Multiple point calibration linear

BCR Analytical Approach for the Certification of PAHs in Natural Matrix CRMs Prior to the certification analyses for the CRM, each participating laboratory has to prepare standard solutions of the analytes to be determined from certified reference compounds (purity >99.0 %) to calibrate their instruments for response and response linearity (multiple point calibration), detection limit, and reproducibility. In the case of PAH measurements, reference compounds of certified purity are used as internal standards, which are not present at a detectable concentration in the matrix to be analyzed (e.g. indeno[i,2,3-cd]fluoranthene (CRM 267), 5-methylchrysene (CRM 081R), benzo[f ]chry-sene (CRM 046), picene (CRM 168), and/or phenanthrene-dio). 

Prerequisites for the Calibration Types. It depends on the design of the analytical procedure as to which regression parameters are meaningful and which results are acceptable. In other words, the model to be used for quantitation must be justified. For a singlepoint calibration (external standardization), a linear function, zero intercept, and the homogeneity of variances are required. The prerequisites for a linear multiple-point calibration are a linear function and in case of an unweighted calibration also the homogeneity of variances. A non-linear calibration requires only a continuous function. With respect to the 100%... 

Standardizations using a single standard are common, but also are subject to greater uncertainty. Whenever possible, a multiple-point standardization is preferred. The results of a multiple-point standardization are graphed as a calibration curve. A linear regression analysis can provide an equation for the standardization. 

Case 3. Linearity demonstrated from 50% of the ICH reporting limit to 150% of the shelf life specification of a related substance, and a significant y-intercept is observed (Figure 3.8). Due to the significant -intercept, a single-point calibration (e.g., high-low or one-point external standard calibration) is not suitable. In this case, multiple-point external standard calibration is the most appropriate. See Section 3.3.3 for more discussion of the significant y-intercept. 

Demonstration of linear response enables the use of single point calibration techniques, which can offer considerable cost savings regarding the number of calibration materials that must be maintained and the time it takes to calibrate the on-line analyzer. A 2000 ppmw standard was made with 7.88 g of pure dibutyl sulfide diluted in 1 L of toluene. This standard was further diluted sequentially 1 1 nine times until the expected sulfur concentration in the sample was down to 3.91 mg/kg. Each of these 10 samples were injected multiple times in a 6000 series analyzer that was set for a 2500 ppmw full scale range and calibrated with the 2000 ppmw sample (see Figs. 8 and 9). Data from the results of the 16.13 ppmw sample were lost due to a data acquisition problem. The data, shown in Table 3, indicate a % RSD of better than 1 % in the sulfur range 62-2000 mg/kg. At lower sulfur levels of 4-31 mg/kg, the %RSD is 11-2 %. 

Some application software programs use methods known as MLR multiple linear regression) that permits the statistical treatment of a large number of data points in order to establish a calibration equation. These chemiometric methods take advantage of all the absorptions measured at different wavelengths, irrespective of their origin analyte to be measured, matrix or artefacts of the instrument. 

RBF networks have seen few chemical applications.They were better than multiple linear regression at predicting boiling points from structural parameters. (Reference 191 also contains a good description of the RBF method.) RBF networks resulted in better calibration than partial least squares in the determination of blood glucose by near-infrared spectroscopy. 

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