What does the $-1$ mean in $2\mu g/L^{-1}$

Question

What does the $-1$ mean in the expression $2\mu g/L^{-1}$? Does it literally mean multiply by $-1$?

Answer 1

$2\mu g/L$ or $2\mu g L^{-1}$ means $2$ micrograms per litre. I don't think anyone would write $2\mu g/L^{-1}$, though that (in the weird situation) would be equivalent to $2\mu gL$.

The $-1$ refers to the exponent of $L$ in the unit. For any nonzero real number $a$, $a^{-1}$ can be taken to mean $\frac1a$.