Nitrous

Sigma usually deals with USP and reagent grade chemicals.. I would see no reason for them to contaminate it with SO2.

I think I can post this, I mean it is legal and all (if not, edit it out):

http://www.sigmaaldrich.com/catalog/ProductDetail.do?lang=en&N4=295590|ALDRICH&N5=SEARCH_CONCAT_PNO|BRAND_KEY&F=SPEC

227G @ $224.00 (USD).. I don't know if that is a good deal or not, don't care to do the math, but that is 99% pure Nitrous Oxide.

I just tried to order it. Just as I thought you need to jump hoops to get it. They don't sell to joe blow off the street. oh we added so2 to the the car boost stuff so peeps couldn't huff it. Don't know that I ever looked at a spec sheet to see the purity. Since we did usp gas we had 02,n2,co2,no2,he and hydogen all at 99.999% pure. Spec gas had all sorts of ultra pure gas for mixes.