Chromatography Forum

The HPLC I'm using is set up with ELSD and UV detector in tandem for detection of fatty acid methyl esters, glycerides and the like. So why, oh why, don't I see the ethyl acetoacetate I've injected from the ELSD while a see a respectable UV peak? The column is at room temp. The detector is at 60C, well below the boiling point. EAA is not volitile. It is thermally unstable but I'd be surprised if it were decomposing at 60C. Yet I can think of no other explanation.

Whatever the reason, can the UV signal be used to track the conversion of the reaction? I thought that the intensity of the peak wouldn't change with the quantity but that the broadness does.

First off, it is the vapor pressure of a compound that determines in volitility in an ELSD, not the boiling point. You can try lowering the temp of the drift tube, most people run them too hot, and see if you can get it to show itself. Depending on the brand of ELSD, flow rate and composition of the mobile phase, you ma be able to go as low as 45 C. Many compounds which you think are not volitile at all can give very poor ELSD signals, caffeine and BHT come to mind right off the bat.

Good Luck

I agree with AA. The compound, eventhough you are below the BP, is evaporating before it gets to the detctor cell. I mean water at RT will evaoprate even though the room is well below the BP!

I removed the column and ran it directly into the detectors. The flow rate is .5mL/min. Mobile phase is 99.6%hexane, .4% isopropanol. I also lowered the temp to 50C and never saw a signal more than a few tenths of a millivolt. The pressure reads 6 mbar. With this info, does it still make sense that it would vaporize?

With this info, does it still make sense that it would vaporize?

Yes.