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

In Positive Material Identification (PMI) the portable XRF analyzer is the reference instrument: fast, non-destructive, able to identify a wide range of alloys in seconds. It has, however, a precise and often misunderstood physical limit: it does not measure carbon. And carbon is exactly what separates metallurgically different grades such as 304 and 304L, or 316 and 316L. When a specification calls for an "L" grade, XRF alone is not enough and another technique is needed: LIBS. Understanding why is a matter of physics, not instrument quality.

Why XRF does not see carbon

XRF (X-Ray Fluorescence) identifies elements by measuring the characteristic X-rays that atoms re-emit after being excited. The problem with carbon is its position in the periodic table: it is a light element, atomic number Z=6. Three effects follow and add up:

• Very low line energy: the characteristic carbon line (C-K) falls around 0.28 keV, far below that of typical metallic elements (iron, chromium, nickel sit between 5 and 8 keV).
• Minimal fluorescence yield: in light elements the excited atoms preferentially de-excite non-radiatively (Auger effect) rather than emitting an X-ray photon. The probability of emitting the useful line is therefore very small.
• Absorption of the signal: the few 0.28 keV photons emitted are easily absorbed by the air path, by the detector window and by the material itself, so they do not reach the detector in a measurable amount.

The result is that XRF does not quantify carbon reliably. This is not a calibration flaw: it is a physical barrier. For very light elements X-ray fluorescence spectrometry loses sensitivity, as illustrated by the fluorescence-yield and attenuation-coefficient data published by NIST.

How LIBS measures carbon

LIBS (Laser-Induced Breakdown Spectroscopy) uses a completely different principle: optical emission instead of X-ray fluorescence. A laser pulse focused on the surface brings a tiny volume of material to a plasma state; as the plasma cools, the excited atoms, carbon included, decay by emitting light at wavelengths characteristic of each element. An optical spectrometer measures these lines and the software derives the composition.

For carbon the main analytical line is C I around 193 nm, in the deep ultraviolet. Because air absorbs at these wavelengths, instruments targeting carbon operate with an inert-gas path, typically argon, to bring out the line. It is this ability to work on light elements, together with optical plasma analysis, that lets LIBS do what XRF cannot: read carbon content.

The critical case: telling L grades apart

The most common austenitic stainless steels have a low-carbon version, indicated by the letter L (low carbon). The chemical difference between a standard grade and its L grade is essentially the maximum carbon limit.

Molybdenum distinguishes the 316 family from 304 and is visible to XRF; but between 316 and 316L, or between 304 and 304L, only carbon changes. This is why XRF, which does not measure it, cannot reliably separate the standard from the L grade: it reads the same metallic spectrum in both cases.

Why carbon matters: sensitization and intergranular corrosion

Choosing the L grade is not a formality. When an austenitic steel is heated in the critical range (roughly 450–850 °C), as happens in the heat-affected zone of a weld, carbon tends to combine with chromium to form chromium carbides at the grain boundaries. This phenomenon, called sensitization, depletes chromium in the zones immediately adjacent to the grain boundaries, locally reducing corrosion resistance and opening the way to intergranular corrosion in service.

L grades, having less available carbon, limit carbide formation and reduce the risk of sensitization: this is why they are prescribed where welding takes place or where the plant operates in aggressive environments (oil & gas, chemical, pharmaceutical). Mistakenly installing a 316 instead of a 316L in a welded plant can therefore compromise the corrosion resistance of the whole line. Hence the importance of verifying carbon, not just the metallic elements. The susceptibility of stainless steels to intergranular corrosion is assessed with standardised laboratory tests, such as those in the ASTM A262 series.

LOD, repeatability and surface preparation

To measure carbon well in the field with LIBS, several practical factors matter:

• Limit of detection (LOD): the 0.03% threshold of L grades requires an instrument with adequate LOD and repeatability on carbon, not just the ability to "see" the element.
• Surface preparation: oxides, paints, oils, greases and carbonaceous surface contamination distort the carbon reading. The surface must be ground or cleaned down to the base metal; the first laser pulses are often used precisely to remove the surface layer (cleaning shots).
• Inert-gas purge: the 193 nm line requires a stable argon flow; its absence or instability degrades the carbon signal.
• Repeating the measurement: averaging several acquisitions on different points improves reliability, especially near the L-grade threshold.

Trade-offs: when LIBS, when XRF

The two techniques are not mutually exclusive: in many departments they coexist. XRF remains irreplaceable for non-destructive screening over a very wide range of elements; LIBS steps in when the objective is carbon or L-grade distinction. The framework of accepted methods for metals identification, grade verification and sorting is described by the guide ASTM E1476 (Standard Guide for Metals Identification, Grade Verification, and Sorting). The reference laboratory method for carbon in carbon and low-alloy steels remains spark optical emission spectrometry, standardised by ASTM E415.

Typical applications

• Oil & gas and energy: L-grade verification on piping, valves and welded components, where intergranular corrosion is a real risk.
• Welding and PWHT: confirming that the welded material is genuinely low-carbon as specified.
• Incoming material control: distinguishing standard grades from L grades on bars, tubes and plates before processing.
• Maintenance and asset integrity: verifying in-service components and reconstructing material traceability.

Common mistakes to avoid

• using XRF to "certify" an L grade: it does not measure carbon;
• not preparing the surface before LIBS carbon measurement;
• neglecting the argon purge for the 193 nm line;
• relying on a single acquisition near the L-grade threshold;
• confusing the presence of molybdenum (which distinguishes 316 from 304) with the distinction between standard and L grade.

To explore the instrument and configurations, see the PITECH XRF/PMI material analysis page. A complementary topic, on the precious-metals side and the XRF surface limit, is covered in the article XRF analysis of gold and precious metals. PITECH supports a neutral technical-commercial assessment, comparing materials, critical elements (carbon included) and regulatory requirements before guiding the choice between XRF, LIBS or a mix of the two.