I am trying to quantify low levels of ammonia with NMR. The good news is that ammonia has a very low T1 (<0.5) which shortens d1 a lot. However, I cannot find any internal standard with a T1 that is as low as ammonia so that I can use absolute qNMR for quantification. For example maleic acid would require around 10 times longer relaxation, because the T1 is around 3-5s which reduces the LOD of cause.

Regards, Martin

  • 1
    $\begingroup$ Can you say more about your sample? You are willing to add an internal qNMR standard, what about a relaxation agent ? $\endgroup$ – Buck Thorn Mar 14 '19 at 19:10
  • 1
    $\begingroup$ You might want to try other relaxation agents and pick your concentration carefully. The combination maleic acid + iron sounds bad because they may form a complex (for quantitation that may not matter depending how the sample is prepared). $\endgroup$ – Buck Thorn Mar 16 '19 at 17:26
  • 1
    $\begingroup$ I take that back: the formation of a complex may be very bad for quantitation if it is a tight complex, resulting in very rapid relaxation of the complexed maleate. There are other choices, however N-14 relaxation will be a problem with ammonia. $\endgroup$ – Buck Thorn Mar 16 '19 at 20:59
  • 1
    $\begingroup$ There is a vast literature on the subject of relaxation agents. Among the common ones in nmr are GdCl3 and Cr(acac)3 and some nickel salts. Their effect may vary but typically insofar relaxation is concerned they shorten T1/2 for small molecules to the same degree (for small molecules T1~=T2). $\endgroup$ – Buck Thorn Mar 17 '19 at 20:30
  • 1
    $\begingroup$ It is difficult to give further opinion for your particular case. The chemistry of combining all of the ingredients you mention may be complex. I am not sure changing relaxation agent would make much of a difference insofar as ammonia versus maleic acid is concerned. Feel free to post a more specific question or even data if you need an opinion. $\endgroup$ – Buck Thorn Mar 19 '19 at 17:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.