Chromatography Forum

I'm trying to get my head around the chemistry of the new BP dissolution method for levothyroxine, specifically the sample pre-treatment step which says:

"To a 5mL aliquot add 200 mcL of 0.1 M NaOH and 125 mcL of a 2% v/v solution of diethylamine"

I understand the purpose of the NaOH, since the dissolution medium is water, but why would you also need to add diethylamine? The mobile phase 30% ACN/70% dilute phos acid in water and suggested column is Spherisorb S5 CN, which as far as I can tell isn't an ion exchange column.

The use of triethylamine in mobile phases I understand. The addition of diethylamine to a sample has me beat, and I have tried googling.

The other thing about the BP method that seems odd is it says to inject 150 mcL which seems a very large volume to me.

By way of background, this is for a student research project so we're not "bound" to follow the BP method for analysis. I'd like to avoid adding diethylamine if it's unnecessary, simply because it's another potential source of contamination when reading at 223 nm. However, since the BP only just this year introduced the dissolution test I'm thinking there must be a reason for it?

Thanks

Hello

Perhaps DEA solution provides efficient extraction.
Regards

Tomasz Kubowicz

Hi Thomas, can you please explain a bit more? I'm not sure if you're referring to the chemistry of the sample when injected (and the manner the 150 mcL aqueous sample mixes with the mobile phase), or if I've worded the question poorly. In case the latter, the DEA and NaOH are both added once the dissolution sample has been centrifuged. The dissolution medium is just water.

Hello

I've checked BP method and I don't really know why DEA is added...
Perhaps it has something to do with compound nature (stability, chiral properties or enantiomers ratio).

Regards

Tomasz Kubowicz

Hi Thomas,

Thanks so very much for spending your time to seek out the BP method. That's so kind of you. I'll try searching you ideas!

DoryFish,

Looking at the structure of levothyroxine there is a chiral amine at the alpha position to the acid group. Speaking as a peptide chemist, those types of stereocenters are prone to racemization under acidic conditions so I imagine the addition of the DEA is to prevent that from occurring.