Calculations in Analytical Methods(HPLC and GC)

I want to write this topic for giving an idea about how to calculate the Assay, related substances and Residual solvents.

Calculations for Assay Method (Tirtimetry)

Titire value(Sample - Blank) x M x Factor x 100 x 100

--------------------------------------------------------

                            Ws x (100-LOD)

M= Molarity of the volumetric solution 

Ws= Weight of the sample 

Factor = Factor for Drug substance 

LOD is Loss on drying or MC is water content 

Calculations for Assay Substances Method (HPLC)

             

                    At x Ws x P x (100-WC of std)

Assay % = ----------------------------------

                    As x Wt x (100-WC of sample)

At = Area due to sample Preparation 

As= Area due to working standard solution 

Wt= Weight of the sample in mg

Ws= Weight of the working standard

P=Potency  or Assay of the standard 

WC=% water content of the sample or standard 

Note: WC may be replaced by LOD based on the drug substance 

Calculations for Related Substances Method (HPLC)

                                          ri

% of Known Impurity = ---x 100 X RF

                                         rs

                                              ri

% of unknown Impurity = ---x 100

                                              rs

Total Impurities = Sum of all known and unknown impurities 

ri =Area of each impurity Peak in the chromatogram of the sample solution preparation

rs=Sum of areas of Main drug and all impurity Peaks in the chromatogram of the sample solution preparation

RF= Response factor 

Calculations for Residual solvents Method (GC)

     

    At        Volume of solvent taken x density     0.1 ml       dilution of sample(ml)

= ------x -------------------------------------------x ---------x-------------------------------x 10 ^ 6

    As                     10 ml                                                                   10 ml        Weight of the sample

At= Area of the solvent in the sample - Blank

As= Area of the solvents in the standard -blank 

Note: Volume of the solvents taken should be mentioned in microliters 

As we are multiplying with 10 power 6 we can get the results in PPM 

Note: Refer Merck index for solvent densities