Mass Spectrometry and Isotopes HL Only
Quick Notes:
- Mass spectrometry is used to identify isotopes of an element and determine their relative abundances.
- Each peak in a mass spectrum represents an isotope.
- The height or area of each peak shows the abundance of that isotope.
- The relative atomic mass (Ar) of the element is calculated as a weighted average of the isotope masses and their abundances.
- You are not required to know how a mass spectrometer works — focus on interpreting the spectra.
Full Notes:
What Is Mass Spectrometry?
Mass spectrometry is an analytical technique used to:
- Identify the different isotopes of an element.
- Measure the relative abundance of each isotope.
- Calculate the relative atomic mass (Ar) based on this data.
How a mass spectrometer works has been outlined in more detail here, however you are not required to know any detail about this, only how to interpret and use mass spectra produced.
Interpreting a Mass Spectrum
A mass spectrum is a graph with:
- x-axis: mass-to-charge ratio (m/z), which usually corresponds to the mass number of each isotope.
- y-axis: relative abundance (often shown as a percentage).
Each peak corresponds to a different isotope of the element.
The m/z value for each peak tells you the mass number of that isotope (assuming a 1+ ion charge).
The height (or area) of the peak tells you how much of it is present in a sample.
Calculating Relative Atomic Mass (Ar)
To find the relative atomic mass, you calculate a weighted average using the isotopic masses and their abundances.
Steps:
- Multiply the mass of each isotope by its % abundance.
- Add all the results together.
- Divide by 100.
Example: Chlorine has two peaks in its mass spectra, from two isotopes:
Use this formula: Ar = (Σ (isotopic mass × % abundance)) / 100
- Cl-35 (75%)
- Cl-37 (25%)
Ar = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 35.5
Summary
- Mass spectra show isotope peaks and their abundances.
- Relative atomic mass (Ar) is the weighted average of isotope masses and abundances.
- You must be able to read spectra and calculate Ar from isotopic data.
- You are not expected to know the internal working of a mass spectrometer.
Linked Questions
How does the fragmentation pattern of a compound in the mass spectrometer help in the determination of its structure?
When a compound is ionised in a mass spectrometer, some molecular ions break apart into smaller charged fragments. Each fragment corresponds to a specific part of the molecule, producing characteristic peaks at different m/z values. By analysing these peaks and matching them to known fragment masses, chemists can identify structural features such as alkyl groups, functional groups, or branching patterns. Together with the molecular ion peak (M⁺), the fragmentation pattern acts like a structural fingerprint that helps deduce the molecule’s overall structure.