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 Mass spectrometry - spectra index

Students and teachers please note my explanation of the mass spectrum of butanoic acid is designed for advanced, but pre-university, chemistry courses.

If M represents the butanoic acid molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of butanoic acid and only the formation of singly charged positive are considered for the mass spectrum of butanoic acid.

I've included a stick diagram and table of m/z ions for the mass spectrum of butanoic acid and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including butanoic acid.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for butanoic acid. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for butanoic acid, but the mass spectrometer software does!

Interpreting the fragmentation pattern of the mass spectrum of butanoic acid

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original butanoic acid molecule minus an electron, [CH₃CH₂CH₂COOH]⁺

It is a very small peak, suggesting the parent molecular ion is not very stable relative to other fragmentation ions.

You might see a tiny M+1 peak at m/z 89, corresponds to an ionised butanoic acid molecule with one ¹³C atom in it i.e. an ionised butanoic acid molecule of formula [¹³C¹²C₃H₈O₂]⁺

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this ¹³C M+1 peak.

Butanoic acid has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

However, it doesn't look in right proportion to the m/z 88 ion, so perhaps there is another M+1 molecular ion.

The most abundant ion of the molecule under mass spectrometry investigation (butanoic acid) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

The base ion peak for the mass spectrum of butanoic acid is the m/z 60 ion [C₂H₄O₂]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of butanoic acid.

Unless otherwise indicated, assume the carbon atoms in butanoic acid are the ¹²C isotope.

Mass spectrum parent molecular ion of butanoic acid is m/z 88: [C₄H₈O₂]⁺ ,[CH₃CH₂CH₂COOH]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of butanoic acid.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of butanoic acid

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1;  C = 12 (~1% 13);  O = 16

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  C=O

Possible equations to explain the most abundant ion peaks of butanoic acid (tabulated above)

Formation of m/z 87 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [C₄H₇O₂]⁺  +  H

C-H bond scission, hydrogen atom loss,

mass change 88 - 1 = 87 (M-1 ion peak)

Formation of m/z 73 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₂CH₂COOH]⁺  +  CH₃

C-C bond scission, loss of methyl group to give the [C₃H₅O₂]⁺ ion.

mass change 88 - 15 = 73 (M-15 ion peak)

The tiny peak for m/z 74 is probably the same fragment ion with a ¹³C atom in it i.e. [¹³C¹²C₂H₅O₂]⁺

Formation of m/z 71 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [C₄H₇O]⁺  +  OH

mass change 88 - 17 = 71 (M-17 ion peak, M-OH)

This small peak is characteristic of carboxylic acid mass spectra.

Formation of m/z 60 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [C₂H₄O₂]⁺  +  C₂H₄

Elimination of ethene, mass change 88 - 28 = 60 (M-28 ion peak)

This is due to the McLafferty rearrangement, details not required, but is a characteristic peak in the mass spectroscopy of carboxylic acid molecules.

The m/z 60 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 61 ion is likely to be formed in the same way, but containing a ¹³C atom.

Theoretically it could also be the [C₂H₅O₂]⁺ ion.

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034, ¹⁶O = 15.9949, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 61: [¹³C¹²C₂H₄O₂]⁺ = 61.0244, [C₂H₅O₂]⁺ = 61.0288, a difference of 0.0044 in relative ion mass.

Formation of m/z 55 ion:

[C₃H₅O₂]⁺  ===>  [C₃H₃O]⁺  +  H₂O

Maybe elimination of water from the m/z 73 ion?

mass change 73 - 18 = 55

Formation of m/z 45 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [COOH]⁺  +  CH₃CH₂CH₂

C-C bond scission of parent molecular ion,

mass change 88 - 43 = 45 (M-43 ion peak)

Formation of m/z 43 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₃CH₂CH₂]⁺  +  COOH

C-C bond scission of parent molecular ion,

mass change 88 - 45 = 43 (M-45 ion peak, M-COOH)

Further hydrogen atom/molecule losses lead to m/z ions of mass 42, 41 and 39.

Formation of m/z 29 ion:

[CH₃CH₂CH₂COOH]⁺  ===>  [CH₃CH₂]⁺  +  CH₂COOH

C-C bond scission of parent molecular ion, mass change 88 - 59 = 29.

Further hydrogen atom/molecule losses lead to m/z ions of mass 28 and 27.

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

The infrared spectrum of butanoic acid

The H-1 proton NMR spectrum of butanoic acid

The 13C carbon-13 NMR spectrum of butanoic acid

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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