Why this matters
Positive Material Identification (PMI) is the non-destructive method buyers use to confirm the alloy grade of a fitting matches the MTC. The two field methods are X-Ray Fluorescence (XRF) and Optical Emission Spectroscopy (OES). Each has hard limits — XRF cannot read carbon, so it cannot distinguish 316 from 316L. Reading PMI reports without understanding those limits is the most common cause of accepted-but-wrong-grade material installed in service. This PMI test guide explains XRF vs OES interpretation and what to write on the PO.
A disciplined PMI test workflow — XRF for screening, OES when carbon matters — protects every alloy fitting purchase.
Method-by-method breakdown
XRF (X-Ray Fluorescence). A handheld XRF gun excites the sample with X-rays and reads the fluorescent emission of each element. It is the most common PMI tool because it is portable, fast (5–30 seconds), and does not damage the sample. Limit: it cannot detect elements lighter than magnesium reliably, so it cannot quantify carbon. That means XRF can confirm 316 chemistry, but cannot tell whether the carbon content is ≤ 0.030% (316L) or up to 0.08% (316). It can read Cr, Ni, Mo, Mn, Cu, Ti, Nb, V, W, Fe.
OES (Optical Emission Spectroscopy). A spark or arc vaporises a tiny crater on the sample surface; the light from the plasma is split into its element wavelengths and intensity is converted to concentration. Portable OES (with argon purge) reads carbon down to ~0.01%, plus B, P, S, Si, Al. The trade-off: OES leaves a small (~3 mm) burn mark and needs an argon bottle.
