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XRF spectrometry is a rapid, multi-element technique that is accuratewith linear calibration and good long-term precision. A non-destructive methodology, XRF requires little or no sample preparation and lendsitself well to automation.
Elemental analysis using X-ray fluorescence (XRF) spectrometry is established as the key to controlling awide range of quality and production processes. The technique is becoming widespread in the chemical, food, pharmaceutical, cosmetic industries, as well as for cement, mining and petrochemical businesses, where it provides a valuable tool for manufacturing and quality assurance,monitoring environmental impact and resource utilization.
XRFis a highly accurate and reproducible technique for identifying and determining the concentrations of the chemical elements present in solids, liquids and powders. An XRF spectrometer measures the individual component wavelengths of the fluorescent emission produced when a sample is irradiated with X-rays. XRF is capable of measuring elements fromBeryllium (Be) to Uranium (U) and beyond at trace levels and up to 100%.
There are two main types: wavelength dispersive XRF (WDXRF), which is achieved by diffraction using an analyzer crystal; and energy dispersive XRF (EDXRF) that does not use a crystal. WDXRF systems provide application versatility, optimal measurement conditions, excellent light element performance, very high sensitivity and low detectionlimits. EDXRF systems generally offer a lower cost alternative for more routine applications, although recent developments in detection technology are bringing significant analytical improvements to these systems.
X-ray fluorescence analysis is non-destructive of the sample. It can be carried out directly on liquid or solid materials - a powder in a sample cup or one that has been pressed into pellets, for example. Simple sample preparation and manipulation deliver significanttime and cost savings. No reliance on chemical pre-treatment or digestion processes ensure that a sample can be recovered intact for repeat,or further, analysis.
XRF exhibits a linear response between count rate and concentration. The data below show how a method developed by Northern Ireland’s Agri-Food and Biosciences Institute (AFBI) inBelfast to monitor the problem of phosphorus fertilizer over-application has an almost perfect straight-line fit.
A wavelength dispersive XRF spectrometer was used for this analysis of phosphorus in grass. The Axios system uses dedicated Pro-Trace software - a solution for calculating net intensities in trace element analysis. A series of advanced algorithms provide accurate background determination, and corrections are performed for matrix effects, spectral overlap and low-level spectral impurities. Figure 1 shows a typical calibration curve forphosphorus analysis.
A) Figure 1: Linear calibration: phosphorus
For quality control purposes the determination of B2O3 concentration in borosilicate glass is critical. The data below illustratethe highly precise analysis that is routine with XRF. Both short- andlong-term stability are presented. Borosilicate glass consists of silica (70-80 %), boron oxide (7-13 %), smaller amounts of alkalis (sodium and potassium oxides) and aluminium oxide. The relatively low alkalicontent gives borosilicate glass its useful properties: high chemicaldurability, high thermal shock resistance and high mechanical strength.
A fully integrated WDXRF spectrometer (Axios-Advanced) wasused for the analysis of borosilicate glass. The instrument was equipped with PANalytical’s most advanced X-ray tube - the SST-mAX. This isa high power, metal-ceramic end window tube with a 160 mA emission current that incorporates ZETA (Zero Evaporation Technology Advantage) cathode technology. This eliminates classical tungsten cathode evaporation, the major contributor to instrument drift to ensure an unprecedented constant output over its entire lifetime.
One Breitlander glass sample was measured repeatedly to assess analytical precision. Measurements were carried out 20 times consecutively and also over a period of five days. Results demonstrate that the analytical precision, repeatability and reproducibility are excellent (Table 1). The 20 consecutive measurements show standard deviations of 0.58 % relative at 4 wt% (3.95 ± 0.02 wt% B2O3). This level of precision is maintained over five days.
The counting statistical error (CSE) - theoreticallythe minimum possible error - is also shown in Table 1. Comparison with the CSE emphasizes the inherent stability of XRF for B2O3 (Figure 2). Measurement time for boron, including the peak and two background positions, is just 300 seconds per sample.
Table 1. Analytical precision
Figure 2. Short- and long-term stability of B2O3 in glasssample
The multi-element capability of XRF, combined with thepossibility to implement sophisticated matrix correction, makes XRF the ideal analysis technique for assessing the use of alternative fuelsin the manufacturing process. In addition, it offers a cost-effectivemeans of ensuring compliance with environmental regulations.
Manufacturers in many industries are relying on alternative fuels to help them control production costs and improve environmental performance– two very significant benefits. If international targets for the reduction of CO2 emissions are to be met, then the wider use of alternative fuels in the future is certain. In most parts of the world the incineration of alternative fuels and the resultant emissions are tightlycontrolled by legislation.
Alternative fuels pose two main challenges: they may contain toxic elements - such as Hg, Cd, Tl, Pb, Crand Zn - that are heavily restricted, and considerable variation can occur in the composition of fuel batches making it hard to standardizeand challenging to monitor
Using a high-performance EDXRF spectrometer (Epsilon 5) a method was designed to analyze different typesof solid alternative fuel samples with just one robust calibration program. As suitable reference materials for alternative fuels are not readily available, CRMs were selected with a wide variety of different matrices. Results demonstrate that all these different matrix types fiton one calibration line, providing proof that it is possible to measure a wide variety of sample types with just a single calibration.
Table 2. Calibration results
Automation & control
All the advantages of XRF analysis reported above can be delivered in a fully automated process control system where appropriate. Here, the systemat a major steel manufacturer is described.
Thyssen Krupp Steel AG is a manufacturer of high-quality carbon steel flat products fora variety of applications. In 2000, the company installed its first fully automated production control laboratory at its Bruckhausen steelworks (OX-1). Five years later a second automated laboratory, at the Beeckerwerth steelworks (OX-2), was commissioned. The laboratories have many similarities: both are situated inside modular container housings;peripheral equipment, such as climate control, compressed air supplyand an airtube system, is mounted on the roof of the container; and both laboratories perform automated analysis of iron, steel and slag samples taken from each treatment point in the steelmaking process. Analysis needs are served by a combination of X-ray Fluorescence (XRF) spectroscopy and Optical Emission spectroscopy (OES). All the analytical equipment is installed inside a 325 m2 container and located directly in the plant.
Conclusion
From economical stand-alone monitoring systems to high-performance, multi-element analysis that provides an insight into complex processes, the intrinsic advantages of XRF offer significant benefits. Ideal for a wide range of industries, and usedfor monitoring, control and quality assurance, this robust techniqueis being adopted for a growing range of applications.
By Dr SimonMilner, Senior Product Manager, PANalytical BV, Almelo / The Netherlands
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