I'm optimizing a method using online SPE (HLB) and there appears to be a carryover problem coming from the cartridges. This disappears after one blank, but I'd like to avoid having to run blanks after every sample. We use acetonitrile and water at 0.1% acidity, and our gradient reaches 90% organic solvent for a total run of 55 minutes. I'm not sure how these are getting trapped like this.
you can try to incorporate a column rinse step in your LC gradient. meaning, increase your organic solvent to 100% at higher flow rate for few minutes. then, resume your gradient. Maybe this is sufficient...
Narrow down exactly where the carryover is coming from. If it is the injector then you can add an injection rinse step, if it is the SPE column then a column rinse as Fabian suggests.
How do you load the SPE? If you have separate loading and eluting pumps, then you may be able to regenerate the SPE cartridge during the analytical run, perhaps by switching the column out of the analytical flow at about 45 minutes and running a gradient just on the SPE cartridge.
Are all your analytes carrying over, or is it just a subset? If it is generally your late eluting ones then a 100% hold could help, but if your earlier ones then a more complex gradient may be needed. If you cannot amend your run gradient, then add a second gradient after the 55 minutes. This can be much shorter so does not use up a further 55 minutes, and because it is built into the run you don't have to remember to run a blank each time.
I am guessing from the 55 minute run time that this is a complex mixture with many analytes? If this is being used as a screen and a "normal" sample would have only a few of the analytes, injecting each sample in duplicate would be a better use of your run time than a blank. You could also add acceptance criteria for duplicates
Good luck
It's the compounds that elute early that I'm having trouble with. They are retained really well, and the system is incredibly sensitive. For now, I'm going to increase my organic %, and increase flow at the end of the run. We do have a separate loading and eluting pumps, so if this new method doesn't work, we will attempt to have a gradient run on just the SPE cartridges.
Currently, we run at 0.3 mL/min. How high should I increase the flow at the end of the run?
Depends on the back pressure of your system. But you can certainly increase your flow to 1-2 mL/min. Maybe divert the flow into waste to save your ESI source...
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