1.  A method for separating cisatracurium in high isomeric  purity from (lR,l’R)-atracurium isomer mixture, which  method comprises eluting the isomer mixture from a Reverse  Phase (RP) stationary phase with a mildly acidic or buffered  mobile phase, to separate the lR-cis,l’R-cis isomer (cisatra› curium) therefrom in high isomeric purity.  2.  The method of claim  1,  wherein the lR-cis,1 ’R-cis iso› mer exhibits a degradation rate in the mobile phase of less  than 2% after 26 hours at room temperature.  4.  The method of claim  3,  wherein the reverse phase sta› tionary phase is a Cl stationary phase, a C3 stationary phase,  a C4 stationary phase, a CS stationary phase, a C 14 stationary  phase, a C18 stationary phase, or a polymeric packing.  5.  The method of claim  4,  wherein the reverse phase sta› tionary phase is a C18 stationary phase.  6.  The method of claim  3,  wherein the mobile phase is an  aqueous phase, which comprises an acid and, optionally, a  salt or an amine.  7.  The method of claim  6,  wherein aqueous phase com› prises a buffer, which a mixture of an acid and a conjugate salt  thereof.  8.  The method of claim  6,  wherein the aqueous phase  comprises a salt, which is ammonium formate, sodium for› mate, ammonium acetate, sodium acetate, sodium nitrate,  sodium chloride, potassium chloride, barium chloride,  sodium bromide, calcium bromide, monopotassium dihydro› genphosphate, mono sodium dihydrogenphosphate, or a com› bination thereof.  9.  The method of claim  6,  wherein the acid is hydrochloric  acid, hydrobromic acid, phosphoric acid, boric acid, nitric  acid, or a combination thereof.  10.  The method of claim  6,  wherein the aqueous phase  comprises nitric acid and sodium nitrate.  11.  The method of claim  6,  wherein the acid is acetic acid,  citric acid, formic acid, camphoric acid, adamantaneacetic  acid or a combination thereof.  12.  The method of claim  7,  wherein the buffer is a mixture  of acetic acid and sodium acetate, a mixture of citric acid and  sodium citrate, a mixture of formic acid and ammonium  formate, or a combination thereof.  15.  The method of claim  7,  wherein the buffer concentra› tion in the aqueous phase is from 20 mM to 40 mM.  16.  The method of claim  3,  wherein the eluent comprises at  least one organic solvent, which is acetonitrile, methanol,  ethanol, isopropyl alcohol, tetrahydrofuran (THF), or a mix› ture thereof.  17.  The method of claim  3,  further comprising performing  an ion exchange step.  24.  The method of claim  23,  wherein the cisatracurium salt  is isolated as cisatracurium besylate having an isomeric  purity of at least 99 .5%.  3.  The method of claim  1,  comprising:  *  *  *  *  *  US 2010/0087650 Al  11  Apr. 8, 2010  loading a solution of a (lR,l’R)-atracurium salt isomer  mixture into a reverse phase HPLC system equipped  with a separating column and a reverse phase stationary  phase;  eluting the column with an eluent, which includes an aque› ous phase, an organic solvent or a mixture thereof, to  separate a cisatracurium salt from the isomer mixture;  collecting at least one fraction comprising the cisatracu› rium salt; and  isolating the cisatracurium salt.  13.  The method of claim  6,  wherein the aqueous phase  comprises a salt of an acid and has a pH of from 1.0 to 5.5.  14.  The method of claim  13,  wherein the aqueous phase has  a pH of from 3.0 to 3.5.  18.  The method of claim  3,  wherein the cisatracurium salt  is isolated from the HPLC elution liquid mixture by a Solid  Phase Extraction (SPE) method, which method comprises:  contacting the HPLC elution liquid with a sorbent; and  eluting the product from the sorbent with an organic sol› vent.  19.  The method of claim  18,  wherein the SPE method  further comprises:  optionally evaporating at least a portion of an organic sol› vent from the HPLC elution liquid;  adding an organic solvent to the HPLC elution liquid and  separating the phases and optionally washing the  organic layer;  optionally changing the anion using a suitable 10n  exchange method; and  isolating the product from the aqueous phase.  20.  The method of claim  19,  wherein the organic solvent  added is ethyl acetate, n-propyl acetate, isopropyl acetate,  n-butyl acetate, isobutyl acetate, toluene, chloroform, dichlo› romethane, or a mixture thereof.  21.  The SPE method of claim  20,  wherein the organic  solvent added is dichloromethane.  22.  The method of claim  3,  wherein the cisatracurium salt  is isolated as cisatracurium besylate having an isomeric  purity of at least 98%.  23.  The method of claim  22,  wherein the cisatracurium salt  is isolated as cisatracurium besylate having has an isomeric  purity of at least 99%. 