How to reduce noise in LC/MSMS???

[catlover]

I would like to know what can I do to reduce the noise in LC/MSMS, the noise level is quite high and signal to noise ratio is around 7 to 9.

I am doing analysis on PAHs (Fluorene, Anthracene, Fluoranthene and Pyrene.)
Instrument used are Waters UPLC and TQ detector.
Column is BEH C18 with 1.7 micrometer.
Solvent is 66% ACN : 34%water
Flow rate is 0.3 mL/min

I tried to increase the flow rate and solvent composition, it only vary the shape and retention time of peaks and the noise is still so high

By the way, I would also like to know why the baseline of chromatogram is sometime drift, I found it better after several sample flows... Are there any good reasons to explain this???

Please help me!! Thank you all.

[lmh]

How are you ionising these things? They absorb UV beautifully but they have no functional groups for ionisation whatsoever. I wouldn't even attempt them with electrospray.

[James_Ball]

These would work best with APPI ( Atmospheric Pressure Photo Ionization) if you have they type of source available.

I have tried them with ESI and APCI an neither give good sensitivity at all, so you will have very high noise levels in relation to your signal. Without APPI you will have much better sensitivity using UV and Fluorescence in tandem.

[catlover]

I see your points, but the APPI is not available in the TQ detector. All I can use is only ES mode...
The detector is PMT, so I think it will be okay.
However, there is a quite strong peak in the blank solution, so I am going to wash the column today, hope everything will get better.

Anyways, thank you all for quick reply.

[lmh]

Seriously, they will never give good results in electrospray; it's not the detector that is limiting them, it's the fact that they won't form any ions to detect! Have you considered a simple hplc approach using UV?

[catlover]

Actually, analyzing PAHs is only the starting part of my training, what I need to do is to get familiar with the LC/MSMS.
I asked my adviser and he also agreed that GC-MS is easier and better method for PAHs.....
But still, I need to make a good result of PAHs through LC/MSMS..

Here are the chromatograms I got, I think it is no that good, right?
Blank:


PAHs mixture with 40 ppb Fluorene: (you can see that LC cannot separate Fluoranthene and Pyrene..)


And this is the zoom in of chromatogram, the calibration result is quite nice, it gives R^2 =0.999, but still there is a peak of background noise..


=======================================================
By the way, I think the order of retention time is completely reverse of expected, is it really normal????
My adviser said it may happen due to the change of physical environment, but it seems contradictory to what I learn....

I would like to make the results better, but somehow my adviser said it is okay and he would like me to try on reserpine for MRM tuning in the coming week....
I don't know if I should give up on PAHs and start the reserpine...
At the same time, the next problem is that amount of reserpine he gave me is only around 2 mL, so I get stuck on how to get the best tuning... (Since there is lack of analyte, I think I cannot use autotune...)
Problems always come again and again....Orz

Anyways, really thank you for your respond.

[Gaetan Glauser]

lmh said it all: even the best expert in mass spec won't get good results on PAHs using LC-MS. If you simply need to get familiar with the technique, forget PAHs and go for reserpine and you can be assured you'll get a nice signal this time! Reserpine is not expensive, you can ask for more to your supervisor who certainly won't get broke! BTW, it depends on the concentration of your solution: if you were given 2 mL of a 1mg/ml solution, you'll have enough for thousands of assays.