Matrix matched calibrators

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. 

It is often difficult to define where sample extraction ends and cleanup procedures begin. Sample extracts may be injected directly into a gas or liquid chromatograph in certain cases, but this will be dependent on the analyte, sample matrix, injection, separation and detection system, and the limit of determination (LOD) which is required. It is also more likely that matrix-matched calibration standards will be needed in order to obtain robust quantitative data if no cleanup steps are employed. 

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. 

Use as a matrix matched calibrant (direct or via working standards) to ensure traceabiiity of results to an external reference (the CRM)... 

Conventional external calibration uses pure standard solutions (single- or multielement) and is therefore unable to compensate for matrix effects, fluctuations or drifts in sensitivity. Matrix effects can be compensated for by using matrix-matched calibration solutions. In this case, the degree of compensation depends on the proper matrix adjustment. 

Despite the wide application of matrix-matched calibration, it is very difficult or even impossible to completely match the matrix of every sample due to diversities of subjects, particularly human subjects, and the dynamic nature of in vivo metabolism. 

The ICP-AES and ICP-MS techniques may also suffer from matrix effects, such as spray chamber effects caused by the different viscosity of the samples and the calibration standards. The careful choice of internal standards can reduce this problem. The effects caused by high amounts of easily ionized elements may be solved by internal standardization or by the use of matrix-matched calibration curves. An additional specific problem with ICP-AES is the risk of spectral overlaps. 

Chromium and Sb have been determined in cachaca by ET-AAS using matrix matching calibration and Ru as a permanent modifier [128], Calibrants containing both elements were prepared in a 40 percent v/v ethanol solution and spiked samples were used to check accuracy. LoDs of 2 and 43 pg were achieved for Cr and Sb, respectively. The results obtained from the analysis of 52 samples varied from nondetectable to 39.1 xg l-1 for Sb and between 0.64 and 1.53 p,g l-1 for Cr. 

M. H. Canuto, H. G. Luna Siebald, G. Magela de Lima, J. B. Borda Silva, Antimony and chromium determination in Brazilian sugar cane spirit, cachaca, by electrothermal atomic absorption spectrometry using matrix matching calibration and ruthenium as permanent modifier, J. Anal. Atom Spectrom., 18 (2003), 1404-1406. 

Several recommendations arose from the interlaboratory smdy to minimize analytical challenges and to ensure data quality. As discussed above, it is recommended that mass labelled PFCs be employed as internal standards [93, 97]. It should be noted, however, that some electrospray ionization suppression may still occur if these internal standards are used at high concentrations [97]. Matrix effects can also be minimized by employing matrix-matched calibration standards in lieu of solvent-based calibration standards [97]. Unfortunately, matrix-matched standards can be impractical when an appropriate clean matrix cannot be found [94]. Other quality assurance and quality control measures, such as spike and recovery analyses of an analyte added to the sample matrix, repetitive analysis of samples to determine precision and comparison of internal standard quantitation to quantitation via standard additions, are also useful in determining data quality [94]. 

Bruguier O., Telouk P., Cocherie A., Fouillac A. M., and Albarede F. (2001) Evaluation of Pb-Pb and U-Pb laser ablation ICP-MS zircon dating using matrix-matched calibration samples with a frequency quadrupled (266 nm) Nd-YAG laser. Geostand. Newslett. J. Geostand. Geoanal. 25(2-3), 361-373. 

Calibration Calibration solutions +/- Matrix effect, work outside linear range of the detector Reagents of suitable purity and stoichiometry where necessary verification of stoichiometry and purity of calibrants different calibration methods when possible calibration graphs, matrix-matched calibration solutions and standard additions. 

From sx the 95% confidence interval on the estimate may be determined by multiplication by the appropriate /-value (/o.o5",n-2)-Standard addition is used when there are potential interferents that would lead to a systematic error that is proportional to concentration. Calculation of the concentration by the standard addition method causes these errors in the measurements to cancel. It is also useful if the analyte cannot be extracted from its matrix, and there is not a matrix matched calibrant available. This may be the case in environmental analysis. Note, however, that standard addition does not compensate for a constant additive interferent. 

Using DCP-AES, Roberts and Williams (1990) determined silicon in serum and urine after simple dilution in a 1% nitric acid solution. In the absence of any spectral or background interference, samples could be measured against aqueous standards. Matrix-matched calibration curves were used by Bercowy et al. (1994) who diluted serum and urine samples in water. Jackson et al. (1998) treated serum samples with highly pure nitric acid (2%) followed by dilution with distilled de-ionized water. 

Two other techniques which warrant some mention are inductively-coupled plasma atomic emission spectrometry (ICP-AES) and inductively-coupled plasma mass spectrometry (ICP-MS). The former is limited in sensitivity. Bussiere et al. (1989) found a detection limit of 2 fig/L which was just sufficient to determine manganese in amniotic fluid directly when matrix-matched calibration and an internal standard of gallium was used. Concentrations in ten samples ranged from 14 to 16 /ig/L. Preconcentration techniques using a poly(dithiocarbamate) resin have been applied to the determination of manganese in urine (Barnes et al., 1983 Van Berkel and Maessen, 1988). Barnes et al. (1983) digested the... 

Ashing may be carried out at temperatures up to 1100°C which enables removal of part of the matrix. The matrix that remains can still be a problem. In the author s experience (Halls and Fell, 1981), the effect of the matrix on the signal can be minimized by control of the ashing temperature and time. In this way, simple aqueous standards can be used for calibration. However, the experience of others, particularly in the determination of manganese in serum, was that aqueous calibration could not be used and they used either matrix-matched calibration with a serum low in manganese (Pleban and Pearson, 1979 Subramanian and Meranger, 1985 N6ve and Leclerq, 1991) or standard additions on each sample (Casey et al., 1987). Smeyers-Verbeke et al. (1976) have studied the... 

Using pyrolytic graphite tubes, platform atomization and integrated absorbance readings a Zeeman-based instrument provides an acceptable situation for selenium determinations and permits most analyses to be performed against matrix matched calibration graphs. In Table 1 recommended procedures for the direct GF-AAS determination of selenium in body fluids are listed. 

Lower results (factor 2) were observed for ICP in comparison to ETAAS. It was assumed that interferences were taken into account in the calibration in the ICP measurement. However, when standard additions were used or Ca was added to the calibrants, ICP results agreed with those of ETAAS, which demonstrated the need to perform a matrix matching calibration or a standard addition procedure. The scatter of results observed in step 2 (0.73 to 1.66 mg kg ) was found to be high but not surprising considering the low Cr content in the extract. 

Turnipseed et al. described a method for the multi-class residue determination of P-Iactams, sulfonamides, tetracyclines, fluoroquinolones, and macroiides in milk and other dairy products. The sample preparation combines extraction with acetonitrile, clean-up with Oasis HLB cartridges, and ultrafiltration using molecular weight cut-off filters to improve the overall performance of the analysis. Acceptable recoveries were obtained for sulfanomides, macroiides, and quinolones (>70%) however, recoveries were rather low for tetracyclines (50-60%) and P-lactams (<50%). Despite the extensive clean-up procedure, significant matrix ion suppression was observed for many compounds, making it necessary to include matrix-matched calibration standards for quantification purposes. 

Steliopoulos P, Estimating the decision limit and the detection capability using matrix-matched calibration data, Accred. Qual. Assur. 2010 15 105-109. 

It can be assumed that with the development and study of new methods, the ability to determine M (S), the method bias component of uncertainty, cannot be done given that it can be evaluated only relative to a true measure of analyte concentration. This can be achieved by analysis of a certified reference material, which is usually uncommon, or by comparison to a well-characterized/accepted method, which is unlikely to exist for veterinary drug residues of recent interest. Given that method bias is typically corrected using matrix-matched calibration standards, internal standard or recovery spikes, it is considered that the use of these approaches provides correction for the systematic component of method bias. The random error would be considered part of the interlaboratory derived components of uncertainty. 

Matrix matched calibration curves for DON, 3-AcDON, DAS, 15-AcDON, nivalenol, neosolaniol, HT-2, and T-2 were tested in the range from 0.050 mg/kg to 1.5 mg/kg. The correlation coefficients were higher than 0.99, thus the linearity was considered appropriate. The procedure using pnire standards calibration tested previously did not give reasonable results. The problem was unreasonably high recoveries (often higher than 110%, 120% or... 

Most commercial XRF systems have a fundamentals parameters software program as part of their data processing package. The advantage to the FP approach is that many industrial materials do not have readily available matrix-matched calibration standards commercially available. Preparation of good, stable calibration standards takes a lot of time and money even when it is possible to make... 

In the quantitation process, isotopic-labeled standards when available and/or certified standards of PFCs in methanol were used [13,17,25,34,35,48,49]. Matrix-matched calibration was also employed [48]. 

There are two broad classes of calibration solutions used in analyses of the kind discussed here. The first class, referred to as calibration solutions, corresponds to solutions of the analyte(s) in clean solvent, possibly also containing internal standard(s) such solutions can be certified calibration solutions (Section 2.2.2) or solutions prepared in the analyst s own laboratory according to procedures determined ahead of time to be fit for the purpose for which the analysis is to be undertaken. The other class of calibration solutions, which will be referred to as matrix-matched calibrators (sometimes just calibrators ), is prepared from aliquots of a blank (or control) matrix (identical or almost so to the matrix composing the analytical samples but devoid of the target anal54 e(s) to within the detection limits of the analytical method, see Section 9.4.7). When a suitable blank matrix is available, this is the preferred approach since many interferences and other effects are largely accounted for automatically. 

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