MOOC: Instrumental analysis of cultural heritage objects
4.1. SEM-EDS
In this lecture, the introduction to scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) will be given, and practical tips for the analysis as well as examples will be given. SEM-EDS (together with XRF) are among the most important elemental methods used for the analysis of cultural heritage objects. During this lecture, the capability and modern approaches (for example, imaging and mapping) of the analysis of various materials (for example, pigments, fillers, construction materials, minerals, clays etc.) will be discussed, and the advantages and disadvantages of the method will be pointed out.
1. General aspects of SEM-EDS
Scanning electron microscope (usually abbreviated SEM) is a high-resolution imaging technique that is routinely used in the investigation of materials’ surfaces. SEM can be coupled with an energy-dispersive X-ray detector, which enables highly local spatially resolved micro- and nano-scale elemental analysis of the surfaces. Due to its excellent spatial resolution, it allows detailed surface analysis at high imaging magnifications and is thus a very suitable method for the analysis of paint cross-sections.
Scanning electron microscope (SEM)
SEM operates by generating a beam of electrons in a vacuum. The beam is focused by electromagnetic lenses within a column and directed downwards onto the sample surface, focusing it on a very small surface area. The interaction of the electron beam with the sample causes several simultaneous processes. The most important of them from the practical analysis point of view are three:
- ionisation of the sample – ejection of electrons from the atoms within the sample (whereby they are converted into excited ions), producing the so-called secondary electrons,
- reflection (back-scattering) of the beam electrons from the sample surface, producing the so-called backscattered electrons and
relaxation of the excited ions formed in process 1 accompanied by emission of X-rays.
Both secondary and backscattered of electrons can be detected, amplified, and their abundance distribution across the surface can be used for image formation. The secondary electrons give an image where the contrast is achieved on the basis of surface topography. The backscattered electrons give an image that is determined by differences in the elemental composition differences between parts of the sample surface. The electron beam can be very narrowly focused, and thereby high surface resolution (down to a few or few tens of nm) and high magnification (up to 100 000) is possible.
Energy dispersive X-ray spectrometer (EDS)
EDS detects and identifies the X-rays (as in the XRF method) that are emitted in the course of the above mentioned process 3 by the excited ions formed upon electron bombardment on the surface of the sample. When the incident electron beam interacts with the inner shell electrons of an atom, then an electron is ejected (this process generates the above-mentioned secondary electrons). Ejection of the electron produces a vacancy in the inner electron shell of the atom. That vacancy is thereafter filled by an electron from an outer shell. The transition results in the emission of X-rays. Each element emits X-ray photons with energies characteristic to that element. Energy dispersive X-ray analysis measures the energy of each emitted X-ray photon and uses this information to identify the elements in the sample (via energies of the photons) and measure the abundance of the specific elements on the sample (via the abundance of the respective photons).
When interpreting the EDS spectra, many elements have lines that overlap with one another (i.e. the energies of their characteristic photons are similar). Sometimes a peak from one element can obscure the peak from another. Some of the notorious overlaps encountered in paint analysis are titanium/barium (Ti/Ba), sulphur/lead/mercury (S/Pb/Hg), chromium/manganese (Cr/Mn), iron/manganese (Fe/Mn) and nikcel/zinc (Ni/Zn).
2. Analysis with SEM-EDS method
In the following video, geologist Marian Külaviir, MSc, introduces SEM-EDS instrument, sample preparation and how to perform analysis with this instrument.
The slides used in the video can be downloaded from here: