mist
2. A high voltage is applied to the tip of the needle
3. This causes each particles to gain a proton as it leaves the needle
4. The solvent evaporates

Question 4

describe electron impact ionisation

Answer 4

the sample is vaporised

an electron gun is used to fire high energy electrons at the sample

this knocks an outer electron off each particle so they become +1 positive ions

Question 5

describe what happens in acceleration

Answer 5

an electric field is applied to give all the ions with the same charge a constant kinetic energy

this electric field accelerates the ions

the velocity of each particle depends on its mass as they all have the same kinetic energy

Question 6

how does the mass of the particles affect its velocity

Answer 6

the lighter the particle, the faster it will move

Question 7

what 2 properties of the ion determines its path through the magnetic field

Answer 7

the mass of the ion
the charge of the ion

Question 8

kinetic energy =

Answer 8

1/2 x mass x velocity^2

Question 9

describe ion drift

Answer 9

-the ions enter a region with no electric field( the flight tube)

-the ions are now separated based on their different velocities

-smaller ions arrive at the detector first as they have more kinetic energy

Question 10

describe detection

Answer 10

when ions reach the detector, their flight times are recorded

the ions generate a small current which is fed to a computer for analysis

the current is produced by electrons transferring from the detector to positive ions

Question 11

what does a TOF mass spectrometer do

Answer 11

identifies different isotopes and determines the mass of separate atoms or molecules

Question 12

what are the uses of mass spectrometry

Answer 12

drug testing in sport
quality control in the pharmacuetical industry

Question 13

suggest a reason why all the peaks in a mass sprectrum are not all the same height

Answer 13

the height of the peak is directly proportional to the abundance of the isotope. One isotope may be more abundant than the other

Question 14

what does the x axis in a mass spectrum show

Answer 14

the mass to charge ratio (m/z)

Question 15

what does the y axis in a mass spectrum show

Answer 15

relative intensity/abundance

Question 16

what does a peak in the spectrum represent

Answer 16

a positively charged ion

Question 17

how can you tell which peak gives you the molecular mass of the compound

Answer 17

it is the last peak, NOT THE HIGHEST

Question 18

What does the tallest peak show you

Answer 18

the base peak represents the most stable fragment

Question 19

why is the mr of ethanol 46

Answer 19

the peak with the highest m/z is the molecular ion

Question 20

how do you calculate the molecular formula from the empirical formula

Answer 20

n x empirical formula = molecular mass (given)
n x (Mr of empirical formula) = molecular mass
xn = molecular mass
n = molecular mass / x

Question 21

why might there be 3 peaks in a mass spectrum

Answer 21

the element has 3 different isotopes

Question 22

why might the height of 2 peaks be the same

Answer 22

there are equal amounts of each isotope. They are equally abundant

Question 23

Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

Answer 23

(Ions will interact with and) be accelerated (by an electric field)

Ions create a current when hitting the detector OR ions create a current in the detector. Allow (ions) can be detected

Question 24

Describe how the molecules are ionised using electrospray ionisation.

Answer 24

(Sample is) dissolved (in a volatile solvent) Allow named solvent (eg water/methanol)

(Injected through) needle at high voltage

Each molecule/particle gains a proton

Question 25

Describe how ions are formed in a time of flight (TOF) mass spectrometer. (for electrospray ionisation)

Answer 25

A high voltage is applied to a sample in a polar solvent

the sample molecule, M, gains a proton forming MH+

Question 26

Describe how ions are formed in a time of flight (TOF) mass spectrometer. (for electron impact ionisation)

Answer 26

the sample is bombarded by high energy electrons

the sample molecule loses an electron forming M+

Question 27

A TOF mass spectrometer can be used to determine the relative molecular mass of molecular substances.
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer.

Answer 27

Ions, not molecules, will interact with and be accelerated by an
electric field

Only ions will create a current when hitting the detector

Question 28

Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.

Answer 28

Positive ions are accelerated by an electric field

To a constant kinetic energy

The positive ions with m / z of 104 have the same kinetic energy as
those with m / z of 118 and move faster

Therefore, ions with m / z of 104 arrive at the detector first

Question 29

what is electrospray ionisation used for

Answer 29

high Mr compounds such as proteins

Question 30

what is electron impact ionisation used for

Answer 30

elements and ions

Question 31

Give 2 reasons why you need to ionise the isotopes of an element before they can be analysed in a TOF mass spectrometer (2)

Answer 31

Ions will interact with and be accelerated by an electric field
Ions create a current when hitting the detector and so can be detected

Question 32

Describe how the molecules are ionised using electrospray ionisation (3)

Answer 32

Sample is dissolved in a volatile solvent
Injected through a needle at high voltage
Each molecule/particle gains a proton