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By Anonymous on Saturday, June 12, 2004 - 09:32 pm:
We have a new LC-MS. When we do FID the MS always give the good response. If we go LC firstly the the mass spectrum often give no or strange answer comparing with FID spectrum. Where is the resolution? Someone said this is caused by diffusion in LC. But this is not a good reply for me.
Linda
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By MG on Monday, June 14, 2004 - 07:04 am:
I'm not sure if I understand your question. FID = flame ionization detector. This will not give a spectrum, only a response. The mass spectrometer will give a response (in the Total Ion Chromatogram), with a spectrum at each time interval in the chromatogram, unless you are operating in Selected Ion Mode.
So, are you speaking of poor resolution in the chromatogram? In that case, state your conditions, and the compounds you are trying to ionize (if possible), and we'll be able to help you out.
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By Anonymous on Monday, June 14, 2004 - 09:58 pm:
Dear MG,
I am developing method for fatty acids analysis. When I did FID, the sample gave a good response in TIC. The spectrum responding this TIC was 199, 227,255, 283, very clear C-Chain distribution. But after injected into LC, I could not find any TIC peak during the run. I don't know where they go? The mobile phase is IPA and Methanol, C18 4.6*25 column.
Very sorry the last poor describtion
Linda
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By MG on Tuesday, June 15, 2004 - 09:33 am:
What is your ionization mode (type of ion source and polarity)? What is your LC flow rate?
Are you sure that you are giving your compounds enough time to elute from the column? I think fatty acids can be quite strongly retained depending on chain length.
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By Overpaid Slacker on Tuesday, June 15, 2004 - 02:00 pm:
I agree with MG's second comment. You see the compounds in your flow injection analysis. I'm guessing that is what you mean by FID. Your compounds may not be eluting from the column. The use of only IPA/Methanol on a C18 is called non-aqueous reversed phase and is used for strongly retained compounds. You may want try using a C8 column with a weaker mobile phase of methanol/water.
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By Anonymous on Saturday, June 19, 2004 - 09:02 pm:
You need to run a gradient from water (maybe with formic acid) to acetonitrile (maybe with formic acid). It appears that you need some basic information on how to do chromatography.
We have a new LC-MS. When we do FID the MS always give the good response. If we go LC firstly the the mass spectrum often give no or strange answer comparing with FID spectrum. Where is the resolution? Someone said this is caused by diffusion in LC. But this is not a good reply for me.
Linda
-------------------------------------------------------------------------------------------------------
By MG on Monday, June 14, 2004 - 07:04 am:
I'm not sure if I understand your question. FID = flame ionization detector. This will not give a spectrum, only a response. The mass spectrometer will give a response (in the Total Ion Chromatogram), with a spectrum at each time interval in the chromatogram, unless you are operating in Selected Ion Mode.
So, are you speaking of poor resolution in the chromatogram? In that case, state your conditions, and the compounds you are trying to ionize (if possible), and we'll be able to help you out.
-------------------------------------------------------------------------------------------------------
By Anonymous on Monday, June 14, 2004 - 09:58 pm:
Dear MG,
I am developing method for fatty acids analysis. When I did FID, the sample gave a good response in TIC. The spectrum responding this TIC was 199, 227,255, 283, very clear C-Chain distribution. But after injected into LC, I could not find any TIC peak during the run. I don't know where they go? The mobile phase is IPA and Methanol, C18 4.6*25 column.
Very sorry the last poor describtion
Linda
-------------------------------------------------------------------------------------------------------
By MG on Tuesday, June 15, 2004 - 09:33 am:
What is your ionization mode (type of ion source and polarity)? What is your LC flow rate?
Are you sure that you are giving your compounds enough time to elute from the column? I think fatty acids can be quite strongly retained depending on chain length.
-------------------------------------------------------------------------------------------------------
By Overpaid Slacker on Tuesday, June 15, 2004 - 02:00 pm:
I agree with MG's second comment. You see the compounds in your flow injection analysis. I'm guessing that is what you mean by FID. Your compounds may not be eluting from the column. The use of only IPA/Methanol on a C18 is called non-aqueous reversed phase and is used for strongly retained compounds. You may want try using a C8 column with a weaker mobile phase of methanol/water.
-------------------------------------------------------------------------------------------------------
By Anonymous on Saturday, June 19, 2004 - 09:02 pm:
You need to run a gradient from water (maybe with formic acid) to acetonitrile (maybe with formic acid). It appears that you need some basic information on how to do chromatography.
