PL - Mass spectrometry

Question 1

What is mass spectroscopy used for?

Answer 1

To identify organic compounds.

Question 2

How is mass spectroscopy used to identify organic compounds?

Answer 2

1) When an OC is bombarded with electrons, it breaks up into different fragments some of which are charged ions. These ions are detected by a mass spectrometer and create fragmentation patterns.
2) The fragmentation pattern is recorded on a mass spectrum, with the mass/charge ratio of the different fragment ions on the x-axis, and the intensity (or relative abundance) of each fragment on the y-axis.
3) The fragmentation pattern of a molecule can be used to determine its structure by working out what fragments are in the molecule. Molecules with different structural formulas will all have unique fragmentation patterns.
4) If the charge of a fragment is +1 (which it normally will be), the the mass/charge ratio recorded for that fragment on a mass spectrum is equal to the mass of the fragment.
5) In general, the peak with the highest mass/charge ratio on a mass spectrum tells you the molecular mass of the original molecule. This peak is called the M peak. There may also be a very small peak with a mass/charge ratio one greater than the M peak called the M+1 peak. This exists because a small proportions of organic molecules in a sample will contain atoms of the isotope 13C.
6) You may also see other peaks that have a mass greater than the M peak, depending on the isotopes of the atoms in the compound. E.g. chlorine has 2 naturally occurring isotopes.

Question 3

What happens when an organic compound is bombarded with electrons?

Answer 3

It breaks up into different fragments some of which are charged ions. These ions are detected by a mass spectrometer and create fragmentation patterns.

Question 4

What are some fragments?

Answer 4

Charged ions.

Question 5

How are charged ion fragments detected?

Answer 5

By a mass spectrometer and create fragmentation patterns.

Question 6

What is a fragmentation pattern recorded on?

Answer 6

A mass spectrum, with the mass/charge ratio of the different fragment ions on the x-axis, and the intensity (or relative abundance) of each fragment on the y-axis.

Question 7

What can the fragmentation pattern of a molecule be used to determine?

Answer 7

Its structure by working out what fragments are in the molecule. Molecules with different structural formulas will all have unique fragmentation patterns.

Question 8

What will molecules with different structural formulas all have in mass spectroscopy?

Answer 8

Unique fragmentation patterns.

Question 9

What is the charge of a fragment usually?

Answer 9

+1.

Question 10

What will there be is the charge of a fragment is +1?

Answer 10

Then the mass/charge ratio recorded for that fragment on a mass spectrum is equal to the mass of the fragment.

Question 11

In general, what does the peak with the highest mass/charge ratio on a mass spectrum tell you?

Answer 11

The molecular mass of the original molecule.

Question 12

What is the peak with the highest mass/charge ratio on a mass spectrum called?

Answer 12

The M peak.

Question 13

What is the very small peak with a mass/charge ratio one greater than the M peak called?

Answer 13

The M+1 peak.

Question 14

Why does the very small peak with a mass/charge ratio one greater than the M peak exist?

Answer 14

This exists because a small proportions of organic molecules in a sample will contain atoms of the isotope 13C.

Question 15

Why might you also see other peaks that have a greater mass than the M peak?

Answer 15

It depends on the isotopes of the atoms in the compound.

Question 16

What is on the x-axis on a mass spectrometer?

Answer 16

Mass/charge (m/z)

Question 17

What is on the y-axis on a mass spectrometer?

Answer 17

Relative intensity/abundance (%)

Question 18

What does a molecule split into when it fragments?

Answer 18

A positively charged ion and an uncharged radical.

Question 19

What are radicals?

Answer 19

Particles with unpaired electrons.

Question 20

What do fragmentations result in?

Answer 20

Some result in either of the particles being charged. Others always result in one of the particles being the cation, and the other always being the radical.

Question 21

What can mass spectrometers only detect?

Answer 21

They can only detect charged particles.

Question 22

What does it mean for radical fragments that mass spectrometers can only detect charged particles?

Answer 22

So fragments that only ever form as radical fragments won’t show up on the mass spectrum of the molecule.

Question 23

What is a feature of some charged fragments?

Answer 23

Some are also unstable.

Question 24

Why won’t some charged particles show up on a molecules mass spectrum?

Answer 24

Because they are unstable and so break down before they have time to be detected.

Question 25

Explain why some fragments aren’t detected by the mass spectrometer

Answer 25

1) When a molecule fragments, it split into a positively charged ion and an uncharged radical.
2) Some fragmentations can result in either of the particles being charged. Other fragmentations always result in one of the particles being the cation, and the other always being the radical.
3) Mass spectrometers can only detect charged particles, so fragments that only ever form as radical fragments won’t show up on the mass spectrum of the molecule.
4) Some charged fragments are also unstable. They break down before they have time to be detected, so won’t be seen on the mass spectrum.

Question 26

What does the difference between peaks tell you about?

Answer 26

‘Lost’ fragments.

Question 27

What can you work out if you look at the difference between the m/z values of two peaks on a mass spectrum?

Answer 27

The mass of the fragment that was ‘lost’.

Question 28

How can you work out the mass of the fragment that has been ‘lost’ on a mass spectrum?

Answer 28

Use the m/z for the molecular peak and then the next peak below. Find the difference between them to find how much less mass the second peak was from the molecular peak.

This value will tell you the mass of the ‘lost’ fragment/the free radical.

Question 29

If a fragment is unstable or doesn’t form with a positive charge easily meaning that there might not be a peak for it at all, what will be the only evidence for that fragment?

Answer 29

The difference between the m/z values of other peaks.

Question 30

What does high-resolution mass spectrometry measure?

Answer 30

Very accurate masses.

Question 31

What does a mass spectrometer normally record the m/z values as on a mass spectrum?

Answer 31

Whole numbers.

Question 32

What does a high-resolution mass spectrometer normally record the m/z values as on a mass spectrum?

Answer 32

To at least 4 decimal places.

Question 33

Why are very accurate measurements of m/z values very useful?

Answer 33

Because they allow you to compare elements and compounds using relative isotopic masses.

Question 34

What is everything measured relative to?

Answer 34

The relative isotopic mass of 12C (which is exactly 12).

Question 35

What is the only relative isotopic mass that is a whole number?

Answer 35

12C.

Question 36

What is the exact relative mass of a single isotope of 1H?

Answer 36

1.0078

Question 37

What is the exact relative mass of a single isotope of 12C?

Answer 37

12.0000

Question 38

What is the exact relative mass of a single isotope of 14N?

Answer 38

14.0031

Question 39

What is the exact relative mass of a single isotope of 16O?

Answer 39

15.9949

Question 40

Why might some molecules mass spectrums have an M+2 peak?

Answer 40

Because there may be 2 common isotopes of that molecule.

Question 41

To how many decimal places can a high-resolution mass spectrometer measure m/z values to?

Answer 41

At least 4.

Question 42

Why do you sometimes need to use high-resolution mass spectrometry to identify a compound?

Answer 42

Because in low-resolution spectrometry, an M+ peak at a given value could be produced by several different molecules when rounded to a whole number. However, if you use accurate masses/an accurate m/z value from a high-resolution mass spectrum, you can easily tell which molecule made the M+ peak.

Question 43

Using the exact m/z values for O, N, C and H, work out the exact m/z value for N2

Answer 43

2 x 14.0031 = 28.0062

Question 44

Using the exact m/z values for O, N, C and H, work out the exact m/z value for CO

Answer 44

12.0000 + 15.9949 = 27.9949

Question 45

Using the exact m/z values for O, N, C and H, work out the exact m/z value for C2H4

Answer 45

(2 x 12.0000) + (4 x 1.0078) = 28.0312