IB Chemistry Topic S1.2.3 Mass Spectrometry Revision Notes (HL)

IB Chemistry Topic S1.2.3 Mass Spectrometry Revision Notes (HL) | RevisionDojo

Written by official IB examiners

Interpretation and Applications of Mass Spectra

You’re handed an unknown element sample and asked to identify it.
What steps would you take? How would you handle the challenge if the element has multiple isotopes?

This is where mass spectrometry becomes an invaluable tool.

Using Mass Spectra to Identify Isotopes and Their Relative Abundances

What Does a Mass Spectrum Display?

Definition

Mass spectrum

A mass spectrum is a graph that plots the relative abundance of ions (y-axis) against their mass-to-charge ratio (m/z) (x-axis).

In most cases, the charge (z) is +1, meaning the m/z value directly corresponds to the ion's mass.
Each peak on the spectrum represents a specific isotope.

Identifying Isotopes

For elements, each peak corresponds to an isotope.
The m/z value of the peak indicates the isotope’s mass number, while the peak height (or intensity) reflects its relative abundance.

Example

Mass Spectrum of Chlorine

The mass spectrum of chlorine reveals two main peaks at m/z = 35 and m/z = 37.
These peaks represent chlorine’s two naturally occurring isotopes: chlorine-35 and chlorine-37.
The relative heights of the peaks indicate their abundances:
- Chlorine-35 (m/z = 35): Higher peak, showing greater abundance.
- Chlorine-37 (m/z = 37): Lower peak, showing lesser abundance.
If the relative abundances are 75.8% for chlorine-35 and 24.2% for chlorine-37, the relative atomic mass of chlorine can be calculated as follows: $$\text{Ar} = \frac{(35 \times 75.8) + (37 \times 24.2)}{100} = 35.45$$
This value matches the relative atomic mass of chlorine listed in the periodic table.

Determining Relative Abundance

To calculate the relative abundance of isotopes:

Measure the height (or area) of each peak.
Divide each peak’s height by the total height of all peaks.
Multiply by 100 to express the result as a percentage.

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Interpretation and Applications of Mass Spectra

You’re handed an unknown element sample and asked to identify it.
What steps would you take? How would you handle the challenge if the element has multiple isotopes?

This is where mass spectrometry becomes an invaluable tool.

Using Mass Spectra to Identify Isotopes and Their Relative Abundances

What Does a Mass Spectrum Display?

Definition

Mass spectrum

A mass spectrum is a graph that plots the relative abundance of ions (y-axis) against their mass-to-charge ratio (m/z) (x-axis).

In most cases, the charge (z) is +1, meaning the m/z value directly corresponds to the ion's mass.
Each peak on the spectrum represents a specific isotope.

Identifying Isotopes

For elements, each peak corresponds to an isotope.
The m/z value of the peak indicates the isotope’s mass number, while the peak height (or intensity) reflects its relative abundance.

Example

Mass Spectrum of Chlorine

The mass spectrum of chlorine reveals two main peaks at m/z = 35 and m/z = 37.
These peaks represent chlorine’s two naturally occurring isotopes: chlorine-35 and chlorine-37.
The relative heights of the peaks indicate their abundances:
- Chlorine-35 (m/z = 35): Higher peak, showing greater abundance.
- Chlorine-37 (m/z = 37): Lower peak, showing lesser abundance.
If the relative abundances are 75.8% for chlorine-35 and 24.2% for chlorine-37, the relative atomic mass of chlorine can be calculated as follows: $$\text{Ar} = \frac{(35 \times 75.8) + (37 \times 24.2)}{100} = 35.45$$
This value matches the relative atomic mass of chlorine listed in the periodic table.

Determining Relative Abundance

To calculate the relative abundance of isotopes:

Measure the height (or area) of each peak.
Divide each peak’s height by the total height of all peaks.
Multiply by 100 to express the result as a percentage.

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S1.2.3 Mass Spectrometry Notes

Interpretation and Applications of Mass Spectra

Using Mass Spectra to Identify Isotopes and Their Relative Abundances

What Does a Mass Spectrum Display?

Identifying Isotopes

Determining Relative Abundance

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Interpretation and Applications of Mass Spectra

Using Mass Spectra to Identify Isotopes and Their Relative Abundances

What Does a Mass Spectrum Display?

Identifying Isotopes

Determining Relative Abundance

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Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics