Matrix matching method

Quantitative Analysis Using Matrix Matching Method... 

The matrix matching method attempts to duplicate the sample matrix by adding the major matrix constituents to the standard and blank solutions. For example, in the analysis of sea water samples for a trace metal, the standards can be prepared in a synthetic sea water containing Na, K, Cl , Ca, Mg " and other components. The concentrations of these species are well known in sea water and fairly constant. In some cases, the analyte can be removed from the original sample matrix and the remaining components used to prepare standards and blanks. Again, we must be careful that added reagents do not contain the analyte or cause extra interference effects. 

Burger, S., Riciputi, L.R., and Bostick, D.A. (2007). Determination of impurities in uranium matrices by time-of-flight ICP-MS using matrix matched method, I. Radioanal. Nucl. Chem. 274,491-505. 

The method of standard additions can be used to check the validity of an external standardization when matrix matching is not feasible. To do this, a normal calibration curve of Sjtand versus Cs is constructed, and the value of k is determined from its slope. A standard additions calibration curve is then constructed using equation 5.6, plotting the data as shown in Figure 5.7(b). The slope of this standard additions calibration curve gives an independent determination of k. If the two values of k are identical, then any difference between the sample s matrix and that of the external standards can be ignored. When the values of k are different, a proportional determinate error is introduced if the normal calibration curve is used. 

Wavelength dispersive x-ray fluorescence spectrometric (xrf) methods using the titanium line at 0.2570 nm may be employed for the determination of significant levels of titanium only by carefiil matrix-matching. However, xrf methods can also be used for semiquantitative determination of titanium in a variety of products, eg, plastics. Xrf is also widely used for the determination of minor components, such as those present in the surface coating, in titanium dioxide pigments. 

Errors due to nonspectral interferences can be reduced via matrix matching, the method of standard additions (and its multivariant extensions), and the use of internal standards. ... 

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

The enormous difference in certified values between methods and between analytes illustrates well how much care is needed in matrix/method matching. Further evidence of the importance of matrix matching is provided by an interlaboratory study on trace elements in soil reported by Maier et al. (1983) and the certification of a sewage sludge described by Maaskant et al. (1998). 

Other ways to minimize matrix effects include improving the sample cleanup, diluting the sample, using labeled internal standards, using standard addition, or using matrix-matched standards. The last approach, however, is not permitted for enforcement methods at present by the US EPA or the US Pood and Drug Administration... 

Analyte dilution sacrifices sensitivity. Matrix matching can only be applied for simple matrices, but is clearly not applicable for complex matrices of varying composition. Accurate correction for matrix effect is possible only if the IS is chosen with a mass number as close as possible to that of the analyte elements). Standard addition of a known amount of the element(s) of interest is a safe method for samples of unknown composition and thus unknown matrix effect. Chemical separations avoid spectral interference and allow preconcentration of the analyte elements. Sampling and sample preparation have recently been reviewed [4]. 

As XRF is not an absolute but a comparative method, sensitivity factors are needed, which differ for each spectrometer geometry. For quantification, matrix-matched standards or matrix-correction calculations are necessary. Quantitative XRF makes ample use of calibration standards (now available with the calibrating power of some 200 international reference materials). Table 8.41 shows the quantitative procedures commonly employed in XRF analysis. Quantitation is more difficult for the determination of a single element in an unknown than in a known matrix, and is most complex for all elements in an unknown matrix. In the latter case, full qualitative analysis is required before any attempt is made to quantitate the matrix elements. 

In addition to statistical peculiarities, special features may also result from certain properties of samples and standards which make it necessary to apply special calibration techniques. In cases when matrix effects appear and matrix-matched calibration standards are not available, the standard addition method (SAM, see Sect. 6.2.6) can be used. 

Brenner et al. [ 169] applied inductively coupled plasma atomic emission spectrometry to the determination of calcium (and sulfate) in brines. The principal advantage of the technique was that it avoided tedious matrix matching of calibration standards when sulfate was determined indirectly by flame techniques. It also avoided time-consuming sample handling when the samples were processed by the gravimetric method. The detection limit was 70 ig/l and a linear dynamic range of 1 g/1 was obtained for sulfate. 

An ideal method for the preconcentration of trace metals from natural waters should have the following characteristics it should simultaneously allow isolation of the analyte from the matrix and yield an appropriate enrichment factor it should be a simple process, requiring the introduction of few reagents in order to minimise contamination, hence producing a low sample blank and a correspondingly lower detection limit and it should produce a final solution that is readily matrix-matched with solutions of the analytical calibration method. 

Matrix-matched Certified Reference Materials or spiked samples should be used to determine the linearity of a method. 

A particular issue that must be considered for all calibration procedures is the possibility of matrix effects on the analyte signal. If such effects are present they may be allowed for in many cases by matrix matching of the standard to the sample. This of course requires an accurate knowledge of the sample matrix. Where this is not available, the method of standard addition is often effective. This involves spiking at least three equal aliquots of the sample with different amounts of the analyte, and then measuring the response for both spiked and unspiked aliquots. A plot of response vs analyte, extrapolated back, will give abscissae intercepts from which the amount of analyte in the sample may be deduced (Figure 2.8). 

Interferences in atomic absorption measurements can arise from spectral, chemical and physical sources. Spectral interference resulting from the overlap of absorption lines is rare because of the simplicity of the absorption spectrum and the sharpness of the lines. However, broad band absorption by molecular species can lead to significant background interference. Correction for this may be made by matrix matching of samples and standards, or by use of a standard addition method (p. 30 et seq.). 

One or more of these bias components are encountered when analyzing RMs. In general, RMs are divided into certified RMs (CRMs, either pure substances/solu-tions or matrix CRMs) and (noncertified) laboratory RMs (LRMs), also called QC samples [89]. CRMs can address all aspects of bias (method, laboratory, and run bias) they are defined with a statement of uncertainty and traceable to international standards. Therefore, CRMs are considered useful tools to achieve traceability in analytical measurements, to calibrat equipment and methods (in certain cases), to monitor laboratory performance, to validate methods, and to allow comparison of methods [4, 15, 30]. However, the use of CRMs does not necessarely guarantee trueness of the results. The best way to assess bias practically is by replicate analysis of samples with known concentrations such as reference materials (see also Section 8.2.2). The ideal reference material is a matrix CRM, as this is very similar to the samples of interest (the latter is called matrix matching). A correct result obtained with a matrix CRM, however, does not guarantee that the results of unknown samples with other matrix compositions will be correct [4, 89]. 

An interference check standard is a standard solution used to verify an accurate analyte response in the presence of possible interferences from other analytes present in the samples. For methods that have known interference problems arising from the matrix or that are inherent in the method, such as ICP-AES (spectral interference lines) and ICP-MS (isotope combinations with similar masses to analyte), these solutions are used in the batch. The interference check standard must be matrix matched to acid content of the samples. Acceptance criteria are set—for example, the magnitude of uncorrected background and spectral interference must not be greater than a stated value. 

Analysis of a matrix-matched reference material, or typical matrix spiked with reference material, or typical test material with quantity value with known measurement uncertainty established by another method... 

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