Stability of acids in methanol [August 12, 2004]
Posted: Sun Aug 29, 2004 6:42 pm
By Jimmy on Thursday, August 12, 2004 - 07:13 am:
I am developing the method for an acid at low pg/mL level. It is not stable in acidic pH and even less stable in neutral and high pH. Could someone tell me how to stabilize it at low pH? Anything that would slow esterification? (lower temperature, what else)?
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By Mark on Thursday, August 12, 2004 - 08:34 am:
Jimmy,
Why not put the sample in an aprotic solvent such as acetonitrile, dichloromethane, etc.? You can add TFA to these and keep the pH low and avoid esterification.
Regards,
Mark
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By Jimmy on Thursday, August 12, 2004 - 10:57 am:
Mark,
The acid is also unstable in MeCN. Not tried Chloroform yet.
Thanks.
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By zelechonok on Thursday, August 12, 2004 - 02:46 pm:
Why do you think it is esterefication? If it goes in ACN it is not esterification, but hydrolysis or oxidation.
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By Jimmy on Friday, August 13, 2004 - 04:19 am:
Zelechonok,
Thanks for your reply.
Alcoholic impurities could pose a lot of problems for a pg/mL acid. Oxidation, hydrolysis, polymerization etc are possible as well.
The sample matrix is plasma. We currently use SPE, so chloroform is not very feasible. Have tried MeOH and MeCN, which are bad for my compound. Any other ideas?
Thanks!
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By Uwe Neue on Friday, August 13, 2004 - 04:15 pm:
If I understand you correctly, the real problem is the esterification with components of your extraction solvent. you may be able to extract your compound from the SPE with other solvents as well, say for example THF....
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By bill tindall on Friday, August 13, 2004 - 08:27 pm:
You need to understand what chemistry is altering your acid. Acids can not esterify if they are present as an anion, so it would seem that esterification is not what is causing loss of this component at high pH. Once you figure out what chemistry is resulting in the loss it may be more obvious how to stop it.
alternatively if it is that easy to esterify, why not esterify it and analyze it as the ester?
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By Jimmy on Sunday, August 15, 2004 - 07:21 pm:
I have found the answer. The acidic analyte (pKa maybe about 4.4 in aq) sticked to the HLB cartridge at pH 3.5. What I did was to disrupt the ion exchange force.
Remember that the analyte is only in the low pg/mL range. The ion exchange may be insignificant for ng/mL or ug/mL, but not with pg/mL. The recovery is very reproducible and high now.
Thanks for everyone!
I am developing the method for an acid at low pg/mL level. It is not stable in acidic pH and even less stable in neutral and high pH. Could someone tell me how to stabilize it at low pH? Anything that would slow esterification? (lower temperature, what else)?
-------------------------------------------------------------------------------------------------------
By Mark on Thursday, August 12, 2004 - 08:34 am:
Jimmy,
Why not put the sample in an aprotic solvent such as acetonitrile, dichloromethane, etc.? You can add TFA to these and keep the pH low and avoid esterification.
Regards,
Mark
-------------------------------------------------------------------------------------------------------
By Jimmy on Thursday, August 12, 2004 - 10:57 am:
Mark,
The acid is also unstable in MeCN. Not tried Chloroform yet.
Thanks.
-------------------------------------------------------------------------------------------------------
By zelechonok on Thursday, August 12, 2004 - 02:46 pm:
Why do you think it is esterefication? If it goes in ACN it is not esterification, but hydrolysis or oxidation.
-------------------------------------------------------------------------------------------------------
By Jimmy on Friday, August 13, 2004 - 04:19 am:
Zelechonok,
Thanks for your reply.
Alcoholic impurities could pose a lot of problems for a pg/mL acid. Oxidation, hydrolysis, polymerization etc are possible as well.
The sample matrix is plasma. We currently use SPE, so chloroform is not very feasible. Have tried MeOH and MeCN, which are bad for my compound. Any other ideas?
Thanks!
-------------------------------------------------------------------------------------------------------
By Uwe Neue on Friday, August 13, 2004 - 04:15 pm:
If I understand you correctly, the real problem is the esterification with components of your extraction solvent. you may be able to extract your compound from the SPE with other solvents as well, say for example THF....
-------------------------------------------------------------------------------------------------------
By bill tindall on Friday, August 13, 2004 - 08:27 pm:
You need to understand what chemistry is altering your acid. Acids can not esterify if they are present as an anion, so it would seem that esterification is not what is causing loss of this component at high pH. Once you figure out what chemistry is resulting in the loss it may be more obvious how to stop it.
alternatively if it is that easy to esterify, why not esterify it and analyze it as the ester?
-------------------------------------------------------------------------------------------------------
By Jimmy on Sunday, August 15, 2004 - 07:21 pm:
I have found the answer. The acidic analyte (pKa maybe about 4.4 in aq) sticked to the HLB cartridge at pH 3.5. What I did was to disrupt the ion exchange force.
Remember that the analyte is only in the low pg/mL range. The ion exchange may be insignificant for ng/mL or ug/mL, but not with pg/mL. The recovery is very reproducible and high now.
Thanks for everyone!