Inter-elemental correction

A major obstacle in implementing double-spike calibration is the difficulty in obtaining an appropriate isotopically enriched calibrant (in addition to the fact that 

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In order to calculate the absorption of X-rays by the matrix using Equation (4.1), composition of the target needs to be known. This apparent dilemma posed by the need to know the composition of the matrix in order to apply inter-element corrections can be solved using iteration techniques. 

Related to the above-described inter-elemental correction method, the need for a proxy element can be obviated at the expense of resorting to sequential analysis of the sample and the calibrator, that is, with external gravimetric calibration. Although this technique is not favored by all, it has played a major role, for example, in the recent re-evaluation of the atomic weight of zinc [13]. 

Peel, K, Weiss, D., Chapman, )., Arnold, T., and Coles, B. (2008) A simple combined sample-standard bracketing and inter-element correction procedure for accurate mass bias correction and precise Zn and Cu isotope ratio measurements. J. Anal. 

In contrast to thermal ionization methods, where the tracer added must be of the same element as the analyte, tracers of different elemental composition but similar ionization efficiency can be utilized for inductively coupled plasma mass spectrometry (ICPMS) analysis. Hence, for ICPMS work, uranium can be added to thorium or radium samples as a way of correcting for instrumental mass bias (e g., Luo et al. 1997 Stirling et al. 2001 Pietruszka et al. 2002). The only drawback of this approach is that small inter-element (e g., U vs. Th) biases may be present during ionization or detection that need to be considered and evaluated (e.g., Pietruszka et al. 2002). 

For complicated samples where matrix or interelement effects are present, a linear calibration curve may not be valid, and one should consider using an empirical model for concentration correction. This usually requires a large set of standards of similar composition to the unknown, which generally makes analysis rather impractical. Inter-element effects can be calculated from a basic knowledge of physical parameters in combination with the appropriate use of samples of known composition, pure elemental standards or composite standards. 

XRF nowadays provides accurate concentration data at major and low trace levels for nearly all the elements in a wide variety of materials. Hardware and software advances enable on-line application of the fundamental approach in either classical or influence coefficient algorithms for the correction of absorption and enhancement effects. Vendors software packages, such as QuantAS (ARL), SSQ (Siemens), X40, IQ+ and SuperQ (Philips), are precalibrated analytical programs, allowing semiquantitative to quantitative analysis for elements in any type of (unknown) material measured on a specific X-ray spectrometer without standards or specific calibrations. The basis is the fundamental parameter method for calculation of correction coefficients for matrix elements (inter-element influences) from fundamental physical values such as absorption and secondary fluorescence. UniQuant (ODS) calibrates instrumental sensitivity factors (k values) for 79 elements with a set of standards of the pure element. In this approach to inter-element effects, it is not necessary to determine a calibration curve for each element in a matrix. Calibration of k values with pure standards may still lead to systematic errors for unknown polymer samples. UniQuant provides semiquantitative XRF analysis [242]. 

The relationship between the weight concentration of an element and the intensity of one of its characteristic lines is complex. Several models have been developed to correlate fluorescence to weak, atomic concentrations. Many corrections have to be made due to inter-element interactions, preferential excitation, self-absorption, and fluorescence yield (heavy elements relax more quickly by internal conversion without emission of photons). All of these factors require the reference sample to be practically the same structure and atomic composition for all elements present as the... 

Where element interference is unavoidable, several possible corrections exist. The interference correction scheme of Roeder (1985) is outlined below, followed by a listing of theoretical and empirical correction factors (Table 3) for inter-element interferences from multiple sources (Amli and Griffin 1975, Roeder 1985, Reed and Buckley 1996, Scherrer et al. 2000, Pyle 2001). 

Electron Microprobe Analysis of REE in Apatite, Monazite and Xenotime 349 Table 3. Correction factors for inter-element interference. 

Correction for inter-element interference requires knowledge of the wavelength of both the desired peak ( id) and interfering peak (Xi), the width of the peak at 1 standard deviation (width at half height, W), and the intensity of each peak. The amount of peak... 

Spectral interferences were corrected by using the experimentally determined inter-elemental coefficients in the following expression ... 

Wang et al proposed a multivariate dominant factor based non-linearized PLS model for LIBS measurements. In constructing such a multivariate model, non-linear transformation of multi-characteristic line intensities according to the physical mechanisms of a laser-induced plasma spectrum were made, combined with a linear-correlation-based PLS method, to model the non-linear self-absorption and inter-element interference effects. Moreover, a secondary PLS was applied, utilizing information from the whole spectrum to correct the model results further. The proposed method showed a significant improvement when compared with a conventional PLS model. Even compared with the already improved baseline dominant-factor-based PLS model, the PLS model based on the multivariate dominant factor yielded the same calibration quality while decreasing the RMSEP. 

Chen, Z. (1999). Inter-element fractionation and correction in laser ablation inductively coupled plasma mass spectrometry./.ydnal./It. Spectrom. 14(12), 1823-1828. 

The complexity of some of these problems of interference, matrix and inter-element fractionation is exemplified by the analysis of Sr isotopes in both solution mode and laser ablation (LA-) MC-ICP-MS. Sr isotope studies have been traditionally carried out using TIMS although MC-ICP-MS can also be used but requires correction (or monitoring) for mass bias, interfering elements, doubly charged ions, argides and dimers, and unidentified interferences. Primarily, isobaric interferences from Rb and Kr (present in trace quantities within the Ar plasma gas) must be corrected but this is not wholly straightforward. 

In most instances, the effect of the common interferences reported for the technique (e.g. inter-element, oxide and Ar-based) can be minimised by careful matching of standard solutions to the values expected to be found in the samples. If rare earth elements are required then consideration must be given to the impact of barium and light rare earth element oxides on middle rare earth elements (most notably Eu, Tb and Gd) and it is common to apply a correction factor to mitigate these interferences. However, where elements such as barium are present at percent levels, automatic correction may not be effective because of the size of the interfering peak relative to the analyte signal. 

Since these early studies, efforts have concentrated on expanding the number of isotope systems to which LA-MC-ICP-MS can be applied, and mechanisms to correct for inter-element interference, mass bias and laser-induced isotopic fractionation. Where possible, internal normalisation of data using fractionation factors determined from a pair of stable isotopes of the element under investigation is the preferred route of data correction. Where this is not possible, external normalisation using another element at similar mass is used but, especially at light masses, this approach may not readily correct for laser-induced preferential volatilisation of lighter isotopes which may vary on an element-by-element basis. ... 

The X-ray determination of REE in geological samples is normally complicated by the relatively low concentrations of the REE, their complex X-ray spectra, the high concentration of matrix elements and the lack of reference standards with certified values for REE. A rapid and sensitive ion exchange and X-ray fluorescence procedure for the determination of trace quantities of rare earths is described. The REE in two U.S.G.S. standards, two inhouse synthetic mixtures and three new Japanese standards have been determined and corrections for inter-rare earth element interferences are made. 

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