Mass Spectrometry m/z Calculator

Calculate m/z values for molecular ion, common losses, and isotope patterns in mass spectrometry.

Molecular weight (nominal, g/mol) (g/mol)

Compound class

Charge state z (usually 1)

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Reading a Mass Spectrum

The molecular ion peak gives molecular weight, but the fragments are where structural information lives. Certain losses and isotope patterns are diagnostic enough to identify functional groups directly.

Loss from M⁺	Fragment lost	Indicates
−15	CH₃	Methyl group present
−17	OH	Carboxylic acid, alcohol
−18	H₂O	Alcohol — very common
−29	CHO or C₂H₅	Aldehyde or ethyl
−31	OCH₃	Methyl ester
−45	COOH	Carboxylic acid

Isotope Patterns Identify Halogens

Chlorine and bromine have two abundant isotopes, producing distinctive M and M+2 patterns that are unmistakable once recognised.

Element	Isotopes	M : M+2 ratio	Appearance
Chlorine	³⁵Cl 76%, ³⁷Cl 24%	3 : 1	Clear satellite peak
Bromine	⁷⁹Br 51%, ⁸¹Br 49%	1 : 1	Twin peaks of equal height
Two chlorines	—	9 : 6 : 1	M, M+2, M+4

The Nitrogen Rule

A compound with an odd nominal molecular weight contains an odd number of nitrogen atoms. An even molecular weight means zero or an even number. This follows from nitrogen being the only common element whose valence and mass parity differ, and it is one of the fastest structural deductions available from a spectrum.

The base peak — the tallest — is the most stable fragment, not necessarily the molecular ion. In some compounds the molecular ion is so unstable it barely appears, which is why soft ionisation methods such as electrospray are used for fragile molecules.

Worked Examples

Example 1: Alcohol M=74 (1-butanol)

Key fragment: M-18=56

Result: m/z=56 dehydration base peak

Alcohols lose water readily

Example 2: Chlorobenzene M=112

M=112 and M+2=114 equal to 75% intensity

Result: 3:1 pattern diagnostic for Cl

35Cl:37Cl = 75.8:24.2 natural abundance

Example 3: Two chlorines

M, M+2 and M+4 in roughly 9:6:1

Result: Two chlorine atoms

The pattern follows binomial expansion of the 3:1 single-chlorine ratio. Dichloro compounds are recognisable at a glance.

Example 4: Applying the nitrogen rule

Molecular ion at m/z 121, odd

Result: Odd number of nitrogens

Combined with a loss of 17, this suggests an amine or amide. Two simple observations narrow the candidates considerably.

Common Mistakes

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Assuming the base peak is the molecular ion

The base peak is simply the most abundant fragment. For many compounds the molecular ion is weak or absent entirely under electron impact.

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Ignoring isotope patterns

A 3:1 M/M+2 pattern means chlorine and a 1:1 pattern means bromine. These are among the most reliable structural clues in a spectrum.

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Forgetting the nitrogen rule

An odd nominal molecular weight indicates an odd number of nitrogens. This immediately narrows the possible formulas.

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Confusing nominal with exact mass

Nominal mass uses integer isotope masses; exact mass uses precise values and can distinguish formulas with the same nominal mass. High-resolution instruments exploit this.

Frequently Asked Questions

Molecular ion recognition?

M+ is always highest mass peak (ignoring isotopes) for pure compound. If no M+, compound may fragment easily (alcohols, branched alkanes). M+ absent = must identify from other fragments.

Nitrogen rule?

Organic molecules with only C,H,O,N: even M+ if N=0,2,4; odd M+ if N=1,3,5. Quick check for molecular formula plausibility.

What does the nitrogen rule state?

A compound with an odd nominal molecular weight contains an odd number of nitrogen atoms. Even molecular weight means zero or an even number.

How do I identify chlorine in a mass spectrum?

Look for an M+2 peak about one third the height of M. Bromine instead gives an M+2 peak of nearly equal height.

Is the base peak always the molecular ion?

No. The base peak is the most abundant fragment, which is often a stable rearrangement product. Some molecular ions barely appear at all.

What does a loss of 18 indicate?

Loss of water, which strongly suggests an alcohol. It is one of the most common and diagnostic fragmentations in electron impact spectra.

Formula Explorer connections

Interpretation: This analytical relationship converts an instrument signal, separation measure or optical response into concentration, identity or performance. Assumption: Calibration, blank correction, linear range, path length, matrix effects and instrument settings must match the sample and method.

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