Introduction

Mass spectrometry enables chemists to determine isotopic masses and abundances, allowing accurate calculation of an element’s relative atomic mass. This subsubtopic explains the principles required for OCR A-Level Chemistry.

Mass Spectrometry: Purpose and Context

Mass spectrometry is an analytical technique used to identify isotopes in a sample and measure their relative abundances.

Schematic diagram of a typical mass spectrometer, showing ionisation, acceleration, deflection in a magnetic field, and detection of ions. Ions with different mass-to-charge ratios follow different curved paths and reach the detector at different positions, which produces the mass spectrum. Extra details such as the vacuum system and gas inlet are shown but are not required by the OCR specification. Source

OCR requires students to understand how mass spectrometry provides isotopic masses and percent or fractional abundances, which are then used to calculate the relative atomic mass (Ar) of an element. Only the conceptual steps are required here; instrumental mechanisms such as ionisation modes and vacuum design fall outside the OCR scope.

Understanding Isotopic Mass and Abundance

When an element contains multiple isotopes, each isotope contributes differently to the overall atomic mass based on how common it is in nature. A mass spectrum displays a series of peaks, each corresponding to an isotope with a particular mass-to-charge ratio

Mass spectrum of boron showing isotope peaks at m/z 10 and 11 with different relative abundances. The taller peak represents the more abundant isotope, and the shorter peak the less abundant one. This type of spectrum is used to calculate relative atomic mass using weighted isotopic contributions. Source

A mass spectrum displays a series of peaks, each corresponding to an isotope with a particular mass-to-charge ratio (m/z) and relative abundance.

Key Features of a Mass Spectrum

• Each peak represents a positive ion formed from an isotope.

• The m/z value on the x-axis corresponds to the mass of the ion (charge is usually +1 at this level).

• The peak height or labelled abundance represents the isotope’s percentage or relative proportion.

• The combined data allow determination of isotopic masses and their relative abundances, which are essential for calculating Ar.

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A mass spectrum provides these relative isotopic masses directly on the m/z scale. It is important to recognise that mass spectrometry compares masses relative to the carbon-12 standard, meaning the scale is consistent across all elements.

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These abundances determine the weighted contribution of each isotope when calculating relative atomic mass.

Calculating Relative Atomic Mass (Ar) from Mass Spectrometry

The relative atomic mass listed in the periodic table is not usually a whole number because it reflects the weighted average of all naturally occurring isotopes. OCR requires that students use isotopic masses and abundances from a mass spectrum to determine this average.

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If abundances are given as fractions rather than percentages, division by 100 is not required.

Interpreting Mass Spectrometry Data

OCR expects students to read and interpret simple mass spectra that show isotopic peaks clearly. The steps generally followed are:

Step-by-Step Interpretation

• Identify all isotopic peaks and record their m/z values.

• Locate the accompanying abundances, which may be numerical labels or proportional peak heights.

• Check whether abundances are percentages or relative values, as this changes how the Ar equation is applied.

• Multiply each isotopic mass by its abundance, ensuring consistency in units.

• Sum all calculated values.

• Divide by the total abundance (e.g., 100% or the sum of all relative abundances).

• Record Ar to an appropriate number of significant figures, usually matching the precision of the data given.

These steps build the conceptual link between experimental mass spectra and theoretical atomic structure.

Ionisation and Detection in OCR Context

Although OCR does not require detailed mechanistic descriptions, it is essential to understand the fundamental processes that make mass spectrometry possible.

Core Stages (Conceptual Only)

• Ionisation: Atoms are converted into positive ions (typically +1) so they can be accelerated and detected.

• Acceleration: Ions are propelled through an electric field so they achieve similar kinetic energies.

• Deflection or separation: Due to magnetic or electric fields, ions separate according to m/z, allowing isotopes to form distinct peaks.

• Detection: Ions strike a detector, generating a signal proportional to abundance.

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Since most ions in A-Level mass spectrometry have a +1 charge, the m/z value often effectively equals the mass of the isotope.

Importance of Mass Spectrometry in Atomic Structure

Mass spectrometry provides evidence for the existence of isotopes and supports the concept that elements consist of atoms with the same number of protons but different numbers of neutrons. Calculated Ar values reflect these isotopic distributions and confirm that atomic masses are not whole numbers. Understanding this method reinforces key ideas from atomic structure, particularly isotopes and atomic mass scales.