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Weighing a single molecule

• In mass spectrometry 质谱, a molecule is ionised and sorted by mass-to-charge ratio 质荷比 ($m/e$).
• The spectrum is a set of peaks.
• From it we get relative masses and structural clues.

Relative atomic mass from isotopes

• An element's isotopes give several peaks. From their abundances 丰度, the relative atomic mass is a weighted average:

• Chlorine (75% $^{35}\text{Cl}$, 25% $^{37}\text{Cl}$): $A_r = \dfrac{35 \times 75 + 37 \times 25}{100} = 35.5$.

A mass spectrum: the molecular ion 分子离子 gives the Mr, fragments show what was lost

Molecular ion and fragmentation 碎裂

• The highest $m/e$ peak is the molecular ion ($M^{+}$) — it gives the relative molecular mass.
• The molecule also breaks into pieces (fragmentation). The gap between peaks = the mass lost: a loss of 15 = a $\text{CH}_3$ group; a loss of 29 = $\text{CHO}$ or $\text{C}_2\text{H}_5$.

A modern mass spectrometer in a laboratory; the sample is loaded at the front and the machine sorts its ions by mass-to-charge ratio

The [M+1] and [M+2] peaks

• a small [M+1] peak (from $^{13}\text{C}$) tells you the number of carbons.
• an [M+2] peak (two units up) shows a halogen: chlorine gives a 3:1 ratio, bromine a 1:1 ratio.

Worked examples

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