Interpret mass-to-charge ratio

m/z is ionic mass divided by charge number z.

GCE A-Level H3 Chemistry 9813-2027

On this page

Learning objectives

Interpret mass-to-charge ratio

Read the ratio literally

m/z is ionic mass divided by charge number z.

Nominal mass 80 at +1 gives 80; mass 100 at +2 gives 50.

The horizontal axis reports mass-to-charge ratio, m/z, not mass alone. Always inspect the charge before turning a peak position into an ion mass.

Sum a formula mass

For C₆H₆⁺•, 6 × 12 + 6 × 1 = 78 and z = 1, so m/z 78.

Use supplied isotope mass numbers rather than average atomic masses for a specified ion.

For an ion of mass number m and charge number z, m/z is their numerical ratio. Most simple electron-ionisation ions are singly charged, so z = 1 and the peak value equals the nominal ion mass.

Handle multiple charge

The same nominal mass 78 at z = 2 appears at m/z 39.

A smaller m/z need not mean a smaller ion.

A doubly charged ion of mass 100 appears at m/z 50, whereas a singly charged ion of mass 50 appears at the same position. The position alone therefore does not identify charge state without further evidence.

Use integer nominal masses for the stated simple fragments unless accurate-mass data are supplied. Do not mix relative isotopic mass, molar mass and mass number without checking the question's convention.

Different formulas can share one nominal m/z. CO⁺•, N₂⁺• and C₂H₄⁺• all appear at m/z 28, so a peak position alone does not fix a formula; the parent molecule, the fragments around it or accurate-mass data decide between them.

Calculate a singly charged ion

³⁵Cl¹⁶O⁺ has nominal mass 51 and z = 1.

Write the isotope mass sum before division.

CH₃CO⁺ has nominal mass 2(12) + 3(1) + 16 = 43 and charge +1, so it appears at m/z 43. If an ion with the same mass carried charge +2, it would appear at 21.5.

Try this

Calculate the m/z value of the ³⁵Cl¹⁶O⁺ ion, showing the mass sum and the charge number.

Check your answer

Add the specified isotope mass numbers: m = 35 + 16 = 51. The charge is +1, so z = 1 and m/z = 51/1 = 51. Using the average mass of chlorine, 35.5, would be wrong because the ion contains the ³⁵Cl isotope specifically.

Calculate a doubly charged ion

An ion of nominal mass 128 with charge 2+ occurs at m/z 64.

Charge magnitude, not the plus sign alone, is the divisor.

Write m and z on separate lines before dividing. This prevents the common mistake of adding the charge number to the mass or ignoring a multiple charge.

Try this

An ion of nominal mass 128 carries a 2+ charge. Write m and z separately, then find the m/z value at which it appears.

Check your answer

m = 128 and z = 2, so m/z = 128/2 = 64. The charge number divides the mass; it is neither added to the mass (130) nor ignored (128).

Resolve isotope variants

³⁵Cl³⁷Cl⁺• has mass 72 and ³⁷Cl₂⁺• has mass 74; both are singly charged.

The isotope label changes numerator, not charge.

For a peak at m/z 36, list more than one possible mass/charge pair, then use the ionisation method and isotope spacing to decide which is chemically plausible.

Try this

A pure Cl₂ sample with no hydrogen-containing contaminant gives peaks at m/z 70, 72 and 74. Assign a singly charged isotopic ion to each peak, then give the ion that could also appear at m/z 36 and decide whether HCl⁺• could explain a peak there instead.

Check your answer

m/z 70 is ³⁵Cl₂⁺• (35 + 35), 72 is ³⁵Cl³⁷Cl⁺• (35 + 37) and 74 is ³⁷Cl₂⁺• (37 + 37); the isotopes change the numerator, not the charge. At m/z 36, the doubly charged ³⁵Cl³⁷Cl²⁺ gives 72/2 = 36. H³⁵Cl⁺• also gives 1 + 35 = 36, but a pure Cl₂ sample contains no hydrogen, so in this spectrum the dication is the plausible assignment.

Common mistake: mass-equals-axis

m/z equals mass only for z = 1.

Always inspect charge notation before assigning a formula.

A higher peak on the page is more intense, not more massive. Ion mass is read from horizontal m/z position; vertical height reports relative abundance.

Try this

Correct this statement: ‘A peak at m/z 50 means the ion has mass 50.’

Check your answer

m/z 50 gives the ratio of mass to charge number, so the ion has mass 50 only if z = 1. A 2+ ion at m/z 50 has mass 100, and a 3+ ion has mass 150. Decide the charge from the ion's formula or other evidence, such as half-integer peaks or isotope spacing, before quoting a mass.

Cross-check formula and spacing

Peaks 39 and 78 can be C₆H₆²⁺ and C₆H₆⁺•, not two unrelated formulas.

Next: molecular ion.

Show the ion formula, nominal-mass sum, charge number and final division. A bare m/z number without an ion assignment is not a complete structural answer.

Try this

The benzene spectrum has peaks at m/z 78 and 39. Explain how one species can give both peaks, then explain why a peak at m/z 28 in another spectrum cannot, on its position alone, be assigned to CO⁺•.

Check your answer

C₆H₆ has nominal mass 6 × 12 + 6 × 1 = 78. The singly charged C₆H₆⁺• appears at 78/1 = 78 and the doubly charged C₆H₆²⁺ at 78/2 = 39: the same mass with charges of 1 and 2. At m/z 28, CO⁺• (12 + 16), N₂⁺• (14 + 14) and C₂H₄⁺• (24 + 4) all have the same nominal mass, so the assignment needs the parent molecule, related fragments or accurate-mass data.