Mass spectrometry

Question 1

4 stages of mass spectrometry

Answer 1

Ionisation
Acceleration
Deflection
Detection

Question 2

Ways to ionise the sample

Answer 2

Electrospray ionisation
Electron impact ionisation

Question 3

Electrospray ionisation

Answer 3

Prevents fragmentation as its more gentle
Involves dissolving the sample + high voltage
Which rips a proton off the solvent and attaches it to the sample molecules = positive charge

Question 4

Electron impact ionisation

Answer 4

Sample is vaporised then hit with electrons from electron gun
Knocks off electrons = positive ions
Causes fragmentation

Question 5

What method of ionisation causes fragmentation?

Answer 5

Electron impact ionisation

Question 6

Acceleration

Answer 6

The positively charged ions are attracted to a negative plate causing them to accelerate
They have same kinetic energy so now their speed is dependant on the mass

Question 7

Which particles move faster during acceleration?

Answer 7

Lighter particles with a smaller mass number
So isotopes which have less neutrons

Question 8

Ion drift

Answer 8

The ions drift down the chamber
Time of flight is dependent on the mass

Question 9

Time of flight for ions

Answer 9

Distance/ velocity

Question 10

Time taken for ions to drift

Answer 10

= √mass
————
2 x kinetic energy

Question 11

Which ions take less time to drift?

Answer 11

Lighter ions

Question 12

Detection

Answer 12

Ions hit a negatively charged plate
Causes a current and the size of it gives a measure of how many ions hit the plate
To give the abundance of the molecule

Question 13

Mass spectrum

Answer 13

Produced as a result of ions being detected
X axis has mass to charge ratio
Y axis has % abundnace

Question 14

Molecular ion peak

Answer 14

The peak with the largest mass to charge ratio aka furthest right
Represents the mass of the molecule we are analysing

Question 15

Peaks clustered around certain mass to charge ratio

Answer 15

Isotopes of the molecule/ element
Have different masses so different mass/charge ratio

Question 16

Fragments

Answer 16

Smaller and lighter peaks that occur as a result of ionisation of the sample breaking it off into smaller molecules
And larger molecules

Question 17

How to use the spectrum to view different isotopes and their relative abundance

Answer 17

Isotopic mass is along the x axis
Isotopic abundance is along the y axis

Question 18

What else could you use instead of ion drift

Answer 18

Deflection of ions using a magnetic field according to mass
Lighter the ions the more they are reflected

Question 19

Molecular/ parent ion

Answer 19

When one electron is knocked off an organic molecule forming a positive ion
Represented as M+ and is situated the furthest right on the mass spectrum graph with highest mass/charge ratio

Question 20

Why does the molecular/ parent ion fragment?

Answer 20

Because it is energetically unstable due to having electron knocked off during bombardment of electrons

Question 21

Why are there so many peaks on a mass spectrum of a molecule?

Answer 21

Because many fragments are possible with the atoms of a parent ion

Question 22

Radical

Answer 22

A fragment that isn’t charged so won’t be detected and as a result lost in the machine

Question 23

Mass spectrum of an organic molecule

Answer 23

Peaks all along the x axis (different mass numbers)
Because they are fragments of the molecular ion so can be varying masses

Question 24

Mass spectrum of a pure element

Answer 24

Peaks represent different isotopes of that element
In a diatomic molecule the peaks will be at the molecule not individual atoms eg around 72 for chlorine
And the different masses of chlorine possible due to isotopes

Question 25

How to work out the ratio peaks will appear in a mass spectrum for diatomic molecules

Answer 25

Turn the percent abundance of each isotope into a fraction
Write out each possible combination of individual atoms can form the diatomic molecule
Multiply fractions together giving ratios peaks will appear in

Question 26

Base peak in a mass spectrum

Answer 26

Most common fragment or multiple ways for fragmentation to result in this molecule

Question 27

How to identify the identity of the molecular ion peak for an organic molecule

Answer 27

Consider any functional groups told about such as alcohol and take away the masses of those elements
Divide by 12 to see how many carbons present
Remainder = number of hydrogen = identified organic molecule

Question 28

M+1 peak

Answer 28

When analysing the fragments of organic compounds, there may be a peak at the end
Which has a m/z ratio one more of molecular ion peak

Question 29

What is the M+1 peak?

Answer 29

When organic compounds have traces of the isotope carbon-13 rather than carbon-12 which increases the mass number of it so it’s more than the molecular ion peak

Question 30

What is the point of mass spectrometry?

Answer 30

A machine that produces positive ions and deflects them in a magnetic field depending on their relative masses
So we can determine the masses of atoms and molecules

Question 31

What could a m/z ratio of 15 indicate

Answer 31

That a CH3+ fragment/ion is present

Question 32

What could a m/z ratio of 17 indicate

Answer 32

That an alcohol group such as OH is present

Question 33

How can we use the mass spectrum of an element provided in calculation?

Answer 33

The y axis shows the exact isotopic abundance
And the m/z charge ratio shows the relative isotopic masses
So we can put that in the calculation to find the relative atomic mass of an element

Question 34

Finding the relative molecular masses of diatomic molecules

Answer 34

For the singular isotopes of the element, work out the ratio of isotopic abundance
Eg Cl-35 to Cl-37 is 1:3
Put as fractions and for each diatomic molecule find the fraction probability eg Cl-70 is 3/4x3/4 = 9/16
(The numerator is the ratio)
Multiply the fraction of calculated calculation to find diatomic molecule’s Mr

Question 35

How to find the relative molecular mass of poly atomic molecules?

Answer 35

Just use the molecular ion peak
CAREFUL NOT TO USE THE M+1 PEAK