How to Identify Chlorine and Bromine in an Organic Molecule: M+2 Peak in Mass Spectrometry
What changes appear in a mass spectrum when chlorine or bromine is present?
Change the atoms (chlorine or bromine). Watch the M, M+2 and M+4 peaks change with the help of the calculator.
Try the Calculator1. What Is the M+2 Peak?
The M+2 peak in mass spectrometry is the peak that appears two m/z units above the molecular ion peak.
Chlorine
Example: chloromethane, CH₃Cl
Ratio 3 : 1
Bromine
Example: bromomethane, CH₃Br
Ratio 1 : 1
Sulfur
Example: dimethyl sulfide, C₂H₆S
M+2 about 4.5%
Molecular ion peak and M+2 peak: the M+2 peak stands two m/z units above the molecular ion peak. It is about one-third of the molecular ion peak for chlorine, almost equal to it for bromine, and very small for sulfur.
The M+2 peak is used to identify chlorine and bromine in an organic molecule, and to indicate sulfur, because each of these elements gives an M+2 peak of a different height.
The M+2 peak appears because some molecules of the compound contain a heavier isotope of one of their atoms, such as chlorine-37, bromine-81 or sulfur-34.
2. How to Identify Chlorine, Bromine and Sulfur from the M, M+2 and M+4 Peaks
Chlorine, bromine and sulfur are identified in an organic molecule from the shape of the molecular ion peaks: the number of peaks, M, M+2 and M+4, and the ratio of their heights.
Ratio 3 : 1
Example: Chloromethane, m/z 50 and 52
Ratio 1 : 1
Example: Bromomethane, m/z 94 and 96
Ratio 9 : 6 : 1
Example: Dichloromethane, m/z 84, 86 and 88
Ratio 1 : 2 : 1
Example: Dibromomethane, m/z 172, 174 and 176
M+2 about 4.5%
Example: Dimethyl sulfide, m/z 62 and 64
M+2 peak of chlorine, bromine and sulfur: one chlorine atom gives M and M+2 peaks in a ratio of about 3:1, one bromine atom about 1:1, two chlorine atoms about 9:6:1, two bromine atoms about 1:2:1, and one sulfur atom a small M+2 peak of about 4.5%.
The shape of these peaks is a diagnostic feature, because each element gives its own pattern.
In some compounds the molecular ion peak is weak, but the same pattern appears in any fragment ion that still contains the chlorine or bromine atoms.
2.1 Molecular Ion Peaks of Chlorine Compounds
A chlorine compound is identified by an M+2 peak that is about one-third of the height of the M peak.
- One chlorine atom: two peaks, M and M+2, in a ratio of about 3:1, as in chloromethane at m/z 50 and 52.
- Two chlorine atoms: three peaks, M, M+2 and M+4, in a ratio of about 9:6:1, with the M peak the tallest, as in dichloromethane at m/z 84, 86 and 88.
- Three chlorine atoms: four peaks, M, M+2, M+4 and M+6, in a ratio of about 27:27:9:1, with the M and M+2 peaks almost equal, as in trichloromethane at m/z 118, 120, 122 and 124.
Ratio 3 : 1
Example: Chloromethane, m/z 50 and 52
Ratio 9 : 6 : 1
Example: Dichloromethane, m/z 84, 86 and 88
Ratio 27 : 27 : 9 : 1
Example: Trichloromethane, m/z 118, 120, 122 and 124
Molecular ion peaks of chlorine compounds: one, two and three chlorine atoms give peaks in ratios of about 3:1, 9:6:1 and 27:27:9:1.
2.2 Molecular Ion Peaks of Bromine Compounds
A bromine compound is identified by M and M+2 peaks of almost equal height.
- One bromine atom: two peaks, M and M+2, in a ratio of about 1:1, as in bromomethane at m/z 94 and 96.
- Two bromine atoms: three peaks, M, M+2 and M+4, in a ratio of about 1:2:1, with the M+2 peak in the middle the tallest, as in dibromomethane at m/z 172, 174 and 176.
- Three bromine atoms: four peaks, M, M+2, M+4 and M+6, in a ratio of about 1:3:3:1, with the two middle peaks the tallest, as in tribromomethane at m/z 250, 252, 254 and 256.
Ratio 1 : 1
Example: Bromomethane, m/z 94 and 96
Ratio 1 : 2 : 1
Example: Dibromomethane, m/z 172, 174 and 176
Ratio 1 : 3 : 3 : 1
Example: Tribromomethane, m/z 250, 252, 254 and 256
Molecular ion peaks of bromine compounds: one, two and three bromine atoms give peaks in ratios of about 1:1, 1:2:1 and 1:3:3:1.
2.3 Molecular Ion Peaks with Chlorine and Bromine Together
A compound that contains both chlorine and bromine gives a pattern different from that of either element alone.
- One chlorine and one bromine atom: three peaks, M, M+2 and M+4, in a ratio of about 3:4:1, with the M+2 peak the tallest, as in bromochloromethane at m/z 128, 130 and 132.
- Two chlorine atoms and one bromine atom: four peaks, M, M+2, M+4 and M+6, in a ratio of about 9:15:7:1, as in bromodichloromethane at m/z 162, 164, 166 and 168.
- One chlorine atom and two bromine atoms: four peaks, M, M+2, M+4 and M+6, in a ratio of about 3:7:5:1, as in dibromochloromethane at m/z 206, 208, 210 and 212.
Ratio 3 : 4 : 1
Example: Bromochloromethane, m/z 128, 130 and 132
Ratio 9 : 15 : 7 : 1
Example: Bromodichloromethane, m/z 162, 164, 166 and 168
Ratio 3 : 7 : 5 : 1
Example: Dibromochloromethane, m/z 206, 208, 210 and 212
Molecular ion peaks with chlorine and bromine together: the peaks stand in ratios of about 3:4:1, 9:15:7:1 and 3:7:5:1.
2.4 Molecular Ion Peaks of Sulfur Compounds
A sulfur compound is indicated by a small M+2 peak beside a tall M peak.
- One sulfur atom: the M+2 peak is about 4.5% of the M peak, as in dimethyl sulfide at m/z 62 and 64.
- Two sulfur atoms: the M+2 peak is about 9% of the M peak, as in dimethyl disulfide at m/z 94 and 96.
- Sulfur and chlorine compared: the M+2 peak of sulfur is much smaller than the M+2 peak of chlorine, which is about 32% of the M peak.
M+2 is about 4.5% of M
Example: Dimethyl sulfide, m/z 62 and 64
M+2 is about 9% of M
Example: Dimethyl disulfide, m/z 94 and 96
Molecular ion peaks of sulfur compounds: one sulfur atom gives an M+2 peak of about 4.5%, and two sulfur atoms give about 9%.
3. What Causes the M+2 Peak?
The M+2 peak is caused by isotopes: chlorine, bromine and sulfur each have a heavier isotope that is two mass units above the common one.
Chlorine
100 atoms of natural chlorine
24 ÷ 76 = about 32%, ratio 3 : 1
Bromine
100 atoms of natural bromine
49 ÷ 51 = about 97%, ratio 1 : 1
Sulfur
100 atoms of natural sulfur
4.25 ÷ 95 = about 4.5%
Cause of the M+2 peak: the lighter isotope gives the molecular ion peak, M, and the heavier isotope gives the M+2 peak. The height of the M+2 peak follows the share of the heavier isotope.
- Rule: the more abundant the heavier isotope, the taller the M+2 peak. Bromine gives the tallest M+2 peak, chlorine a medium one, and sulfur the smallest.
- No M+2 peak: fluorine and iodine each have only one stable isotope, so they give no M+2 peak.
4. Why Two Chlorine or Two Bromine Atoms Give an M+4 Peak
A molecule with two chlorine or two bromine atoms gives three molecular ion peaks: the M peak, the M+2 peak and a third peak, the M+4 peak. The M+4 peak appears because both atoms can be the heavier isotope at the same time.
Two chlorine atoms
57.4 : 36.7 : 5.9 = about 9 : 6 : 1
Two bromine atoms
25.7 : 50.0 : 24.3 = about 1 : 2 : 1
Cause of the M+4 peak: with two chlorine or two bromine atoms, both atoms can be the lighter isotope (M), one of each (M+2), or both the heavier isotope (M+4).
- Two chlorine atoms: 57.4% of the molecules give the M peak, 36.7% the M+2 peak and 5.9% the M+4 peak, a ratio of about 9:6:1.
- Two bromine atoms: 25.7% of the molecules give the M peak, 50.0% the M+2 peak and 24.3% the M+4 peak, a ratio of about 1:2:1.
5. M+1 Peak and M+2 Peak Compared
The M+1 peak and the M+2 peak are different peaks with different causes. The M+1 peak comes from carbon-13, and the M+2 peak comes from chlorine-37, bromine-81 or sulfur-34.
Benzene, C₆H₆
No chlorine, bromine or sulfur
M+1 peak only: no M+2 peak
Chlorobenzene, C₆H₅Cl
One chlorine atom
M+1 peak and M+2 peak together
M+1 peak and M+2 peak: the M+1 peak comes from carbon-13 and appears in every organic compound. A clear M+2 peak appears when chlorine, bromine or sulfur is present.
- M+1 peak: one m/z unit above the M peak. Each carbon atom adds about 1.1%, so the M+1 peak shows the number of carbon atoms.
- M+2 peak: two m/z units above the M peak. It shows the presence of chlorine, bromine or sulfur.
6. M+2 and M+4 Peak Ratios at a Glance
The table gives the shape of the molecular ion peaks and their height ratio for each combination of chlorine, bromine and sulfur atoms.
| Chlorine | 1 chlorine 3 : 1 | 2 chlorine 9 : 6 : 1 | 3 chlorine 27 : 27 : 9 : 1 |
|---|---|---|---|
| Bromine | 1 bromine 1 : 1 | 2 bromine 1 : 2 : 1 | 3 bromine 1 : 3 : 3 : 1 |
| Chlorine and bromine | 1 chlorine + 1 bromine 3 : 4 : 1 | 2 chlorine + 1 bromine 9 : 15 : 7 : 1 | 1 chlorine + 2 bromine 3 : 7 : 5 : 1 |
| Sulfur | 1 sulfur M+2 about 4.5% | 2 sulfur M+2 about 9% |
- Number of peaks: each extra chlorine or bromine atom adds one more peak, two m/z units higher.
- First peak, M: all the atoms are the lighter isotope (chlorine-35, bromine-79). For two atoms: 35 + 35 = 70 for chlorine, and 79 + 79 = 158 for bromine.
- Second peak, M+2: one atom is the heavier isotope and the other the lighter. For two atoms: 35 + 37 = 72 for chlorine, and 79 + 81 = 160 for bromine.
- Third peak, M+4: both atoms are the heavier isotope (chlorine-37, bromine-81). For two atoms: 37 + 37 = 74 for chlorine, and 81 + 81 = 162 for bromine.
- Tallest peak: the tallest peak belongs to the most common combination. Chlorine-35 is three times as common as chlorine-37, so the M peak is the tallest for chlorine. Bromine-79 and bromine-81 are equally common, and the mixed pair can form in two ways, so the M+2 peak is the tallest for two bromine atoms.
7. M+2 and M+4 Peak Calculator: Identify Chlorine, Bromine and Sulfur in a Molecule
The M+2 and M+4 peak calculator draws the molecular ion peaks for any combination of chlorine, bromine and sulfur atoms, so a pattern can be checked in seconds instead of worked out by hand.
What you can practise with the calculator
- How the molecular ion peak and the M+2 peak appear when one chlorine atom is present.
- How they appear when one bromine atom is present.
- How the M+4 peak is added when two chlorine atoms or two bromine atoms are present.
- Result: when you read an actual mass spectrum, you will have the visual impact, and you will identify clearly which element is present and how many atoms of that element are present in the compound.
M+2 and M+4 peak calculator: the calculator set to two chlorine atoms and one bromine atom, showing the peaks, their ratio of 9:15:7:1 and the isotopes in each peak.
How to use the calculator
- Choose a common case, such as 1 Cl, 2 Br or 1 Cl + 1 Br, with one click.
- Or set the number of chlorine, bromine, sulfur and carbon atoms with the plus and minus buttons.
- Read the graph, the ratio and the table.
What the calculator gives
The calculator gives the graph of the molecular ion peaks (M, M+2, M+4 and M+6), the ratio of the peak heights, such as 9:15:7:1, the exact height of each peak in percent, and the isotopes that make up each peak. It also shows combinations not covered in this article, such as four chlorine atoms, or chlorine with sulfur.
Open the calculator: M+2 and M+4 Peak Calculator
8. Glossary of Terms for Identifying Chlorine and Bromine in Mass Spectrometry
M+2 peak: The M+2 peak is the peak that appears two m/z units above the molecular ion peak in a mass spectrum. It is caused by a heavier isotope, such as chlorine-37 or bromine-81.
Example (chlorine): chlorobenzene gives the molecular ion peak at m/z 112 and the M+2 peak at m/z 114, in the ratio 3:1.
Example (bromine): bromobenzene gives the molecular ion peak at m/z 156 and the M+2 peak at m/z 158, in the ratio 1:1.
M+4 peak: The M+4 peak is the peak that appears four m/z units above the molecular ion peak in a mass spectrum. It appears when a molecule contains two chlorine atoms, two bromine atoms, or one of each.
Example (chlorine): dichloromethane gives peaks at m/z 84 (M), 86 (M+2) and 88 (M+4), in the ratio 9:6:1.
Example (bromine): dibromomethane gives peaks at m/z 172 (M), 174 (M+2) and 176 (M+4), in the ratio 1:2:1.
Molecular ion peak: The molecular ion peak is the peak in a mass spectrum given by the whole molecule after it has lost one electron. Its m/z value equals the mass of the molecule made of the lightest isotopes of its atoms, and it is also called the M peak.
Example: chloromethane, CH₃Cl, has its molecular ion peak at m/z 50.
Isotope: An isotope is one of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons, so they differ in mass.
Example: chlorine-35 and chlorine-37 are isotopes of chlorine.
Natural abundance of isotopes: Natural abundance is the percentage of each isotope of an element as it occurs in nature.
- Chlorine: chlorine-35, 75.8%; chlorine-37, 24.2%.
- Bromine: bromine-79, 50.7%; bromine-81, 49.3%.
- Sulfur: sulfur-32, 95.0%; sulfur-33, 0.75%; sulfur-34, 4.25%.
- Carbon: carbon-12, 98.9%; carbon-13, 1.1%.
Isotope peak: An isotope peak is a peak in a mass spectrum given by ions that contain a heavier isotope of an element.
Example: in the mass spectrum of chloromethane, the peak at m/z 52 is an isotope peak caused by chlorine-37.
Height ratio: The height ratio is the ratio of the heights of two or more peaks in a mass spectrum, written with the smallest peak as 1.
Example: a compound with one chlorine atom gives its molecular ion peak and its M+2 peak in a height ratio of about 3:1.
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 50 with a single positive charge appears at m/z 50.
Diagnostic feature: A diagnostic feature is a feature of a mass spectrum that points to a particular element or group in the molecule.
Example: M and M+2 peaks of almost equal height are a diagnostic feature of bromine.
Mass spectrum: A mass spectrum is a graph that shows the ions formed from a compound, with the m/z value along the x-axis and the height of each peak along the y-axis.
Mass spectrometry: Mass spectrometry is a technique that turns molecules into ions, separates the ions by their m/z values and records the amount of each.
Molecular ion: The molecular ion is the ion formed when a molecule loses one electron without breaking apart. It is written M⁺•.
Relative abundance: Relative abundance is the height of a peak expressed as a percentage of the tallest peak in the mass spectrum.
Halogen: A halogen is an element of group 17 of the periodic table, such as fluorine, chlorine, bromine or iodine.
Monoisotopic element: A monoisotopic element is an element with only one stable isotope, so it gives no isotope peak in a mass spectrum.
Example: fluorine occurs only as fluorine-19, and iodine only as iodine-127.