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. 

3. The method of claim 1, comprising: 

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. 

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. 

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. 

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. 

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%. 

24. The method of claim 23, wherein the cisatracurium salt is isolated as cisatracurium besylate having an isomeric purity of at least 99 .5%. 

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