Carryover Problem
Posted: Fri Mar 29, 2013 8:51 pm
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What happens if you inject *two* MeCl2 blanks after a sample? Is the amount of carryover in the second blank 3% of 3% (i.e. 0.09%) or is it higher?
Next question: do you see the same carryover when your run standards (I'm assuming you're trying to quantitate)?
If you see the same % carryover with standards, and if the second blank is very low (under 0.1%), then the easiest fix might be to simply run a MeCl2 blank in between each run as a "cleanup". Your recovery will only be about 97%, but if the standards behave the same way, that will cancel out.
If the carryover in the second blank is much higher (say 1% or so), that suggests that you have stuff adsorbing *somewhere* in the system that will come off in MeCl2 but not in DMSO. If you have a ternary or quaternary gradient system, you might add a final gradient segment where you switch to a MeCl2/iPrOH blend. Again, less than 100% recovery is a problem, but if the standards behave the same way it might be OK.
If you don't see the carryover with standards, then quantitation will be difficult and you will have to explore a different sample prep.
A big part of your problem may be that you are injecting MeCl2 into a highly aqueous system. One work-around might be to add some iPrOH as a co-solvent before you inject (and double the injection volume if detectability is an issue).