El sistema internacional de unidades estable que mili se representa por la m minuscula por lo que es correcto mg/L que es equivalente mg *L-1 pero es incorrecto mg/L-1
First of all, it is 'm'; 'M' stands for mega. Now, L-1 =1/L, so 1/L-1=1/1/L=L. So, if you write mg/L-1, it will become mgL means mg x Litre instead of mg/L. So the correct form is mg/L or mgL-1. Mg/L or mg/L-1 are incorrect.
To express concentration of materials (be it element or compound) present/dissolved in a medium (water, soil or solution) we have to state weight per volume. It must be mg/L or mgL-1 (-1 in superscript) or ppm, but the unit can't be written in the form of mg/L-1, which is incorrect and meaningless!. mg/l may be written in alternative forms of microgram per millilitre, .......
The notations mg/L or mg*L-1 or simply mg L-1 are the same. However, the notation "mg/L-1" is wrong (at least it does not mean concentration of something). Mathematically, it is equal to "mg*L".
To express the concentration of materials (be it element or compound) present/dissolved in a medium (water, soil, or solution) we have to state weight per volume.
You can write mg/L or mg* L-1, both are the same (is a mistake to write mg/L-1).
So, if you write mg/L-1, it will become mg L means mg x Litre instead of mg/L. So the correct form is mg/L or mgL-1. Mg/L or mg/L-1 is incorrect.