Calibration cross-contributions

Figure 18 Calibration curves using an internal standard (IS). Analytes are quantified against an IS that has been added as early as possible in the analytical procedure. The ratios of detector responses for the analyte (fiA) and IS (R S) are plotted against the ratio of known amounts of analyte (A) and IS. When a sample is analyzed, the ratio Ra/Ris is measured. Then knowing the amount of IS added into the sample, the amount of analyte present in the sample can be estimated. Curves that do not pass through the origin of the graph or which are nonlinear are diagnostic of (a) chemical interference or sample carryover, (b) sample loss during the assay due to adsorption, and (c) saturation or cross-contribution between the IS and the analyte.

Figure 8.15 Examples of GC/EIMS calibration curves that are nonlinear as a result of cross-contributions between the analyte (butalbital) and SIS (butalbital-D5), both as the methylated derivatives. The SIS was present in all calibration solutions at a concentration of 200 ng mL . The mjz values (all for fragment ions) monitored were (a) analyte 196, SIS 201 (b) analyte 138, SIS 143. See main text for data on cross-contributions. In both cases the curves represent nonlinear least-squares regression fits of the data to Equation [8.89], accounting for cross-contributions in both directions. Reproduced from Whiting, /. Anal. Toxicol. 25, 179 (2001), with permission of Preston Publications, A Division of Preston Industries, Inc.

FDA work will tolerate an SIS cross-contribution of up to 20 % of the response of the analyte being quantified at the LLOQ concentration. Note that these fitness for purpose guidelines are based largely on practical experience without (thus far) any statistical justification. Ultimately this question should be settled by visual examination of the experimental calibration curve together with careful evaluation of the accuracy and precision over the entire range of analyte concentration for the specified SIS concentration used to generate the calibration. In any event, the cross-contributions (if any) must be carefully monitored during all phases of method validation and sample analysis and also must be fully discussed in the method description and final report. 

Normally a calibration curve—molar mass against the total retention volume—exists for every GPC column or column combination. As a measure of the separation efficiency of a given column (set) the difference in the retention of two molar masses can be determined from this calibration curve. The same eluent and the same type of calibration standards have to be used for the comparison of different columns or sets. However, this volume difference is not in itself sufficient. In a first approximation the cross section area does not contribute to the separation. Dividing the retention difference by the cross section area normalizes the retention volume for different diameters of columns. The ISO standard method (3) contains such an equation... 

It should be mentioned that another validation technique, called leverage correction [1], is available in some software packages. This method, unlike cross validation, does not involve splitting of the calibration data into model and test sets, but is simply an altered calculation of the RMSEE fit error of a model. This alteration involves the weighting of the contribution of the root mean square error from each calibration... 

The leverage expressed by an object provides information on its importance in contributing to the calibration model by relating the position of its independent variables relative to others. It is related to the Mahalanobis distance and is derived from the cross product matrix of factors used in the calibration model. 

The handbooks and operation manuals provided by tube suppliers usually give detailed information about the various reaction mechanisms [6-99]. Because of the chemical nature of detection mechanisms, with few exceptions they do not have absolute selectivity toward one single substance. In many cases, cross-sensitivities to chemically similar substances can be observed, which means that chemicals showing the same chemical reaction behavior will contribute to the indicated result As the calibration is performed only using the pure substance, the true concentration cannot be concluded from the indicated result if an interfering chemical is present in the air. Such kinds of cross-sensitivities are not easily foreseen. The true concentration of a contaminant is often lower than that indicated by the reading on the tube, which means the result is false positive. However, with... 

Theoretical reaction dynamics can contribute on many levels to our understanding of chemical phenomena. Occasionally, for example, theory can calculate an observable quantity which cannot be measured accurately. Since most dynamics experiments measure relative populations, theory can calibrate them by calculating the absolute cross section for a particular transition. On a more important level, theory contributes to our understanding of chemical phenomena by direct comparison with experiment. Comparisons with approximate calculations are indispensable in determining which aspects of the underlying physics control the reactivity. Alternatively, comparisons with accurate calculations help to ensure that our picture of the chemical reaction is complete. In this dissertation, we develop and apply accurate theoretical methods for describing electronically adiabatic atom—diatom reactive scattering. 

The analyte s concentration ranges are important to identify, in consideration of the sensitivity of the instrument, to establish whether or not a dilution or enrichment step is necessary. In good analytical practice, the calibration range should cover the analyte concentration present in each sample. Additionally further decisions in terms of equipment have to be made. For working in the mg/kg range, conventional glassware for nebuliser and spray chamber can be used, but to go down to the pg/kg or ng/kg level requires the sample introduction system to be made from quartz or PFA (perfluoroalkoxy polymer) to minimise blank contribution, memory effects and cross contamination. 

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