Chromatography Forum

Hi! Everyone, I recently did HPLC-ELSD. I injected benzyl bromide to the LC. I could found a big peak at 210nm with VWD detector, but there is no peak for benzyl bromide on ELSD detector at the same retention time with the same LC condition. B.P. of benzyl bromide is 200 degree celsius. I don't understand why no sign for benzyl bromide on ELSD detector because B.P. of benzyl bromide is high enough?

Two possible explanations: you are not eluting benzyl bromide or you ELSD temperature is too high. Remove column and inject BB without column, measure peak area in UV and ELSD, compare peak area in UV and see if it matches. Reduce temperature to 35*C on your ELSD.

Two possible explanations: you are not eluting benzyl bromide or you ELSD temperature is too high. Remove column and inject BB without column, measure peak area in UV and ELSD, compare peak area in UV and see if it matches. Reduce temperature to 35*C on your ELSD.

My sample enters to vwd detector and then enter to ELSD (VWD and ELSD connect). My ELSD temperature is only 40*C. ELSD is Agilent 1260 infinity ELSD.

just measure peak area in UV with column and without column, if there is mismatch, then you are not looking at benzyl bromide, because it is not eluting from the column or you have problem with injection

This is my experience with ELSD: phenanthrene, triphenylene, benzoic acid and pseudoephedrine are solid and can be detected by UV. But all of them are too volatile to be detected by ELSD. Benzylbromide may be simply too volatile. If possible, set your nebulizer temperature to sub-ambinet temperature and the evaporation temperatue the lowest possible, although the chance of detection is low.

Hi

Might be a stability issue as well here.

Some of these compunds and related are quite reactive or/or instable depending of dissolution media, for instance benzyl chloride (GC with organic solvent seems to work fine for that) reacts readily with water/moist and form benzyl alochol and HCl (might have been used in war gases as its a bit volatile). Benzyl bromide is less volatile but Br is a better leaving group from an organic chemstry point of view.

So instinctively I would consider injecting benzyl alcohol and benzyl bromide to insure that a) they separate b) the benzylbromide do not degrade into the alcohol or use MS to insure the same thing.

On the MSDS, the vapor pressure is listed as 1 mmHg at 32C. I agree that benzyl bromide is too volatile for ELSD at 40C. Lower evaporator temperature (preferably, subambient) is likely the only way this will work with ELSD.

Thank you very much! I also a question about ELSD.
We did HPLC-ELSD to get purity for a sample. We often get a sample purity by peak area normalization like using UV dectector. The same situation as ELSD detector. The purity of a impurity in our sample is about 0.5% by HPLC-ELSD. But from HNMR, the purity is about 5%. How to explain this result and what kind of relationship between peak area and sample concentration? I'm a freshman for ELSD and please give me some suggestions! Thanks!

Absent volatility issues, the mass response of an ELSD is more or less independent of structure (you're measuring dust particles and "dust is dust"). That said, the number and size of the dust particles (and hence, the response) will depend on what happens in the nebulizer, and thus may vary as a function of things like the surface tension of the mobile phase.

What are your chromatogrphy conditions? If using gradient RP, early eluting compounds give a significantly lower response than late eluting compounds. This will complicate area based calculations if the mobile phase composition is not taken into account.

Anthony

In ELSD the relationship between peak area and sample concentration is log dependent.

log(area) = log(a) + xlog(c)

a= response factor
x = gradient of response line
c = sample conc

A good explanation is covered here:

http://books.google.co.uk/books?id=yKdI ... LC&f=false

If you are interested in determining concentrations from ELSD data, we sell FRAC software which can perform these calculations for you. You need to perform a calibration using generic compounds from which we calculate a surface against which concentrations can be calculated from the RT and peak area. Please get in touch with me via Info@chemtechsoftware.com if you are interested in this.

Anthony.

Head of Chemistry Products
ChemTech Software Ltd
http://www.chemtechsoftware.com