Interpret M+1, M+2 and M+4 isotope patterns

One chlorine gives M:M+2 about 3:1; one bromine gives about 1:1.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Interpret M+1, M+2 and M+4 isotope patterns

Read one-halogen signatures

One chlorine gives M:M+2 about 3:1; one bromine gives about 1:1.

The two-unit spacing follows isotope mass difference.

Read a cluster in this order: spacing, number of members and relative intensities. A single M + 2 line without its partners is not enough to identify a halogen.

Build two-halogen patterns

Two chlorine atoms give about 9:6:1 at M, M + 2 and M + 4; two bromines give 1:2:1.

Binomial combinations count zero, one or two heavy isotopes.

One chlorine plus one bromine gives about 3:4:1, because (3 + 1)(1 + 1) = 3 + (3 + 1) + 1 once the two mixed combinations of equal mass are collected.

Chlorine occurs mainly as ³⁵Cl and ³⁷Cl in roughly a 3:1 abundance ratio; bromine occurs as ⁷⁹Br and ⁸¹Br in roughly a 1:1 ratio. Replacing a light isotope by its heavy partner raises m/z by two for a singly charged ion.

Interpret M+1

Natural ¹³C produces M+1; its relative size grows approximately with carbon count.

M+1 is distinct from halogen M+2 evidence.

For two halogen atoms, expand the combinations just like a binomial distribution. Two chlorines give light–light, two light–heavy arrangements and heavy–heavy, producing approximately 9:6:1 at M, M + 2 and M + 4.

The same expansion handles mixed halogens. With one chlorine and one bromine, ³⁵Cl⁷⁹Br gives M, ³⁵Cl⁸¹Br and ³⁷Cl⁷⁹Br together give M + 2, and ³⁷Cl⁸¹Br gives M + 4, in about 3:4:1. The middle peak is tallest, which separates this pattern from 9:6:1 for two chlorines and 1:2:1 for two bromines.

Real spectra list intensities, not ratios. Scale the members of a cluster to the same base before comparing: peaks at 77, 100 and 24 become about 3:4:1, and small peaks one unit above each member come from ¹³C, not from a halogen.

The M + 1 peak is commonly influenced by ¹³C. Its approximate size relative to M can estimate carbon count, but apply that estimate cautiously when other isotopes or overlapping clusters contribute.

Count one chlorine

A cluster 112:114 = 3:1 supports one Cl atom.

It does not support one Br, whose pair is nearly equal.

A pair at m/z 112 and 114 in a 3:1 ratio supports one chlorine-containing ion. The two-unit separation identifies the isotope mass difference and the unequal heights distinguish chlorine from bromine.

Try this

A molecular-ion cluster has peaks at m/z 112 and 114 with relative intensities 75 and 25. Identify the halogen pattern and justify the assignment from both spacing and ratio.

Check your answer

The peaks are two m/z units apart and have a 3:1 ratio, supporting one chlorine atom. One bromine would give an approximately 1:1 pair, while two chlorines would add an M + 4 member.

Count two chlorines

A 9:6:1 triplet at M/M+2/M+4 supports two Cl atoms.

The middle member contains one ³⁵Cl and one ³⁷Cl, with two arrangements.

For two chlorines, write (3 + 1)² = 9 + 6 + 1. The middle coefficient includes two ways to choose which chlorine atom is ³⁷Cl.

Try this

Derive the M:M + 2:M + 4 intensity ratio for an ion containing two chlorine atoms.

Check your answer

Using ³⁵Cl:³⁷Cl ≈ 3:1, the combinations are 3²:2(3)(1):1² = 9:6:1. The factor of two counts the two light–heavy arrangements.

Count two bromines

A 1:2:1 triplet supports two Br atoms.

Equal natural Br abundances yield coefficients 1,2,1.

For two bromines, (1 + 1)² gives 1:2:1. A one-bromine ion would instead show only an approximately equal M/M + 2 pair.

Try this

A molecular-ion cluster shows peaks at m/z 200 (relative intensity 51), 201 (1.7), 202 (100), 203 (3.2), 204 (49) and 205 (1.6), with nothing above m/z 205. Decide which halogen atoms it supports and contrast it with the one-bromine pattern.

Check your answer

The members two units apart, m/z 200, 202 and 204, are in about 51:100:49, close to 1:2:1, so the ion contains two bromine atoms. The peaks at 201, 203 and 205 are ¹³C contributions. One bromine would produce only an almost equal M/M + 2 pair, with no M + 4 member.

Common mistake: M+2 absolutism

M+2 evidence must use ratios and formula context; Cl, Br and other isotopes can contribute.

One M+2 stick alone cannot establish exact identity.

M + 2 means a peak two mass-to-charge units above the chosen M peak; it is not automatically chlorine and it is not a doubly charged ion label.

Try this

Correct this statement: ‘Any M + 2 peak proves that the compound contains chlorine.’

Check your answer

An M + 2 peak is only an observation. Identify the whole cluster and its relative intensities: one chlorine gives about 3:1 for M:M + 2, one bromine about 1:1, and two halogens add an M + 4 peak. Other isotopes can also contribute, so check the proposed formula.

Combine carbon and halogen evidence

A 3:1 M/M+2 pair plus M+1 about 6.6% supports one Cl and roughly six carbons.

Next: fragments.

An exam answer should quote the full cluster, assign the isotope count and explain the ratio. Add M + 1 carbon evidence only as a separate, clearly qualified inference.

Try this

A molecular-ion cluster shows peaks at m/z 120 (relative intensity 100), 121 (6.5), 122 (32.0) and 123 (2.1). Explain the carbon and halogen evidence separately.

Check your answer

The peaks at m/z 120 and 122 are about 3:1, supporting one chlorine atom. The M + 1 peak at m/z 121 is 6.5% of M, consistent with roughly six carbon atoms because each carbon contributes about 1.1% from ¹³C. Treat the carbon count as an estimate and check it against the molecular formula.