is it normal to have splitted peaks on ELSD?
Posted: Thu Jan 22, 2009 11:53 pm
by chromlc
I've been using UV detectors for a long time and seldom see splitted peaks caused by the detector. But for ELSD, i am seeing peak splitting very often. Is this normal? I know the peak shape depends on gas flow, temperature etc. But according to the theory of evaporative light scattering, it's essential to make sure the system is absolutely stable, which is much harder than UV. What do you guys think? Thanks
Posted: Fri Jan 23, 2009 9:38 am
by HW Mueller
What do you consider a split peak? I can imagine strange shapes of peaks if detection fluctuates during the elution of the peaks, but that would change the area as well. The real beginning of the peak and its end wouldn´t change. The baseline would be weird also if the detector malfunctions.
Proper Set-up of the ELSD is key
Posted: Thu Jan 29, 2009 2:53 pm
by HPLCCONSULT
If you see split peaks with your ELSD and not with a "regular" detector (indicating that your chromatography is OK), then you have not set up or properly optimized your ELSD.
ELSD detectors must be optimized for a given flow rate, solvent composition, nebulizer gas flow rate and drift tube temperature (and if a gradient is used, then this is more complex as you must find a compromise in settings). An idea balance between these parameters must be found for each method used. Too high a flow rate relative to the drift tube temperature and/or gas flow rate will cause poor nebulization or even droplets to form which will result in strange signals. In my experience, "split" peaks are usually caused by a clogged or partially plugged nebulizer "spitting" sample out. It can also occur when too high a sample concentration has been injected.
ELSD units require a great deal of maintenance, understanding and optimization time to use. They should never be used like a "plug and play" UV detector which is far easier to use, maintain and clean. That said, ELSD's are useful for sample types where nothing else works well (i.e. some lipids, fats, triglycerides, sugars, polymers...). They key to being successful with an ELSD is spending the time to learn how to use it properly (many test runs to optimize the many parameters stated before as well as regular cleaning). Last Point: They are NOT Universal Detectors (that is an old advertising/marketing phrase only and unscientific).
Good luck and remember to consult the instrument manufacturer, when possible, for advice on cleaning and using the detector.