Each and every analysis requires choice of a method that will help you achieve the required objectives. In case an established method is available you should consider yourself fortunate as this will save you the effort and time required to validate the method before adopting it.
Several options are available to you for selection of the method to suit your analysis requirements. Such options include survey of standard analytical textbooks, research journal publications, Google search, etc. You will come across several methods in your Google search but this can add to further confusion as the method selected may not be suitable for your analysis requirements.
In making your method choice you also have to keep into consideration other factors such as:
- Cost of analysis
- Availability of required reagents and standards
- Availability of required instrumental facilities
- Availability of trained manpower
- Time requirement for analysis
- Simplicity of method
It is essential to keep in mind that the simplest method may not be the ideal one so you have to carefully evaluate the choice keeping in view the considerations listed above.
Let us assume that you have decided on the method of your choice and are planning to put it into routine use. However, at this stage it is essential to validate it so as to gain confidence that the data obtained using the method will be above question and will meet the prescribed regulatory requirements.
What is method validation and why it is required?
Method validation is a series of experimental studies that establish that the method under consideration will meet the requirements of the intended application. This requirement arises on account of several considerations such as:
- The selected method is not specific to the analyte(s) of interest
- The matrix composition of the sample is different from the composition in the method of choice
- The required concentration range is different from the one in the method selected or detection limits required are different
- The method is to be adapted to a different make of instrument
- Changes have been incorporated in the existing method to meet the requirements of the application
What all is covered under method validation?
Before undertaking the validation process it should be ascertained using reference standards that the response of the analytical measurement is on account of the analyte of interest and not due to any other chemical species. After this has been established you may proceed with the validation which evaluates the following method specific features:
Selectivity and Specificity
Selectivity and Specificity are often used synonymously but these are different terms. Specificity is the unequivocal response of the technique to a specific analyte present in the sample whereas selectivity can be applicable to a collective response to a group of analytes having similar chemical and physical characteristics. The method selected should meet the criteria for selectivity and sensitivity in presence of deliberately added interferents that can be expected in the sample under investigation
Accuracy is the degree of closeness of the observed value with the accepted or referral value. For this purpose the sample can be spiked with the analyte so that the base sample matrix composition remains unchanged.
Precision is a measure of reproducibility of test results. It should be kept in mind that a high degree of precision does not necessarily imply a high degree of accuracy as due to errors inherent in the measurements the same results can be repeated a number of times but the results could be removed from accurate value.
Limit of detection and limit of quantitation
The limit of quantitation is the lowest amount of the analyte which can be determined quantitatively with an acceptable level of precision. On the other hand limit of detection is the lowest concentration which can be detected by the method but not necessarily quantified. Limit of detection is always lower than the limit of quantitation.
Linearity is the ability of the method to provide a response which is in direct relation to the analyte concentration. The linearity range should be as large as possible and should be established with the help of multipoint calibration with at least 6 to 10 points
Ruggedness is the confidence that you can place on the selected method. In other words the results should be reproducible in your laboratory or in any other laboratory or even when an analysis is carried out by a different set of analysts.
Robustness examines the effect of change in operating parameters such as minor changes in pH or temperature on the capability of method to provide results within specified tolerance limits.
Method validation may appear to be a tedious and time-consuming activity but once the method is validated before adoption it will not let you down under any circumstances.