[Eogle]

Okay folks, I have a units issue that I need to resolve with the least amount of complication possible.
I know that mg/kg = ppm by weight just like mg/L = ppm by volume.  But here's my problem:
I have to have an outside lab test my ethanol and denaturant for sulfur content.  I neglected to review the ASTM method and see that the results would likely be reported in mg/kg.  So when I got my results I'm like daaaaang 
Because when I prepared several samples of neat ethanol to which I added specific amounts of denaturant I did it by volume; 1 mL in 50 mL for example.

Now I can't just subtract the sulfur contribution of the denaturant (natural gasoline btw) because in no way is the density of ethanol, natural gasoline, or any blend of the two equivalent to water.

So what I think I have to do is take the specific gravity of my denaturant (0.659 avg) to calculate the sulfur content per volume.
Then I would have to determine the density of my denatured ethanol solutions and do the same thing.  Then I can subtract the contribution of the denaturant.

Is this the correct approach?

[Borek]

mg/L = ppm by volume

That's w/v, not v/v.

Can't say I understand the rest of your post.

What you have and in what units?

What you did (1 mL + 50 mL) is more or less clear.

What you want and in what units?

[Eogle]

I'm sorry. You're of course correct, it's weight per volume.
What I need to be able to do is calculate the amount of sulfur my neat ethanol, but since I couldn't send that to an outside lab without paying various taxes I had to add denaturant.  So the results I received, in mg/kg, are the amount of sulfur in the denaturant and in the ethanol samples, to which I added known amounts of denaturant.  Because I have certificates of analysis telling me the specific gravity of the denaturant can convert the sulfur result in mg/kg to mg/mL.  Likewise, if I make up more denatured samples and determine their density, I can use the sulfur results I have obtained to calculate the probable sulfur content of the ethanol before I added anything. 
Mostly I just like to have someone to check my math and I'm the only chemist in the building today.