Reading a Spectrum: Axes, Peaks and Units

Introduction

Every spectrum is a graph, and like any graph it can only be read correctly if you know what its axes mean. Spectroscopists have developed conventions that can seem odd at first: IR spectra run "backwards" from high to low wavenumber, NMR spectra increase from right to left, and some spectra plot dips instead of peaks. Learning these conventions, and what each feature of a signal tells you, is the first practical skill in spectral interpretation.

Core explanation

Three properties of every signal.

- Position tells you what is responsible — which bond vibrates, which electronic transition occurs, which hydrogen environment is present or which ion mass is detected. - Intensity tells you how much or how strongly — the strength of absorption, the number of equivalent nuclei, or the abundance of an ion. - Shape (width and splitting) tells you about the surroundings — hydrogen bonding broadens IR bands; coupling to neighbours splits NMR signals.

Conventions for each technique:

Spectrum Horizontal axis Direction Vertical axis Signals appear as --- --- --- --- --- IR Wavenumber / cm⁻¹ 4000 on left, 400 on right % transmittance Downward dips UV-visible Wavelength / nm Increases left to right Absorbance Upward peaks (broad bands) ¹H NMR Chemical shift δ / ppm 0 on right, increasing to left (typically up to 12) Signal intensity Upward peaks, often split ¹³C NMR Chemical shift δ / ppm 0 on right, up to about 220 on left Signal intensity Single sharp lines Mass spectrum m/z Increases left to right Relative abundance / % Vertical lines

Intensity measures. In IR, a strong absorption dips close to 0% transmittance; a weak one barely dips. In UV-visible spectra, absorbance is a logarithmic measure and relates directly to concentration. In ¹H NMR, the area under a signal (its integration), not its height, is proportional to the number of hydrogens. In mass spectra, the tallest line — the base peak — is set to 100% and others are scaled relative to it.

Shape. An O–H stretch in an alcohol is broad because hydrogen bonding gives a spread of bond strengths. A C=O stretch is sharp and strong. UV-visible bands are broad because vibrational and rotational changes accompany the electronic transition. NMR signals may be singlets, doublets, triplets and so on, depending on neighbouring hydrogens.

Noise and baseline. Small random wiggles are noise and should not be interpreted. Always identify the baseline first, then look for signals that rise clearly above it.

Step-by-step reasoning

When you meet any spectrum:

1. Read the axis labels and units to identify the technique. 2. Note the direction of the horizontal axis. 3. Locate the baseline and ignore noise. 4. Pick out the most intense or most diagnostic signals. 5. Record each signal's position, intensity and shape before interpreting.

Visual explanation

Picture an IR spectrum as a line hanging from the top of the chart at 100% transmittance, with downward "icicles" wherever radiation is absorbed. Picture a mass spectrum as a bar chart of tall and short sticks standing on the axis. Both encode the same idea: something happens at particular positions.

Real-world analogy

Reading a spectrum is like reading a railway timetable in an unfamiliar country. Once you know which column shows times and which shows platforms, the information becomes easy to use; misread the columns and every conclusion is wrong.

Real-world example

Environmental analysts compare IR spectra of unknown oil spills with reference spectra of crude oils and fuels. Matching the positions, relative intensities and shapes of bands across the whole spectrum helps trace a spill to its source.

Why?

Why do IR spectra show dips rather than peaks? Traditionally they plot transmittance — the percentage of radiation passing through. Where the sample absorbs, less radiation is transmitted, so the trace falls. UV-visible spectra usually plot absorbance instead, so absorption appears as a peak.

Common misconception

"The tallest NMR peak always represents the most hydrogens." In ¹H NMR, peak area, not height, is proportional to the number of hydrogens. A split signal spreads its area over several lines, so an individual line may look short.

Worked example

Question: An IR spectrum shows a strong, sharp dip reaching 10% transmittance at 1715 cm⁻¹ and a weak, narrow dip at 2250 cm⁻¹. Describe each signal in terms of position, intensity and shape.

Reasoning: The first signal lies in the carbonyl region; it is strong and sharp. The second lies in the triple-bond region; it is weak and narrow.

Answer: 1715 cm⁻¹: strong, sharp, consistent with C=O. 2250 cm⁻¹: weak, narrow, consistent with a C≡N or C≡C stretch.

Quick check

1. In which direction does the chemical shift scale increase on a standard ¹H NMR spectrum? Answer: From right to left, with 0 ppm at the right-hand side.

Exam focus

Examiners often print spectra without much explanation. Check the axis direction before reading positions — misreading a reversed IR axis is a common error. When describing signals, give the position with units, then comment on intensity and shape.

Advanced insight

Modern IR instruments often use attenuated total reflectance (ATR), where the sample is pressed against a crystal and radiation reflects inside it. ATR spectra closely resemble transmission spectra, but band intensities vary slightly with wavenumber, so software corrections are applied before comparing with libraries.

Summary

Every signal has a position, an intensity and a shape. IR spectra plot % transmittance against wavenumber, decreasing from left to right; UV-visible spectra plot absorbance against wavelength; NMR plots signals against chemical shift, increasing right to left; mass spectra plot relative abundance against m/z. Reading axes and conventions correctly comes before interpretation.

Practice questions

1. What are the units on the horizontal axis of an IR spectrum? Answer: Wavenumber, in cm⁻¹. 2. Why are IR absorptions shown as dips? Answer: The vertical axis is transmittance, which falls where radiation is absorbed. 3. What does the area under a ¹H NMR signal indicate? Answer: The relative number of hydrogen atoms in that environment. 4. Explain why an alcohol O–H band in IR is broad. Answer: Hydrogen bonding between molecules produces a range of O–H bond strengths, so a range of wavenumbers is absorbed.