By C.Allan (stile.ggr.co.uk - 193.128.25.11) on Tuesday, July 20, 1999 - 06:40 am:
Not too sure if I put this in the right place to be read. It's a similar problem to a query someone had last month.
We've been having similar problems with a method which is CH3CN:0.05M
NH4H2PO4, initial conditions 35:65 increasing to 70:30 over 15min gradient then
held for 15mins, 239nm, 1.5ml/min, 25cm x 0.46cm spherisorb ODS2 column. it
varies from a 'peak' to humps around the 15-20 min mark!! We thought it's
probably a combination of 'contaminants' from the buffer salt/glassware AND
water !! I've used a prep guard column to flush the aqueous part to gain some
success. However, there is still interference around the 15-19min mark ( max
1mV)even starting a run without injection. This may still be due to contaminants
somewhere in the glassware/tubing, flush solution (Gilson injector used). Any
further ideas would be much appreciated !!
By Jambo on Thursday, August 12, 1999 - 05:25 am:
We struggle with this too...
1) If you are filtering your buffer after the prep step, discard the first 100 mL. We've been surprized by the amount of extractives in mobile phase filters.
2) If 1 is not applicable, the we'll assume your mobile phase is clean, it's not your injector, and the contamination is in your system (degasser tubing or coming from He sparge, soap on your mobile phase jugs (as you suggest). Try purging your system with ACN, Isopropanol, ACN, and see if that helps. You can also rinse your system with nitric acid (1-3 N) for 15 minutes but never mix let the acid mix with your organic solvents or you can cause (a sometimes violent) reaction that emits poisonous gases. I can't say that we've had 100% clean baselines but every bit helps.
By BILL on Monday, August 16, 1999 - 10:16 am:
Look at you system and be sure you have no unswept volumes.If the system is a high pressure
gradient,you might look at the mixer.Older dynanic mixers use a stir bar inside a high pressure enclosure.If the teflon wears off the bar over the years,the steel is exposed.Low pressure systems can expose portions of the switching valves because the small membranes leak.Three and four valve systems have may have
mobile phase on unused valves that is unintentionally drawn into the pump.Knowing what type of system you have might help. GOOD LUCK
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