Chromatography Forum

Hi, all,

I am a new CAD user. I'd appreciate any input on the following two questions

1). Sensitivity. Can enough sensitivity be achieved for impurity method, especially for those polar impurities resulting from a non-polar compound? CAD seems to have better sensitivity with high organic MP.

2). The statement that the response factor is independent of analyte's chemical structure. How much we could use this? Any applications against this statement?

thanks

Lijun

Dear Duantech,

I have used a CAD-Detector for pharmaceutical impurity analysis. We wanted to come up with an assay/purity method for an API.

I found that it was not possible to detect 0,05% impurity in a 100% sample of the API. This was due to the fact that the attenuation/range was not dynamic enough (not to be confused with sensitivity!). This means either I could detect the impurities and the main compound was cut (note the column was not overloaded, it was only the signal was too high fopr the detector), or I could detect the main compund well, but then the noise was too high.

Therefore switching the attenuation during a run would be nescessary.

As for the response factors, this is right for isocratic elution. It is however not right for gradient elution. It would therefore be nescessary to calibrate at different organic modifyer oncentration.

Else, the detector worked very well. It is easy to handle and detects a lot. In our case I could prove that we had substantial amounts of an impurity without chromophor in our sample. This way I was able to close the mass ballance,...

Hope this helps,
Daniel.

It is possible to dtermine relative response factors (RRF's) with the CAD although it does need a constant mobile phase composition. Search out a paper in analytical chemistry recently from Pat Sandra's team (Belgium) that used a second compensating pump to determine %w/w from gradient LC analyses.

The CAD is also seeing use for picking up components not detectable with UV/Vis.

From what I have seen so far, if the compounds elute under the same %B, they will have the same peak area. Although the instrument response gets saturated as more analyte is on column. To get a decent signal, you have to have at least ~35%B. I had a couple of compounds elute at 20%B and their peak area was 1/10 the peak area of compounds eluting at 40%B. I am planning to use the inverse compensation pump technique to normalize the signal.