Validation parameters for specific tasks

Validation parameters for specific tasks

The validation experiments should be carried out by an experienced analyst to avoid errors due to inexperience. The analyst should be very well versed in the technique and operation of the instrument. Before an instrument is used to validate a method, its performance specifications should be verified using generic chemical standards. Satisfactory results for a method can be obtained only with equipment that is performing well. Special attention should be paid to those equipment characteristics that are critical for the method. For example, if detection limit is critical for a specific method, the instrument’s specification for baseline noise and, for certain detectors, the response to specified compounds should be verified.

Any chemicals used to determine critical validation parameters, such as reagents and reference standards, should be

1. available in sufficient quantities,
2. accurately identified,
3. sufficiently stable and
4. checked for exact composition and purity.

Any other materials and consumables, for example, chromatographic columns, should be new and be qualified to meet the column’s performance criteria . This ensures that one set of consumables can be used for most experiments and avoids unpleasant surprises during method validation.

Operators should be sufficiently familiar with the technique and equipment. This will allow them to identify and diagnose unforeseen problems more easily and to run the entire process more efficiently.

If there is little or no information on the method’s performance characteristics, it is recommended to prove the suitability of the method for its intended use in initial experiments. These studies should include the approximate precision, working range and detection limits. If the preliminary validation data appear to be inappropriate, the method itself, the equipment, the analysis technique or the acceptance limits should be changed. Method development and validation are, therefore, an iterative process. For example, in liquid chromatography, selectivity is achieved through the selection of mobile phase composition. For quantitative measurements, the resolution factor between two peaks should be 2.5 or higher. If this value is not achieved, the mobile phase composition needs further optimization. The influence of operating parameters on the performance of the method should be assessed at this stage if this was not done during development and optimization of the method.

There are no official guidelines on the correct sequence of validation experiments, and the optimal sequence may depend on the method itself. Based on the author’s experience, for a liquid chromatographic method, the following sequence has proven to be useful:

1.  Selectivity of standards (optimizing separation and detection of standard mixtures if selectivity is insufficient)
2.  Linearity, limit of quantitation, limit of detection, range
3.  Repeatability (short-term precision) of retention times and peak areas
4.  Intermediate precision
5.  Selectivity with real samples
6.  Trueness/accuracy at different concentrations
7.  Ruggedness (interlaboratory studies)

The more time-consuming experiments, such as accuracy and ruggedness, are included toward the end. Some of the parameters, as listed under (2) to (6), can be measured in combined experiments. For example, when the precision of peak areas is measured over the full concentration range, the data can be used to validate the linearity.

During method validation, the parameters, acceptance limits and frequency of ongoing system suitability tests or QC checks should be defined. Criteria should be defined to indicate when the method and system are beyond statistical control. The aim is to optimize these experiments so that, with a minimum number of control analyses, the method and the complete analytical system will provide long-term results to meet the objectives defined in the scope of the method.

Once the method has been developed and validated, a validation report should be prepared that includes the following:

•  Objective and scope of the method (applicability, type).
•  Summary of methodology.
•  Type of compounds and matrix.
•  All chemicals, reagents, reference standards, QC samples with purity, grade, their source or detailed instructions on their preparation.
•  Procedures for quality checks of standards and chemicals used.
•  Safety precautions.
•  A plan and procedure for method implementation from the method development lab to routine analysis.
•  Method parameters.
•  Critical parameters taken from robustness testing.
•  Listing of equipment and its functional and performance requirements, e.g., cell dimensions, baseline noise and column temperature range. For complex equipment, a picture or schematic diagram may be useful.
•  Detailed conditions on how the experiments were conducted, including sample preparation. The report must be detailed enough to ensure that it can be reproduced by a competent technician with comparable equipment.
•  Statistical procedures and representative calculations.
•  Procedures for QC in routine analyses, e.g., system suitability tests.
•  Representative plots, e.g., chromatograms, spectra and calibration curves.
•  Method acceptance limit performance data.
• The expected uncertainty of measurement results.
• Criteria for revalidation.
• The person(s) who developed and validated the method.
• References (if any).
• Summary and conclusions.
• Approval with names, titles, date and signature of those responsible for the review and approval of the analytical test procedure.