The mass spectrometer gives accurate information about relative isotopic…

The idea

A mass spectrum reports two things for each isotope. The position of a peak on the mass-to-charge axis gives the relative isotopic mass, the mass of one atom of that isotope compared with one-twelfth the mass of an atom of carbon-12, and for a singly charged mononuclear ion that reading is the isotopic mass itself. The height of the peak gives the relative abundance, which comes physically from the size of the current generated as those ions arrive at the detector. Because the technique resolves individual isotopes rather than averaging them, it is far more precise than any bulk measurement, and it lets you work backwards: given the relative atomic mass and all but one isotopic mass, the missing value can be calculated.

What earns the marks

• Reading the relative isotopic mass from the mass-to-charge value of the peak.
• Reading the relative abundance from the height of the peak.
• Stating that abundance is measured from the size of the current at the detector.
• A correct weighted-mean expression linking isotopic masses, abundances and relative atomic mass.

Common mistakes and how to put them right

• Reading the tallest peak as the relative atomic mass of the element.

• Assuming abundances must be percentages when the axis is labelled relative abundance.

• Forgetting that a 2⁺ ion appears at half its true mass on the mass-to-charge axis.

• Rounding a calculated relative isotopic mass to a whole number when the question says calculate.

Exam tips

• Add up the abundances first; that total is the denominator of every weighted mean you write.
• Label each peak with isotopic mass and abundance before doing any arithmetic.
• When solving for an unknown isotopic mass, write the weighted-mean equation in full before rearranging.