Conversion help! ppmv to mM

[jearles]

I have two different sets of dissolved gas data, one collected using passive diffusive samplers down groundwater wells, and reported in ppmv. Another data set is headspace analysis on groundwater samples and reported in mM. So of course I have someone who wants the ppmv data converted into mM. I've read a couple ways of doing this online, had two methods given to me from other researchers, and none of it agrees. So I'm hoping someone here would know where to start?

[rb6banjo]

I take that you're calling ppmv = µg/mL?

If that's true, you need to know the molecular weight (MW) of the analyte. For a 10 ppmv solution of toluene (MW = 92 g/mole) in water you convert it like this:

10 µg/mL x 1g/10^6 µg x 1 mole/92 g x 1000 mL/L x 1000 mM/1 M

M = moles/L and there are 1000 mM in 1 M. The 2 factors of 1000 combine to make 10^6 so they divide out the µg-to-g conversion.

Ultimately, if you divide ppmv by the molecular weight of your analyte, you get mM.

If ppmv means something other than µg analyte per mL of solvent (like water), then these calculations may change. Gas-phase ppm is not the same as ppm in the condensed phase.