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Chemical Stability and Compatibility - Digesting an organic with Os Spike

 
I am developing a method for a 24 ICH metal analysis of Hydroxypropyl Methylcellulose.  I cannot get Os spikes to recover and I understand it is because we are microwave digesting the sample in HNO3.  From everything I've read on your site and others, the problem is osmium tetroxide formation.  However, the only thing that's going to digest the sample is an oxidizing acid (as far as I know).  I'm sure HClO4 or H2SO4 will be just as detrimental.  I can't get it into solution with just HCl.  Everyone says use an HCl matrix with Os but how do you reconcile that with a need to spike and then digest?  I can't digest in nitric and then dilute in HCl because the sample must be spiked before any preparation steps.  Thanks for any assitance!   Dan
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Brian Alexander, PhD Total posts: 12

Through discussions and work with pharmaceutical and microwave manufacturers it is possible to obtain good Os recoveries for excipients and drug product analyses. The key appears to be mimimizing the HNO3 and using HCl in its place if and where possible. An example of a whole tablet microwave digestion that used the following procedure resulted in a 94% spike recovery for Os:

  1. Add 2 mL H2SO4 and 1 mL HF
  2. Rest 15 minutes
  3. Add 4 mL HCl and 3 mL HNO3
  4. Rest 15 minutes
  5. Digestion - 20 minute ramp to 210 °C and hold 1 hour

If you can avoid HNO3, or at least maximize the HCl/HNO3 ratio, then it should be possible to obtain adequate Os recoveries. You'll quickly know if OsO4 is forming as the volatility of the tetroxide results in very efficient nebulization/transport in the ICP sample introduction system, with the end result being very high Os signal intensities (i.e., extermely high recoveries). You can also mitigate OsO4 formation post-digestion by adding hydroxylamine hydrochloride to the samples, which will help prevent Os oxidation.

Posted: 11/12/19 13:13:37

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