Wally,

These are very different concepts and can't be compared directly. Micrograms per liter $(\mu g/l)$ is is the weight of a material in a liter of liquid, probably water. For parts per billion (ppb) you need to think of a mixture with so many parts of the material in a billion parts of the mixture. $\mu g/l$ is a weight and ppb is a count.

I hope this helps,
Penny