Stability in Solution amounts versus concentrations

If you are doing a study where you want to ascertain the stability in solution of standards and you prepare standards Day 0, then assay these standards with fresh standards on Day 5, Day 10 etc etc, is it better to compare areas or concentrations? For instance, lets say you prepare Std on Day 0 by weighing 10mg into 100mls and diluting 5 into 100 to get a final concentration of (10/100) * (5/100) * 1000 ie 5 mcg/mL. So this is the concentration of my Standard Day 0. 

Come Day 5, I prepare fresh standards and use this standard to quantify my original Day 0 standard such that I run the Day 0 Standard bracketed by the fresh standards. My component editor has in it the concentration of the new fresh standard (for example 4.998 and units mcg/mL) and nothing for the Day 0 Standard so im using Empower and the calibration curve to get an amount. So lets say I get an amount of 4.997 for the Day 0 Standard- an excel sheet putting this value over the Day 0 value and multipled by 100 gets my percent recovery ie 4.997/5 * 100 ie 99.94%. And I continue with this up to Day 10, 15 etc. 

Is there any issues calculating amount like this or should I set the Sample value type to concentration but doesn't that take into account injection volume? I don't want to complicate the study. Is this approach ok or is it better to use areas and compare areas of Day 0 to that of subsequent days?

Best Answer

  • Accepted Answer

    You want to compare stability time points to the calculated concentration of the standard.

    You don't need to account for injection volume unless you want to consider mass load on column. However, this does assume that the injection volume for samples and standards are the same.

Answers

  • Hi ,you could carry out a complete stability study in one sample set

    ie .. Day 0 , day 1 , day 4 day 10 and day 15 where day 15, 10,4 and 1 have been stored at certain conditions and day 0 prepared on day of analysis .Though you have to be confident in system or 15 days could be wasted

    You could use 'Create process only sample set' if you required quantification v std analysed on a different day/sample set  

  • Thanks for the replies they are very helpful. 
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