Matrix effects preparation

Domingo et al. [107, 108] used ultrapure nitric acid for sample digestion, performed all specimen manipulation in a laminar flow hood, carried out readings of a blank (reagents only) for each group of samples and, to minimize matrix effects, prepared the standard curves for aluminum determination by the standard addition method. 

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... 

Other effects. In addition to the compound formation and ionisation effects which have been considered, it is also necessary to take account of so-called matrix effects. These are predominantly physical factors which will influence the amount of sample reaching the flame, and are related in particular to factors such as the viscosity, the density, the surface tension and the volatility of the solvent used to prepare the test solution. If we wish to compare a series of solutions, e.g. a series of standards to be compared with a test solution, it is clearly essential that the same solvent be used for each, and the solutions should not differ too widely in their bulk composition. This procedure is commonly termed matrix matching. 

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. 

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). 

The use of standards prepared in control matrices is typically not allowed for determinative procedures because control tissues are not routinely available to regulatory laboratories. When a matrix effect alters the spectrum or chromatography of an analyte relative to the pure standard, so that confirmatory criteria cannot be met, a control extract containing standard may be substituted for pure standard. Justification, with CVM concurrence, should be provided for confirmatory methods that use fortified control extracts. 

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. 

Two general approaches have been used to overcome matrix effects (1) partial purification of the analyte prior to analysis by immunoassay ( cleanup methods) and (2) the use of a matrix blank when preparing the calibration curve. Both options are widely used, but each has its individual limitations. 

Because of the possibility that the herbicide alachlor could adulterate food if either poultry or livestock consumed contaminated materials, Lehotay and Miller evaluated three commercial immunoassays in milk and urine samples from a cow dosed with alachlor. They found that milk samples needed to be diluted with appropriate solvents (1 2, v/v) to eliminate the matrix effect. One assay kit (selected based on cost) was also evaluated for use with eggs and liver samples from chickens. Egg and liver samples were blended with acetonitrile, filtered, and diluted with water. Linear calibration curves prepared from fortified egg and liver samples were identical... 

Sample preparation consists of homogenization, extraction, and cleanup steps. In the case of multiresidue pesticide analysis, different approaches can have substantially different sample preparation procedures but may employ the same determinative steps. For example, in the case of soil analysis, the imidazolinone herbicides require extraction of the soil in 0.5 M NaQH solution, whereas for the sulfonylurea herbicides, 0.5M NaOH solution would completely decompose the compounds. However, these two classes of compounds have the same determinative procedure. Some detection methods may permit fewer sample preparation steps, but in some cases the quality of the results or ruggedness of the method suffers when short cuts are attempted. For example, when MS is used, one pitfall is that one may automatically assume that all matrix effects are eliminated because of the specificity and selectivity of MS. 

HPLC/MS and HPLC/MS/MS analyses are susceptible to matrix effects, either signal enhancement or suppression, and are often encountered when the cleanup process is not sufficient. To assess whether matrix effects influence the recovery of analytes, a post-extraction fortified sample (fortified extract of control sample that is purified and prepared in the same manner as with the other samples) should be included in each analytical set. The response of the post-extraction fortified sample is assessed against that of standards and samples. Matrix effects can be reduced or corrected for by dilution of samples, additional cleanup, or using calibration standards in the sample matrix for quantitation. 

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]. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

It has to be stressed that selectivity of both separation methods depends on the matrix effect and the additives used, which can strongly influence chromatographic or electrophoretic equilibria. For that reason proper sample preparation plays a crucial role in the analytical process. 

The spectrum of GeF2 trapped in a neon matrix is shown in Fig. 3. The ratio of GeF2/rare gas in the matrix was 1 1000. When new matrices were prepared similar spectra were obtained, even when the ratio of diluent was changed or the temperature of deposition was altered. This indicated that the splitting seen in the spectrum was due to isotope effects and was not due to matrix effects. As can be seen the intensities of the various peaks are in the same ratio as the abundant isotopes of germanium, providing additional evidence that the splitting is due to isotope effects. 

Matrix effect — To demonstrate that the assay performance was independent from the sample matrix, QC samples were prepared using two different lots of matrix. The QC samples were evaluated using the same calibration curve. With regard to analytical recovery, no significant difference was observed for the QCs prepared in two lots of plasma. 

Matrix effect is a phrase normally used to describe the effect of some portion of a sample matrix that causes erroneous assay results if care is not taken to avoid the problem or correct for it by some mechanism. The most common matrix effects are those that result in ion suppression and subsequent false negative results. Ion enhancement may lead to false positive results.126 127 Several reports about matrix effects include suggestions on what can cause them and how to avoid them.126-147 While various ways to detect matrix effects have been reported, Matuszewski et al.140 described a clear way to measure the matrix effect (ME) for an analyte, recovery (RE) from the extraction procedure, and overall process efficiency (PE) of a procedure. Their method is to prepare three sets of samples and assay them using the planned HPLC/MS/MS method. The first set is the neat solution standards diluted into the mobile phase before injection to obtain the A results. The second set is the analyte spiked into the blank plasma extract (after extraction) to obtain the B results. The third set is the analyte spiked into the blank plasma before the extraction step (C results) these samples are extracted and assayed along with the two other sets. The three data sets allow for the following calculations ... 

Hexachloroethane can be detected in tissues at levels as low as 0.001 pg/g (Nolan and Karbowski 1978) and recoveries range from 50 to 130%. Prior to analysis, hexachloroethane must be separated from the biological sample matrix and prepared for introduction into the analytical instrument. Separation may be effected by purging with an inert gas (helium), and trapping on an adsorbent cartridge (Tenax GC ), followed by... 

With the application of FIA in the mixture analytical mode for the analysis of environmental samples and after a marginal sample pretreatment by SPE, matrix effects are a high probability. But, as cited previously [27—31], matrix effects were not only observed with FIA but also in LC-MS and MS—MS modes. Advice to overcome these problems by, e.g. an improved sample preparation, dilution of the analyte solution, application of stable isotopic modification of LC conditions [29] or even application of two-dimensional LC separations [27], postcolumn standard addition [29], addition of additives into the mobile phase (e.g. propionic acid, ammonium formate) [34,35] or even matrix compounds [32] were proposed and discussed. 

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