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Why is no UV peak detected by LCMS?
Posted: Fri Mar 31, 2006 6:04 pm
by sxg
One compound (MW--870) was analyzed by HPLC at 250nm with gradient ( H2O/ACN 0.1%TFA). The HPLC method is good. I found one new peak and want to check it by LCMS.
I transferred this HPLC method to the LCMS ( HCOOH replaced TFA). No UV peaks were detected. Anyone knows what is the problem?
I am a newcomer to the LCMS area.
Posted: Fri Mar 31, 2006 7:52 pm
by tom jupille
Four possibilities:
1. Your compound is unretained using the acetate in place of the TFA
2. Your compound is irreversibly retained using the acetate in place of the TFA
3. The absorbance spectrum of your compound changes when acetate replaces TFA
4. Your original "peak" was an artifact.
So, some questions:
No peaks at all? not even at t0?
How high did you take the %ACN?
Posted: Fri Mar 31, 2006 11:08 pm
by Kostas Petritis
In addition to Tom's suggestions... TFA is compatible with the MS so I wouldn't change anything in my original LC method. If you have significant ion suppression problems you can try then to change the mobile phase additives but until then I would go with your original method and LC-MS...
Posted: Sat Apr 01, 2006 5:20 pm
by KC
Maybe I'm missing something here. You see this peak in the UV and take it to LCMS detector and don't see it. If your compound does not ionize you will not see it.
Posted: Sat Apr 01, 2006 8:20 pm
by JMB
sxg,
You should remember that you have four common options when you run an LC/MS experiment, namely ESI under +ve and -ve and APCI under +ve and-ve ionization.
ESI is better for polar and/or ionized compounds, while APCI is generally recommended for non-polar, non-ionizable compounds (BUT may cause thermal degradation).
Therefore pick first the appropriate ionization technique (ESI/APCI), then consider the likely/possible functional groups present in the the new LC/UV and set the ionization mode accordingly.
Furether, use all available information ---do you have a diode array UV spectrum---is it similar/dissimilar to that of the parent compund ??
From the RRT, is the new peak more/less hydrophilic than the parent ??
It may be as simple as expanding the mass range that is scanned.
Let us know the outcome and Good Luck !!
JMB
Posted: Mon Apr 03, 2006 9:16 am
by libo
Sometimes we will get a lower efficiency using HCOOH instead of TFA.
This might cause poor peak shape even "lost" at low concentraction.
Use a larger injection volume might be helpful.
Posted: Tue Apr 04, 2006 2:25 pm
by sxg
Thans all. I will transfer the original HPLC method to the LCMS.
Posted: Tue Apr 04, 2006 9:12 pm
by sxg
Kostas Petritis, Libo,
All,
Thank you very much for your help. It works when I transferred the original HPLC method to the LCMS/w 0.1%TFA. I got the peaks.