Mass Spectrum Interpretation: What Information Can Be Obtained from a Mass Spectrum?

1. What Is a Mass Spectrum?

Every organic compound, when ionised inside a mass spectrometer, breaks apart in its own characteristic way, and the mass spectrum is the record of that event.

A mass spectrum is a simple bar graph: the position of each peak gives the mass-to-charge ratio (m/z) of an ion, and its height gives the relative abundance of that ion.

Mass spectrum interpretation, also called the interpretation of mass spectrum data, is the skill of reading the peaks of a mass spectrum to work out what the compound is.

From a single mass spectrum, recorded on less than a milligram of sample, a chemist can read:

• the molecular mass from the molecular ion peak
• the most abundant ion from the base peak
• the presence of chlorine or bromine from the isotope peaks
• the presence of nitrogen from the nitrogen rule
• the structural units from the fragmentation pattern
• the molecular formula

This guide explains what a mass spectrum is in chemistry, how to read it, and how each piece of information is obtained.

1.1 Mass Spectrum Explained

A mass spectrum is a graph of relative abundance against mass-to-charge ratio (m/z) for the ions formed from a sample.

• In simple words: a mass spectrum is a record of the masses of the ions formed from a compound and their relative amounts.
• How it is drawn: a mass spectrum is drawn as a bar graph, with one vertical line for each ion.
• Plural: mass spectra is the plural of mass spectrum, so the spectra in mass spectrometry are a set of such graphs.

1.2 What Information Can Be Obtained from a Mass Spectrum?

Mass spec is used for identifying compounds and determining their structures, and each kind of information is read from a different feature of the mass spectrum.

The molecular ion peak gives the molecular mass of the compound, and the base peak shows its most abundant ion.

The isotope peaks, called the M+1 peak and the M+2 peak, reveal the number of carbon atoms and the presence of chlorine, bromine or sulfur, while the nitrogen rule shows whether nitrogen is present.

The fragmentation pattern, the diagnostic fragment peaks and the McLafferty rearrangement peak identify the structural units and functional groups, and the exact mass leads to the molecular formula.

Information obtained	Feature of the mass spectrum	Explained in
Molecular mass	Molecular ion peak	Section 4
Most abundant ion	Base peak	Section 5
Number of carbon atoms; chlorine, bromine or sulfur	Isotope peaks (M+1, M+2)	Section 6
Presence of nitrogen	Nitrogen rule	Section 7
Structural units	Fragmentation pattern	Section 8
Carbonyl group with a γ-hydrogen	McLafferty rearrangement peak	Section 9
Functional groups	Diagnostic fragment peaks	Section 10
Molecular formula	Exact mass and isotope peaks	Section 11

2. How to Read a Mass Spectrum

Reading a mass spectrum starts with its two axes, because every peak is defined by a position on the x-axis and a height on the y-axis.

A mass spectrum graph is read in three steps:

1. Read the x-axis to find the m/z value of each peak.
2. Read the y-axis to find the relative abundance of each peak.
3. Identify the three kinds of peak: the molecular ion peak, the base peak and the fragment peaks.

2.1 The X-Axis: Mass-to-Charge Ratio (m/z)

The mass spectrum x axis shows the mass-to-charge ratio (m/z) of each ion.

• Scale: m/z values increase from left to right, so heavier ions appear further to the right.
• Position of a peak: the position of a peak on the x-axis gives the m/z value of the ion that produced it.
• Meaning of the value: most ions carry a single positive charge, so the m/z value of a peak equals the mass of the ion.
• Molecular ion peak: the molecular ion peak comes from the whole molecule after it has lost one electron, so its m/z value equals the molecular mass and it is normally found at the right-hand end of the mass spectrum.
• Fragment peaks: fragment peaks come from pieces of the molecular ion and appear at lower m/z values, to the left of the molecular ion peak. In the mass spectrum of n-pentane, m/z 57 and m/z 29 are fragment peaks.
• Example: in the mass spectrum of n-pentane and in the mass spectrum of isopentane, the molecular ion peak stands at the same position on the x-axis, m/z 72, because both isomers have the same molecular mass.

2.2 The Y-Axis: Relative Abundance

The mass spectrum y axis shows the relative abundance of each ion, also called its relative intensity.

• Scale: relative abundance runs from 0 to 100%.
• Height of a peak: the height of a peak shows how abundant the ion is, so a taller peak means more ions of that m/z value reached the detector.
• Base peak: the tallest peak in the mass spectrum is the base peak and is given the value 100%. In the mass spectrum of n-pentane, the base peak is at m/z 43.
• Other peaks: the height of every other peak is measured as a percentage of the base peak.
• Relative, not absolute: relative abundance compares the ions with one another and does not give the actual number of ions.
• Example: the peak at m/z 57 appears at the same position in both graphs but at different heights on the y-axis, which is how the mass spectrum distinguishes n-pentane from isopentane.

Together, the mass spectrum x and y axis fix every peak: the x-axis says which ion it is, and the y-axis says how much of that ion is present.

3. m/z Meaning in Mass Spectrometry

In chemistry, the m/z meaning is the mass-to-charge ratio of an ion: its mass (m) divided by the number of charges it carries (z).

• m: the mass of the ion in atomic mass units.
• z: the number of charges on the ion, counted as 1, 2, 3 and so on.
• How it is read: in mass spec, m/z is read aloud as “m over z”.
• Why z is usually 1: electron ionization removes one electron from a molecule, so the ion carries a single positive charge and its m/z value equals its mass.
• Worked example: the molecular ion of n-pentane has a mass of 72 and a charge of 1, so its m/z value is 72 ÷ 1 = 72.
• Higher charge: an ion of mass 72 carrying two charges would appear at m/z 36, because 72 ÷ 2 = 36.
• Unit: the m/z ratio is written as a plain number, without a unit.
• m/z calculator: an m/z calculator works out the m/z value of the molecular ion and of each fragment from its formula and charge.

4. Molecular Ion Peak and Molecular Mass

The molecular ion peak is the peak given by the whole molecule after it has lost one electron, and it is the most useful single peak in a mass spectrum because it gives the molecular mass.

4.1 What Is the Molecular Ion?

The molecular ion is the ion formed when a molecule loses one electron without breaking apart.

• Formation: M + e⁻ → M⁺• + 2e⁻
• Symbol: the molecular ion is written M⁺•, where the plus sign shows one positive charge and the dot shows one unpaired electron.
• Type of ion: the molecular ion is a radical cation, because it is both a radical and a positive ion.
• Other name: the molecular ion is also called the parent ion.

4.2 Molecular Mass from the Mass Spectrum

The m/z value of the molecular ion peak gives the molecular mass of the compound, because the mass of the lost electron is negligible.

• Example: n-pentane, C₅H₁₂, has a molecular mass of (5 × 12) + (12 × 1) = 72, and the mass spectrum of n-pentane shows the molecular ion peak at m/z 72.
• How to find it: the molecular ion peak is the last significant peak at the right-hand end of the mass spectrum.

4.3 Weak or Absent Molecular Ion Peak

The molecular ion peak is weak or absent when the molecular ion is unstable and breaks into fragments before it reaches the detector.

• Strong molecular ion peak: aromatic compounds, such as benzene (m/z 78).

• Weak molecular ion peak: branched alkanes and alcohols.
• Absent molecular ion peak: tertiary alcohols, such as 2-methylpropan-2-ol, which shows no peak at m/z 74.

4.4 Types of Molecular Ion Peak

The molecular ion peak changes its appearance when certain elements are present in the compound.

• Nitrogen: a compound with one nitrogen atom gives a molecular ion peak at an odd m/z value, such as aniline at m/z 93.

• Chlorine: a compound with one chlorine atom gives two molecular ion peaks, M and M+2, in a height ratio of about 3:1, such as 2-chloropropane at m/z 78 and m/z 80.

• Bromine: a compound with one bromine atom gives two molecular ion peaks, M and M+2, of almost equal height, such as bromoethane at m/z 108 and m/z 110.

• Two chlorine atoms: a compound with two chlorine atoms gives three molecular ion peaks, M, M+2 and M+4, in a height ratio of about 9:6:1, such as dichloromethane at m/z 84, 86 and 88.

• Two bromine atoms: a compound with two bromine atoms gives three molecular ion peaks, M, M+2 and M+4, in a height ratio of about 1:2:1, such as dibromomethane at m/z 172, 174 and 176.

5. Base Peak in a Mass Spectrum

The base peak is the tallest peak in a mass spectrum, and it shows which ion is formed in the greatest amount from the compound.

5.1 Base Peak vs Molecular Ion Peak

The base peak and the molecular ion peak are identified in different ways, so they are usually two different peaks.

• Base peak: the base peak is identified by its height, as the highest peak in the mass spectrum.
• Molecular ion peak: the molecular ion peak is identified by its position, at the m/z value equal to the molecular mass.

Three examples show how the two peaks compare:

• Base peak and molecular ion peak of n-pentane: the base peak of n-pentane is at m/z 43 and the molecular ion peak of n-pentane is at m/z 72, so the two are different peaks.
• Base peak and molecular ion peak of 2-hexanone: the base peak of 2-hexanone is at m/z 43 and the molecular ion peak of 2-hexanone is at m/z 100, so the two are different peaks.
• Base peak and molecular ion peak of phenol: the molecular ion peak of phenol at m/z 94 is also the base peak of phenol, because the molecular ion is very stable.

5.2 What the Base Peak Tells About Structure

The base peak belongs to the ion that is both stable and easily formed, so it points to a structural unit in the molecule.

• Base peak of acetone: the base peak of acetone at m/z 43 is the acetyl ion, CH₃CO⁺, which indicates a CH₃CO group. The base peak of 2-hexanone is at m/z 43 for the same reason, so both are methyl ketones.

• Base peak of acetophenone: the base peak of acetophenone at m/z 105 is the benzoyl ion, C₆H₅CO⁺, which indicates a C₆H₅CO group.

• Base peak of acetaldehyde: the base peak of acetaldehyde at m/z 29 is the formyl ion, CHO⁺, which indicates an aldehyde group.

Note: the same m/z value can belong to different ions. In the mass spectrum of acetone and in the mass spectrum of n-pentane, the base peak is at m/z 43. The base peak of acetone at m/z 43 is the acetyl ion, CH₃CO⁺, while the base peak of n-pentane at m/z 43 is the propyl ion, C₃H₇⁺. The base peak in a mass spectrum must therefore be read together with the rest of the fragmentation pattern.

6. Isotope Peaks: M+1 and M+2

Isotope peaks appear one or two m/z units above the molecular ion peak, because some molecules contain a heavier isotope of one of their atoms. The height of an isotope peak depends on how abundant the heavier isotope is.

• M+1 peak: the M+1 peak comes from carbon-13, which is only about 1% of natural carbon, so the M+1 peak is small. The height of the M+1 peak can be used to calculate the number of carbon atoms in the molecule.

• M+2 peak: the M+2 peak comes from chlorine-37 or bromine-81, which are abundant isotopes, so the M+2 peak is large. Chlorine is about 75% chlorine-35 and 25% chlorine-37, as in the mass spectrum of chloromethane, and bromine is about 50% bromine-79 and 50% bromine-81, as in the mass spectrum of bromomethane. The appearance of an M+2 peak is used to check for the presence of chlorine or bromine.

• Isotope pattern: the isotope pattern of a compound with two halogen atoms has three peaks, M, M+2 and M+4, as in the mass spectrum of dichloromethane, the mass spectrum of dibromomethane and the mass spectrum of 1,3-dibromobenzene. The pattern of the M, M+2 and M+4 peaks confirms the presence of two chlorine or two bromine atoms.

• Isotope peaks of elements: the mass spectrum of an element shows one peak for each isotope, as in the mass spectrum of boron, neon, magnesium and chlorine, and gives the relative atomic mass of the element.

• Isotope pattern calculator: an isotope pattern calculator predicts the M, M+1 and M+2 peaks of a compound from its formula.

7. The Nitrogen Rule

The nitrogen rule in mass spectrometry links the m/z value of the molecular ion peak to the number of nitrogen atoms in the compound.

• Nitrogen rule: a compound with an odd number of nitrogen atoms has a molecular ion peak at an odd m/z value, and a compound with no nitrogen or an even number of nitrogen atoms has a molecular ion peak at an even m/z value.
• Nitrogen rule with one nitrogen atom: the molecular ion peak of pyridine is at m/z 79, an odd value, because pyridine contains one nitrogen atom.

• Nitrogen rule with two nitrogen atoms: the molecular ion peak of pyrazine is at m/z 80, an even value, because pyrazine contains two nitrogen atoms.

• Nitrogen rule in aniline: the molecular ion peak of aniline is at m/z 93, an odd value, because aniline contains one nitrogen atom, as in the mass spectrum of aniline.
• Compounds without nitrogen: the molecular ion peaks of benzene (m/z 78), phenol (m/z 94) and acetone (m/z 58) are all at even m/z values, because these compounds contain no nitrogen.
• Use of the nitrogen rule: the nitrogen rule is used to check for the presence of nitrogen from the molecular ion peak alone.

8. Fragmentation Pattern in a Mass Spectrum

The fragmentation pattern is the set of fragment peaks in a mass spectrum, formed when the molecular ion breaks into smaller ions. The fragmentation pattern is used to work out the structural units of a molecule.

The main types of fragmentation are:

• Alpha cleavage: alpha cleavage breaks the bond next to a carbonyl group, as in the mass spectrum of 2-butanone, where the molecular ion at m/z 72 gives the acetyl ion at m/z 43 and the propanoyl ion at m/z 57.

• Beta cleavage: beta cleavage breaks the bond one position away from a benzene ring, as in the mass spectrum of ethylbenzene, where the molecular ion at m/z 106 loses a methyl group to give a peak at m/z 91.
• Tropylium ion: the tropylium ion, C₇H₇⁺, appears at m/z 91. The tropylium ion is the base peak of ethylbenzene, and of toluene, where it forms by loss of one hydrogen atom. A peak at m/z 91 is used to identify a benzyl group in the molecule.

• Allylic cleavage: allylic cleavage breaks the bond one position away from a carbon–carbon double bond, as in the mass spectrum of 1-butene, where the molecular ion at m/z 56 loses a methyl group to give the allyl ion at m/z 41.

• Neutral loss: the neutral loss is the difference in m/z value between the molecular ion peak and a fragment peak, and it is used to identify the group that has left the molecule. In ethylbenzene, the loss of 15 from m/z 106 to m/z 91 shows a methyl group.
• Other types of fragmentation: benzylic cleavage and retro-Diels–Alder fragmentation are further ways in which a molecular ion breaks apart.

9. McLafferty Rearrangement

The McLafferty rearrangement is a fragmentation of carbonyl compounds in which a hydrogen atom moves to the carbonyl oxygen and a neutral alkene molecule is lost.

• McLafferty rearrangement mechanism: a hydrogen atom on the gamma carbon moves to the carbonyl oxygen through a six-membered ring, and the bond between the alpha and beta carbons breaks.
• McLafferty rearrangement of butanoic acid: the molecular ion of butanoic acid at m/z 88 loses ethene (28) to give the McLafferty peak at m/z 60, which is the base peak.

• McLafferty rearrangement of butanal: the molecular ion of butanal at m/z 72 loses ethene (28) to give the McLafferty peak at m/z 44, which is the base peak.

• McLafferty rearrangement of 2-hexanone: the molecular ion of 2-hexanone at m/z 100 loses propene (42) to give the McLafferty peak at m/z 58, as in the mass spectrum of 2-hexanone.
• McLafferty peak by functional group: the McLafferty peak appears at m/z 44 in aldehydes, m/z 58 in methyl ketones, m/z 60 in carboxylic acids and m/z 74 in methyl esters.
• McLafferty rearrangement vs alpha cleavage: in 2-hexanone, alpha cleavage gives the acetyl ion at m/z 43, and the McLafferty rearrangement gives the peak at m/z 58.
• Use of the McLafferty rearrangement: a McLafferty peak is used to identify a carbonyl compound that has a hydrogen atom on the gamma carbon.

10. Diagnostic Fragment Peaks

A diagnostic fragment peak is a fragment peak whose m/z value points to a particular group in the molecule.

• Diagnostic peak and base peak: a diagnostic fragment peak is often the base peak, as with the benzoyl ion at m/z 105 in acetophenone and in benzoic acid, but not always, as with the propanoyl ion at m/z 57 in 2-butanone, which is a small peak.
• Same m/z, different ions: a diagnostic peak must be read with the rest of the mass spectrum, because m/z 43 is the acetyl ion in acetone but the propyl ion in n-pentane.
• Fragmentation of functional groups: each class of compound has its own diagnostic fragment peaks, such as ketones (acetone, acetophenone), aldehydes (acetaldehyde, benzaldehyde), carboxylic acids (benzoic acid, salicylic acid) and phenols (phenol, resorcinol, α-naphthol, β-naphthol).

The mass spectrum peaks table below lists the characteristic fragment ions, the group each diagnostic ion indicates, and the compounds whose mass spectrum shows it.

m/z	Fragment ion	Indicates	Seen in the mass spectrum of
15	CH₃⁺, methyl ion	A methyl group	Acetone
29	CHO⁺, formyl ion	An aldehyde group	Acetaldehyde
41	C₃H₅⁺, allyl ion	A carbon–carbon double bond	1-Butene
43	CH₃CO⁺, acetyl ion	A methyl ketone	Acetone, 2-butanone, acetophenone
44	McLafferty peak	An aldehyde with a gamma hydrogen	Butanal
57	CH₃CH₂CO⁺, propanoyl ion	An ethyl ketone	2-Butanone
58	McLafferty peak	A methyl ketone with a gamma hydrogen	2-Hexanone
60	McLafferty peak	A carboxylic acid with a gamma hydrogen	Butanoic acid
77	C₆H₅⁺, phenyl ion	A benzene ring	Acetophenone, benzoic acid, benzaldehyde
91	C₇H₇⁺, tropylium ion	A benzyl group	Ethylbenzene
105	C₆H₅CO⁺, benzoyl ion	A benzoyl group	Acetophenone, benzoic acid
115	C₉H₇⁺	A naphthol ring	α-Naphthol, β-naphthol
120	C₇H₄O₂⁺•	A salicylic acid unit	Salicylic acid, aspirin

11. Molecular Formula from the Mass Spectrum

The molecular formula is worked out from the molecular ion peak, the isotope peaks and the exact mass.

• Nominal mass: the whole-number mass of the molecular ion peak, such as 58 for acetone, which limits the possible formulas.
• Exact mass: the mass calculated to four decimal places from the masses of the isotopes, which high-resolution mass spectrometry can measure, so that acetone, C₃H₆O (58.0419), is separated from butane, C₄H₁₀ (58.0783), although both have the nominal mass 58.

• Monoisotopic mass: the exact mass calculated from the most abundant isotope of each element.
• Isotope peaks: the M+1 peak gives the number of carbon atoms, and the M+2 peak shows chlorine or bromine.
• Nitrogen rule: an odd nominal mass shows an odd number of nitrogen atoms.
• Degree of unsaturation: the number of rings and double bonds, calculated from the formula, such as one for acetone and five for benzoic acid.
• Molecular formula calculator: a tool that lists the possible formulas for a given exact mass.

12. Worked Example: Interpreting the Mass Spectrum of 2-Hexanone

Mass spectrum interpretation follows the same steps for every compound, and the mass spectrum of 2-hexanone shows each step.

1. Molecular ion peak: the molecular ion peak at m/z 100 gives a molecular mass of 100.
2. Nitrogen rule: the even m/z value of the molecular ion peak shows that the compound contains no nitrogen, or an even number of nitrogen atoms.
3. Isotope peaks: the absence of an M+2 peak shows that the compound contains no chlorine or bromine.
4. Molecular formula: the M+1 peak, about 6.6% of the molecular ion peak, shows six carbon atoms, so the molecular formula is C₆H₁₂O, with one degree of unsaturation.
5. Base peak: the base peak at m/z 43 is the acetyl ion, CH₃CO⁺, formed by alpha cleavage, which shows a methyl ketone.
6. McLafferty peak: the peak at m/z 58 is the McLafferty peak, which shows a hydrogen atom on the gamma carbon.
7. Neutral loss: the peak at m/z 85 is formed by the loss of a methyl group (15) from the molecular ion.
8. Structure: the compound is 2-hexanone, CH₃COCH₂CH₂CH₂CH₃.

Note: an isomer can give the same main peaks, so the identification is confirmed by comparing the mass spectrum with a reference spectrum in a mass spectrum library.

13. Applications of Mass Spectrometry

Mass spectrometry is used wherever a compound has to be identified or measured from a very small sample.

• Structure determination: identifying an organic compound from its molecular ion peak and fragmentation pattern, as in the mass spectrum of 2-hexanone.
• Molecular mass and molecular formula: finding the molecular mass from the molecular ion peak and the molecular formula from the exact mass.
• Isotope analysis: measuring the isotopes of an element and its relative atomic mass, as in the mass spectrum of chlorine.
• GC-MS: gas chromatography–mass spectrometry, which separates a mixture and records the mass spectrum of each compound in it.
• LC-MS: liquid chromatography–mass spectrometry, used with electrospray ionization (ESI) for compounds that cannot be vaporised.
• Pharmaceutical analysis: confirming the identity and purity of a drug, such as aspirin.
• Forensic and environmental analysis: detecting drugs, poisons and pollutants at very low concentrations.
• Proteomics: identifying proteins from the masses of their fragments.
• Quantification: measuring how much of a compound is present in a sample.

14. Summary: Information Obtained from a Mass Spectrum

A mass spectrum gives several kinds of information, and each kind is read from one feature of the spectrum.

The molecular ion peak gives the molecular mass of the compound, and the base peak shows its most abundant ion.

The M+1 peak gives the number of carbon atoms, the M+2 peak shows the presence of chlorine or bromine, and the M, M+2 and M+4 pattern shows two chlorine or two bromine atoms.

The odd or even m/z value of the molecular ion shows whether nitrogen is present, by the nitrogen rule.

The alpha cleavage peaks, the tropylium ion, the allyl ion and the McLafferty peak identify the structural units of the molecule, and the neutral loss identifies the group that has left it.

The exact mass of the molecular ion gives the molecular formula.

The mass spectrum table below summarises the information obtained from each feature of a mass spectrum, with an example from this article.

Feature of the mass spectrum	Information obtained	Example in this article
Molecular ion peak	Molecular mass	2-Hexanone, m/z 100
Base peak	Most abundant ion	Acetone, m/z 43
M+1 peak	Number of carbon atoms	Naphthalene, m/z 129
M+2 peak	Presence of chlorine or bromine	Chloromethane; bromomethane
M, M+2 and M+4 pattern	Two chlorine or two bromine atoms	Dichloromethane; 1,3-dibromobenzene
Odd or even m/z of the molecular ion	Presence of nitrogen	Pyridine, m/z 79; pyrazine, m/z 80
Alpha cleavage peaks	A carbonyl group	2-Butanone, m/z 43 and m/z 57
Tropylium ion, m/z 91	A benzyl group	Ethylbenzene
Allyl ion, m/z 41	A carbon–carbon double bond	1-Butene
McLafferty peak	A carbonyl group with a gamma hydrogen	Butanal, m/z 44; butanoic acid, m/z 60
Neutral loss	The group that has left the molecule	Ethylbenzene, loss of 15
Exact mass	Molecular formula	Acetone, 58.0419

15. Mass Spectrum: Key Terms and Definitions

Mass spectrum: A mass spectrum is a graph of the ions formed from a sample, with the m/z value on the x-axis and the relative abundance on the y-axis.

Mass spectrometry: Mass spectrometry is an analytical technique that converts molecules into ions, separates the ions by their m/z values, and measures how many of each are present.

m/z (mass-to-charge ratio): The m/z value is the mass of an ion divided by the number of charges it carries.

Example: an ion of mass 58 with a single positive charge appears at m/z 58.

Relative abundance: Relative abundance is the height of a peak in a mass spectrum, expressed as a percentage of the tallest peak.

Molecular ion: The molecular ion is the ion formed when a molecule loses one electron without breaking apart. It is written M⁺• and is also called the parent ion.

Molecular ion peak: The molecular ion peak is the peak given by the molecular ion. Its m/z value equals the molecular mass of the compound.

Example: acetone, C₃H₆O, has its molecular ion peak at m/z 58.

Radical cation: A radical cation is a species that carries both a positive charge and an unpaired electron. It forms when a neutral molecule loses one electron.

Base peak: The base peak is the tallest peak in a mass spectrum. It is given the value 100%, and all other peaks are measured against it.

Example: the base peak of acetone is at m/z 43.

Fragment ion: A fragment ion is a smaller ion formed when the molecular ion breaks apart.

Example: the molecular ion of acetone breaks to give the fragment ion CH₃CO⁺ at m/z 43.

Diagnostic fragment peak: A diagnostic fragment peak is a peak at a characteristic m/z value that indicates a particular structural unit in a molecule.

Example: a peak at m/z 105 indicates the benzoyl group, C₆H₅CO.

Isotope peak: An isotope peak is a peak given by ions that contain a heavier isotope of an element, such as carbon-13, chlorine-37 or bromine-81.

M+1 peak: The M+1 peak is the peak one m/z unit above the molecular ion peak. It is caused mainly by carbon-13.

Example: each carbon atom adds about 1.1% to the M+1 peak, so benzene, with six carbon atoms, has an M+1 peak about 6.6% of the molecular ion peak.

M+2 peak: The M+2 peak is the peak two m/z units above the molecular ion peak. It is prominent when a compound contains chlorine or bromine.

Example: chloromethane has peaks at m/z 50 and 52 in a ratio of about 3:1; bromomethane has peaks at m/z 94 and 96 in a ratio of about 1:1.

Isotope pattern: An isotope pattern is the group of peaks (M, M+1, M+2 and so on) whose relative heights are fixed by the natural abundance of the isotopes in the ion.

Example: dibromomethane has peaks at m/z 172, 174 and 176 in a ratio of about 1:2:1.

Natural abundance: Natural abundance is the percentage of each isotope of an element as it occurs in nature.

Example: chlorine is about 75.8% chlorine-35 and 24.2% chlorine-37.

Relative atomic mass: Relative atomic mass is the weighted average mass of the atoms of an element, taking all its isotopes into account, compared with one-twelfth of the mass of a carbon-12 atom.

Example: the relative atomic mass of chlorine is 35.45.

Nitrogen rule: The nitrogen rule states that an organic compound with an odd number of nitrogen atoms has an odd molecular mass, and a compound with zero or an even number of nitrogen atoms has an even molecular mass.

Example: pyridine, with one nitrogen atom, has a molecular mass of 79; pyrazine, with two, has a molecular mass of 80.

Nominal mass: Nominal mass is the mass of a molecule or ion calculated with the whole-number mass of the most abundant isotope of each element.

Example: the nominal mass of acetone, C₃H₆O, is 58.

Exact mass: Exact mass is the mass of a molecule or ion calculated from the precise masses of its isotopes, given to several decimal places.

Example: acetone, C₃H₆O, has an exact mass of 58.0419; butane, C₄H₁₀, has an exact mass of 58.0783.

Monoisotopic mass: Monoisotopic mass is the exact mass of a molecule calculated with only the most abundant isotope of each element.

Example: the monoisotopic mass of chloromethane, CH₃Cl, is 49.9923, calculated with chlorine-35.

Degree of unsaturation: The degree of unsaturation is the total number of rings and pi bonds in a molecule. It is calculated as (2C + 2 + N − H − X) ÷ 2.

Example: benzene, C₆H₆, has a degree of unsaturation of 4: one ring and three double bonds.

Fragmentation pattern: A fragmentation pattern is the set of fragment peaks in the mass spectrum of a compound. It is characteristic of the compound and serves as its fingerprint.

Alpha cleavage: Alpha cleavage is the breaking of the bond next to a functional group, such as a carbonyl group or a heteroatom.

Example: 2-butanone loses an ethyl radical by alpha cleavage to give CH₃CO⁺ at m/z 43.

Beta cleavage: Beta cleavage is the breaking of the bond between the alpha and beta carbon atoms, counted from a functional group or an aromatic ring.

Example: ethylbenzene loses a methyl radical by beta cleavage to give a peak at m/z 91.

Allylic cleavage: Allylic cleavage is the breaking of the bond one position away from a carbon–carbon double bond, which gives a resonance-stabilised allyl cation.

Example: 1-butene loses a methyl radical to give C₃H₅⁺ at m/z 41.

Neutral loss: A neutral loss is an uncharged fragment lost from an ion. It is not detected, but is found from the mass difference between two peaks.

Example: a difference of 18 shows loss of water; a difference of 15 shows loss of a methyl radical.

McLafferty rearrangement: The McLafferty rearrangement is a fragmentation of carbonyl compounds in which a gamma hydrogen atom moves to the carbonyl oxygen through a six-membered transition state, and a neutral alkene is lost.

Example: 2-hexanone loses propene (mass 42) to give a peak at m/z 58.

Acetyl ion: The acetyl ion is the acylium ion CH₃CO⁺, which appears at m/z 43. It is characteristic of methyl ketones.

Benzoyl ion: The benzoyl ion is the acylium ion C₆H₅CO⁺, which appears at m/z 105.

Example: the benzoyl ion gives the base peak of acetophenone.

Tropylium ion: The tropylium ion is the aromatic seven-membered ring cation C₇H₇⁺, which appears at m/z 91. It is characteristic of alkylbenzenes.

Electron ionization: Electron ionization is a method of ionization in which gas-phase molecules are struck by high-energy electrons, usually of 70 eV, which remove one electron to give the molecular ion.

High-resolution mass spectrometry: High-resolution mass spectrometry measures m/z values to four or more decimal places, which allows the molecular formula of an ion to be determined.

Example: it distinguishes acetone (58.0419) from butane (58.0783), which both have a nominal mass of 58.