Matrix effects methods

From the various results of the homogeneity and stability studies, it was concluded that the part of the uncertainty that cannot be evaluated by the statistical analysis (e.g. related to matrix effects, method stability, blank variations etc.) was in the order of 10% (expressed as standard uncertainty). The uncertainty related to the certification exercise was equal to Ucerj = 0.9166/ 6 = 0.0374 (or 2.0%), whereas the uncertainty factor related to homogeneity is 4.6% (see section 8.9.5). The combined uncertainty was thus estimated as follows U. = [(2.0) + (4.6) + (10) )] = 11%... 

The generalized standard addition method (GSAM) extends the analysis of mixtures to situations in which matrix effects prevent the determination of 8x and 8y using external standards.When adding a known concentration of analyte to a solution containing an unknown concentration of analyte, the concentrations usually are not additive (see question 9 in Chapter 5). Conservation of mass, however, is always obeyed. Equation 10.11 can be written in terms of moles, n, by using the relationship... 

Most ion sources produce singly charged ions, i.e., z = I and the ranges shown here apply to such ions. Matrix assisted methods may produce ions with r > 1. When = 1, m/z. = m, viz., mass can be measured directly. An ES ion source produces ions with z > 1 and this effectively extends the mass ranges that can be examined. For example, with z = 1 and m = 10,000, the m/z value is 10,000 and this would be beyond tbe capabilities of a quadrupole instrument. 

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

Standards used to constmct a cahbration curve must be prepared such that the matrix of the standard is identical to the sample s matrix because the values of the parameters k and b associated with a linear cahbration curve are matrix dependent. Many areas of chemical analysis are plagued by matrix effects, and it is often difficult to duphcate the sample matrix when preparing external standards. Because it is desirable to eliminate matrix effects, cahbration in the sample matrix itself can be performed. This approach is called the standard addition method (SAM) (14). In this method, the standards are added to the sample matrix and the response of the analyte plus the standard is monitored as a function of the added amount of the standard. The initial response is assumed to be Rq, and the relationship between the response and the concentration of the analyte is... 

The usage of the ratio of chai acteristic lines as analytical parameter in the process of formation of the calibration curve provides a significant decrease of the residual error. In Realization of this method simultaneously with the decrease of the matrix effects causes some decrease or even full compensation of the fonu and condition of the measured surface. 

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

The most accurate - and most popular - method of quantifying matrix effects is to analyze the unknown sample with a similar sample of known composition. The relationship between measured intensity and the content of each sample is, usually, defined by the relative sensitivity factor (RSF) ... 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

The Method of Standard Additions to Overcome Matrix Effects... 

The issue of selectivity is one that is often difficult to address. Initial method development is invariably carried out by using standards made up with pure solvents, i.e. free from any matrix effects. It is often only when real samples are analysed that the true extent of interference becomes apparent and the value of the method can be properly assessed. An added complication is that interferences , by their very nature, are not constant and a number of samples may have a combination of interferences that defy analysis by a method that is otherwise successful on a routine basis (another example of Murphy s law ). 

In many cases when methods involve internal or external standards, the solutions used to construct the calibration graph are made up in pure solvents and the signal intensities obtained will not reflect any interaction of the analyte and internal standard with the matrix found in unknown samples or the effect that the matrix may have on the performance of the mass spectrometer. One way of overcoming this is to make up the calibration standards in solutions thought to reflect the matrix in which the samples are found. The major limitation of this is that the composition of the matrix may well vary widely and there can be no guarantee that the matrix effects found in the sample to be determined are identical to those in the calibration standards. 

Table 5.17 Quantitative results obtained for the determination of four diarrhetic shellfish poisons (DSPs) using external standards and the method of standard additions. Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), with permission from Elsevier Science...

The fact that APCl and electrospray are soft ionization techniques is often advantageous because the molecular ion alone, in conjunction with HPLC separation, often provides adequate selectivity and sensitivity to allow an analytical method to be developed. Again, method development is important, particularly when more than one analyte is to be determined, when the effect of experimental parameters, such as pH, flow rate, etc., is not likely to be the same for each. Electrospray, in particular, is susceptible to matrix effects and the method of standard additions is often required to provide adequate accuracy and precision. 

Standard additions A method of relating the intensity of signal from an analyte measured in an unknown to the amount of analyte present. This technique is designed to take matrix effects into account. 

A fully automated instrumental procedure has been developed for analyzing residual corrosion inhibitors in production waters in the field. The method uses ultraviolet (UV) and fluorescence spectrophotometric techniques to characterize different types of corrosion inhibitors. Laboratory evaluations showed that fluorescence is more suitable for field application because errors from high salinity, contamination, and matrix effect are minimized in fluorescence analysis. Comparison of the automated fluorescence technique with the classic extraction-dye transfer technique showed definite advantages of the former with respect to ease, speed, accuracy, and precision [1658],... 

In Section 8.2.8 we have discussed the standard addition method as a means to quantitate an analyte in the presence of unknown matrix effects cf. Section 13.9). While the matrix effect is corrected for, the presence of other emalytes may still interfere with the analysis. The method can be generalized, however, to the simultaneous analysis of p analytes. Multiple standard additions are applied in order to determine the analytes of interest using many q > p) analytical sensors. It... 

An important prerequisite for the use of CRMs as calibrants, at least for optical methods and particularly all AAS modes, is that they should match the matrix and level of analyte contents of the materials to be analyzed as closely as possible, so that potential matrix effects will be compensated if calibrant and sample material are affected by the applied method, e.g. the temperature program for furnace techniques, in the same way. Further it is very important for all methods that the CRMs used should not show a nugget effecf, i.e. particles with extremely high analyte content that can lead to a high analyte heterogeneity (Kurfiirst 1991 Kurfiirst et al. 

In trace organic analysis there is usually an extraction or clean-up process, rather than a sample dissolution. Here not only must the matrix effect be considered, but also the recovery yield of the extraction. Frequently an external spike standard is added, but there is often no way of knowing if the recovery of the spike standard matches the analyte in question. There is considerable evidence that the U S E P A method for VOA analysis (Minnich 1993) is subject to such error, as reported by Schumacher and Ward (7997). The analyst must always consider the possibility of such an error, especially when using CRMs to control methods that are applied in routine mode. 

Nappom WT, Laborde H, Leger J-M, Lamy C. 1995. Flectro-oxidation of Ci molecules at Pt-based catalysts highly dispersed into a polymer matrix Effect of the method of preparation. J Electroanal Chem 404 153-159. 

It is important to note that the matrix effects, interferences, and variability in method efficiency are to be factored in when determining the MDL. If this was not done then only the background noise (see Figure 2, peak 13) would be considered in the definition of the MDL. In real-life samples there is a good possibility that matrix component peaks would either co-elute or elute at retention times close to... 

Method Simple and easy to apply Considers variability of cah-bration curve Considers method efficiency and matrix effects Variability between laboratories and analysts Good for estimating LOD/LOQ Comments... 

Although accepted by lUPAC and ACS, the k s /m" definition is hard to implement and does not take either variability in method efficiency or matrix effects into consideration. This would be rectifiable if the calibration curves were prepared from control matrix samples fortified at different concentrations (within one order of magnitude of an estimated LOD). 

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