We typically just process water samples but after our LC-MS run, we always observe white substances at the ESI nozzle. Some are saying that it may be because of the sample preparation but our samples are just water with very low concentration of pharmaceuticals.
We use water and acetonitrile with formic acid as our mobile phase in our runs.
Hi Carl,
the amount of the solid substances depends on how much samples and how often you do sample runs. Due to the high temperature in the ESI source (for us it is ~300 °C Nitrogen), the main part of the solvent evaporates immediately - which it should, because the ESI instrument can only handle free ions in the gas flow, not liquid drops. Some molecules of the sample will ionize, some will go into the waste and some do not ionize but get dry in the ESI source - this is the substance you can observe. It is a kind of mixture of everything. Although you rate the sample concentration as "low" - with time and more and more samples - it will be still enough to get sufficient amounts of solid to the ESI source. Accumulations of solid around the nozzle tip is not normal. If the nozzle is adjusted right, a really fine spray with small drops will be produced. The ESI nozzle consists of two components - an inner and an outer metal tubing (kind of two syringe needles) - the inner tubing can be adjusted in its position - under the microscope. The tip from the inner tube should be barely seen when looking from the top.
Just look at the produced spay - do you have some tubulences etc.?
If the nozzle gets dirty really quick, I strongly recommend you to adjust the nozzle position into the ESI chamber - it coud be too far from the hole, may have some scratches (you can have a look under the microscope) so that you do not have a fine spray, but bigger drops/turbulences, which accumulate next to the nozzle tip - much more substance could precipitate then. We do not observe any substance on the nozzle tip, even after days of intensive analysis. We have 0.5 ml/min flow rate - if the flow rate you use is too high, this could be reason for a bad spray quality - it is simply too much for the nozzle.
A general tip for your ESI source chamber - clean it frequently (weekly or daily, depends on the sample thoughput) with 1/1 iPrOH/water - it will dry immediately during the Nitrogen gas flow. Only if necessary, gently clean it with lint-free paper (e.g. Kimtech precision wipes). Do not use standard tissue paper, you may get some fibers from the tissue into the nozzle/instrument.
Are you running a gradient with 100% Water on a standard C18 column? I had once white residues due to a C18, which is not made for 100% water. It is way more likely that the substances you obsoved origin in your solvent than in your sample in my opinion.
I don't agree with Marcus (I am sorry). I think residues are not a problem of bad ESI conditions. If you change the position of your spray you might just cause the powder to be formed in or at the inlet rather than on the tip. Observing scratches at the tip is normal after using it for some time, especially if you have a metal tip which gehts consumed due to electrochemical processes caused by the high voltage. Haowever, a bad spray will cause bad and non stable signals (which is not your problem right?). I would have a more detailed look at your solvents and samples.
What quality of solvent and acid do you use? Whats the volume you inject? How many samples do you run per day and whats the flow rate of your method? What coulmn do you use?
These points may help to get an idea about the sources of the white powder you mentioned.
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