Zero standard for sulfur measurement

I'm looking for a good zero standard to use to calibrate a sulfur analyzer which has a range of 0-30 ppm. Currently we have been looking at Decane, but in a recent test we determined that the ULS No2 diesel (so Cal. winter blend if there is one) sample had a lower sulfur measurement. This is a problem. Do you have a recommended petroleum product for my standard? My calibration standards are manufactured by added sulfur to the zero standard. I have been using Decane as a base for the standard. It is possible that the Decane has been contaminated or is bad. It is or was 99% pure. The problem is the sulfur level is not stated and likely not measured.

Thanks.