I must say that HPLC-MS method is not a quantitative method at all. Further, notorious ELISA method, which is also not a quantitative method at all, should not be utilized.
HPLC-fluorometric method (at an excitation wavelength of 365nm and an emission wavelength of 412 nm) may give the quantitative results. Therefore, please derivatize ADMA by ADAM (9-Anthryldiazomethane) reagent, which tags ADMA's carboxyl residue, and please use the HPLC-fluorimetric method.
Alternatively, PITC (p-phenylene diisothiocyanate; tags to amine residue) may be used as a good derivatization reagent for amino acid (please see file; PTH Roma RP-HPLC). In this case, HPLC-photometric detection at 269 nm is used.
An example of biotin determination is attached (please see file; Netherlands Biotin).