ICP-MS method for sulfur

Question

I've been asked to develop an IPC-MS method for sulfur content in a pharmaceutical drug substance. We have an Agilent 7800 G8421A system. Based on my preliminary literature search it appears development for such a method may be difficult . Would appreciate suggestions. regards,

carwogill · Answer

Hi Kurt. I analyse sulphur all the time. However, in my case in oleochemicals. 
 Typically I will digest 0.5 g of my sample with nitric acid in a microwave. Afterwards diluting in milliQ to 50 mL. Keep in mind that some sulphur compounds in your ample may be volatile! 
 I use the same ICP as you. I analyze sulphur at mass 32 or 34. Sometimes small amounts are more accurate at mass 34. Sc is the internal standard, and I also use Xe as a collisiongas to remove O2 interference. (Xe is required, without it you can't measure S). Maybe instead of Xe you can add O2 to your plasma gas so that SO ifs formed. And then you can analyse mass 48. But if your samples contain significant amount of Ca and/or Ti this mass can be problematic. 
 Like this I can measure approx 20 ppb in solution. Because I am stuck with a dilution factor for the samples of 100, my LOQ is approx 2 ppm S in the sample. 
 For very low amounts of S (in my case 2- 10 ppm) I will also add a significant amount of IPA to my ISTD solution. This will slightly elevate the S signal and so decrease detection limits. 
 Hope this helps. Good luck!