by Tom. L.
In order for us to create a mass spectrum, a mass spectrometer is used; the whole purpose of mass spectrometry is to try to establish, firstly the molecular mass of a compound, but also the possible structure of this compound using the fragmentation peaks on the mass spectrums. Mass spectrums are normally used in co-ordination with infrared spectrums, so that you can establish not just the structure of the compound but the functional groups present in the compound therefore proving further evidence for the possible structure of the compound present.
- Mass spectrometry – identifying possible structure of the compound
- Infrared spectrometry – identifying possible functional groups in the compound
How is a mass spectrum made? :
- When a compound is placed in a mass spectrometer it is ionised, meaning that the compound looses one electron to form a positive ion.
- This positive ion formed can be detected; this positive ion will have a molecular mass equal to the molecular mass of the compound and is called the molecular ion.
- There are also other peaks detected on the mass spectrum; these are caused by the compound fragmenting, and these are what are referred to as the fragmentation peaks.
- The mass spectrum is recorded as a line diagram (see below) showing m/e ratio against relative abundance.
- m/e = mass to charge ratio
There are 2 important peaks to identify in the mass spectra, these are always found at the highest mass to charge ratio of the spectra (i.e. furthest to the right):
- The molecular ion peak (M), which provides us with the molecular mass of the compound present
- M+1 peak – which shows the existence of 13C – note this may not be present in all spectra, it is sometimes left out of exams, but also may be present so must be aware of this and the reasons why it is present, if it is present it is obvious to see.
The M+1 peak exists in many spectra’s and at very small amounts, hence is very small in height compared to the peak representing the molecular mass of the compound (M peak). The reason we see this M+1 peak is due to presence of carbon-13, is makes up 1.11% of all carbon atoms hence why it’s present in very small amounts.
Mass Spectrum: - example question worked through
1.) Above is the mass spectrum of compound A, with an empirical formula of C2H4O
a.) what is the relative molecular mass of compound A?
- The answer is 88 – can see that the M peak is at 88 on the M/z ratio axis, which is the same of the molecular mass of the compound.
- Note that the M+1 peak is present at 89 on the axis, but is obviously much smaller than the molecular ion peak
b.) molecular formula of compound A?
- The molecular formula for compound A is C4H8O2
- As we know the empirical formula is C2H4O, we work of the mr of the empircal formula which is 44, we then divide 88 the mr of compound a by 44, which gives us answer of 2.
- Use this answer to times the empirical formula by this factor, so in this case 2, so molecular formula is C4H8O2
c.) write an equation to show the formation of the molecular ion?
- C4H8O2 + e- à C4H8O2+ + 2e-
d.) suggest possible structures for the ions that give rise to the peaks at the following m/z values:
29 – CH3CH2+
43 – CH3CH2 CH2+
45 – CHO2+
Now that you have given possible structures of the fragmentation peaks, you can begin to piece together these fragments of the molecule to suggest a possible structure of this molecule, could be:
- Identify molecular ion peak and M+1 peak if present
- Identify fragmentation peaks and structures
- Piece together to form possible structure of the compound present
However you would normally then go on to use an infrared spectrum of the compound to help identify the functional groups present which would confirm whether it has the functional groups of a carboxylic acid present
Once you know what you’re looking for within a mass spectra you just need to keep practicing, and the best way if to do past papers, as most of these questions are used within the long answer questions, linked with empirical formulas and infrared spectroscopy
Uses of mass spectrometry:
- Differentiate between 2 different compounds – 2 similar compounds with the same atoms but different structural arrangement, will have the same mr hence Molecular ion peak, however there fragmentation peaks will be different as they will break down into different ions, hence the two mass spectrums will be different
- Probes to mars – study composition of surface of mars and look for particular molecules
- Levels of pollutants in the environment – can look for presence of certain pesticides of lead within a food chain.