The sensitivity of triple quad is not stable.

[wan]

Hi Ken,

The other scientist using the same autosampler did not experience any problems, so I doubt it is due to the autosampler.

We ran a few samples yesterday and mysteriously, the problem seemed to have been solved (abundance from run to run differs by at most 0.6E6 unlike before which is at least 2.0E6, most of the time it differs by 0.3E6.). I'm doubtful, so we'll try again today to see if the MS is really okay.

There is the suggestion to try changing the LC column in a different thread I posted regarding jagged/tailing peaks. We will try that today if possible.

Regarding the GCMS problem I stated, we will clean the ion source and see if that helps, as it might not be due to excess BSTFA/pyridine.

Thanks! I'll post again!

Regards,
Wan

[Ken]

Hi Wan, you mentioned that the other user did not experienced the problem using the same autosampler; are you saying that the other user is using the same configuration ie same instrumentation for other applicationS and they dont see this problem?

If that's the case, wont it be obvious that there might be some discrepancies in your application setup ie buffers, columns rather than problems with the MS?

Unless the user used another instrument with the same brand, than this will not eliminate the possibility that the MS is having a problem.

If you say that the instrument somehow mysteriously 'performed' at it is, can you then verify by:

Multiple injections of same concentration of melamine standard ie 10X replicates at 0.1ppm concentration?

The signal should not be varying much; else this will be caused by ionisation issue with the MS source; or it's from the LC side ie column, pump causing differential buffer concentration, injector etc.

Let us know more in details so that we can help you out ^^

Cheers~

[wan]

Hi Ken,

Sorry for the confusion. I didn't think there is a problem with the MS, but more of my MS settings and LC parameters.

We changed the column, flush our old one (since the new one has to go back to its owner), changed the ionspray current, did multiple injections of same concentration of melamine standard as you said and now obtained almost the same abundance.

We can only use the LCMSMS when the scientist using it most of the time allow us to. So we have to squeeze many things into that tiny timeslot and thus, it gets really confusing at times. I appreciate all the help. Thank you.

I still have one problem. Does anyone who did melamine analysis on LCMSMS, observed polymerization of melamine on the ionspray needle tip? Is there any way to overcome that?

We have to run a very long sequence to account for instrumental drift, so we are worried about the melamine polymer plugging and clogging the needle tip.

Thanks again!

Regards,
Wan

[wan]

Hi Ken,

Sorry for the confusion. I didn't think there is a problem with the MS, but more of my MS settings and LC parameters.

We changed the column, flush our old one (since the new one has to go back to its owner), changed the ionspray current, did multiple injections of same concentration of melamine standard as you said and now obtained almost the same abundance.

We can only use the LCMSMS when the scientist using it most of the time allow us to. So we have to squeeze many things into that tiny timeslot and thus, it gets really confusing at times. I appreciate all the help. Thank you.

I still have one problem. Does anyone who did melamine analysis on LCMSMS, observed polymerization of melamine on the ionspray needle tip? Is there any way to overcome that?

We have to run a very long sequence to account for instrumental drift, so we are worried about the melamine polymer plugging and clogging the needle tip.

Thanks again!

Regards,
Wan

[Ken]

Hi Wan, I believed that you have sorted out the issues with the inconsistencies in signal for your analyses.

As for your second concern, we did not experienced any polymerization of melamine on the ionisation tip; I assumed that what you are trying to say is salt precipitation on the ionisation needle/tip, am I right?

As you may have known, melamine is a very polar yet 'sticky' compound; if you have a very sensitive LCMS/MS instrument, and you somewhat accidentally injected in a very high concentration of melamine ie >1ppm, then you will tend to have a high background during your analyses; so make sure you did not inject a very high concentration of melamine samples - polymerization or precipitation of melamine from liquid form to its powdery form is very rare unless your LCMS/MS source settings are wrong ie desolvation temperature or gas parameters.

In term of melamine analyses; my lab has the highest number of samples to run in my country; in short, it's actually the designated government lab to test for melamine and cyanuric contamination in food samples and I've never experienced any issues in regards to 'melamine polymerization on ionisation source'.

Let me know if you need additional advice. All the best !

[wan]

Hi Ken,

We thought it was salt precipitation on the ionisation tip initially and tried to rinse it out with water, but we could not clean it off, even after trying a few different solvents. A application scientist then told us to increase the distance of the tip away from the nebulizer gas. We did so, but still obtain tiny amount of precipitates on the tip.

We're injecting less than or equal 20 ppb of melamine standard. I'll play around with the source settings and see how it goes.

Thank you so much for your help!

Regards,
Wan