Interview with: Dr. Ravi Yellepeddi from Thermo Fisher Scientific about wavelength dispersive x-ray fluorescence (WDXRF) spectroscopy.
Please start by introducing yourself, and telling us about your background.
Dr Ravi Yellepeddi:
Okay, thanks Stewart. So I started with Thermo Fisher Scientific about twenty-five years ago, after completing my PhD in physics (solid-state spectroscopy and materials science), which is a very relevant area of specialization for what we do in Écublens, Switzerland. I was in the University of Geneva in chemical physics as a faculty member for about eight years, doing research and teaching spectroscopy courses, and joined as Chief Scientist in R & D what was called at that time ARL, Applied Research Laboratories, before we became part of Thermo Fisher Scientific in 1996. Initially, I was leading the new product developments related to X-ray technologies, their applications in industrial markets via customer interactions and marketing before taking the responsibility for all products made in Switzerland. Here in Ecublens, we develop, manufacture and commercialize three product lines: the X-ray Fluorescence (XRF) and Diffraction (XRD), Optical Emission Spectrometry (OES) and Laboratory Automation. We will be covering mostly XRF this interview as a focus point. OES is used predominantly for metals analysis with the spark excitation and this was the first analytical technique developed within our business unit. Laboratory Automation combines XRF/XRD and OES analyzers with the robotics for a completely automated operation providing chemical analysis. Currently we are organized under Vertical Markets with focus on Metals, Cement, Minerals, Coal, Materials Science and non-metallic gauging and I am the Global Director of Marketing for this business unit called Materials and Minerals. As you probably know, Thermo Fisher Scientific offers analytical technologies and solutions in three areas of any industrial market: field, lab and line based analyzers. Indeed, as part of Chemical Analysis Division, we manufacture and commercialize portable instruments for field applications, on-line analyzers for real time process control and laboratory instruments for more flexible analysis which also serve as reference for the field and on- line. Our goal is to provide complete workflow solutions integrating our lab and on-line to the end markets with a view to improve material and energy efficiency, productivity, higher quality and reliable analysis.
That’s fantastic, thank you. You mentioned wavelength-dispersive x-ray fluorescence there. So to begin, just what is wavelength-dispersive x-ray fluorescent spectroscopy?
Dr Ravi Yellepeddi:
Yeah, so wavelength-dispersive x-ray fluorescence is one of those fundamental spectroscopy techniques developed in the first half of last century already, to identify and quantify the chemical elements in a sample. X-ray fluorescence comes in two flavors: one is the Wavelength-Dispersive X-ray Fluorescence, which we will be focusing on today; and the other is called Energy-Dispersive X-ray Fluorescence- in short, WDXRF, and EDXRF. Both are complementary; the fundamental physics is the same. They differ in the way the sample fluorescence is dispersed and detected. Like in any spectroscopy technique used for chemical analysis, or elemental analysis, one needs (i) a suitable source to excite the sample, (ii) then to disperse the response from the sample (the secondary emission or fluorescence) into discrete wavelength-dispersive x-ray spectrum, (iii) a means to detect the intensities at those wavelengths, and (iv) quantify them into concentrations. The excitation is done using a source of X-rays, an X-ray tube or a radioactive isotope although a great e majority of the WDXRF or EDXRF instruments use X-ray tubes which are controllable sources. The X-ray tubes can have different powers, typically from 1W used in portable/hand-held XRF instruments to several kilowatts used in high performance Laboratory XRF instruments. These X-ray are then absorbed by the sample which then emits fluorescence spectrum with characteristic wavelengths or energies corresponding to the chemical elements present in the sample. These wavelengths are dispersed by using crystals, single crystals of a specific structure. The phenomena of this dispersion is governed by Bragg’s Law which relates the wavelength dispersed to a specific angle of the crystal and as one rotates the crystal, different wavelengths get diffracted at different angles. This device, also known as Goniometer, is commonly used in WDXRF spectrometers. One can also fix the position of the crystal to measure one specific wavelength and this device is called a Monochromator. Some of the WDXRF instruments can have Goniometers and Monochromators, the former giving flexibility and sequential measurement while the latter providing simultaneous and rapid measurement. The final step in the measurement scheme is the detection of each wavelength/energy using specific detectors. Different types of detectors are used for X-rays: gas detectors based on ionization, scintillation detectors and semi-conductor based solid state detectors. The output of these detectors is then processed to obtain, by calibration and calculation, concentrations of the elements in the sample analyzed.
So it’s as simple as excitation, dispersion and detection – three steps in the process which compose the physics of any WDXRF spectrometer. It’s one of the most universal techniques with the capability to handle multi-element, multi-matrix samples: organic materials, inorganic materials; solids, or liquids; loose powders, or compact, rigid solids; small samples, or big-sized objects; granules, or solid, rigid samples. It is therefore a very versatile technique that can measure the chemical analysis, or element analysis, in a variety of materials, both in an industrial environment or in an investigative laboratory.
That’s fantastic, thank you. So, what specifically have you been working in the context of WDXRF?
Dr Ravi Yellepeddi:
So we have been doing the development and manufacturing of WDXRF for nearly fifty years now. Thermo Fisher Scientific is one of the leading suppliers of X-ray fluorescent instruments covering a broad product portfolio from field portable XRF through Laboratory XRF to on-line XRF and we are continuously improving our products with innovative technologies and analytical expertise. In general, in WDXRF, there is really not a revolution- it’s much more an evolutionary development over years. So most of the time, the new instruments are developed with the availability of new crystals, new power sources, x-ray tubes, and certainly different geometries of the instrument for convenience and performance. As the industry demands grew alongside the multiple analytical tasks that the chemical laboratories needed to ensure, the X-ray instrumentation and analytical software programs had to become more flexible with continuous improvements in performance. Indeed, the full scale of WDXRF versatility began to unfold with new generation instruments in order to accommodate solids and liquids, loose powders, small samples, thin films, coated substances, and all kinds of organic and inorganic materials. The performance of the WDXRF was also getting better and better. When we say analytical performance, it’s about the element coverage in the periodic table from ultra-light elements to the trans-uranium series, their limits of detection, i.e. the lowest concentration that we can detect and quantify statistically at the three-sigma level in a given sample etc. Today, we are able to detect sub-parts per million, sub-ppm levels, say from 0.1ppm upwards for most of the elements in a given sample. So, various factors influence the new product development: analytical performance, instrument reliability, repeatability (precision), element coverage, sample handling convenience, cost per analysis (cost of ownership) and peripheral dependence etc. are some of the criteria that help design a new instrument.
That’s great, thank you. Now, what kinds of materials and application areas particularly benefit from this kind of study?
Dr Ravi Yellepeddi:
WDXRF established itself as a matured technique in industrial markets such as cement, metals, minerals, glass, ceramics, chemicals and petrochemicals, polymers etc. Most of the time, it was limited to inorganic materials in early fifties and sixties. The technique became almost routine in industrial markets, in terms of controlling the raw material to the intermediate products, to the final products. Highly respected for its reliability and repeatability as an analytical technique, WDXRF has become a reference method in process control and quality control in cement, metals, minerals, coal, glass, ceramics, refractories and associated materials. Then, a nice evolution, and a great boon to the WDXRF, happened in the early Sixties and Seventies, when physicists developed programs, algorithms, where an intensity measured on a totally unknown sample can be processed to quantify the concentrations. These programs are popularly known as “Standard-less” analysis programs.
Let me explain: in majority of the industrial applications such as cement and metals, one can easily calibrate the WDXRF instruments using a series of certified reference materials (standards) for specific elements in a specific matrix and within a specific concentration range. So if I’m talking about a cement plant, I would look at the cement materials like clay minerals, limestone, dolomites and gypsums, clinkers and so on. I would then look for internationally-referenced materials that match those types of materials, use those standards to calibrate my X-ray spectrometer, and those calibration curves are used to analyze the samples across the cement plant. Now, there are number of other areas or applications where such reference materials are not always available or they are too expensive or too few to establish a decent calibration curve. The so called “Standard-less” programs or Universal Quantitative programs are very helpful in handling such unknown or varied samples without having to build specific calibration curves for each element in each matrix. This development has opened many avenues for WDXRF from routine analysis in industrial applications to a central laboratory handling all kinds of materials.
So let me give you some examples on that. One of them is the environmental sample analysis: air, water and soil contamination can be monitored using XRF either to detect toxic elements or to track industrial pollution etc. Forensic analysis is another example in which the physic-chemical nature of the sample is not always known and XRF technique comes handy for a quick, non-destructive chemical characterization of the samples taken from an accident site or crime scene. So the applications we are covering today with XRF in general have grown tremendously in the last fifteen to twenty years, thanks to the sensitivity improvements, limits of detection improvements, the reliability and ease of use of the modern XRF instruments, but more so with the software packages (matrix-independent, of “standard-less” analysis) that can handle all types of materials, irrespective of their source, their origin and their chemistry.
Another advancement in recent WDXRF instruments is the ability to identify and quantify defects or imperfections, inclusions or anomalies in a solid sample. Historically WDXRF technique was used to get the overall chemistry of the sample in the sense that it was a bulk elemental analysis. Indeed, in a typical industrial process control, the purpose of WDXRF instrument in the lab is to get as representative analysis as possible which reflects the process and quality of the product being manufactured. In recent years, there has been increasing interest to identify, wherever necessary and possible, the elemental distribution in a given sample using WDXRF. Typically, in a metallic or glass sample, which goes through melting, fusing or other physic-chemical treatments, it is important to ensure that all the elements are uniformly distributed and that there are no undesirable precipitations or aggregates that can adversely affect the final product quality. This is referred to as elemental “spotting” or “mapping” in contrast to the bulk or averaged analysis. We have been developing WDXRF instruments which can handle both the bulk sample analysis as well as provide that complementary information about the elemental distribution within the same sample. These “elemental distribution maps” help the process chemist or metallurgist to identify rapidly and in a manufacturing environment, any unexpected changes or reactions leading to quality deviations. In few minutes, the chemist in the lab, with the help of these 2D or 3D elemental maps, can determine whether the product coming out of that furnace, or that melting process, is indeed homogeneous and corresponds to the overall chemistry and structure, so that its physical and chemical properties can be guaranteed. Such elemental mapping or spotting is a valuable piece of information in some critical applications.
That’s fantastic, thank you. So finally, as always, I’d like to finish by asking, in your opinion, what are the other hot topics in materials science right now?
Dr Ravi Yellepeddi:
Okay, thank you, Stewart – I think that’s a very pertinent, valid question. As we speak today, the last ten years have seen a tremendous pressure on industrial markets and forced the industries to develop materials or processes that consume less energy, pollute less, yet obtain very reliable products for final application. Some of these industries are also being increasingly regulated for safety, environmental control and quality. Chemicals, petrochemicals, polymers, food products, waste products etc. have all been subject to tighter controls and more efficient production. In that sense, energy and materials have become central themes in our modern life and industrial environment. So as these industries and research scientists work on alternate materials, new areas of applications, new or advanced materials and a demanding process control, the analytical techniques such as XRF are being challenged to provide comprehensive and adoptive analysis.
An example which is very relevant today is the cement industry. Cement and Metals industries are energy intensive industries and they are under pressure to reduce the energy consumption or to use alternative fuels while maintaining tighter quality controls of the end products. In addition, the quality of the raw materials is also degrading as most of the good quality ores are depleted. With the urbanization and tremendous industrial growth we have been witnessing in emerging economies, these energy-materials balance has become even more relevant. With strong desire to limit the greenhouse gases, lesser consumption of natural resources and more efficient process control, the analytical techniques such as XRF are being increasingly solicited to help meet those targets. Integrated technologies (XRF/XRD) are being deployed to not only control the overall chemistry of the process but also specific compounds or minerals to improve the energy efficiency and product quality.
So the hot topics in which XRF technique can play an important role are related to the manufacturing of industrial processes and materials, linked with very demanding environmental restrictions, material quality of the incoming raw materials, and tighter specifications of the final product quality.