ISOTOPES AND MASS SPECTOMETRY

Question 1

Isotopes

Answer 1

isotopes of an element are atoms with same number of protons but different number of neutrons.

Question 2

relative isotopic mass

Answer 2

the mass of an isotope measured in atomic mass units or AMU is called relative isotopic mass.

Question 3

mass spectometry

Answer 3

mass spectometry is an analytical technique used to identify different isotopes and find overall relative atomic mass of an element.

Question 4

principle of mass spectometry

Answer 4

an ionised sample is accelerated through the spectometer, producing a mass spectrum which are being separated based on the ratio of their charge to mass.

Question 5

molecular ion peak

Answer 5

the peak with the highest mass is called molecular ion peak.

Question 6

base peak

Answer 6

the peak with the largest abundance or the tallest peak is called the base peak.

Question 7

time of flight mass spectometry

Answer 7

this is a common form of mass spectometry where all particles of the sampleto be analysed are ionised to 1+ ions.

Question 8

why is the whole apparatus kept under vaccum

Answer 8

the whole apparatus is kept under vaccum to prevent any ions that are produced from colliding with molecules in the air.

Question 9

stages in tof spectometry

Answer 9

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

Question 10

ionisation

Answer 10

2 ways in which the sample can be ionised.
1. electron impact or electron ionisation
2. electron spray ionisation.

Question 11

electron impact

Answer 11

used for elements or substances with a lower molecular mass.
- the sample is vaporised and bombarded with high energy electrons. The electrons are fired from an electron gun. an electron gun is just a hot wire filament firing electrons when current is passed through it.
- as the sample is bombarded with high energy electrons, an electron is knocked off each particle, forming a 1+ ion.
- these 1+ ions formed are called molecular ions or M+ ions.
- these positive ions are attracted to negative charge which makes it drifted through the whole spectrometer. these molecular ions can be fragmented.

Question 12

electronspray ionisation

Answer 12

for the particles with a higher molecular mass
- unlike electron impact, fragmentation doesnt happen. also called as soft ionisation technique.
- sample is dissolved in a volatile solvent and then injected into the spectometer using a hypodermic needle which then forms aerosol or mist.
- the needle is attatched to a high voltage power supply so whenever the sample is injected, the particles gets ionised by gaining a proton. the XH+ ion is then attracted to the negative charge.

Question 13

acceleration

Answer 13

these 1+ ions are attracted by the metal plates in the mass spectrometer having a negative charge.
-they all are accelerated to have the same kinetic energy no matter the mass.
-lighter ions move faster and heavier ions move slower.

Question 14

ion drift

Answer 14

the ions pass through a hole and moves into negatively charged tube called the flight tube