Limit of qunatitation-Requirements

The Quantitation limit or Limit of quantitation in short LOQ:

LOQ is the least level of concentration of an analyte(eg: Impurity-A) That an analtyical method can quantify precisely and accurately.

How to estatblish LOQ :

Method 1: Slope and standard deviation(=Steyx in Microsoft excel) method.

Inject low concentrations of an analyte  from the detection level about 3-4 concentrations and record the absorbances.

X= Concentration(% of impurity with respect to test sample concentration)

Y= Absorbances.

Now calculate slope and Standard deviation(SD) using Microsoft excel.

% LOQ = 10 x Slope/ SD

Method 2: Signal  to noise ratio method.

Prepare the analyte about 0.1% level and find the S/N ratio of the peak.

now estimate the concentration to get the  analyte peak signal to noise ratio is about 10:1

Inject the concentration to check the S/N is about 10:1

Now the estimated concentration is called LOQ concentration.

After obtaining the LOQ, we shall  prove this is precise and accurate.

Inject the LOQ concentration six times and check the  repeat-ability.( Precision)

Spike The LOQ concentration of impurity to the test sample and  verify the % recovery.(Accuracy)

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Limit of detection is 3.3 times less to the Limit of quantitation.

simply dilute 3.3 mL of LOQ solution to 10 mL with the diluent and analyse. the peak shall be visually detected and Signal to noise ration is about 3.

Usually LOD is not required for Quantification methods. It applies to the limit tests like TLC..