Calibration strategy

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

To overcome these matrix influences, other calibration strategies can be used (Danzer et al. [1998]). According to one of them, a specialized three-dimensional calibration model, the relation between signal intensity and both sample weight and content is evaluated. The relationship between the three quantities, y = f(x,w) is demonstrated in Fig. 6.12 by the example of Zn determination represented above. 

Perkins, W. T., Pearce, N. J. G., and Westgate, J. A. (1997). The development of laser ablation ICP-MS and calibration strategies examples from the analysis of trace elements in volcanic glass shards and sulfide minerals. Geostandards Newsletter 21 175-190. 

Two procedures for improving precision in calibration curve-based-analysis are described. A multiple curve procedure is used to compensate for poor mathematical models. A weighted least squares procedure is used to compensate for non-constant variance. Confidence band statistics are used to choose between alternative calibration strategies and to measure precision and dynamic range. 

Implications For B ethod Development. The effects of the calibration process on precision suggest the need for an additional step in the development of an analytical method. A suggested flow chart is shown in Figure 9. The analyst should first develop a method of adequate accuracy and precision without using calibration curves. The calibration step is then added, and the precision is rechecked. If precision has been excessively degraded, the analyst can choose among alternative calibration strategies, such as use of more standard measurements and use of the multiple-curve procedure. 

Of course, the ultimate goal is to geuerate calibratiou data that is both highly accurate and highly relevant. However, in PAT this can be a rather difficult goal, as it seems that one often trades off accuracy for relevance, and vice versa. For example, consider two of the most common calibration strategies in PAT ... 

Figure 12.30 Illustration of the calibration strategy space, where any calibration strategy can be mapped by its relevance (x-axis) and it accuracy (y-axis). In this figure, several calibration strategies are mapped onto this...

In Section 12.3.2, the fundamental differences between direct and inverse modeling methods were discussed. As will be discussed here, this distinction is not just a convenient means for classifying quantitative regression methods, but has profound implications regarding calibration strategy and supporting infrastructure. 

Extended-CLS (ELS)-type modeling, can be supported with the same calibration strategies as those mentioned above for direct methods, with an additional consideration one must also collect data that can be used to estimate unknown contributions to the spectrum. Such characterization of the unknown contributors often requires the collection of sufficiently relevant process data, or at least reasonable a priori estimates of expected interference effects, such as baseline effects and water vapor interferences, in the process spectra. 

Regarding relevance, the spectral miscibility of the data obtained from these two different sources can be readily observed by doing a PCA analysis of the combined spectral data. The scatter plot of the first two PC scores obtained from PCA of such a data set for one of the process analytes is shown in Figure 12.31a. Note that there is considerable common space for the two data sources in the PC1/PC2 space, and there are some regions of this space where only samples from the old calibration strategy lie. A similar pattern is observed in the later PCs of this model. This result indicates that the on-line spectra contain some unique information, but that the on-line and injected-standard spectra are generally quite similar. 

Figure 12.31 Illustration of a hybrid calibration strategy. (A) Scatter plot of first two PCA scores obtained from a process analytical calibration data set containing both synthesized standards (circles) and actual process samples (triangles). (B) Results of a PLS regression fit to the property of Interest, using all of the calibration samples represented In (A).

Several PAT calibration strategies, especially those that are intended to support inverse calibration methods, rely heavily on data that is routinely collected from the deployed analyzer, as opposed to data collected from carefully designed experiments. Such data, often called happenstance data , can be very inexpensive. 

Analgin Powder Non-destructive drug determination by means of artificial neural network. Different calibration strategies compared. Relative predictive error lower than 2.5% 149... 

ISO 8466 describes how to perform calibration. Part 1 is covering the linear regression and part 2 the second order calibration strategy. 

Part 2 Calibration strategy for non-linear second order calibration functions... 

There also might be requirements on the uncertainty of the measurement, which might not be fulfilled at the lower end of the preliminary working range. If we want to use a linear calibration strategy we have to ensure that the variances are sufficiently homogeneous and that there is no significant deviation from linearity. 

For routine use calibration strategies with less effort are used... 

ISO 8466-2 2001 - Calibration and evaluation of analytical methods and estimation of performance characteristics - Part 2 Calibration strategy for non-linear second-order calibration functions... 

Certified reference materials (CRMs) are mainly applied to validate the analytical procedure developed for routine analysis in order to determine the accuracy of analytical data, the recovery for selected elements, the uncertainty of trace element determination and the detection limits. Otherwise, in solid-state mass spectrometric techniques, such as SSMS, LA-ICP-MS, GDMS, SNMS or SIMS, one point calibration using CRMs has been established as an important calibration strategy to obtain reliable analytical data. The one point calibration is performed using the experimentally determined relative sensitivity coefficients (RSCs) on a suitable CRM with a similar trace/matrix composition. An RSC of a chemical element is defined as the ratio of the measured element concentration (experimentally determined) divided by the certified element concentration (accepted or recommended value of element concentration) in a given matrix. 

An easy calibration strategy is possible in ICP-MS (in analogy to optical emission spectroscopy with an inductively coupled plasma source, ICP-OES) because aqueous standard solutions with well known analyte concentrations can be measured in a short time with good precision. Normally, internal standardization is applied in this calibration procedure, where an internal standard element of the same concentration is added to the standard solutions, the samples and the blank solution. The analytical procedure can then be optimized using the internal standard element. The internal standard element is commonly applied in ICP-MS and LA-ICP-MS to account for plasma instabilities, changes in sample transport, short and long term drifts of separation fields of the mass analyzer and other aspects which would lead to errors during mass spectrometric measurements. 

Three different calibration strategies for solution based calibration in LA-ICP-MS have been developed in our laboratory. These are similar to the solution calibration in solution analysis by ICP-MS external calibration if a high purity matrix-matched blank target is available,29,71 the standard addition technique (e.g., for high purity platinum)76 or the isotope dilution technique.43... 

---