The proximity histogram (proxigram) is a key method in APT for analyzing compositional changes near phase interfaces. Taking a specific interface or isoconcentration surface in the 3D reconstruction as a reference, it re-bins atoms by their distance from the interface, yielding the variation in elemental concentration with distance from the interface.

In simple words, first of all APT obtains the elemental species and spatial position of each detected ion inside the material through reconstruction process. And then determines the boundary of the target phase using methods such as isoconcentration surfaces. Subsequently, taking this interface as the zero point, the distances from atoms on both sides to the interface are calculated. Atoms within different distance ranges are classified and counted. A curve of elemental concentration versus distance from the interface is observed ultimately, namely the proxigram.

The significance of this method is that it converts complex 3D atomic distributions into a 1D quantitative composition profile. A proxigram provides an intuitive view of how elements partition between the matrix and precipitate phases, and whether there is obvious elemental enrichment, depletion, or a chemical gradient near the interface. In nickel-based superalloys, for instance, it can further reveal how the concentrations of elements such as Ni, Al, Ti, Cr, Co, and W change as they move from the matrix into the γ′ phase, enabling quantitative assessment of interphase partitioning behavior.

The proxigram is one of APT's signature and unique capabilities. Conventional techniques such as EDS and EPMA can also provide local chemical composition, but they usually cannot establish such a detailed 3D interface–element concentration relationship at the nanoscale. By combining atomic-scale 3D spatial coordinates with elemental identity, APT enables researchers to perform statistical analysis around specific nanoscale interfaces, turning "where the interface is" into "what is on each side, how the composition changes, and how elements partition."

For complex alloys, nanoscale precipitates, and multiphase materials, the proxigram is especially suited to studying precipitate/matrix interfaces, cluster boundaries, elemental segregation, and interphase partitioning. Nova Scientific integrates 3D atomic reconstruction, isoconcentration surface construction, and quantitative composition analysis to perform proxigram analysis on APT data, providing atomic-scale insight into interfacial chemistry within materials.