Chromatography Forum

Hello,

I am trying to solve a problem. I am analysing for glyphosine by ELSD. At higher concentrations the analysis works perfectly. However, I now need to go to lower cocentrations, e.g. down to 0.1 % and I find that signal to noise is very bad. I use Alltech 3300 instrument. The mobile phase is 100 % aqueous with 0.2 % TFA. The ELSD method wizard suggests drift tube temp. of 40 deg. C and nitrogen flow of 1.5 L/ min, however I find that too noisy and use 85 deg. C and 2.0 L/min. Manufacturer's settings never worked for me. How can I increase the sensitivity and S/N in this situation?

Thanks a lot

Bangkok34

Hi, Bangkok 34

I have the same instrument as you have. If you look in the manual, the procudure to fine tune the operating conditions are described there.
First, you need a high enough temperature in the drift tube to evaporate the eluent. I think that you can go to a high temperature, since your analyte is not very volatile.
Second, the gas flow is of importance, and my understanding is to have a as low gasflow as possible. A lower gasflow created larger particles and thus increases the sensitivity.
Check the manual for more details and the procedure.

Hi, Bangkok 34

I have the same instrument as you have. If you look in the manual, the procudure to fine tune the operating conditions are described there.
First, you need a high enough temperature in the drift tube to evaporate the eluent. I think that you can go to a high temperature, since your analyte is not very volatile.
Second, the gas flow is of importance, and my understanding is to have a as low gasflow as possible. A lower gasflow created larger particles and thus increases the sensitivity.
Check the manual for more details and the procedure.

I appreciate your help on this. Thanks a lot! I will give it a go.
B

One thing to keep in mind with the ELSD is that the signal is very non-linear. Doubling analyte concentration will far more than double the signal strength. Conversely, halving the analyte concentration will far more than halve the signal strength. This gets to be quite a problem with low sample concentrations. You would do best to reoptimize detector conditions for the low concentrations you now have to deal with.

One thing to keep in mind with the ELSD is that the signal is very non-linear. Doubling analyte concentration will far more than double the signal strength. Conversely, halving the analyte concentration will far more than halve the signal strength. This gets to be quite a problem with low sample concentrations. You would do best to reoptimize detector conditions for the low concentrations you now have to deal with.

Thanks, Yes, it's certainly non-linear. I optimised the conditions for my concentration range. Yet, still a bit unhappy with the amount of noise I am getting, but the method is workable now.

Regards
B