Matrix-matched calibration standards

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. 

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]. 

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. 

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. 

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... 

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. 

An ICP-MS instrument with an MC array system can easily improve the precision of the isotope ratio measurements. However, stability of the signal intensity profile, correction for isotope fractionation during LA and ionization, and correction of the mass bias effect using a matrix-matched calibration standard are also very important for improving the trueness of the isotopic data. 

A typical GC-MS batch consisted of matrix-matched calibration standards, samples, one matrix blank and one recovery sample for performance check after a set of every six samples. 

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. 

Pd removal was determined as follows. An aliquot of a representative liquid or solid sample was accurately weighed and subsequently digested by refluxing in nitric and/or hydrochloric acid using a closed vessel microwave procedure (CEM MARS5 Xpress or Milestone Ethos EZ). Cooled, digested samples were diluted, matrix matched to standards, and referenced to a linear calibration curve for quantitation an internal standard was employed to improve quantitation. All samples were analyzed by an Inductively Coupled Plasma Mass Spectrometer or ICP/MS (Perkin Elmer SCIEX Elan DRCII) operated in the standard mode. 

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

Figure 9.38 Concentration profile of copper and zinc in the scanned area of interest measured by LA-ICP-MS on brain samples (hippocampus). Calibration was performed via synthetic matrix matched laboratory standards for 1, 5 and 10 fxgg-1 of analyte (see inserted figures on left). Bottom histologically processed brain tissue in which cell bodies were stained (cresyl violet staining) in order to demonstrate the layered structure of the analyzed region. (]. S. Becker, M. Zoriy, C. Pickhardt, N. Palomero-Gallagher and K. Zilles, Anal. Chem., 77, 3208 (2005). Reproduced by permission of American Chemical Society.)...

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. 

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. 

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... 

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. 

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... 

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