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.

May be this method is less practiced one among all methods proposed for the determination of limit of detection (LOD) and limit of quantitation (LOQ). I got interested, as I think there might be a possibility to use the results obtained from the concentrations (in replicates) used for the determination of Linearity of a method.

I am confused whether ‘RSD% of series of concentrations’ means RELATIVE standard deviation (in percentage) of the replicates values at different concentrations or ‘residual standard deviation’ measured from the data concentrations(x), Absorbance(y) ?

I am confused whether ‘RSD% of series of concentrations’ means RELATIVE standard deviation (in percentage) of the replicates values at different concentrations or ‘residual standard deviation’ measured from the data concentrations(x), Absorbance(y) ?

Thanks
ahma[/quote]

Don’t confuse.

%RSD Stands for Relative standard deviation only.

While establishing LOQ Residual std deviation is used.

Once LOQ Concentration is established. Prepare and inject six replicates Than Check % RSD(Relative standard deviation)