Portable X-Ray Fluorescence - Research Paper Example

Table of Contents 3 Introduction 4 Theory 4 How the instrument works 4 Methodology 5 Experiment 5 Experiment 2 11 Conclusion 16 Abstract The techniques adopted in this lab test include XRF, with the main objective of the study being to develop a critical understanding of the application of these techniques. The experiment entails preparation and application of instruments, samples, and analysis of data. The outcome of the experiment highlights advantages and limitations to each technique and instrument applied during the lab exercise. In which case, three experiments were set up with the aim of exploring the concept of XRF. For the first experiment, there is analysis of gold in plating solutions in the presence of several additive ions by portable X-ray fluorescence (XRF) revealing an optimum potassium gold cyanide concentration of 2 - 3 g/L for maximum cathode efficiency. Further, there is exploration of utility of XRF to determine the thickness of gold plated on copper substrates up to 6 microns. For the second experiment, the objectives were to determine the detection limits, accuracy, repeatability and efficiency of a X-ray fluorescence spectrometer (Niton XRF analyzer) in comparison with the traditional analytical methods, inductively coupled plasma optical emission spectrometer (ICP-OES). Further, the experiment also involved applying inductively coupled plasma optical emission spectrometer (ICP-MS) in screening of major and trace elements of environmental samples including estuary soils and sediments, contaminated soils, and biological samples. For the third experiment the major objective was to compare lead concentrations in surface peat samples from the South Pennines (UK) derived using (a) FPXRF in the field, (b) FPXRF in the lab on dried samples and (c) ICP-OES analysis. Introduction Theory X-ray fluorescence spectrometry is a core aspect in analysis especially in the field of science and industry application. The concept of XRF lies on the principle that individual atoms, wherever excited as a result of contact with an external energy source, ends up emitting X-ray photons characterized by a a wavelength or energy. In which case, on this basis one can define the identity and quantification of an element present therein by simply counting the number of photons contained in energy from the given sample. Henry Moseley coined the technique, after being led by a discovery of X-ray tube that he applied in bombarding samples containing high energy electrons. Intuitively, Henry ended up with a mathematical relationship that could related the atomic number to the element’s emitted X-ray frequency. The years were followed by eventual development of the instrument leading to a modern XRF instrument that is reliable for analysis of solid, liquid, and thin-film samples for both major and trace components. How the instrument works The characteristic radiation associated with an atom, arising from the inner electronic shells, forms the background for the practice of using X-ray method in identification. In which case, the emitted ration from the electronic shells of the atoms are associated with specific energies which facilitates the identification process. Consequently, understanding the mechanism for using X-ray spectroscopy requires a bit of knowledge in how the x-rays are generated. The process begins with a high-energy electron beam striking a material thereby resulting to an interaction that yields emission of photons. A broad range of energies usually characterizes the photons. The radiation therein, referred to as bremsstrahlung, arises as the result of deceleration of electrons that are identified in the material. The bremsstrahlung range is a function of acceleration of electron especially for a molybdenum target. Another assuring result from the interaction between the material and electron beam is the ejection of photoelectrons form the inner shells of the atoms characteristics of the material. The principle of operation is that X-rays from an excitation source interact with the test surface causing the emission of secondary fluorescence X-rays that have energies characteristic of the atoms of the excited material. Contrary to ICP-OES and ICP-MS, XRF analyzer is characterized by the limited preparation required for solid samples, non-destructive analysis, increased total speed and high throughout, the decreased production of hazardous waste and the low running costs as well as multi-elemental determination and portability in the fields. The current comparative study demonstrates that XRF is a good rapid non-destructive method for contaminated soils, sediments and biological samples containing higher concentrations of major and trace elements. Unfortunately, XRF does not have sensitive detection limits of most major and trace elements as ICP-OES or ICP-MS but it may serve as a rapid screening tool for locating hot spots of uncontaminated field soils and sediments. Methodology Experiment 1 Figure 1, indicates the experimental XRF emission spectra for a gold plated substrate (figure 1a) and for gold plating solution(figure 1b). The figure shows that the bands are well-defined and resolved from copper and the brightening agents in the plating solution. Figure 1: Experimental XRF emission spectra for a) gold plated substrate and b) gold plated solution From the above model, calibration plots were created from analyzing standardized PGC solutions by XRF, yielding linear results in the concentration range investigated as seen in the figure 2 below. Figure 2: Calibration plots obtained for gold (as PGC) in plating solutions. The above Calibration of the XRF instrument was accomplished by dissolving solid PGC standard at concentrations of 0 - 12 g/L. . Intensities were obtained by integrating the gold (a) Lα and (b) Lβsignals. From these principles, a plot was then obtained for cathode efficiency against PGC as shown in the following figure 3 Figure 3: Cathode efficiency vs PGC concentration The plot above shows that action of copper substrate exhibited a plateau phase at concentrations of PGC higher than 3 g/L in the plating solution. As seen in Figure 4, at about 0 - 3 g/L of PGC in the plating solution, the cathode efficiency increases significantly with PGC concentration (24.0% increase between 0.39 g/L and 2.76 g/L). However, above this concentration of PGC, the efficiency of gold deposition remains around 35% - 38%. Therefore, addition of excessive PGC in plating operations is unwarranted. Optimum plating efficiencies are achieved at PGC concentrations between 2 - 3 g/L, in which the cathode efficiency is above 30% and before it begins to plateau (6.2% increase between 2.76 g/L and 7.80 g/L). Figure 4 shows that only negligible amounts of the brighteners were observed on the gold plated copper substrate, which is consistent with earlier reports of using brighteners in gold plating processes decreasing emission from copper. It was observed that the concentration of the brightening agents in the plating solution decreased minimally, compared to the rate of gold depletion. Figure 4: Calibration plot of copper and gold XRF emission intensities from plated substrate. Note: red squares represent copper (Kα , blue diamonds (copper Kβ), green triangles (gold Lα) and orange triangles (gold Lβ) Figure 5: Calibration plot of the difference between copper emission signals (Kα- Kβ) (red squares) and gold emission signals (Lα- Lβ) (blue diamonds). . The figure 5 above shows that the difference between the copper emission intensities (Kα - Kβ) and the gold emission intensities (Lα - Lβ) offers additional options for determining the thickness of the gold plate. Experiment 2 An X-ray fluorescence spectrometer (XRF) (Thermo Scientific Niton XL2) was used to determine major and trace elements in soils and sediments and oyster samples. Samples were introduced, via an open-ended sample plastic cell. An acid-purified sand (SiO2) was used as the media blank for determining the detection limits of major and trace elements and heavy metals. Figure 1: The comparison of major elements measured with XRF and with ICP-OES after acid digestion Figure 2: The comparison of major elements measured with XRF and with ICP-OES after acid digestion Figure 1 and 2 above shows the result of comparison of major elements when using XRF. The accuracy was determined by testing two certified reference soil standards containing major and trace elements and heavy metals. NIST SRM 2710 (Montana soil) basically contained higher amounts of most trace elements (Cu, Zn, Pb, U, Hg, and Mo et c.) than NIST SRM 2711a (Montana II soil). Five to eight replicates were run on both soils. Figure 3: XRF used to record recovery (%) of major and trace elements over the standard values for two standard soils: Figure 3 shows that, for both major elements (Fe, Mn, Ti, Ca, and K) and trace elements (Pb, Cr, Rb, Zn, Cu, V, Mo), XRF generated high recoveries of 80%–120% mostly, while the recoveries of As, Hg and U gave the overestimate compared to the certified values. To further check the precision and accuracy as well as reproducibility of XRF. Figure 4: CV % of three measurements of trace and major elements for two standard soils with XRF To further check the precision and accuracy as well as reproducibility of XRF measurements, measurements, the two standard soil samples were examined with three measurements. We calculated relative errors among three measurements for all major and trace elements (Fig. 4). The CV% for all major elements (Fe, Mn, Ti, Ca, K, S) and trace elements (Zr, Pb, Sr, U, Rb, Zn, Cu, V, Hg and Sc) were all below 10%, indicating that XRD achieved a good reproducibility among measurements. However, CV% for Th, Mo and As were in range of 18–28% Experiment 3 The set up was obtained by selecting a stratified random subset of 40 samples for ex situ chemical analysis. In order to represent the full range of Pb concentrations found across the fieldsite, the 159 in situ measurements were sorted by Pb concentration, divided into quartiles and 10 samples were selected at random from each quartile. The figure 3 below shows that Figure3shows the effect of increasing analytical time on the CV produced by the FPXRF when analyzing samples containing varying Pb concentrations. The CV decreases as a power function of analysis time for all samples, producing regressions with power functions of approximately−0.5 (statistically similar to each other, pairedt test, p=0.025). Figure 3: Coefficient of variation produced for peatsamples containing various concentrations of Pb with increasing ex situ FPXRFanalysis time. Conclusion The execution of the XRF in the process of measuring the metal within a sample can take less than an hour thus the quickest means in relation to speed as shown in experiment 1. The experiment shows that these instruments are capable of quantitatively testing gold in liquid solutions, such as plating solutions in the presence of other elements such as nickel, chromium, and cobalt. In addition, the same instrument can be successfully utilized to measure the thickness of gold plated substrates such as copper in ranges that are commonly encountered in commercially plated products. XRF is also cost-effective in operation in comparison to AA and AE. The devices or instruments are easy to use and requires minimal time for the researcher to obtain the required information in relation to the experiment as shown by experiment 2. Running samples using XRF is rapid (less than 2 min per sample). XRF has the highest throughout per day, especially for a laboratory with a large amount of samples and it is durable, portable, and cost efficient. It is a good monitoring tool for contaminated soils, sediments and biological samples containing higher concentrations of major and trace elements. Some of the limitations of the XRF relate to the object, instruments, or both in the process of executing crucial experiments. XRF instruments are frequently affected by high temperature and humidity. This includes limitation to the analyzer and the substrate. The instruments are most efficient in the soil and scrap industry due to density and geometry of the sample. There is also essence of external interference by absorption and scattering or improvement of fluorescence. Detection is also a limitation as it hinders achievement of accurate information in relation to element of interest. The elements of interest are dissimilar in distribution within or on the artifact thus inaccuracy of data during the measurement process (Beckhoff p. 435). References McComb, J., Rogers, C., Han, F. & Tchounwu, P., (2014) Rapid screening of heavy metals and trace elements in environmental samples using portable X-ray fluorescence spectrometer, A comparative study. Water Air Soil Pollut.; 225(12): . doi:10.1007/s11270-014-2169-5 Shuttleworth, E., Evans, G., Huchtison M. & Rothwell, J., (2014). Assessment of Lead Contamination in Peatlands Using Field Portable XRF. Water Air Soil Pollut. 225:1844 DOI 10.1007/s11270-013-1844-2 Brandon J., Andrew G., Arthur L.,Terry D. & Edward W., (2014). Rapid Determination of Gold during Plating Operations by Portable X-Ray Fluorescence. American Journal of Analytical Chemistry, 2014, 5, 1178-1183 http://www.scirp.org/journal/ajac http://dx.doi.org/10.4236/ajac.2014.517125 Read More