X-ray fluorescence (XRF) and X-ray diffraction (XRD) are complementary, nondestructive techniques for analyzing the chemical composition of materials.
On X-ray excitation of electrons from lower to higher orbitals, the fluorescence emitted as the electron returns to its unexcited state is diagnostic for specific elements, and signal strength is proportional to concentration. XRF is independent of the element’s chemical form.
XRD occurs when X-rays incident on rigid chemical structures diffract, or scatter, characteristically to reveal an element’s chemical and physical state: for example, oxide vs. sulfide or crystalline vs. amorphous.
While XRD quantifies concentrations of specific minerals, peaks often overlap in very complex samples like cement, making determination of specific phases difficult. Absolute concentrations of simple compounds like calcium oxide, however, are straightforward.
Laboratory-based XRF/XRD analysis traditionally requires two separate instruments. Combining the two modalities offers the advantages of reduced floor space, a single user interface, and a combination of diffraction and fluorescence data from a single sample.
XRF analyzes elements irrespective of sample type or chemical form, while XRD provides insights into structure, crystallography, and minerology. Current XRF capability spans nearly 90 elements, from beryllium to uranium. When incorporated within a single instrument, XRF and XRD provide detailed characterization of materials. “It’s like your left and right eyes, viewing objects from different angles, and the brain processing those different perspectives into one image,” says Ravi Yellepeddi, director of marketing and business development for elemental and structural analysis at Thermo Fisher Scientific.
XRF typically detects elements at concentrations ranging from parts-per-million to 100 percent, while XRD quantifies phases or minerals at concentrations down to around 0.1 percent. Since both XRF and XRD are mostly nondestructive and usually require little or no sample preparation compared with other analytical techniques, their applications range from routine industrial process control (for cement, minerals, metal, polymers, or petrochemicals) to investigative laboratories practicing forensics, archaeology, and gemology.
The complementarity of XRF/XRD and their co-location in one package sold by Thermo Fisher Scientific are unique in the world of instrumental analysis. Together, the methods help elucidate and predict physical, mechanical, electrical, and pharmacologic properties from structural and chemical attributes. While XRD and XRF cover quite a bit of analytical territory, users should view these methods as just two pieces of what is normally a complex, many-faceted material characterization puzzle that normally includes additional spectroscopy, microscopy, and separations (e.g., chromatography).
INDUSTRIAL AND RESEARCH CATEGORIES
Yellepeddi assigns XRD/XRF to two significant application categories: industrial and investigative/research, for which some overlap exists. Research applications include discovery, basic science, and analysis of samples with novel or unknown chemical composition. Industrial uses include quality control, process monitoring, and field analysis in mining, geology, and environmental industries.
The relationship between chemistry and structure is complex. In cement, the world’s most-produced material, chemical composition is critical, but concrete’s structural performance depends on the constituent elements’ mineralogical forms, crystalline structures, and physical phases.
Composition and structure are key quality attributes for all materials, including polymers, cement, semiconductors, foods, composites, pharmaceuticals, and glasses. For cement, these properties determine a formulation’s compressive strength, volume expansion, and setting time. “XRF unveils the basic chemistry, but a material’s ultimate performance is reflective of its minerology, which is the realm of XRD,” Yellepeddi says.
The same base characteristics governing the suitability for cement for building also dictate the charge density and number of lifetime recharge cycles for advanced energy storage batteries or the dopant levels in semiconductors. The real-time nature of XRF/XRD helps process engineers monitor those properties during the manufacture of these products.
Similarly, macro and micronutrients are regulated or registered for most food products, particularly those carrying nutritional or health claims. XRF (in addition to inductively coupled plasma and mass spectrometry) guarantees elemental concentrations, whereas XRD confirms texture, crystallinity, and other physical properties related to taste, solubility, or homogeneity.
In pharmaceuticals, XRF/XRD assume even greater significance, for example, in assessing the quality of starting materials, the progress of synthetic or formulation processes, the quality attributes of drug substances, and the integrity or suitability of formulated packaged drugs on pharmacy shelves.
For solid drugs, assurance of suitable dissolution and distribution through the body requires drug compounds to exist in a preferred physical form with the required degree of crystallinity. Crystalline drugs are less immediately dissolved and somewhat less bioavailable compared with amorphous compounds, a fact that plays into controlled release formulations for many pharmaceutical products.
Solid or liquid suspensions of crystals or amorphous compounds, or solid solutions of either, also require precise knowledge not just of drug loading (e.g., as measured by fluorescence) but of the fraction of active ingredient that exists in the desired crystalline form. XRF/XRD serves as a rapid technique for quantifying identity, concentration, degree of crystallinity, and overall formulation quality.
Additionally, XRD can determine the crystallite size distribution for solid drug molecules and their suitability for dosing as tablets, pills, oral suspension, time-release oral or depot formulations, or as liquid suspensions or injectables. Particle size also affects flow, rheology, absorption, and in vivo dissolution. Simultaneous with these measurements, XRF will detect impurities whose maximum concentrations fall under regulatory scrutiny.
While XRF has been a mainstay of chemical analysis since the 1960s, diffraction has traditionally been relegated to research activity. “That has changed,” says Yellepeddi. “XRD has, like XRF, become industrialized, highly automated, and integrated into the control of many industrial processes.”
In this regard, XRD and XRF have followed a path to accessibility similar to that of mass spectrometry, high-performance liquid chromatography, and nuclear magnetic resonance. “XRD and XRF have been demystified and democratized. They’ve become routine tools that no longer require a PhD to operate. Instrument software has built-in control files with predefined calibration programs specific to industries or products,” Yellepeddi says. “We’ve come a long way from research to industrial solutions.”
Recent developments in XRF and XRD have made possible the quantification of elements and crystallinity without the need to generate standard concentration curves based on reference materials. Thanks to fundamental X-ray physics, fluorescence and diffraction responses can be predicted through algorithms integrated into instrument software, thus broadening XRF and XRD capabilities into the realm of unknown or one-off samples.
“Most diffractometers today are equipped with universal phase analysis packages, which are based on fundamental parameters like crystallography data, a plus when working with unknowns,” Yellepeddi explains.
Users can purchase reference libraries for specific applications like minerals or polymers or pharmaceuticals. For example, the International Centre for Diffraction Data (ICDD) houses crystal structures of organic and inorganic compounds, including structural data for ceramics, metals, glasses, polymers, chemicals, and catalysts.
Thermo Fisher Scientific sells diffractometers with such libraries as an option. The libraries help identify phases within samples even when the chemistry is completely unknown. “But if you know the chemistry, this feature becomes even more accurate,” Yellepeddi says. “What used to be a painful data processing and interpretation exercise has become extremely easy thanks to modern computing power, libraries, and time-tested algorithms for detecting amorphous content, crystallinity, and crystallite size.”
PURPOSEFUL APPLICATION CATEGORIES
Within those application categories, Yellepeddi further delineates applications as field, lab, and line. Each is served by XRF/XRD instruments of varying power and sophistication, ranging from handheld to transportable to process-dedicated instruments and laboratory-based analyzers with high performance and flexibility.
Research labs demand the ultimate in analysis power and flexibility, with complete XRF/XRD coverage. These labs may analyze just a few samples per day, but sample types constantly change. “These labs don’t necessarily analyze the same elements or structures, or concentrations repeatedly,” Yellepeddi says, “which is why for this group, I recommend a full-size floor-standing instrument with high sensitivity, precision, reliability, and—above all—flexibility.”
Industrial process control, the midpoint on the application spectrum, involves monitoring expensive production methods for alloys, polymers, thin films, pharmaceuticals, and other high-value (and many relatively low-value) products. “Here, you want to be sure the instrument adequately and reliably measures established process parameters 24/7, reliably and accurately—and often in regulated environments and under harsh conditions, such as near a blast furnace or inside a petroleum refinery. For these customers, reliability and robustness are prime concerns, as are pre-calibration and preconfiguration for the task at hand.” Users also need to consider the cost of ownership, including acquisition costs, consumables, maintenance, and repair.
Field-worthy instruments for geology, mining, and environmental testing are either handheld or portable. Since these tests are conducted on-site, far from central laboratories, purchasers should consider instrument size and weight, and how they will deploy the XRF equipment, for example from a backpack, handheld, or from the back of a vehicle. Battery life, weight, network connectivity, remote controls, and global positioning are highly desirable. Users should understand that features that enable portability often come at the cost of somewhat compromised detection limits, elemental coverage, and reliability for reporting quality data from the field.
XRD is not generally available in handheld devices, although handheld fluorescence-based instruments are sold by several companies, including Olympus, Thermo Fisher Scientific, and Hitachi.
“Handheld XRF is used in the field for real-time geochemical, geological, and environmental analysis, and also as point-of-use analyzers in scrapyards, pawnshops, and refineries,” explains Mathieu Bauer, PhD, senior applications scientist at Thermo Fisher Scientific’s Munich, Germany, facility. “These systems are easy to operate and help users without extensive background in analytical chemistry make quick defensible decisions, whereas laboratory XRF instruments are used in combination with extensive sample preparation to generate more precise and accurate results.”
Handheld XRF analyzers use lower power and therefore have higher limits of detection compared with full-featured lab systems. “Still, their performance is comparable to laboratory energy-dispersive instruments of just 10 to 15 years ago,” Bauer adds.
In the lab, XRF operates through wavelength dispersion or energy dispersion, but handheld devices use only energy dispersion. Compared with wavelength-dispersive methods, energy-dispersive XRF has lower acquisition and maintenance costs and consumes less electrical power but is less sensitive for light elements.
In many instances, handheld XRF provides the answer to a field investigation, for example, alloy composition or the distribution of a toxic metal at a Superfund site. In other situations, the method serves as a screen for particular analytes, which are further and more comprehensively studied by transporting samples back to laboratories.