[How Dont]

Hi,

I have a related substances method whereby I use a certified reference standard (of the API) to quantify related substances in a sample, and correct with an RRF.

I prepare a sample solution of 100%w/w
I prepare a reference standard at 0.5%w/w

The calculation I propose to use is as follows:

Impurity (%w⁄w)=((Peak Area Impurity in 100% Sample)/(Mean PA Active.in 0.5% Std.))×((Theoretical Std.Weight (mg))/(Actual Std.Weight (mg)))×((%Purity)/100)×((Actual Sample Weight (mg))/(Theoretical Sample Weight (mg)))×RRF×Cr

Where Cr = (0.5% CRS/ 100%) =0.005

Its the first time I have used a CRS to quantify related substances (previous to this I would have used 0.5% dilution of the 100% sample solution).

Would anyone be able to tell me if I have this correct?

[pr0poon]

I don't have access to a pen or paper and i am on my phone but the general calculation for such would be as follows:

Lets suggest you prepare a sample solution of 100mg of sample in 100mL of diluent. This gives you a working solution of 1mg/mL. you would then prepare your standard solution as 0.5% of that, so 0.5mg/100mL. ( i know this isnt GLP due to the low weight but for example only...).

From this, i will split the calculation down, for simplicity, your calculation would be. . .

(Area Imp / Area Standard) x (Wt Std (mg) / 100) x P    =    mg/mL of imp in Sample solution

Therefore by then multiplying by your sample dilution you can find the concentration of imp in mg in your sample. So...

mg/mL of imp in sample solution (from above) x 100   = mg imp in sample.

Divide this by your weight to calculate mg/mg or %w/w

mg imp in sample / Sample weight    = %w/w.

Hope that helps...?

[How Dont]

Hi pr0poon,

Thanks for taking the time to respond to my query.

Certainly helps, much appreciated.

How dont.