[Hana]

I scientific papers the concentration is often expressed as "mu g/L";
what exactly does it means? Does it stands for micro-gram per liter? Can I always use the word "mu" instead of the lower case Greek letter Mu?

[Fluorine]

Yes, it's a prefix for "micro". Using the Greek letter μ (mu) is the most appropriate abbreviation as far as I'm aware. I've see "mcg/L" used in some places - never "mu g/L" - though I think it's safest to stick with "μ" or "micro" whenever possible.

[Hana]

I see recently that the "mu g/L" is common in environmental chemistry. I know very well the use in the Greek letter, but not written simply as "mu".
just need to be sure in the exact translation of these measures.