Converting pmol/kg to ppb

[blairxoxo]

Hello,
I need some help with the unit conversion from pmol/kg to ppb.

For example, I have 28 pmol/kg of Lanthanum (138.91 amu) and 50 pmol/kg of Cerium (140.12 amu). How am I going to convert these to ppb?

What I did for these, but still not sure:

28 pmol/kg La * (1/10^12) * (138.91 g/mol) * (1000) = 3.89*10^-6 ppm La * (1000) = 3.89*10^-3 ppb La

50 pmol/kg Ce * (1/10^12) * (140.12 amu) * (1000) = 7.01*10^-6 ppm Ce * (1000) = 7.01*10^-3 ppb Ce

Let me know if I did something wrong or if you have any concerns about my calculation. Thank you.

[JohnS]

Hello,
I need some help with the unit conversion from pmol/kg to ppb.

For example, I have 28 pmol/kg of Lanthanum (138.91 amu) and 50 pmol/kg of Cerium (140.12 amu). How am I going to convert these to ppb?

What I did for these, but still not sure:

28 pmol/kg La * (1/10^12) * (138.91 g/mol) * (1000) = 3.89*10^-6 ppm La * (1000) = 3.89*10^-3 ppb La

50 pmol/kg Ce * (1/10^12) * (140.12 amu) * (1000) = 7.01*10^-6 ppm Ce * (1000) = 7.01*10^-3 ppb Ce

Let me know if I did something wrong or if you have any concerns about my calculation. Thank you.

Those are correct. Let me bring up two minor concerns which are, frankly, picky and you can consider or ignore. If these are homework, you may want to stick with what the instructor says.

First, given the magnitudes, it might be more readable to use parts per trillion, so 3.89 ppt La, 7.01 ppt Ce.

Second, for international readability, the SI discourages terms like parts per million, billion or trillion. The terms do not translate well across languages; some countries use short scale and some use long scale for large counting words like billion and trillion. In addition parts per {whatever} are a ratio, but a ratio of what. They can be mass, volume, or molar rations, or even mass/volume for solutions. Unless the basis is clear, the term is ill-defined. The SI favors retaining (cancelling) units that explicitly show the basis. In your case, they are mass ratios so 3.89 ng/kg La, or 3.89 pg/g. It is clear that these are a mass ratio and 12 orders of magnitude apart.