LC-MS: excessive contamination ion source

[Maus]

Hi all,

I have been trying to quantify a mixture of antibiotics in human plasma using UPLC-MS/MS. During method development, I am always left with excessive (somewhat crystalline) contamination of the ESI-chamber and cone. I have tried rinsing all tubing with IPA/MeOH/AcN/H2O and preparing fresh mobile phase before each run. I have avoided the use of non-volatile buffers and have diluted plasma to minimize the level of endogenous salts. What do you think: is it my sample prep? Should I go over to SPE or should this protein prec have worked? Are there any other causes I haven't thought of?

Sample prep:
Tenfold dilution of plasma with aqueous 0.1% FA in water, followed by precipitation with 10% TCA, containing IS (1 part dil plasma to 1 part 10%TCA). after that left for 20 min at 4C, followed by 5 min centrifugation at 14000rpm and transfer of supernatant to autosamplervial.
I've tried different protein precipitation methods, but TCA gives the best chromatograms; a higher conc is not an option since ion suppression starts to kick in.

LC: BEH-column with in-line filterunit (0.2um), flow=0.5mL/min, column temp=50C
Gradient: A = 0.1% FA in water, B = MeOH, 10% to 90% B over a period of appr. 2.5 min. Injection volume = 30 ul

Thanks in advance
Maurice

[bert]

Hi Maurice,

we have some bad experiences with TCA in combination with MS. Acetonitrile precipitation of proteins can work well. But I would go for SPE to minimize the fauling of the source.

regards Bert

[Uwe Neue]

A plasma sample with just a protein precipitation will sooner or later clog your analytical column. I do not recommend this. I suggest to look into more sophisticated sample prep methods, such as SPE.