16.2 Mass spectrometry

Question 1

Mass spectrometry can be used to find the relative molecular mass of organic compounds
Describe the beginning stages of it

Answer 1

• The compound enters the mass spectrometer in solution
• It is ionised by electrospray or electron impact
• Then the positive ions fragment but are accelerated through the instrument as a beam of ionised molecules

These then fly through the instrument towards a detector

Their times of flight are measured
These depend on mass to charge ratio m/z of the ion

Question 2

How are the results of their times of flight recorded on a graph

Answer 2

Relative abundance is y axis
Mass/charge ratio is x axis

As the ions are all +1 charge, the x axis is basically just relative mass

Question 3

Describe process of Fragmentation in ethanol

Answer 3

. During mass spectrometry, some of the ions in the sample break up or fragment as they pass through the instrument
. So the fragmented ions are accellerated by electric field
. Smaller and more positively charged fragments will arrive first at detector

. SO the mass spectrum of ethanol contains many lines and not just one as we may expect
. When ethanol is ionised it forms the ion C2H5OH+
This is a molecular ion
However many of these ions will then break up because when they are ionised, their bonds break

So there are ions of smaller molecular mass
This is fragmentation

Each fragment ion produces a line in the mass spectrum

Question 4

Describe how the graph works

Answer 4

. Each fragment will correspond to a specific peak on the spectrum which will have a particular m/z value
. The base peak corresponds to the most abundant ion

. The peak with the largest m/z value is the mr of the whole ionised compound

Question 5

What is high resolution mass spectrometry

Answer 5

Mass spectra can show masses to the nearest whole number only
. However many mass spectrometers can measure mass to 3 or 4 dp

So this method lets us work out the formula of the original parent ion
. Uses fact that isotopes of atoms don’t have exact whole numbers (except carbon 12)

A parent ion of mass 200, to the nearest whole number could have molecular formula
C10H16O4 or C11H4O4 or C11H20O3 Because adding up the atomic masses give the molecular mass 200

And these can be easily distinguished using high resolution mass spectrometry

Question 6

How do we use high resolution spectrometry to differentiate between CO2 and N2O

Answer 6

Both substances have an mr of 44 if you are rounding to nearest whole number
If you get data from high resolution mass spectrometer which gives the Ar of each atom to 5 decimal points, you can find a more precise value

So CO2 is mr 43.98982
N2O is 44.00777 so is more

Question 7

How could high resolution mass spectrometry prove that a sample of propane gas C3H8 is contaminated with carbon dioxide

Answer 7

Use high resolution mass spectrometer to prove that there are two mr values.

Despite both values being 44.0 to 1dp, use high resolution spectrometry;
So C3H8 has mr 44.06352
And CO2 has mr 43.98982

So this shows that there are two substances in the sample, so propane is contaminated

Question 8

Explain why determining the precise relative atomic mass of propanal and
prop-2-en-1-ol by mass spectrometry could not be used to distinguish between samples of these two compounds.

Answer 8

• The two compounds have the same mr of 58
• Mass spectrometry determines relative mr of a compound by measuring mass to charge ratio of the ions produced when the compound is ionised
• Prop-2-en-1-ol and propanal will produce ions with the same mass to charge ratio during ionisation
• So they will give same molecular mass value so mass spectrometry can’t be used alone to distinguish between them