Materials testing: how does spectrometry work?

Spectrometry is used in many areas of physics, chemistry and engineering, from investigating atomic structures to analysing chemical reactions. This article focuses on spectrometric methods for determining the chemical composition of materials, particularly optical emission spectrometry (OES) and X-ray fluorescence analysis (XRF).

To understand how it works, it is useful to begin with the atomic fundamentals.

The atomic model and electron shells

Consider a simplified atomic model in which electrons surround the nucleus in defined orbits, known as shells. These shells have a limited capacity. A magnesium atom with 12 electrons illustrates this: two electrons occupy the innermost shell. The second shell is divided into two subshells with maximum occupancies of 2 and 6 electrons respectively. The third shell consists of three subshells with capacities of 2, 6 and 10 electrons; in magnesium, only the first of these subshells is occupied, by two electrons.

The maximum number of electrons per shell cannot be exceeded, although a shell may contain fewer electrons, for example after excitation. The basic sequence of possible occupancies is identical for all chemical elements (2, 2+6, 2+6+10, …). The hydrogen atom also has all these electron shells available, but its single electron is “normally” in the innermost shell.

Energy states and stability

Differences arise from the actual number of electrons and the energy characteristics of the respective shells. The key point is that each electron shell is associated with a particular energy level. These energies are element-specific and therefore characteristic of each chemical element.

The farther an electron is from the nucleus, the higher its energy level. Electrons, however, tend towards energetically stable states, meaning the lowest possible energies. The number of places in each shell is strictly limited. Once the inner shells are fully occupied, additional electrons necessarily remain in outer, higher-energy shells.

These relationships form the atomic-physics basis of spectral analysis. But how are they used in practice?

Spectrometry, step 1: exciting the sample

The first step is excitation. Energy is supplied to the sample to raise electrons from stable states to higher energy levels or remove them from inner shells. The way in which energy is supplied depends on the method:

• In ICP methods (inductively coupled plasma), excitation is produced by electromagnetic fields.
• In OES, by spark or arc excitation using an electric current.
• In XRF, by high-energy X-rays.
• In flame spectrometry, by thermal energy from a flame.

The aim is to create energetically excited or unstable states.

Spectrometry, step 2: emission of characteristic radiation

When a vacancy arises in an inner electron shell, it is quickly filled by an electron from an outer shell. In doing so, the electron releases the energy difference between the two levels.

This energy is emitted as electromagnetic radiation. The energy, or corresponding wavelength, of this radiation matches the energy difference between the shells involved. Since these energies are element-specific, the emitted radiation can also be uniquely assigned to a chemical element. It can be regarded as the electromagnetic fingerprint of that element.

Spectrometry, step 3: resolving spectral lines

After excitation, the emitted radiation contains contributions from all the elements present in the sample and their possible electron transitions. This superposition must be resolved spectrally so that the contributions can then be “sorted” by energy and thus by chemical element.

In a spectrometer, this is typically achieved by diffraction at an optical grating. The radiation is split into its individual wavelengths, the spectral lines. Each line represents a specific electron transition in a particular element.

When complex materials such as steel are analysed, thousands of spectral lines arise, for example from iron, carbon, silicon and manganese. Modern high-performance spectrometers record these simultaneously and at high spectral resolution.

Spectrometry, step 4: energy and intensity analysis

Two quantities are evaluated in the next step:

• The energy (or wavelength) of the spectral lines provides qualitative information: which elements are present.
• The intensity of each line provides quantitative information: the concentration of the element.

In physical terms, intensity is proportional to the number of photons emitted by a particular transition. The higher the concentration of an element in the sample, the more corresponding electron transitions occur and the more intense the associated spectral line becomes.

Spectrometry, step 5: calibration and presentation of results

Initially, the spectrometer measures only physical quantities: energies and intensities. The user, however, needs element names and concentration values.

These are assigned using stored calibration curves (programs). The instrument’s database contains reference and calibration curves that represent the following relationships:

• Energy → chemical element (for example, using the atomic number)
• Intensity → concentration

The measured data are compared with these calibration functions. The quality of the calibration and how up to date it is are decisive for analytical precision and reproducibility. The more precise the calibration curves, the more precise the chemical analysis displayed on the spectrometer’s monitor.

Conclusion: from electron transitions to materials analysis

Spectrometry uses a fundamental physical effect: electrons are excited to higher energy states and subsequently return to stable states. The radiation emitted in the process carries all the necessary information about the types and quantities of elements present.

Figuratively speaking, spectrometry is like collective bungee jumping for electrons. We set electrons in motion and read the chemical composition of a material from their “return signals”. This principle is one of the central analytical foundations of modern metallography and materials testing.

W.S. Werkstoff Service – expertise in materials engineering

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