The concept of the limit of detection (LOD) and limit of quantitation (LOQ) has been, and still is, one of the most controversial in validation of analytical methods. The multiple definitions and calculation methods proposed have contributed to this situation. Although in the last years, several international organizations, such as ISO or IUPAC, have tried to reach a consensus in their definitions and have issued guidelines for the estimation of this important parameter in validation of analytical methods, the subject is still a matter of scientific debate.
As per one of the proposed methods for the determination of LOQ and LOD, the quantitation limit may be determined by the analysis of samples with known concentrations of analyte and by establishing the minimum level at which the analyte can be quantified with acceptable accuracy and precision. If the required precision of the method at the limit of quantitation has been specified, 5 or 6 samples with decreasing amounts of the analyte are injected six times. The amounts range from the known LOD as determined above to 20 times the LOD.
The calculated relative standard deviation (RSD) percent of the precision of six repetitive injections is plotted against the analyte amount. The amount that corresponds to the previously defined required precision is equal to the limit of quantitation. It is important to use not only pure standards for this test but also spiked matrices that closely represent the unknown samples.
I want to know more details about the above method for the determination of LOQ and LOD. I would appreciate if anyone can let me know some practical examples of calculation LOQ and LOD as per the above methods. References of the applications of this method will also be helpful.
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