ELSD sensitivity

[Bangkok34]

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

[gstaepels]

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.

[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.

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

[freemab222]

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.

[Bangkok34]

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