Ion source contamination, help please

[Jasmine]

Hi All,

My LCMS is producing spurious peaks when injecting blank solution at the retention times which I needed for my compunds of interest, could anyone please help me? How do I clean the ion source? I have already done the elimination part between LC and MS and it seems my contamination is in the MS.

Thanks!

[Kostas Petritis]

Jasmine,

If the contamination was on the MS you should had a constant increased background all around the chromatogram. The fact that you see peaks at certain retention times implies that this contamination comes more likely from the LC part/injection.

What did you do exactly to eliminate the LC as a source of contamination?

[Jasmine]

Dear Kostas,

Thanks for your reply. I think I explained very badly in my post, so you misunderstood me a little bit. What I meant was that consistent peaks come out at certain retention time. They are very reproducible in terms of time and the areas only vary a little bit between injections. I first isolated the autosampler and the peaks still appear. I used another pump and the peaks disappear. Also, can you suggest any good basic buffer which can be used for ESI with pH 9-11?

Thanks again.

[Kostas Petritis]

Jasmine,

Indeed it was a little bit unclear .

If the contamination does not insist when you pump with your new pump then you do not have to worry. Otherwise cleaning the source with solvents of different polarity should do the job (for minor contaminations).

Now about your request, 1 methyl piperidine (with formic or acetic acid) is exactly at the range you request (9.1-11.1) and it should be quite volatile.

Of course ammonia is pretty good for pH range of 8.2-10.2.

[Jasmine]

Dear Kostas,

We don't have methyl piperidine in our lab. You mention Ammonia as a good choice for pH 8-10. So, can I use dilute ammonia in aqeous solution only as my buffer?

Thanks

[Kostas Petritis]

Jasmine,

You could but that wouldn't be a buffer (no or extremely small buffer capacity). You need to make a real buffer with the help of an acid as well.

As an example I make one for you with the help of the software pHoEBus.

Let's say for a buffer of ammonium acetate PH =10, and ionic strength 10 mmole/L. If your stock solutions are of 1 mole/L and you will need to make 1 L of buffer you need to mix 59.99 mL of your ammonia stock solution with 9.89 mL of your acetic acid solution and then add water to 1L. This will give you a buffer capacity of 19.3 mmol/L.pH. Normally you should adjust the pH as necessary but normally the solution should be very close to what I mentioned above.

[Jasmine]

Thanks Kostas!