In-situ isomerisation: calibration curve and LOD/LOQ

[JMDV]

I have a compound that is isomerising in solution and the cis and trans isomers can be clearly separated by HPLC. However the trans isomer is about 98% while the cis isomer is about 2%. At low concentrations I am able to integrate the trans isomer easily but not the cis. From literature the equilibrium between the two isomers always stays the same providing the solvent system does not change. Is it acceptable for me to use only the larger trans isomer peak when generating a calibration curve and when determining my LOQ and LOD? If I can't my LOQ/LOD values are going to be very high!

[tom jupille]

If you are quantitating the total amount of the compound, and if, indeed, you can demonstrate that the equilibrium is 98/2, then yes, you should be able to use just the major isomer for LOD & LOQ estimation. LOQ (S/N = 10) effectively implies a CV of +/- 5%. LOD (S/N = 3) implies a CV of +/- 20%.

[danko]

If I had a method that was unable to detect 2% of my load, I would seriously consider a comprehensive revision of the method.

Best Regards

[aceto_81]

If I had a method that was unable to detect 2% of my load, I would seriously consider a comprehensive revision of the method.

Best Regards

But if you have to determine LOD and LOQ for impurity profiling, and should reach 0.05%, then it's possible that 0.05% from 2% isn't visible while 0.05% from 98% still visible.

Ace

[sam.pedraglio]

But if you have to determine LOD and LOQ for impurity profiling, and should reach 0.05%, then it's possible that 0.05% from 2% isn't visible while 0.05% from 98% still visible.

Ace

I'm not able to understand your thought to say this.
0.05% of 98% is always less than 2% of 100%.
If you're not able to see the 2% you'll never be able to see 0.05%.

[aceto_81]

But if you have to determine LOD and LOQ for impurity profiling, and should reach 0.05%, then it's possible that 0.05% from 2% isn't visible while 0.05% from 98% still visible.

Ace

I'm not able to understand your thought to say this.
0.05% of 98% is always less than 2% of 100%.
If you're not able to see the 2% you'll never be able to see 0.05%.

What I meant to say: if you have to look for peaks of 0.05% from your main peak, then it's possible that you can't see the 0.05% peaks from your main peak if the 2% peak is the main peak for your degradants you're searching for. So actually you are searching for peaks of 0.05% * 2% = 0.001% instead of 0.05% * 98%= 0.049%

I hope it's a bit clear now
Ace