Chromatography Forum

Hi all,

We are running melamine on a triple quad (6410) in MRM mode. Now we have this problem which annoys me a lot.
The sensitivity of the instrument is quite good, but after about 10 runs, it starts to reduce quite rapidly (by half), then after some runs, it's returns to normal value.
I have been using this instrument just 2 months so have not much experience of that. Please help me with this.

Thanks.

Hi all,

We are running melamine on a triple quad (6410) in MRM mode. Now we have this problem which annoys me a lot.
The sensitivity of the instrument is quite good, but after about 10 runs, it starts to reduce quite rapidly (by half), then after some runs, it's returns to normal value.
I have been using this instrument just 2 months so have not much experience of that. Please help me with this.

Thanks.

Thohry,

Just few thoughts..

Vacuum variations in your MS?

Ion suppression from something slowly eluting from your column?

Does this sensitivity drop occur after the standards or after your samples?

Thanks Bhuvfe for the input

I am sure that the vacuum is OK.
The problem happens when we run the samples, not standards.
The sensitivity reduces suddenly a long with the baselines level reduced.

If the performance remains the same after 10 runs of standards, it is not a MS problem, it is has do with your samples, probably how clean or dirty they are... Try to increase the washing time or add a blank in between the samples you observe the decrease in sensitivity. I will also try to take the 12th sample or so and inject as first (in order to see if it is accumulation of specific sample problem)...

If the performance remains the same after 10 runs of standards, it is not a MS problem, it is has do with your samples, probably how clean or dirty they are... Try to increase the washing time or add a blank in between the samples you observe the decrease in sensitivity. I will also try to take the 12th sample or so and inject as first (in order to see if it is accumulation of specific sample problem)...

When the problem occurs, I inject the standards and the sensitivity is as low as well. So it may not due to the sample and/or sample matrix.

What is your washing method exactly at the end of each gradient and what type of samples are you analyzing (in terms of complexity)? When the problem occurs I would try to:

1) Clean well the column (including a blanck) and try a standard
2) Try to aquire in full spectra mode during a blanck to see what might be eluting in the background (when you have the problem) that you might not be able to see during MRM and compare it with the same analysis when everything is working well...

Other things that you want to maybe consider if the problem ends up to be uncorrelated with the sample is sudden changes in the temperature around the MS etc...

As was mentioned, it could be ionization suppression from something slowly eluting from your column. Our service engineer told us that with very dirty samples you can get charging on the quads (esp Q0) which will cause signal suppression. Are you using a switching valve to divert most of the garbage away from the mass spec while your peak is not coming out?

As was mentioned, it could be ionization suppression from something slowly eluting from your column. Our service engineer told us that with very dirty samples you can get charging on the quads (esp Q0) which will cause signal suppression. Are you using a switching valve to divert most of the garbage away from the mass spec while your peak is not coming out?

Thanks sassman and others for your advice. I will try to use the switching valve diverting garbage out. What I am concerned is the not smooth baseline.

Divert to waste whatever you don't want to scan in your MS. I'm doing melamine as well, matrix biscuits and pastry. God knows what a mudcake it created on my sweep cone when the sample prep does not include an SPE and without diverting the garbage.

I'm also trying out GCMS analysis of melamine and am experiencing a drastic decrease in signal.

However, it happens only for my calibration standards and not my samples, which I would expected instead due to matrix effect. Thus, I'm puzzled.

Does anyone has similar experiences?

I'll post the conditions I used tomorrow when I have access to the equipment desktop.

Thanks!

Regards,
Wan

Here are the conditions I used:

Injection mode: splitless 280oC
Temp program: 75oC, 1min, 15oC/min to 300oC, 5min
Column: DB5MS
Ionization mode: EI, 70eV
Monitoring 327, 333, 342 and 348 m/z

I'm a newbie to GCMS, so if there are more conditions that should be stated, do let me know.

Thanks!

Regards,
Wan

I guess negative ions being built up on the surface of the transfer tube, ion source etc causing repulsion of precursor ion (M+H)+ from entering the Q1. You may need to clean the transfer tube, ion source thoroughly and I think the problem will go away.

Hubert

Similar problem here.
Sensitivity of our 6410 lc ms ms for Thiram running down like crazy.
Few days ago abundance for 0,01ug/ml Thiram was like 1600 now its 50.
I run checktune and get such resaults.

m/z m/z Exp Tolerance Delta Result
118.05 118.09 0.10 -0.04 Pass
Abundance Minimum1 Result
1 0 Pass

Abundance for 118.05 ion -> 1

And now the checktune that i run few weaks ago.

m/z m/z Exp Tolerance Delta Result
118.09 118.09 0.10 0.00 Pass
Abundance Minimum1 Result
129906 0 Pass

Abundance for 118.09 ion -> 129996

We are using quechers with some minor modifications
EDTA-2NA is used during extraction process and Acetic acid.
Also acetic acid is in our solvets 0,1%

Hi again,

Sorry to confuse. The qns I posted was for GCMS analysis. I relook at the data and realised signals for both calibration stds and samples are drastically reduced. I suspect this has got to do with the excess BSTFA I had used in diluting the stds and samples when I need to.

We are also running LCMSMS for the sample calibration stds and samples and getting fluctuating signal intensity. By the 3rd run, it had dropped by almost 2/3 of its initial intensity obtained in the first run. Then increase back again by the 4th run.

Typically, what can cause this type of phenomenon?

Regards,
Wan

have you check whether your autosampler is giving you this problem?

And can you check the instrument calibration and tuning to eliminate the problem whether it's from the MS or the LC side?

Cheers~