We are developing a method to analyse some low molecular weight compounds in plasma using LC-MS/MS. The sample preparation is a simple protein crash with methanol with 0.1 % formic acid, together with ascorbic acid and DTT as stabilisers. When running around 40-60 plasma extract samples, we get a build up of a lot of soft material around the cone of the inlet - inside the ionisation chamber. We though it was the DTT, but we still get the same problem even after removing it. The 'gunk' is dirty yellow coloured and smells a bit sulphurous.
The system is a Shimadzu LC-MS/MS and we're running it in ESI +/-.
Has anyone ever had the same problem, and might know of a solution?
Hi,
do you also observe an increase in backpressure? How long is your gradient? Do you observe ion-suppression at any timepoint?
U can try to divert the first minutes of your gradient into waste to get rid of all compounds which do not interact with your column. I guess this will save your source a bit.
Use a divert valve to divert the first few minutes of your column eluate to waste when salts and hydrophilic compounds like urea, glucose and creatinine are coming off then turn the divert valve back to the source to analyze for the compounds of interest. The Shimadzu 80XX sources are very prone to this type of fouling, but I find they are easy to clean. The cone is only held on with 2 screws and I typically clean it by sonicating in 50% concentrated nitric acid. The desolvation line is a bit more tedious to remove and clean; I draw the diluted nitric acid through it with a piece of tight fitting tubing connected to a syringe. I also polish the cone and outside of the desolvation line with Wenol or Pikal metal polish.
Thanks for the tips! We've got an 8030+ (basically an 8040). No change in backpressure anything LC side. We divert when we can to waste, and normally we've been able to run 3000+ samples without noticing any drop off in performance. However, new method, new problems! Will try with MeCN, and yes, it's good that the inlet is easy to clean! Just very surprised by how quickly and how much muck is building up.
Ascorbic acid is so prone to oxidation and these by products are yellow to brown in color; could this be the cause of the source fouling? Perhaps try a fresh batch of ascorbic acid.
Hi all,
Thanks for your input - in the end it does seem to be ascorbic acid - for some reason the retention time shifted, so it was coming out while the flow was going to the MS. We've sorted out the valve timing and all is well again!
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