Chromatography Forum

So I have this group who does research on metathesis - in the process of their reactions, they often end up with a bunch of oligomers that they would like to quantify. The obvious problem here with UV/Vis and FID and MS detection (which is what we currently have) is that to get quantitation, you must get the response factor for *each* component, which is not terribly practical in their case.

Are there any detectors that can help you quantify compounds in a more 'absolute' manner? I heard that ELSD (evaporative light scattering) was largely based on Raleigh scattering, so that the response should be proportional to the mass of solute present.

Is this correct? Any other detectors that could be suitable for our purposes...?


Thank you,

Trishia.[/i]

If your oligomers contain nitrogen then your detector of choice should be chemiluminense nitrogen detector (CLND). You just calibrate with any other nitrogen compound (e.g. caffein) and you can have pretty accurate quantitative results of any other nitrogen containing compound (with a few exceptions if I remember well which are pretty rare).
I have used it before, have a look at the link below:
http://www.expotechusa.com/catalogs%5CA ... 200701.pdf

You are correct that ELSD measures a physical property (rather than a chemical one), so response factors are more uniform. However, they are not necessarily the same. But if you have a series of related oligomers, then the estimates for those oligomers would be reasonably accurate. However, comparisons with other components would be subject to some error.

If you had a representative compound in the same class, you could use that as a standard, and then all oligomers would be based on that response factor. It might not be accurate, but it would allow comparisons between different batches of samples. That is, the relative numbers would be useful, but I would be careful when interpreting the absolute values.

The corona discharge (CAD) detector is also reported to produce more universal response factors.

Merlin, are you saying that generally the aerosols, or dust, or whatever, which result from different oligomers are so similar that they scatter light almost identically?

There is at least one paper on the use of ELSD in combinatorial chemistry and if memory serves they reported semi-quantitative results (around 20%?) by using one standard in order to quantify other compounds . Another thing to pay attention with the ELSD is that the response will change depending on the concentration of organic solvent (i.e. different point in gradient elution might give different responses -the potential solution is to use another pump that delivers an inverse gradient post-column-).

HW M:
The detector nebulizes the column effluent, then evaporates the liquid droplets. Anything non-volatile should be left as a stream of particles, which then scatter the light at the end of the drift tube. The scattering is based on size and number of particles (i.e., physical characteristics), and "in theory" is independent of the type of material (i.e., chemical composition). Of course, you can imagine that this is only true to a certain extent. Without knowing all the specifics, I would expect differences in particle size and shape for different molecules, and certainly as a function of the solvent being evaporated; hence the differences noted by Kostas. So, these variables might indeed cause differences in scattering, resulting in different response factors. But even then, the differences would be much less than what is observed by absorbance, as we have discussed in other threads.