LIBS vs XRF in PMI: Measuring Carbon and Telling L Grades Apart

Point an XRF analyzer at a bar of 316 and then at a bar of 316L and it gives the same answer twice: same chromium, same nickel, same molybdenum, same alloy match. The two materials stop behaving alike as soon as someone runs a torch over them, and the whole difference sits in an element that spectrum doesn't contain. Carbon is capped at 0.030% in 316L, while 316 allows up to 0.08%. That is five hundredths of a percentage point, and it decides whether, after welding, the grain boundaries stay protected by chromium or start corroding from the inside. XRF can't read that number, and the reason lies in the physics of carbon rather than in how the instrument is built.

Why the XRF spectrum has no carbon in it

Carbon fails X-ray fluorescence three times over, and each failure compounds the last. The first failure concerns energy. The carbon K absorption edge sits at 284.2 eV in the tables of the X-ray Data Booklet from Lawrence Berkeley National Laboratory, so its characteristic line falls at about 0.28 keV. Iron, chromium and nickel in the same sample answer between 5 and 8 keV, more than twenty times the energy.

The second concerns fluorescence yield. A carbon atom ionized in its K shell hardly ever pays out in light: the compilation by Krause in the Journal of Physical and Chemical Reference Data (1979) gives it a fluorescence yield of 0.0028, fewer than three photons per thousand vacancies, against 340 for iron. In light elements the energy almost always leaves as an Auger electron. The third failure is absorption, because the few photons that are emitted have to cross the alloy they came from, the air gap and the detector window. The attenuation coefficients published by NIST show that absorption is strongest exactly at those energies, and almost nothing reaches the detector.

No calibration recovers a signal that never arrived. So a better XRF spectrometer doesn't fix this, and a more expensive one doesn't either.

How a LIBS spectrometer reads carbon, and what it asks in return

LIBS, laser-induced breakdown spectroscopy, gets around the obstacle by changing the physics. A focused laser pulse turns a tiny volume of metal into plasma. As the plasma cools, the excited atoms, carbon included, emit light at wavelengths specific to each element, and an optical spectrometer measures the lines. The review by Hahn and Omenetto in Applied Spectroscopy (2012) covers the instrumentation, methods and applications to materials analysis.

For carbon the reference line is C I at 193.09 nm, listed in the NIST Atomic Spectra Database, in the deep ultraviolet. Air absorbs at that wavelength, so instruments built for carbon run an argon-purged optical path. Here the limit comes from the instrument more than from physics. In the laboratory, with a vacuum spectrometer, Sturm, Peter and Noll (Applied Spectroscopy, 2000) measured carbon, phosphorus and sulfur in low-alloy steels with detection limits below 10 µg/g, or 0.001%. A portable metal analyzer works in less favorable conditions than the lab; the threshold that matters in PMI, however, is thirty times higher, at the 0.030% of the L grades.

Five hundredths of a point: where L grades part ways

The most common austenitic stainless steels come in a low-carbon version marked by the letter L, and the chemical difference from the standard grade comes down almost entirely to the carbon ceiling. For plate it is set by ASTM A240/A240M-26 and, in Europe, by EN 10088-2:2024.

Molybdenum separates the 316 family from 304 and shows up clearly in XRF. Between 316 and 316L, or 304 and 304L, only carbon changes, and the metallic spectrum is identical in both cases. That gives a fairly clean division of labor: sorting between alloy families is an XRF job, while L-grade compliance requires measuring carbon.

Why those five hundredths matter in welding

When an austenitic steel spends time at temperature, as in the heat-affected zone of a weld or in a poorly run heat treatment, carbon combines with chromium to form chromium carbides at the grain boundaries. The adjacent zones lose chromium, lose corrosion resistance locally and open the way to intergranular corrosion in service. The phenomenon is called sensitization, and carbon sets its pace. A technical note from ASM International (2016) places it, for higher carbon contents, in prolonged exposure between 600 and 850 °C. In grades with carbon below 0.030%, about eight hours of exposure are needed before it appears, and that margin allows excursions of a few hours into the critical range without losing corrosion resistance.

For this reason L grades are specified wherever there is welding or an aggressive service environment, as in oil & gas, chemical and pharmaceutical plants. A 316 installed in place of a 316L on a welded line can compromise the corrosion resistance of the whole circuit. Susceptibility is then checked in the laboratory with the practices of ASTM A262-15(2021), from the oxalic acid etch to the ferric sulfate and copper sulfate tests.

Measuring 0.030% in the field: surface, argon and number of readings

Seeing carbon and measuring it with the repeatability needed near 0.030% are two different capabilities, and the second has to be asked for explicitly. Detection limit and repeatability on carbon, stated on the materials you will actually test, count for more than the list of detectable elements. The rest is down to the operator's hand, and the mistakes cluster on LIBS, because of the two techniques it is the one that demands a precise routine.

On an unprepared surface the laser reads paint, grease and oxide before the metal, and surface carbon is precisely the contaminant that pushes the result the wrong way, toward the standard grade. So the spot is ground locally down to base metal, and the first pulses, the so-called cleaning shots, remove the remaining layer before acquisition. Without a stable argon flow the 193 nm line is lost in air absorption. And near the 0.030% threshold a single acquisition decides nothing: two or three spots on different areas of the part are the minimum for signing off conformity.

LIBS or XRF: what the test report has to prove

The table says what the two techniques do and leaves the choice open. One question closes it: what does your test report have to prove? If it has to say "this alloy is 316", XRF proves it in a few seconds, without touching the part, across an element range LIBS doesn't cover as conveniently. If it has to say "this 316 is 316L", XRF doesn't prove it at all, because any L-grade conclusion drawn from the metallic spectrum remains an inference, and it falls apart in a dispute. Anyone working on welded lines in oil & gas or chemical plants has to prove the second thing more often than they expect. In the field, without taking samples, the instruments that close the sentence are handheld LIBS and mobile spark OES.

The classic mistake concerns the question more than the technique: the right instrument gets asked for something it can't deliver. An XRF PMI report that "confirms" a 316L confirms a 316 and says nothing more. The ASTM E1476-04(2022) guide describes the methods for metals identification, grade verification and sorting with their fields of use, while the acceptance criterion is written in the purchase specification. For plant material verification programs, API RP 578 (4th edition, February 2023) recommends that the owner set extent and method based on risk. When the order requirement is an L grade, that method has to measure carbon.

Where measuring carbon changes the outcome

The most common ground is oil & gas, with L grades on piping, valves and welded components where intergranular corrosion is a real risk. The case is covered in the article on PMI of piping and welds in oil & gas plants. The same check applies in welding and after heat treatment, to confirm that base metal and filler are genuinely low carbon. It also applies to incoming inspection of bars, tubes and plates, where the carbon measurement becomes one link in metal material traceability from heat to finished part. In maintenance, finally, LIBS reconstructs the grade of in-service components whose documentation has been lost.

The rest of the material mix, meaning different stainless families, nickel, copper and titanium alloys, is identified with XRF. The criteria for choosing configuration and detector are in the guide on how to choose a portable XRF analyzer. If you need to cover both questions, there is an overview of XRF and PMI analyzers for material analysis and a page dedicated to handheld LIBS spectrometers for carbon and L grades.

What to send us for an assessment

To tell you whether XRF is enough, whether LIBS is needed or whether the two work best together, we need a few precise details:

• grades to distinguish (for example 304/304L, 316/316L, carbon steels) and the product standard called out in the order;
• part geometry and size: pipe, flanges, bars, plate, weld beads;
• surface condition (painted, oxidized, machined) and whether local grinding is allowed;
• number of parts or spots to check per day or per campaign;
• where the measurement happens: receiving, shop floor, construction site or plant in service;
• required documentation: PMI report, EN 10204 3.1 or 3.2 certificates, customer specification;
• available samples, ideally with a known laboratory analysis.

You can send them by email to info@dipitech.com or on WhatsApp, with photos of the parts and the certificates.

The rule to take back to the shop is short. If your report has to name an alloy, XRF is enough; if it has to guarantee a carbon content, you need an instrument that measures it. The choice between LIBS in the field and the laboratory then depends on how many parts you have and where they are. If yours is the second case, the next step is a test on your own material, at no obligation, with a tailored quote within 24 hours.