Decrease in standard potency

[Chris]

We are a contract lab running an HPLC assay for a customer. There is very little of the primary standard (approx 93% purity) so we have had to quantify a raw material (ca 69% purity) and use this for analysis. The raw material is supposedly stable, however we have stored the material in a screw top glass conatiner in the fridge and found the potency to decrease to about 67% after only 6 weeks and this is the second time this has occured (i.e. probably not contamination). The primary degradation products have not increased therefore leading me to think the cause is intake of moisture.

The standard is used nearly every day so we keep having to remove it from the fridge use it and put it back, is this likely to increase the risk of moisture intake? Unfortunately we did not have the forsight to perorm a moisture conetnt analysis at the point of initial quantification.

Anyone else seen anything similar to this or have any comments?

[Einar Ponten]

It is fundamental to perform any assay based on a dry sample. You may well have a water uptake in your sample. The relative humidity in your facilities will influence the uptake and it is sometimes reversible, i.e., if you put a sample dessicated the water content will decrease.

An option to determine the dry weight ("content of water ") in case you only have a small amount of substance is to use a TGA (thermo gravimetric analyzer).