EPMA

Electron Probe Micro-Analyzer (EPMA) is an analytical instrument that performs quantitative chemical analysis on small volumes of solid materials by bombarding the sample with a focused electron beam and measuring the characteristic X‑rays emitted. It typically combines a scanning electron microscope with one or more wavelength-dispersive spectrometers (WDS) or energy-dispersive spectrometers (EDS). WDS offers higher spectral resolution and lower detection limits, while EDS provides faster qualitative and semi-quantitative analysis.

How it works: An electron beam is focused on the specimen surface, causing inner-shell ionization and emission of characteristic X-rays specific to elements present. Detected X-ray intensities are corrected for atomic number, absorption, and density effects (ZAF or more modern φ(ρz) corrections) and compared to standards of known composition to yield quantitative concentrations, usually expressed in weight percent or atomic percent.

Sample requirements: Samples are typically prepared as polished, flat sections. Conductive coatings may be needed for non-conductive materials to reduce charging in SEM-based EPMA.

Capabilities and limits: EPMA provides quantitative elemental analysis with micrometer-scale spatial resolution. Detection limits range from fractions of a weight percent for major elements to tens of ppm for some elements with WDS; EDS is better for rapid qualitative work but generally has higher detection limits. Quantification depends on matrix effects, beam conditions, and calibration.

Applications: EPMA is widely used in geology (minerals and rocks), materials science (ceramics, metals, semiconductors), archaeology, and metallurgy for phase characterization, compositional zoning, and microanalysis.

History and variants: The technique emerged in the mid-20th century and evolved with the advent of modern SEM hardware and spectrometers, with continual improvements in detectors, software, and standards.