loss peak intensity on Sedex 55 ELSD?

[Cathy]

I am using a RPLC method with Sedex 55 ELSD to seperate some lipids. My mobile phase is ammonium acetate/ACN, my samples are prepared in 50%propylene glycol/50% water.
The method has good linearity, accuracy, precision.
The only problem I have is lossing peak intensity, the noisey level doesn't change, but peaks get smaller and smaller.
I did nubulizer cleaning, changing new lamp and PMT on ELSD, none of them help. BTW, I have to stick with Sedex 55 ELSD.
Would anybody give any suggestion on this issue? really appreciated.

[Mark Tracy]

Check the gas filter; it may be loaded up with dirt and restricting the flow. Check the drain; the serpentine trap should have liquid in it, and the waste line should flow freely. Check the O-ring seals.

[Alfred88]

Did you try swapping column? Change guard?

Alfred

[Cathy]

Mark and Alfred,
Thanks for the suggestions.
I used new columns and running the same method on multiple ELSD. All of them shows loss of signal intensity.
I checked the gas filter. it looks okay. The drain and waste line was fine when ELSD were running.
I don't know if it is related to my sample preparation. Is 50% propylene glycol:50% water a good solvent for ELSD? will it cost any contamination on detector?
by the way, my ELSD condition is drift tube temp: 40C, gas: 2.2 bar.
cathy

[Mark Tracy]

Lipids are hopelessly insoluble in water/glycol. I would start with EtOH for the sample, and if that doesn't do it, you can use acetone, butanone, ethyl acetate, chloroform, THF, etc. The ELSD won't care about any of those solvents. Most likely your sample is precipitating on the walls of the sample vial.

[Kostas Petritis]

Actually THF will produce a constant background noise, probably due to the non volatile additives they add for the THF stabilization. Also if you will use normal phase solvents you might experience miscibility problems depending on the solvents used. This happens as during the evaporation of the normal phase solvents the nebulizer gets very cold and what was miscible in room temperature are now non-miscible. That can create very high background noise but it will go away over time once you flush with isopropanol.

I did some work with temperature controlled nebulizers that took care of the non-miscibility problem...