Trace Analysis Guide
The process of solving a problem, whether involved or relatively straightforward, involves a logical process. The phases of this process are as follows:
|Phase 1: Problem Definition and Planning|
|Phase 2: Method Selection|
|Phase 3: Method Development|
|Phase 4:Method Validation|
|Phase 5: Method Application|
|Phase 6: Data Evaluation|
|Phase 7: Data Published|
This chapter dealing with method validation will conclude the Reliable Measurements guide. As shown above, method validation is the last phase in the process of establishment of a method in your laboratory.
Purpose of Method Validation
The purpose of method validation is to demonstrate that the established method is fit for the purpose. This means that the method, as used by the laboratory generating the data, will provide data that meets the criteria set in the planning phase. There is not a single accepted procedure for conducting a method validation. Much of the method validation and development are performed in an iterative manner, with adjustments or improvements to the method made as dictated by the data. The analyst's primary objective is to select an approach that will demonstrate a true validation while working in a situation with defined limitations, such as cost and time.
Analysts often wonder if a published 'validated method' must be validated in their own laboratory. It is considered unacceptable for the analyst to use a published 'validated method' without demonstrating their capability in the use of the method in their laboratory. This does not mean, however, that the analyst must repeat the original validation study. It is therefore important for the analyst to be familiar with the method validation process to enable the selection of the validation approach that's appropriate for the situation.
References: The Validation Process
There are numerous publications addressing this issue. Following are some references you may find useful:
Trace Analysis: A structured approach to obtaining reliable results; Prichard, E., Mackay, G. M., Points, J., Eds.; The Royal Society of Chemistry: 31-39; 1996.
A Practical Guide to Method Validation; Green, J. Mark, Ed.; Analytical Chemistry (68): 305A-309A; 1996.
Swartz, Michael E.; Krull, Ira S. Analytical Method Development and Validation; Marcel Dekker, Inc.: 1997.
Confirm Basic Performance Criteria
The method must 'fit the purpose' as agreed upon between the client and the analyst. In the case of trace analysis, the following criteria are typically evaluated as part of the method development process:
- Specificity involves the process of line selection and confirmation that interferences (of the types discussed in part 15 and part 16) for the ICP-OES or ICP-MS measurement process are not significant. A comparison of results obtained using a straight calibration curve (without internal standardization to that of internal standardization and/or to the technique of standard additions) will give information concerning matrix effects, drift, stability, and the factors that influence the stability. The various types of spectral interferences encountered using ICP-MS and ICP-OES (see above links) should be explored.
- Accuracy or Bias can be best established through the analysis of a certified reference material (CRM, or SRM if obtained from NIST). If a CRM is not available, then a comparison to data obtained by an independent validated method is the next best approach. If an alternate method is not available, then an inter-laboratory comparison, whereby the laboratories involved are accredited (ISO 17025 with the analysis on the scope of accreditation) is a third choice. The last resort is an attempt to establish accuracy through spike recovery experiments and/or the use of standard additions.
- Repeatability (single laboratory precision) can be initially based upon one homogeneous sample and is measured by the laboratory developing the method. The repeatability is expressed as standard deviation.
- Limit of Detection (LOD) is a criterion that can be difficult to establish. The detection limit of the method is defined as 3*SD0, where SD0 is the value of the standard deviation as the concentration of the analyte approaches 0. The value of SD0 can be obtained by extrapolation from a plot of standard deviation (y axis) versus concentration (x axis) where three concentrations are analyzed ~ 11 times each that are at the low, mid, and high regions of interest. This determination should be made using a matrix that matches the sample matrix.
- Sensitivity or delta C = 2 (2)1/2 SDc, where SDc is the standard deviation at the mid point of the region of interest. This represents the minimum difference in two samples of concentration C that can be distinguished at the 95% confidence level.
- Limit of Quantitation (LOQ) is defined as 10 SD0 and will have an uncertainty of ~ 30% at the 95% confidence level.
- Linearity or Range is a property that is between the limit of quantitation and the point where a plot of concentration versus response goes non-linear.
Robustness is a term that is commonly used in publications dealing with method validation. Robustness testing deals with the critical operational parameters and the tolerances for their control. Robustness is the capacity of a method to remain unaffected by deliberate variations in method parameters. In the case of trace analysis using ICP, parameters such as:
- temperature (laboratory and spray chamber)
- concentration of reagents
- RF power
- nebulizer, spray chamber, and torch design
- torch height
- sampler and skimmer cone design
- sampler and skimmer cone construction material
- integration time
- detector design
- reaction/collision cell type or conditions
- resolution capability
These are all examples of parameters that could be easily altered, either intentionally or unintentionally, that could significantly affect the reliability of the determination. The fact that many procedures specify operational parameters or accessory designs/types is a result of robustness testing where the developing laboratory recognizes that critical parameters must be identified, specified, and controlled for the measurement procedure to be used reliably.
Collaborative and Cooperative Testing
This is an activity or component of method validation that is performed by organizations that develop standard methods of chemical analysis such as ASTM and AOAC. It is also an activity that is performed by large corporations with multiple testing locations. The term reproducibility is used to describe interlaboratory precision and is expressed as standard deviation. Different organizations use different processes, some more convenient than others. For this reason, refer to the following references:
Validation of Analytical Methods; Taylor, J.K., Ed.; Analytical Chemistry (55): 600A-608A; 1983.
The Role of Collaborative and Cooperative Studies in Evaluation of Analytical Methods; Taylor, J.K., Ed.; J. Assoc. Off. Anal. Chem. (69): p. 398; 1986.
Youden, W.J.; Steiner, E. H. Steiner Statistical Manual of the AOAC; AOAC: Arlington, VA, 1975; 5th printing 1987.
ASTM Method E691-92; Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method; ASTM: Philadelphia, PA.
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