Quantitative analysis matrix matching method

Quantitative Analysis Using Matrix Matching Method... 

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 ... 

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. 

Whenever quantitative analysis is desired, care must be taken to use proper standards and account for interelement matrix effects since the inherent sensitivity of the method varies greatly between elements. Methods to account for matrix effects include standard addition, internal standard and matrix dilution techniques as well as numerous mathematical correction models. Computer software is also available to provide semi-quantitative analysis of materials for which well-matched standards are not available. 

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. 

The existence of reference materials and appropriate calibration procedures are two essential issues to be considered in quantifying the components of a sample. Quantitation has been substantially improved by the commercial availability of an increasing number of certified solid reference materials, especially for low concentration levels. Recently [42], NIST archival leaf standards were used as matrix-matched standards for reliable quantitative elemental analysis of Spanish moss samples. LA-ICP-MS was used with mixing standards in order to produce at least three data points for each calibration curve the results thus obtained were compared with those provided by microwave digestion ICP-MS/AES. Standard addition was also examined and found to be an effective method in the absence of matrix-matched standards. 

While the selection of an isotopically-labelled or analogue internal standard is relatively easy in quantitative bioanalysis, the situation is more complicated in multiresidue analysis. It is difficult to select appropriate analogue standards for a wide variety of target compounds, while isotopically-labelled standards are often not available for all target compounds. In addition, if one would introduce one standard per target compound, this would seriously limit the sensitivity of the method as it doubles the number of SRM transitions that have to be monitored. Another problem in multiresidue analysis is the selection of appropriate blanks for the production of the matrix-matched standards and the number of matrices that might have to be studied. When no adequate blank matrix is available, the standard addition method is the only way to achieve sufficiently accurate and precise results [123]. This method is time-consuming and labourious. 

Arc excitation may be used for either qualitative or quantitative analysis. The precision obtainable with an arc is, however, generally poorer than that with a spark and much poorer than that with a plasma or Hame, Furthermore, emi.ssion intensities of solid samples are highly sample dependent. As a eonsequence matrix matching of standards and samples is usually required for satisfactory results. The imevnal-standard method can be used to partially offset this problem,... 

In arc- and spark-emission spectroscopy, one of the critical aspects of quantitative analysis is the need to match the standard as closely as possible to the sample. Dilution of sample and standards by a common matrix in DC-arc methods somewhat reduces the dependence upon exact matches. Gordon and Chapman devised a common matrix-dilution technique for DC-arc analysis which is almost totally independent of the forms of the sample and the standard [4]. 

The analytical signal obtained in SIMS depends on a number of factors, namely, the abundance of the isotope examined on the surface, the properties of the surface, and the bombarding ion. The relationship is complex calibration curves may or may not be continuous. Certihed calibration standards that are matrix-matched and cover the analytical range are required. For bulk analysis using dynamic SIMS, certified standards can be obtained from sources such as NIST or commercial standards firms for many metals and alloys and for some ceramics and glasses, but such standards are rarely available for surfaces, thin layers, and multilayered materials. The excellent sensitivity of SIMS allows it to distinguish between ppb and ppm concentrations of analyte in solids. SIMS is not used for quantitative analysis at the % level XPS or methods such as XRF are better suited to this purpose. 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

AU ionization techniques used for quantitative analysis present the possibility of ionization suppression (and sometimes enhancement) of the analyte(s) by co-eluting compounds arising from the sample matrix or elsewhere this phenomenon is a form of matrix effect. This problem is much more serious for API techniques than the others presented in this chapter, with ESI presenting the highest occurrence of matrix effects. Matrix effects are also of concern in MALDI analyses, especially in the quantitation of small molecules. It is always advisable to be aware of the importance of matrix effects in any proposed analytical method and to minimize them as far as possible. Use of an appropriate isotope-labeled standard in conjunction with matrix matched calibrant standards should give reliable results if all other appropriate precautions are taken, but it is important to investigate the possibility of relative matrix effects if one suspects that the matrix in the analytical sample might differ appreciably from that used to make the matrix matched cahbrants. 

Typical sample preparation steps include homogenization, extraction (liquid—liquid extraction, LLE, or instrumental based techniques), cleanup (usually by solid-phase extraction, SPE), and concentration of extracts. Sometimes, derivatization has to be incorporated into sample preparation (e.g., release of bormd residues or deconjugation). For quantitative analysis, the preparation of adequate calibration standards also may be a key aspect in some cases, matrix-matched standards or the standard additions method may be necessary, as well as the use of suitable internal standards (e.g., isotopically labeled compormds) [19]. Matrix-matched calibration is now preferred, as it is the best compromise in terms of speed and cost of analysis, taking into consideration the features of the MS analyzers. 

This type of calibration is often called external standardization and is usually used when there is very little difference between the matrix components in the standards and the samples. However, when it is difficult to closely match the matrix of the standards with the samples, external standardization can produce erroneous results, because matrix-induced interferences will change analyte sensitivity based on the amount of matrix present in the standards and samples. When this occurs, better accuracy is achieved by using the method of standard addition or a similar approach called addition calibration. Let us look at these three variations of quantitative analysis to see how they differ. 

In general, it is very difficult reliably to extract and quantitate multiple vitamins from complex food systems, due to their diverse physical and chemical properties. Consequently, the extraction of the vitamins from the food matrix is usually the greatest challenge of vitamin analysis. This is especially true for the naturally occurring vitamins, which are often bound to other food constituents, such as carbohydrates or proteins. To prevent vitamin degradation or loss, the extraction conditions should complement the labile nature of the vitamins. Indiscriminate mixing and matching of extraction and quantitation methods is not recommended, since the extraction conditions can affect subsequent separation and quantitation steps. 

An example of the application of the wide isolation window in tandem mass spectrometry is the detection and quantitative imaging of cocaine in postmortem human brain tissue (Reich et al., 2008 Reich et al., 2010). In this instance, it was determined necessary to develop an MS wide isolation window method because of interfering background ions. Cocaine detected in brain tissue was confirmed by matching six MS product ions with those from a cocaine standard. For the quantitation of cocaine analyzed from the tissue, the trideuterated ( Hs) internal standard was spiked beneath the tissue at known concentrations before matrix application to develop a calibration curve. The MS wide isolation method was then employed for the analysis of cocaine and cocaine-fi 3. Cocaine was analyzed successfully and quantified using this approach. 

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