AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Mass spectrometry can be used to identify elements.
Full specification wording
Mass number and isotopes: Mass spectrometry can be used to identify elements.
Every element produces a characteristic pattern of peaks, so a mass spectrum acts as a fingerprint.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every element produces a characteristic pattern of peaks, so a mass spectrum acts as a fingerprint. Two features identify the element: the mass-to-charge values, which tell you which isotopes are present, and the ratio of the peak heights, which is fixed for that element. Chlorine, for example, gives peaks at 35 and 37 in a ratio close to 3:1, and bromine gives peaks at 79 and 81 in a ratio close to 1:1. Combining the isotopic masses with their abundances gives a relative atomic mass that can be matched against the Periodic Table. The identification must use the whole pattern, because a single peak could belong to several different elements or to a multiply charged ion.
What earns the marks
- Identifying the isotopes present from the mass-to-charge values of the peaks.
- Using the relative abundances to calculate a relative atomic mass.
- Matching that relative atomic mass to an element in the Periodic Table.
- Using the characteristic ratio of peak heights as supporting evidence.
Common mistakes and how to put them right
Naming the element from the largest peak alone and ignoring the other isotopes.
Forgetting that a 2⁺ ion appears at half the mass-to-charge value of the 1⁺ ion.
Treating the mass-to-charge axis as a mass in grams.
Quoting the relative atomic mass as a whole number so that two neighbouring elements cannot be told apart.
Exam tips
- Always calculate the relative atomic mass to at least one decimal place before naming the element.
- Check whether the number of peaks matches the number of isotopes the element is known to have.
- If a peak sits at an unexpectedly low value, test whether it is a 2⁺ ion by doubling it.
Try the lesson
The start of the Mass number and isotopes: Mass spectrometry can be used to identify elements. lesson, made from checked teaching notes for this specification point.
Have a go first
Which feature of an atomic mass spectrum is primarily used to identify an unknown element?
Listen
Every element produces a characteristic pattern of peaks, so a mass spectrum acts as a fingerprint. Two features identify the element: the mass-to-charge values, which tell you which isotopes are present, and the ratio of the peak heights, which is fixed for that element. Chlorine, for example, gives peaks at 35 and 37 in a ratio close to 3:1, and bromine gives peaks at 79 and 81 in a ratio close to 1:1. Combining the isotopic masses with their abundances gives a relative atomic mass that can be matched against the Periodic Table. The identification must use the whole pattern, because a single peak could belong to several different elements or to a multiply charged ion. A strong answer earns marks for these points. identifying the isotopes present from the mass-to-charge values of the peaks. using the relative abundances to calculate a relative atomic mass. matching that relative atomic mass to an element in the Periodic Table. using the characteristic ratio of peak heights as supporting evidence. Exam tip: Always calculate the relative atomic mass to at least one decimal place before naming the element.
See it
Key words
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peak patterns
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abundance ratios
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identifying elements
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multiply charged ions
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See every definition
- peak patterns
- Every element produces a characteristic pattern of peaks, so a mass spectrum acts as a fingerprint.
- abundance ratios
- The ratio of the peak heights is fixed for that element, and combining the isotopic masses with their abundances gives a relative atomic mass.
- identifying elements
- The element is identified from the mass-to-charge values of the peaks and the ratio of the peak heights, using the whole pattern.
- multiply charged ions
- A single peak could belong to several different elements or to a multiply charged ion, so the whole pattern must be used.
Label the diagram
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1Box 1
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