Could you list all of the metric prefixes? I consulted several online lists, and I could not find a prefix of "u." (my chart says ug/L) Thanks for clarifying this.

Answer 1

The "mu" stands for micro so your symbol is micrograms/L. Here is a complete list. Good site. Bookmark it.
http://physics.nist.gov/cuu/Units/

Answer 2

u should really be the greek letter mu, meaning micro, or 10^-6

If it appears here, it should be written as μ