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By Linda on Thursday, August 26, 2004 - 04:42 pm:
Hi, everybody,
Could somebody there explain if Ammonium Trifluoroacetate and Trifluoroacetic Acid have the same ion pairing and ion suppression impact on LC-MS analysis? Thanks a lot.
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By Uwe Neue on Thursday, August 26, 2004 - 09:23 pm:
The problem is supposed to be associated with the TFA. No TFA, no ion suppression from TFA. Maybe a MS expert can explain why TFA causes ion suppression, and formic acid or acetic acid don't...
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By Kevin on Friday, August 27, 2004 - 12:08 pm:
Ammonium trifluoroacetate and trifluoroacetic acid will both produce the trifluoroacetate ion in aqueous solutions. As noted, the trifluoroacetate ion is a strong ion pairing agent. This is advantageous for chromatographing positively charged species by reverse-phase LC, as the TFA anion creates a net zero charged species as the compound passes though the column.
While this is advantageous for RPLC, it is problematic for mass spectrometry. This is due to the fact that a mass spectrometer detects ions, and those ions must be unassociated from any counterions. A trifluoroacetate anion paired with a positively charged compound has a net zero charge and hence cannot be detected by mass spectrometry.
Formic acid and acetic acid are preferred acid modifiers for LC/MS because they are much weaker ion pairing agents than TFA, and readily dissociate from positively charged compounds in the ion source.
Kevin
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By Linda on Saturday, August 28, 2004 - 09:31 am:
Many thanks to Uwe and Kevin for your reply.
The lab I worked with before routinely usese ATFA in their LC-MS mobile phase for ESI+ mode and the response was much higher than using Formic acid/ammonium formate or acetic acid/ammonium acetate. I couldn't understand the phenomenon because I thought the TFA- ion in ATFA would suppress positive ions through ion pairing the same way as TFA, but I was told TFA and ATFA are working different way.
Hi, everybody,
Could somebody there explain if Ammonium Trifluoroacetate and Trifluoroacetic Acid have the same ion pairing and ion suppression impact on LC-MS analysis? Thanks a lot.
-------------------------------------------------------------------------------------------------------
By Uwe Neue on Thursday, August 26, 2004 - 09:23 pm:
The problem is supposed to be associated with the TFA. No TFA, no ion suppression from TFA. Maybe a MS expert can explain why TFA causes ion suppression, and formic acid or acetic acid don't...
-------------------------------------------------------------------------------------------------------
By Kevin on Friday, August 27, 2004 - 12:08 pm:
Ammonium trifluoroacetate and trifluoroacetic acid will both produce the trifluoroacetate ion in aqueous solutions. As noted, the trifluoroacetate ion is a strong ion pairing agent. This is advantageous for chromatographing positively charged species by reverse-phase LC, as the TFA anion creates a net zero charged species as the compound passes though the column.
While this is advantageous for RPLC, it is problematic for mass spectrometry. This is due to the fact that a mass spectrometer detects ions, and those ions must be unassociated from any counterions. A trifluoroacetate anion paired with a positively charged compound has a net zero charge and hence cannot be detected by mass spectrometry.
Formic acid and acetic acid are preferred acid modifiers for LC/MS because they are much weaker ion pairing agents than TFA, and readily dissociate from positively charged compounds in the ion source.
Kevin
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By Linda on Saturday, August 28, 2004 - 09:31 am:
Many thanks to Uwe and Kevin for your reply.
The lab I worked with before routinely usese ATFA in their LC-MS mobile phase for ESI+ mode and the response was much higher than using Formic acid/ammonium formate or acetic acid/ammonium acetate. I couldn't understand the phenomenon because I thought the TFA- ion in ATFA would suppress positive ions through ion pairing the same way as TFA, but I was told TFA and ATFA are working different way.
