Using XRF + LIBS Together: When and How It Makes Sense

Introduction

In the digital age, technology such as handheld X-ray Fluorescence (XRF) analyzers have been used to analyze and verify materials in the supply chain quickly and effectively, reducing our reliance on external laboratories. This ability for immediate and fast analysis has improved productivity and safety in many industries and therefore increased profitability and saved lives. The popularity of XRF in industrial environments is driven by ease of use, low cost and its ability to verify that materials, primarily metals, comply with ever increasing regulatory controls.

Handheld XRF can measure all the elements of the periodic table from magnesium to uranium. However, the determination of elements lighter than magnesium such as carbon is not possible with this technique. Until recent years, mobile Optical Emission Spectrometry (OES) has always been the method of choice to determine carbon in materials such as low alloy carbon steels and stainless steels. Mobile OES instruments are generally made of two modules and are much larger and heavier and do not offer the same ease and range of movement than handheld instruments.

More recently, Handheld Laser Induced Breakdown Spectrometry (LIBS) analyzers appeared as a promising technology with the potential to detect and quantify light elements (including carbon) on the periodic table, whilst having a similar size, shape and portability to handheld XRF.

Working Principle and Sample Presentation

With handheld XRF technology the surface of the sample is directly irradiated by a miniaturized x-ray tube and that causes elements contained in the sample to emit specific secondary x-rays (fluorescence). The x-rays emitted by the sample are detected using a semiconductor detector. Handheld XRF can be mostly used in “point and shoot“ mode and results are relatively insensitive to the sample geometry or surface. The spot diameter is generally 3 to 9 mm and there is minimal sample preparation needed to obtain accurate results within seconds.

With Handheld LIBS technology, a laser beam strikes the surface and creates a plasma that vaporizes and reduces the metals into atoms. Those atoms are excited by the high energy of the plasma and emit element specific light in the UV-Visible range. The laser creates a small burn at the surface of the sample, the spot size in LIBS analysis is typically 50 to 100 µm. The properties of the plasma depend on the state and roughness of the sample surface. Grinding the metal samples is generally necessary to obtain accurate results.

When is it best to use handheld XRF?

Handheld XRF delivers outstanding accuracy for most types of metallic materials, especially those containing high levels of alloying transition metals or refractory metals. Such alloys are stainless steel, titanium, nickel, cobalt based alloys as well as special alloys made of zirconium, tungsten or tantalum.

For aluminum and magnesium alloys, despite low sensitivity for magnesium, handheld XRF still delivers very accurate results.

By normalizing results to 100%, handheld XRF results are insensitive to sample size, shape and roughness facilitating accurate measurement of small samples down to millimeters in size. Single and multi-layer coating thickness and composition analysis facilitates quality and cost control over a wide range of industries and products.

XRF analysis is completely non-destructive, leaving the surface of the sample untouched and it is therefore perfectly suitable for quality control of finished goods or to determine the thickness of coatings.

When is it best to use handheld LIBS?

Handheld LIBS is capable of detecting lighter elements such as carbon and is therefore the method of choice in applications requiring the quantification of carbon for metal fabrication or positive material identification of low alloy carbon and stainless steels. Indeed, the content of carbon and a few other elements are crucial to predict the weldability of carbon steels and avoid cold cracking.

Handheld LIBS also delivers outstanding sensitivity for targeted elements at low concentration such as nickel, chromium and copper, elements which are critical in carbon steel piping used in petrochemical processes and in nuclear power plants. Similarly, when welding of stainless steel is required, handheld LIBS can be utilized to determine carbon and other alloying elements to ensure that stainless steel L-grades such as 304L or 316L with carbon content lower than 0.03% are used.

Using XRF and LIBS in Tandem

Positive material identification programs contribute to ensure the safety and integrity of critical assets such as pipeline or processing units in oil refineries, chemical plants, or power plants. Oil and Gas chemical and power generation industries use various materials ranging from carbon steel through stainless steel to superalloys. Hence both handheld XRF and LIBS have to be used by these industries or third party inspection companies to verify the compliance of material either prior to commissioning or retroactively testing. Using handheld XRF and LIBS in tandem also allows inspection companies to achieve ultimate maneuverability and mobility, for example utilizing ladders or rope-access methods to test materials in hard to reach spots, while providing full analytical capability for positive material identification of all type of alloys.

Industries such as aerospace, automotive, naval, or oil & gas have been relying for a long time on handheld XRF for quality control of a large variety of metal and alloys. Using handheld LIBS, those industries can now measure at line, low levels of carbon in stainless steel and make sure that critical parts for items such as jet engines, exhaust manifolds, boat propeller shafts or piping are manufactured to the correct grade.

Conclusion

Handheld XRF and Handheld LIBS are more complementary than competing techniques, each excelling in determining the composition and in verifying the grades of different families of alloys.

Handheld XRF over the last 15 years has revolutionized the way non-ferrous metals are analyzed. Handheld LIBS is tipped to replicate this success over the next 15 years in carbon and low alloy steels along with its ability to differentiate L and H grades of stainless steel in the field due to its small size, low weight and ability to reach the least accessible locations.

Additional Resources: