Chromatography Forum

USP gives analysis of Isopropyl Alcohol purity by Thermal Conductivity Detector ? What is the advantage of TCD over FID in IPA Analysis ?
Whereas USP itself says FID Sensitivity is better than TCD than why USP gives TCD analysis ?

First of all, don't always believe that USP (or any other source of official methods) are the best or easiest available. Sometimes, there is no need or resources or push to update them. To USP, one advantage of TC detector might be that such a GC setup needs only carrier gas, not hydrogen or air as does FID. We do ethanol assays and use FID. I'd guess is that if you wrote a letter to your own cGMP file stating why do are using FID (even reference USP stating better sensitivity), you'll be all right. One can make tweaks and modifications to USP procedures, just document and use good science.

One very likely impurity in IPA is H2O, water.

Since another likely impurity is ethanol, usually a gas solid packed or capillary column will be used for the separation of trace impurities.

This implies a large sample may be placed on column. So TCD may easily perform the analysis in the most effective manner for all the probable impurities in one analysis, using perhaps a R type porous polymer packed column.

Of course you could inject onto a 1701 or 1301 type capillary to a FID and ignore water by GC and/or do water by titration.