1.2.3 (HL)—Mass spectrometry

In a mass spectrum, peak position identifies an isotope's mass-to-charge value and relative peak height represents its abundance in the sample. Use those two pieces of evidence together to identify isotopes and determine relative atomic mass.

Read	Infer	Use
Peak position	Isotope mass	Label the isotope
Relative peak height	Relative abundance	Weight that isotope's contribution
All peaks together	Isotopic composition	Calculate the weighted relative atomic mass

The syllabus assesses interpretation of mass spectra, not operational details of how the spectrometer works.

$A_r = (m_1×f_1) + (m_2×f_2) + …$

If singly charged isotope peaks occur at m/z 35 and 37 in a 3:1 height ratio, use fractional abundances 0.75 and 0.25 to obtain Aᵣ = 35.5. First confirm the charge state: m/z is a ratio, so a multiply charged ion cannot be read as mass alone.

Retrieve the sequence: use nuclear notation to count particles, distinguish isotopes by neutron number, then read mass-spectrum positions and heights as isotope mass and abundance evidence.

When checking an answer, ask: Did I separate A, Z, and charge? Did I explain isotope properties through electron arrangement? Did I weight each isotope by its abundance?

1.2.3 (HL)—Mass spectrometry

Reading a Mass Spectrum

The Nuclear Atom Summary