Matrix-matched standards

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 effect of co-extracted matrix components on the analyte response in the final determination step should be assessed. Normally, this is done by comparing the response of standards in solvent with matrix-matched standards, i.e., standards prepared in the extract of a control sample without residues. Because matrix effects tend to be inconsistent, the guidelines propose the general use of matrix-matched calibration unless it is demonstrated to be unnecessary. 

L. Alder, G. Kempe, and P. Baumann, Need of Matrix Matched Standards-Conclusion from a German Ring Test, Presented at 3rd European Pesticide Residue Workshop, York, UK, July 3-5, 2000. 

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... 

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. 

Inorganic mass spectrometry requires the development of suitable reference materials, such as matrix matched standard reference materials for trace, surface (including depth profiling and microlocal) analysis and/or the creation of matrix independent calibration procedures. The development of species specific standards will be intensified for speciation studies in the future. 

Griffiths et al. [92] quantified Pt, Pd and Rh in autocatalyst digests by ICP with a CCD detector array. They compared univariate techniques (pure standards, pure standards with interelement correction factors and matrix-matched standards) and PLS, the latter being superior in general, although less effective... 

In inductively coupled plasma-mass spectrometry, isobaric interference occurs between species with the same mass and charge. Interference can be eliminated if the mass spectral resolution is sufficiently great or by dissociating an interfering polyatomic species with a collision cell. When laser ablation is used to sample a solid, matrix-matched standards are often necessary for quantitative analysis. 

The table in this appendix recommends primary standards for many elements. An elemental assay standard must contain a known amount of the desired element. A matrix matching standard must contain extremely low concentrations of undesired impurities, such as the analyte. If you want to prepare 10 ppm Fe in 10% aqueous NaCl, the NaCl must not contain significant Fe impurity, because the impurity would then have a higher concentration than the deliberately added Fe. 

Due to numerous interelemental matrix effects, matrix matched standards including a blank are necessary for accurate quantitative analysis. The detection limits for XRF are not as low as other spectrometric methods and a noticeable drop-off in sensitivity is noted for light elements such as magnesium. 

Calibration using simple solutions of well-characterised pure substance standards or matrix matched standard solutions. Calibration solutions are prepared from materials whose identity and purity have been established to an appropriate level of uncertainty and where the effects of any impurities have been evaluated. Where appropriate and where available, standards provided by metrology institutes with demonstrated capability are used. In other cases, materials from other reputable suppliers or prepared in-house are used after appropriate characterisation. Where necessary, professional judgement is used to estimate the uncertainty associated with chemical standards. The target uncertainty of the identity is for practical purposes zero and for purity less than one-fifth of the desired overall uncertainty. 

To analyze metals and alloys directly without dissolution, both spark ablation [349] and laser ablation [61,211] dry aerosol generation systems have been used to introduce samples into an ICP-MS. These approaches often require matrix-matched standards, although several active research groups are focusing on techniques to reduce that requirement. The amount of material ablated depends on the sample type. Fractionation of elements can also be a problem, depending on the sample, the laser fluence, the laser wavelength, and the number of laser pulses used to sample from a fixed location. Volatile elements that are segregated in the samples appear to be most prone to fractionation problems [61],... 

Lelievre et al. [7] developed a rapid method for the direct determination of Cu and Fe in butter by ET-AAS. Butter was dissolved in a butylamine (BTA)D waterDtetrahydrofuran (THF) mixture. The solution (10 xL) was pipetted directly into the graphite tube. The calibrations were compared with the matrix-matched standard and standard addition calibration. 

Method of Standard Addition - Matrix-Matched Standards 63... 

A simplified version of the method of standard additions can be used when a longer series of samples with the same matrix, volume, and sample mass is to be analyzed. Then the standard addition curve is shifted to pass through zero and hence becomes a matrix-matched standard calibration curve. 

Using standard solutions for quantifying concentrations in an unknown sample may give rise to measurement errors due to the influence of food matrix remnants in the injection solution. Ion suppression or enhancement is a typical matrix effect seen in mass spectrometry. Matrix-matched standards are generally used in order to avoid such possible matrix interferences. Standard addition is a valid alternative for dealing with matrix effects. On the other hand, standard addition or matrix based calibration curves require more manipulation, which is time- and money-consuming, whereas a greater risk of manipulation errors ensues. 

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