Drawing of Mass in Science: How to Draw a Mass Spectrum

When people ask about a drawing of mass in science, they usually mean a mass spectrum: a bar chart that shows what a molecule or sample is made of by plotting mass against abundance. To draw one, label the horizontal axis as mass-to-charge ratio (m/z), the vertical axis as relative abundance, and draw a vertical line (a “stick”) at each m/z value, with its height showing how common that fragment is. The tallest line is the base peak, usually set to 100 percent.

Drawing of mass in science: what a mass spectrum shows

A mass spectrometer ionises a sample, separates the resulting ions by their mass-to-charge ratio and counts how many of each reach the detector. The result is a graph rather than a picture of an object, and it acts like a fingerprint of a compound. Chemists use it to find the relative molecular mass, work out formulas and clue into structure.

In most school and university examples the charge on each ion is +1, so the m/z value is the same as the mass of the ion. That makes reading the graph straightforward.

What you need before you start

• Graph paper or a spreadsheet (a simple ruler and pencil will do).
• A table of m/z values and their relative abundances.
• A fine pencil and ruler for bars.
• A clear scale for each axis.

A typical data table might look like the one below. These numbers are only an example to practise drawing.

m/z	Relative abundance (%)
—	—
15	20
29	100
43	45
58	30

Step-by-step: how to draw a mass spectrum

1. Draw the axes. Put m/z on the horizontal axis and relative abundance (%) on the vertical axis.
2. Choose a sensible scale. The x-axis should run from zero up to a little beyond the largest m/z. The y-axis usually runs from 0 to 100 percent.
3. Mark the tick positions evenly and label them clearly.
4. Convert abundances to relative values if they are given as raw counts: divide each by the tallest peak and multiply by 100.
5. Draw a vertical line at each m/z value, up to the right abundance. Use a ruler for clean sticks, not wide bars.
6. Label the key peaks, particularly the base peak and the molecular ion.
7. Add a title and axis labels, including units where relevant.
8. Check that no peak is left out and that the tallest line reaches 100.

How to read the peaks

The molecular ion peak

This peak (written M+) shows the whole molecule that has lost one electron. It is usually, though not always, at the highest m/z in the spectrum. Its m/z gives you the relative molecular mass. Some molecules fall apart so readily that the molecular ion peak is tiny or missing.

The base peak

The tallest peak is the base peak. It represents the most abundant ion, and it is assigned a value of 100 percent. All other peaks are shown relative to it.

Fragment peaks

Peaks at lower m/z come from fragments created when the molecular ion breaks. The difference between peaks can tell you which piece was lost. For instance, a loss of 15 often corresponds to a methyl group, and a loss of 29 may correspond to an ethyl group or CHO, depending on the compound.

The M+1 and isotope peaks

Small peaks one unit above the molecular ion arise from natural isotopes such as carbon-13. Compounds containing chlorine or bromine show distinctive isotope patterns, because these elements have common isotopes with different masses. This is why chlorine-containing compounds show a pair of peaks, with a height ratio of about three to one.

A worked example idea

Imagine you have a small organic molecule whose spectrum has a peak at m/z 58 and a large peak at 43. The peak at 58 is likely the molecular ion, so the molecule has a relative molecular mass of 58. The peak at 43 is 15 lower, which suggests that a methyl group has been lost. Combine this with other information, like infrared spectra or the sample’s formula, to make a stronger case. In exams you will normally be asked to identify the species responsible for specific peaks, so practise by writing the formula of each fragment next to its m/z.

Common mistakes

• Using wide bars or smooth curves rather than thin vertical lines.
• Forgetting to scale to the base peak at 100 percent.
• Mixing up m/z with mass in grams.
• Treating the highest peak as the molecular ion; the base peak and molecular ion are often different.
• Leaving out axis labels or units.
• Ignoring small isotope peaks that give valuable information.

Tools for drawing spectra digitally

Spreadsheet programs such as Excel or Google Sheets can create a column or scatter chart with thin bars. Specialist software, like ChemDraw or open-source tools, can plot predicted spectra from a structure. If you use a spreadsheet, set the gap width low and the bars narrow so that the result looks like a spectrum. For more background reading on the technique, the Wikipedia article on mass spectrometry covers how instruments work and where they are used.

If you are building maths and graph skills along the way, our guides on the four quadrants on a graph, the C2H4 Lewis structure and the semicircle formula explain the building blocks you will meet in chemistry and physics.

Frequently asked questions

Conclusion

A drawing of mass in science is really a mass spectrum: a simple stick graph that turns a hidden molecule into readable data. Label the axes, scale to the base peak, draw each ion as a thin line and read the molecular ion and fragments to find the story behind the sample.