Matrix matched calibration solutions

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. 

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. 

Calibrators (Matrix Matched Calibration Solutions) Samples of blank matrix to which known amounts of the analytical standard (and of SIS if available) have been added before extraction and clean-up used to determine the concentration range of the complete assay and to calibrate the instrument response for analyte concentration while correcting for variations in extraction... 

As emphasized in Section 8.5, use of a control ( blank ) matrix in the preparation of matrix matched calibrator solutions provides several significant advantages over use... 

Use of matrix-matched calibration solutions. Either protein is added to the solutions or certified reference materials are used (if available). 

An ideal method for the preconcentration of trace metals from natural waters should have the following characteristics it should simultaneously allow isolation of the analyte from the matrix and yield an appropriate enrichment factor it should be a simple process, requiring the introduction of few reagents in order to minimise contamination, hence producing a low sample blank and a correspondingly lower detection limit and it should produce a final solution that is readily matrix-matched with solutions of the analytical calibration method. 

Calibration using simple solutions of well-characterised pure substance standards or matrix matched standard solutions. Calibration solutions are prepared from materials whose identity and purity have been established to an appropriate level of uncertainty and where the effects of any impurities have been evaluated. Where appropriate and where available, standards provided by metrology institutes with demonstrated capability are used. In other cases, materials from other reputable suppliers or prepared in-house are used after appropriate characterisation. Where necessary, professional judgement is used to estimate the uncertainty associated with chemical standards. The target uncertainty of the identity is for practical purposes zero and for purity less than one-fifth of the desired overall uncertainty. 

The ion chromatograph is calibrated with standard solutions containing known concentrations of the target ions. Calibration curves are constructed from which the concentration of each ion in the unknown sample is determined. It is strongly recommended to match the calibration solutions with the sample matrix. Five calibration solutions and one zero standard (blank, normally water) are needed to generate a suitable calibration curve. The range to be used will depend on the concentration range for the different samples. 

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. 

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. 

FIA techniques for AAS are described in detail in the reference by Tyson. In addition to automated CVAAS and HGAAS, FIA has been used to automate online dilution for the preparation of calibration curves, online matrix matching of solutions, online preconcentration and extraction for GFAAS, automated digestion of samples, and much more. Commercial FIA systems are available for most AAS instmments. 

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. 

Calibration (Standard) Curve A plot of instrument response (e.g., LC/MS peak area) vs concentration or amount (mass or moles) of analyte injected. The sensitivity (or response factor) is best defined as the slope of the calibration curve (change in signal for unit change in quantity/concentration), but sensitivity is often used in a colloquial sense to imply low LOD and/or LLOQ-When a SIS is used for maximum accuracy and precision, the ratios of instrument response (analyte SIS) are plotted vs the corresponding concentration ratio. The standard solutions used for calibration can be clean solutions of the analytical standard (possibly plus SIS), or matrix-matched calibrators, i.e., analytical extracts of a blank matrix spiked with known amounts of analytical standard (plus possibly also SIS)... 

Stock and spiking solutions (see below) are eventually used to make the cahbration solutions and matrix matched calibrators that are the basis for the comparative measurement of analyte concentration in the unknown analytical sample (Sections 2.6 and 8.5), so the reliability of their concentrations is directly related to that of the measured concentrations. Accordingly, meticulous care and monitoring of these primary solutions is every bit as important as that expended in weighing the analytical standard in the first place (Sections 2.3). 

Sub-stocks are also used for multi-analyte assays where a mixture of analytes in a single solution is prepared for subsequent preparation of QCs, calibration solutions or spiking solutions used for the preparation of matrix matched calibrators. In this case, individual stock solutions are combined to make one or more sub-stock solutions containing all of the analytes in one solution. This practice will reduce the number of steps that would be required versus making an individual spiking solution for each analyte. However, for methods that required a different LLOQ for each analyte, the sub-stocks and spiking solutions need to be prepared at the corresponding appropriate concentration for each analyte. 

For the preparation of matrix matched calibrators, the same general principles used for the preparation of calibration solutions apply except that a fixed known volume of one or more spiking solutions is added to each aliquot of blank (control) matrix (fixed amount, either weighed or possibly dispensed by volume for liquid matrices such as plasma, urine etc.), followed by the fixed volume of SIS solution. 

Matrix matched calibrators are often prepared Ifesh on the day of analysis using spiking solutions of known stability. However, if the stability of the analyte in matrix has been established (Section 10.2.8), cabbrators may be prepared in bulk, divided into smaller abquots and then thawed and used on the day of analysis. 

General principles for preparation of calibration solutions and matrix matched calibrators are described in Section 9.8.1 and similarly for QC samples in Section 9.8.2. Records to support the preparation, storage conditions (location and temperature) and use of these samples must be maintained at the analytical site, together with SOPs and information for all relevant apparatus including freezers and refrigerators, analytical balances, volumetric equipment etc. 

Matrix matched calibrators were prepared in blank rat plasma for each enantiomer separately and also for racemic propranolol. Prior to online extraction-analysis each 25p,L plasma sample was spiked with an equal volume of a solution (250ng.mL ) of the internal standard (d7-propranolol) in 20% methanol 80% 0.5 M formic acid in water, so the internal standard concenh ation in all anal54ical samples was 125ng.mL. To monitor the accuracy and precision of the assay, fom different sets... 

For calibration, microlitre amounts of a gravimetrically prepared standard solution of DMS in ethanediol are injected via the T-fitting into the helium line (see Fig. 24-1), as a sample of degassed (DMS-free) seawater is loaded into the purge vessel. This procedure provides a matrix-matched calibration and minimizes systematic errors by automatically correcting for degassing efficiency and potential DMS losses within the purge and trap unit. To avoid subsequent DMS release from particulates, the DMS-free seawater should be prepared from a filtered sample, ideally from deep water of low DMS concentration. 

However, calibration of LA-ICP-MS measurements for accurate determination of trace elements is still a problem, which is mainly due to fractionation processes and lack of certified standard reference materials with a matrix composition that is sufficiently similar to that of the various sample types that need to be analyzed [39]. A great variety of different calibration methods have therefore been developed, including matrix-matched and non-matrix-matched calibration techniques [39-42], but also techniques relying on the use of standard solutions, with corresponding aerosol introduction or direct ablation of the liquid, have been suggested [43, 44]. As a result of the different difficulties and limitations of these calibration methods, LA-ICP-IDMS is a promising alternative for calibration of measurements in powdered and liquid samples. 

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