Glutamic acid and monosodium glutamate, if they are dissolved together in water, you will have glutamate ion and sodium ion in the solution. Glutamate ions either from glutamic acid  or from monosodium glutamate can't be separated, because of no difference between them. You can just analyze total glutamate of the food with and without added monosodium glutamate, separately, then calculate the difference, which means the number of monosodium glutamate added in the food.

Hi,

in addition to the prior answers the question is, if you are only interested in the free amino acid or in the total content of glutamic acid (free + peptide/protein bound).

To meassure free glutamic acid, you can use an extract after extraction with water.

For total glutamic acid a hydrolysis of peptides/proteins is necessary:

(A) using 6 molar (or higher concentration) hydrochloric acid (disadvantage: under these condition glutamine is partly or completely transformed to glutamic acid)

(B) using total enzymatic digestion with proteases (advantage: no transformation of glutamine to glutamic acid expected; disadvantage: often no entire release from peptide/protein)

Dr. Patrick, Dr. Hanifah and Dr. Jurgen all answers are well taken. I'll try to follow according to the needs