Chromatography Forum

I am doing ethanol analysis from about 0.5 to 15 % in water. The column is a DB-5, 0.25mm x 30 M x 0.25 um film thickness. All flows have been checked and injector/FID detector has been cleaned, with new septum, glass insert (split/splitless with cup, no packing) and 0-ring.
I have done manual injections for external calibration with great peak shape and a corr. coefficient of 0.9993. When I attempt to use the autosampler on fast plunger speed, i get a series of sharp peaks on a hump. When I use slow injection speed I get a large tailing peak about 1.5 minutes long. I have cleaned the rinse/waste vials and can use the autoinjector syringe manually and get excellent results.

does anyone have any suggestions? I need to use the autosampler, as I have many samples.

Ricardo

Ricardo, as you've probably experienced, HP-5 is not the greatest column for low MW alcohols, but I doubt that's your issue (we use PEG-type columns with n-propanol internal standard, water solvent). We use SGE focusliners and split injection, and never inject more than 0.5ul of water, and we do use autosampler. HP5890 can accomodate 5ul and 1ul syringes and be set to 0.5ul injection size. Obviously we rinse the syringe with water between injections.

Ricardo,
what is your pre and post injection dwell time ? Probably you are working in split mode and these values should be 0 min. combine with fast plunger.