Philip J. Potts

Microprobe techniques in the earth sciences

Microanalysis from 1950 to the 1990s. Electron microprobe analysis. Analytical electron microscopy. The nuclear microprobe - PIXE, PIGE, RBS, NRA and ERDA. Synchrotron X-ray microanalysis. Ion microprobe analysis in geology. Mineral microanalysis by laserprobe inductively coupled plasma mass spectrometry. Ar-Ar dating by laser microprobe. Stable isotope ratio measurement using a laser microprobe. Micro-Raman spectroscopy in the Earth Sciences. Index.

An objective assessment of analytical method precision: comparison of ICP-AES and XRF for the analysis of silicate rocks

Abstract The precision of an analytical method has been evaluated objectively by applying the method of Thompson and Howarth (1976) to the analysis in duplicate of 55 igneous rocks covering a range of silicate matrix types and analyte concentrations. Results were analysed 1.0 characterise the change in precision ( s c ) of the analytical method with concentration ( c ) according to the equation s c = s o + kc , where the k parameter represents the limiting high-level precision and s o , the precision at zero concentration, which is related to the method detection limit (MDL). Test materials were analysed using four analytical methods based on two analytical techniques, inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray fluorescence spectrometry (XRF), as operated under routine working conditions in the two participating laboratories. The two XRF methods were major elements on fused glass discs and trace elements on powder pellets, and the two ICP-AES methods were major elements after a fusion decomposition technique and trace elements together with selected major elements, after an acid attack. Statistical evaluation of the data showed that significant changes in precision as a function of concentration (i.e. the k factor) were determined in 34 cases out of 78 analyte-method combinations. In cases where no significant change in precision could be detected, a grand mean precision, representative of the concentration range analysed was calculated. The s o parameter was found to be significantly different from zero in 36 cases out of 72. To allow evaluation of the detection limit performance of all data, a maximum method detection limit (MMDL) was calculated, which was estimated to be on average 1.62 times greater than the MDL derived from significant values of s o In terms of the four methods studied, median high-level precision of the techniques used to determine major elements were found to be 0.23% relative (XRF/glass discs), 0.43% (ICP-AES/fusion decomposition) and 0.70% (ICP-AES/acid attack). Typical precision values in the determination of trace elements by both techniques was 1.5%, providing elemental concentrations extended over a significant range. MMDLs varied from element to element but for XRF/powder pellet data were found to be approximately equivalent to instrumental detection limits (IDLs) calculated from background count rates. However, for trace elements determined by ICP-AES/acid attack, MMDL were found to be on average three times larger than IDLs measured from repeated analysis of an aqueous blank. As a result of an evaluation of these data, it is proposed that appropriate figures of merit to describe the analytical performance of a technique are: (1) median precision in the determination of major elements; and (2) the number of trace elements that can be determined to MDLs of less than one-tenth the crustal abundance of the element. These factors should then be evaluated in conjunction with logistical factors including the rate at which samples can be analysed and the cost per determination. The influence of these factors on applications of the techniques studied in pure and applied geochemistry are discussed.

Portable X-ray Fluorescence Spectrometry Capabilities for In Situ Analysis

Portable X-ray fluorescence (PXRF) instrumentation has some unique analytical capabilities for the in situ analysis of samples in the field. These capabilities have been extended in recent years by the continuing development of solid state detectors, surface mounted electronics, digital signal processing technology, Li-ion batteries combined with a choice of rugged sealed radioisotope sources or miniature X-ray tubes that provide lightweight hand-held devices. As well as opening up new applications, in situ measurements by PXRF, where the instrument is placed in direct contact with the object to be analysed, involve the complete integration of sampling and analysis. Careful interpretation of results is therefore required, particularly when the analysis is used to estimate the bulk composition of a sample. In this monograph, an overview is given of instrumentation, analytical capabilities, and limitations in the interpretation of results, sampling considerations and applications where PXRF offers substantial advantages over conventional analytical techniques. The aim is to give the reader an insight into the capabilities of the technique and to demonstrate the contribution it can make to a range of areas of contemporary scientific interest. Chapters are written by internationally recognised scientists with practical experience of in situ analysis using portable X-ray fluorescence and demonstrates the wide range of applications for the technique. The topics covered are illustrated with diagrams and photographs where appropriate and each chapter includes supporting references to enable the reader to gain a greater understanding of a particular application. Topics include: -analytical capabilities -instrumentation -quantification -correction procedures -sampling considerations -future developments Applications include: -the assessment of contaminated land -surfaces -coatings and paints -workplace monitoring -metal & alloy sorting -geochemical prospecting -archaeological investigations -museum samples & works of art -extraterrestrial analysis The work is aimed at scientists who have some knowledge of analytical techniques and/or the applications covered, but are not fully familiar with the capabilities of PXRF. It offers a general introduction to the technique and its applications rather than a research monograph. As such, it is aimed at analytical scientists, environmental and geological scientists, industrial hygienists, industrial and plant scientists, archaeometrists and museum researchers, research scientists and research students with projects in the applications covered. Undergraduate students studying associated degree courses will also benefit from the work.

Determination of the rare-earth element abundances in 29 international rock standards by instrumental neutron activation analysis: A critical appraisal of calibration errors

Abstract A standard additions method has been used to calibrate an in-house multi-element rock standard for the analysis of the rare-earth elements (REE), and Th, Ta, Hf and Co by instrumental neutron activation analysis. This preliminary calibration, with a precision of typically better than 7% (1σ) has been refined by comparing our analyses of the four U.S.G.S. standard rocks AGV- 1 , BCR- 1 , G- 2 and GSP- 1 with compiled analyses obtained by a critical survey of over 70 published references. Our standard calibration was adjusted by the average discrepancy between our analyses and published analyses for each element for all four U.S.G.S. standards. The accuracy of this final calibration is estimated to be better than 5% absolute and it has been used to determine the above elemental abundances in 29 international rock standards. Five of the REE have been determined by interpolation from chondrite-normalised plots.

High-precision instrumental neutron-activation analysis of geological samples employing simultaneous counting with both planar and coaxial detectors

Abstract An improved method of rock analysis by instrumental neutron-activation analysis is described in which irradiated samples are counted simultaneously using both planar and coaxial Ge detectors. All suitable photopeaks in the 60–1600-keV region of the spectrum are analysed from samples counted within 10 days and again within 30 days after irradiation. Data are presented on the relative sensitivity of individual photopeaks and the precision and accuracy of the technique in routine analysis. New analyses are included for six U.S.G.S. geochemical exploration samples (GXR- 1 to GXR- 6 ).

Determination of platinum, palladium, ruthenium and iridium in geological samples by isotope dilution inductively coupled plasma mass spectrometry using a sodium peroxide fusion and tellurium coprecipitation

A method was developed for the determination of Ru, Pd, Ir and Pt in geological samples by isotope dilution inductively coupled plasma mass spectrometry. After fusion of the sample with sodium peroxide, the platinum group elements were preconcentrated by Te coprecipitation. Results obtained for the reference materials WGB-1, TDB-1, UMT-1, WPR-1, WMG-1 and SARM-7 are in excellent agreement with the recommended values for elements above the detection limit level of 0.3–2.0 ng g–1(whole rock). Although the method used only 0.5 g of sample, no errors were found that could be associated with sample inhomogeneity effects in the analysis of the above reference materials. Further measurements indicated that the technique could be extended to the determination of Rh and Au by external calibration.

Analysis of silicate rocks using field-portable X-ray fluorescence instrumentation incorporating a mercury(II) iodide detector: a preliminary assessment of analytical performance

An assessment is reported of the performance of a field-portable energy dispersive X-ray fluorescence analysis system using 55Fe, 109Cd and 241Am excitation sources and a high resolution mercury(II) iodide detector. Seventy international reference materials, mainly of silicate composition, were analysed as compressed powder pellets. Results from spectra recorded for live times of 200 s (per source) showed that the major elements K, Ca, Ti, Mn and Fe and the trace elements Ba, Nb, Rb, Sr, Y and Zr could be determined routinely in ‘normal’ silicate rocks and in addition Co, Cr, Cu, Ga, La, Nd, Ni, V and Zn at higher concentrations or in mineralized samples. Detection limits for the most sensitively measured trace elements (Rb, Sr, Y, Zr, Nb) were found to be in the range 6–14 µg g–1 and for Ba (K-line), 21 µg g–1. Precision in the determination of the major elements was generally in the range 0.45–2%(relative standard deviation) and a high degree of accuracy was achieved when judged from a self-consistent comparison of analysed values with the expected compositions of the 70 reference materials. Having characterized performance in the laboratory with ‘ideal’ control over sample presentation, work is now in progress to evaluate additional discrepancies that will be encountered in the analysis of geological and archaeological samples in the field.

Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt

Abstract The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria). The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas. While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin.

2013 Atomic spectrometry update—A review of advances in X-ray fluorescence spectrometry

This review describes advances in the continued expansion of work using the XRF group of techniques published approximately between April 2013 and March 2014. Specialised laboratory instrumentation, X-ray sources, detector development and data processing continue unabated. It is remarkable how quickly hand-held XRF instrumentation has developed to the point where many examples of its use are included in the various application sections of this review, rather than in the instrumentation section. Several new beam lines and their new end-stations were described in publications from SR research centres around the world. More analysts are attracted to TXRF and related techniques with new sample preparation techniques offered for an expanding range of applications. Nanoparticles and nanomaterials feature throughout this review particularly in clinical, biological and environmental studies. A novel approach for the reuse of industrial by-products described how acid mine drainage sludge and coal fly ash facilitated the problem of the high levels of phosphate present in waste waters (cow dung) from the dairy industry. The archaeological and cultural heritage section often includes good news stories. This year we learn how an XRF mapping technique was used to show that Pablo Picasso used a popular brand of French house paint in his works of art.

Investigation of a Correction Procedure for Surface IrregularityEffects Based on Scatter Peak Intensities in the Field Analysis ofGeological and Archaeological Rock Samples by Portable X-ray FluorescenceSpectrometry

Discrepancies arise in the analysis by portable XRF of geological and archaeological rock samples that have irregular shaped surfaces, because the instrument is calibrated for the quantitative analysis of flat samples. A simple correction procedure was investigated to overcome these discrepancies in which the measured intensity is normalised by the ratio of the scatter peak intensity from a compositionally similar flat reference sample to the scatter peak intensity measured from the sample itself. The scatter peak data were obtained from the 55 Fe, 109 Cd and 241 Am sources incorporated in the instrument used for this investigation. Under controlled conditions, this correction has proved to be successful in compensating for effective air gaps of up to 3 mm in the analysis of the K lines of higher atomic number elements (Rb, Sr, Y, Zr, Nb, Ba) and up to 1 mm for the Fe K line. Low energy K lines are affected by air attenuation in the air gap, which is not accounted for in this simple model. The scatter peak from the 55 Fe source is preferred for the correction because its intensity is least dependent on sample composition, but the 109 Cd scatter peak can be used instead with more careful matching of the composition of the flat sample used to derive the reference scatter peak intensity. Apart from additional air attenuation, the principle limitations to the application of this method to larger air gaps were ( i ) the change in scatter angle and, therefore, relative scatter intensity as the air gap is increased and ( ii ) the increasing contribution from scatter in air, particularly to the measured 55 Fe scatter peak at larger air gaps between sample and analyser.