Analysis Of Silicate Rocks Research Articles

Inductively coupled plasma-mass spectrometric (ICP-MS) analysis of silicate rocks and minerals

The accurate measurement of low levels of trace elements is now necessary in most petrological studies. In this paper, we present the methods used in our laboratory ("Collectif interinstitutionnel de géochimie instrumentale") to analyse simultaneously, with the ICP-MS instrument, 26 trace elements in silicate rocks and minerals. Whole-rocks are prepared using a lithium metaborate fusion in order to assure dissolution of resistant mineral phases, whereas mineral separates (plagioclases, pyroxenes, apatites) are prepared using an open acid digestion. Calibration of the ICP-MS is made with international reference materials for whole-rocks and with synthetic aqueous solutions for mineral separates. The accuracy of the data as well as limits of quantification (LOQ) vary among elements but are usually very good (accuracy better than 6%, LOQ usually below 1 microg/g in solids). Combination of XRF and ICP-MS capabilities enables us to determine 38 elements (major and trace) on a routine basis.

The Bulk Analysis of Silicate Rocks by Portable X‐Ray Fluorescence: Effect of Sample Mineralogy in Relation to the Size of the Excited Volume

This paper examines the limitations arising from sample mineralogy when a portable X‐ray fluorescence instrument is applied to the direct in situ analysis of silicate rocks. Estimates were made of the size of the excited volume from which the X‐ray fluorescence signal originates by calculating the critical penetration depth for selected X‐ray lines. Measurements were made of the variations in detected intensities over the area of the P‐XRF analyser window and showed that, using radioisotope sources incorporated in the instrument used in this study (Spectrace TN9000), excitation intensities were six to ten times greater at the centre compared to the edge of the analyser window. These data indicated that the region of the sample at the centre of the window will make an enhanced contribution to detected spectra. Replicate measurements on slabs of rock selected to represent fine‐to coarse‐grain size textures indicated the magnitude of the sampling precision that can be achieved in the direct analysis of silicate rocks. Typical values were better than 5% relative standard deviation of the mean from an average of five determinations on fine‐ to medium‐grained rocks and better than 10 % relative standard deviation in a single determination on these samples.

Geoanalysis: Past, Present and Future†

A review of trends in the use of techniques for the chemical analysis of geological samples has been undertaken by abstracting details from reference material characterisation programmes published over the period 1980–1997 and from papers published in Geochimica et Cosmochimica Acta over a similar period. An analysis of these details shows that whereas during the 1980s, XRF and INAA were the two most popular techniques chosen for the major and trace element analysis of silicate rocks, this role continues to be filled by XRF but now complemented by ICP-MS. An equivalent survey of the geochemical use of isotope measurements shows that improvements in the sensitivity and precision of mass spectrometry instrumentation has led to new analytical capabilities: for example, in the application to the Sm–Nd and U–Pb isotope systems in the 1980s and to Re–Os in the 1990s. If these instrument developments continue, the measurement of new isotope systems may become possible including, for example, those related to nucleosynthetic processes. However, it is argued that the greatest influence in future years will arise from the further development of microprobe techniques. Already some of these techniques have demonstrated the importance of providing data that takes into account zoning effects in individual minerals, as in the U–Pb dating of zircon by ion probe and the dating of selected minerals using the laser Ar–Ar technique. Indeed, in these applications, equivalent determinations on bulk samples are now largely discredited, because they mask variations at the mineralogical level. It is likely, therefore, that with further developments in microprobe instrumentation and refinements in geochemical models for interpreting microanalysis data, this area of endeavour will play an increasingly important role in future geochemical studies.

GeoPT - A Proficiency Test for Geoanalysis†

GeoPT is a new proficiency test for silicate rock analysis. Participants’ results are processed by the formation of a z-score, according to the Harmonised Protocol on Proficiency Testing. Two classes of performance criteria are available for participants to work to. In the second round conducted in 1997, 44 analytes were given z-scores and 13 reported without z-scores. Of the results to which z-scores were assigned, 74.4% were classified as satisfactory.

Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis

A fusion procedure commonly used for major element analysis of silicate rocks (1:5 sample to flux ratio, using 0.7 g of sample) was used to determine a small group of trace elements (Cu, Zn, Rb, Sr, Y, Zr, Nb) by X-ray fluorescence. Compton scatter peak ratioing was successfully used for matrix corrections, despite the thickness of the glass discs being only 1.7 mm. Precision and detection limits were compared with pressed pellet values. For most elements, routine analytical precision is better with powder pellets and detection limits are worse in glass discs, by an average factor of 2, considering all elements analyzed. Accuracy was evaluated with international reference materials and for most of the determinations, good or excellent agreement with recommended values was obtained. The main advantages of using glass discs for the trace element determination are the possibility of analyzing small amounts of sample and the suppression of any mineralogical effects in the glass discs, which can also be used to determine major elements.

Selection of internal standards for analysis of silicate rocks and limestones by laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES).

Included (Si, Ca) and added (Li, B) internal standards (IS) have been used comparatively in LA-ICP-AES of silicates and limestones to improve both precision and accuracy. A Q-switched Nd:YAG laser (355 nm, 10 Hz, 10 mJ per shot) has been applied for ablation. Samples have been prepared by fusion with Li(2)B(4)O(7) and measured using a Perkin-Elmer Optima 3000 ICP system. Both types of IS have given a relative standard deviation (RSD) less than 2-1%, which improved the repeatability by a factor of 2-10 and calibration graphs have been linear over the whole concentration range.

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

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. MMDL's varied from element to element but for XRF/powder pellet data were found to be approximately equivalent to instrumental detection limits (IDL's) 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 IDL's 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 MDL's 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.

X-ray fluorescence analysis of silicate rocks using fused glass discs and a side-window Rh source tube: accuracy, precision and reproducibility

In this paper, we provide a detailed account of our sample fusion, calibration and instrumentation methods for major-element whole-rock analysis by XRF, and we discuss several aspects of sample preparation and instrument performance that are important considerations for accurate analysis. The fusion procedure involves moderate capital costs and is easy to apply, yielding flat, polished, homogeneous glass discs as cast. The calibration method utilizes a least-squares procedure that rigorously fits data according to both compositional and counting statistical uncertainties. We use a side-window Rh tube for analyzing major elements (including Na) and employ real-time testing for constant count rate to reject spurious results. The methods result in excellent analytical precision and reproducibility. The standards used for calibration lie within compositional and counting statistical uncertainties of best-fit straight lines. Analyses of replicate discs and repeated analyses of single discs show excellent long-term reproducibility over several months, approaching counting statistical uncertainties in several cases. Comparison with independent measurements made by other laboratories using instrumental neutron activation and X-ray fluorescence analyses shows excellent agreement with our results. A side-window Rh tube gives increased detection limits for most major elements, but otherwise shows little difference in precision compared to a Cr tube. This means that major and trace elements can be analyzed without changing X-ray sources, which provides saving in terms of time and money, as well as being a convenience to the analyst.

Extraction of tetrafluoroborate with crown ethers and its application to silicate rock analysis

The extraction behaviour of tetrafluoroborate with crown ethers was studied. A high distribution ratio of tetrafluoroborate is obtained by extraction with dicyclohexano-18-crown-6 (DC18C6) in an organic solvent of high dielectric constant from potassium fluoride solution. The molar ratios of crown ether to KBF4 in the extracted species are probably 1:1 for DC18C6, dibenzo-18-crown-6 and 18-crown-6, and 2:1 for benzo-15-crown-5 and 15-crown-5. The flow-injection extraction-spectrophotometric determination of tetrafluoroborate with Brilliant Green was worked out. Many rock reference samples were analyzed for boron (1–150 ppm).

Geochemical analysis of silicate rocks and soils by XRF using pressed powders and a two-stage calibration procedure

A two-stage calibration procedure has been developed for major-element analyses of silicate rocks and soils using XRF and pressed powders. The first step involves fitting the composition-intensity data matrix of the standards to the Lucas-Tooth-Pyne (LTP) model using stepwise regression. Compositions of the standards are then regressed stepwise against the composition predicted by the LTP regression using a linear model of the form C i = f( C' 1, C' 2,…, C' i ,…, C' n ) where the primes denote the compositions predicted by the LTP model. The second-stage regression significantly improved the correlation coefficient ( R 2) of the calibrations for SiO 2, Al 2O 3 and Na 2O over those produced by the LTP model alone. The procedure has been used to analyze 185 sample unknowns covering a wide range of composition (granite, diabase, sericite, biotites and soils developed thereon including laterites). Of these 95% had sum of constituents between 98.75% and 101.25%. Eight of these samples have been analyzed by independent methods. In general the agreement between pairs of analyses are very good. Thus, the calibration procedure appears to produce analyses over a wide range of compositions that are sufficiently accurate for routine work. Although the procedure requires a large number of standards (∼ 50), once the initial investment has been made the calibration can be upgraded and its range of application expanded as new standards become available.

Use of theoretical accurate binary influence coefficients with Tertian's equation in x‐ray fluorescence analysis of silicate rocks in borax glass beads

AbstractA study of the suitability of the Tertian algorithm using new ‘accurate’ binary influence coefficients and a comparison standard for x‐ray fluorescence analysis of geological materials prepared as borax glass beads was performed. The algorithm was found to give satisfactory results for the analysis of four major components in geological materials. Even in the worst case, when there is a factor of 20 difference in the concentration in the specimen and a comparison standard, the algorithm gives results which differ from the true results by a factor of about 2. This makes the algorithm attractive as a diagnostic tool in analyses of unknown specimens and in the identification of appropriate standards. The binary coefficients are easily calculated so that the algorithm can be implemented on a minicomputer.

Versatile and accurate trace element determinations in iron‐rich and other geological samples using x‐ray fluorescence analysis

AbstractThe reliable, routine XRF determination of trace elements in iron‐rich geological samples, such as banded iron formations, oolitic ironstones and ferromanganese nodules, has until recently been limited. The scattered radiation method (using Compton scattering), well established in normal silicate rock analysis, was developed for the determination of trace element concentrations in diverse rock compositions. Single calibration lines are used for each trace element and serve to provide data over broad sample compositions, ranging from 0 to 70% Fe2O3. A critical evaluation of the calibration lines obtained and the results of trace analyses of synthetic mixtures showed the method to be readily capable of providing accurate chemical data.

Loss‐on‐ignition corrections in the XRF analysis of silicate rocks

AbstractAn empirical method for loss‐on‐ignition (LOI) corrections in XRF analysis is presented. It is based on a polynomial relationship between the disc weight ratio (D/Dz) and the analyte concentration ratio (Ciz/Ci) for a series of synthetic standards with various known LOI contents, where Dz and Ciz are disc weight and analyte concentration, respectively, at zero LOI. The coefficients derived from regression analysis of the relationships are unique to each analyte element, and may be used to adjust the disc weight ratio. The adjust disc weight ratios are than applied to correct the apparent concentration to a standard measurement condition, which is zero LOIThe effectiveness of this LOI correction method was verified experimentally by the analysis of some well characterized geochemical reference samples with various LOI contents and the results were found to be in good agreement with the recommended values.

Book Review

A HANDBOOK OF SILICATE ROCK ANALYSIS by P.J. Potts, Blackie and Ltd, Glasgow, 1987, 622 p., pD128.00 (ISBN 0‐216‐91794‐8)Reviewed by Iwan Roelandts Geology, Petrology and Geochemistry, University of Liège, B‐4000 Sart Tilman, Liège 1, Belgium

Calculation of calibration line parameters from reference material data in the analysis of silicate rocks: Theil's incomplete method compared with least-squares regression

The reliability of Theil's incomplete method is compared with that of a conventional linear least-squares regression in the calculation of the calibration line from reference material data for trace elements determined by energy-dispersive X-ray fluorescence analysis. A conventional linear least-squares regression procedure generated a reliable calibration line, provided the reference material data set was first examined critically to eliminate outliers and isolated data points. Published values judged to be acceptable for such a calibration procedure are listed. By comparison, Theil's incomplete method gave calibration parameters that were relatively insensitive to the presence of outliers, so avoiding the requirement of making sometimes arbitrary decisions about the acceptability of individual calibration points. Theil's incomplete method represents, therefore, a robust procedure for calculating calibration lines and overcomes many of the discrepancies that may be encountered in fitting calibration data for elements such as Cu, Zn, Sr, Zr, Nb, Pb and U using a least-squares regression.

Spectrofluorimetric Determination of Nanogram Sulfide by Using Copper(II) and 2-(o-Hydroxyphenyl)benzoxazole

A spectrofluorimetric method for the microdetermination of sulfide was established. This will eventually be applied to silicate rock analysis. An excess amount of Cu2+ is added to precipitate copper(II) sulfide with sulfide ion. The remaining Cu2+ forms a chelate with 2-(o-hydroxyphenyl)benzoxazole (HPB) and quenches the fluorescence of HPB. The amount of sulfide is determined by measuring the fluorescence of the remaining HPB. The optimum reaction conditions, in which all the concerned reactions proceed stoichiometrically, were examined. By the proposed method, 5-640ng sulfide can be determined with a relative standard deviation of 1.14% at 250ng S2- level.

High-accuracy analysis by inductively coupled plasma atomic emission spectrometry using the parameter-related internal standard method

The precision of analyses normally achieved by inductively coupled plasma atomic emission spectrometry (ICP-AES) is not sufficient for certain applications. Frequent re-calibration is shown not to improve the precision for a typical sample in routine analysis. The dominant factor limiting the precision is found not to vary systematically with time, i.e., drift, but to be random. The bias of the analysis was found to be small (ca. 1%) and is caused by a combination of matrix effects and drift. Both systematic and random errors in the analysis of silicate rocks are substantially reduced by the application of the parameter-related internal standard method (PRISM). A statistical method (the maximum likelihood functional relationship) is used for the assessment of the accuracy of the analysis, in comparison with certified reference materials.

Silicate rock analysis by energy-dispersive X-ray fluorescence using a cobalt anode X-ray tube. Part 2. Practical application and routine performance in the determination of chromium, vanadium and barium

This paper investigates the practical determination of the elements Cr, V and Ba by energy dispersive X-ray fluorescence using the excitation conditions described in Part 1. Particular attention was paid to the selection of a reliable set of geochemical reference materials to form a coherent calibration set. Discrepancies caused by the overlap of calcium-aluminium and calcium-silicon sum peaks on Ba L and Cr K lines in those samples containing more than 10% CaO were eliminated by reducing spectrum data accumulation rates. New data are presented for the elements Cr, V and Ba (and Ti and Mn) in nine reference materials recently distributed by the Geological Survey of Japan and these results show satisfactory agreement with other published values.

Combination of neutron activation, atomic absorption spectroscopy and conventional methods in the elemental analysis of silicate rocks

Instrumental neutron activation, atomic absorption spectroscopy and conventional methods of analysis were used on eight different silicate rocks and two minerals. Trace elements and major constituents were determined. It was considered that the methods should be regarded as complementary analytical techniques.

Silicate rock analysis by energy-dispersive X-ray fluorescence using a cobalt anode X-ray tube. Part I. Optimisation of excitation conditions for chromium, vanadium, barium and the major elements

A cobalt anode X-ray tube has overcome many of the deficiencies of other tubes in exciting the trace elements Cr, V and Ba in silicate rocks when analysed by energy-dispersive X-ray fluorescence (XRF). The optimum excitation conditions derived for the determination of chromium were an operating potential of 20 kV using a 12.5-µm iron foil filter in the primary X-ray beam. Under these conditions the fluorescence of iron K lines in samples is selectively suppressed. This has the effect of reducing both the proportion of analysis time expended unnecessarily in accumulating iron X-rays and the magnitude of the spectral background caused by incomplete charge collection of Fe Kα/Kβ events in the Si(Li) detector. Detection limits (6σ) for Cr, V and Ba of 6, 9 and 25 p.p.m., respectively, approach those values expected by routine wavelength-dispersive XRF in geological applications.