wherein, A=% area of the lR-cis, 1 ’R-cis isomer, 8=% area of  the lR-cis,1 ’R-trans isomer and C=% area of the IR-trans,  1 ’R-trans isomer.  [0030]  Thus, the present invention provides a chromato› graphic method for separating the isomers of (IR, 1 ’R)-atra› curium salt (e.g., the besylate salt), which uses a Reverse  Phase HPLC (RP HPLC) method for obtaining a highly pure  product, that is lR-cis,l’R-cis isomer salt (e.g., the besylate  salt), having isomeric purity higher than 99.5%.  [0031]  According to one embodiment of the present inven› tion, separating the desired 1 R-cis, 1 R’-cis isomer from a mix› ture of isomers by a method, which includes:  [0032]  loading a solution of (IR, 1 ’R)-atracurium salt ( e.g.,  the besylate salt) into a Reverse Phase HPLC system,  equipped with a suitable separating colunm comprising a  stationary phase;  [0033]  eluting the colunm with an eluent mobile phase that  includes an aqueous phase, an organic solvent or a mixture  thereof;  [0034]  collecting at least one fraction comprising the  desired product; and  [0035]  isolating the product.  [0036]  The lR-cis,1 ’R-cis isomer obtained in accordance  with the present invention preferably is substantially free of  other geometrical and optical isomers. As is evident from the  experimental section of the present application, the obtained  lR-cis,l’R-cis isomer salt (e.g., the besylate salt) contains  less than about 1 % of the lR-trans,1 ’R trans and/or lR-cis,  1 ’R-trans isomer, preferably less about than 0.1 % of the  lR-trans,l’R-trans and/or lR-cis,l’R-trans isomer. Thus, the  highly pure 1 R-cis, 1 ’R-cis isomer is obtained having isomer  purity of at least about 98.5%, and preferably having an  isomer purity higher than 99.5%.  [0037]  In accordance with the present invention, a suitable  RP stationary phase column can include Cl stationary phase,  C3 stationary phase, C4 stationary phase, CS stationary  phase, C 14 stationary phase, C 18 stationary phase, other  polymeric packing, e.g., polyamide, polymethacrylate, poly› styrene, and the like.  [0038]  The RP HPLC separation method of the present  invention can be utilized on analytical, semi-preparative and  preparative scales. Preferred organic bonded reverse phases  for obtaining the lR-cis-l’R-cis isomer include Cl, C4 and  Cl 8 phases. The results of exemplary RP methods for isolat- [0040]  In a preferred embodiment, the aqueous phase fur› ther includes a buffer, e.g., a mixture of a weak acid and its  conjugate salt (e.g., acetic acid and sodium acetate or citric  acid and sodium citrate). Suitable buffers include, for  example, mixtures of acetic acid and sodium acetate, citric  acid and sodium citrate, formic acid and ammonium formate,  and the like. The acid can be either an organic or inorganic  acid. Preferred organic acids include, for example, acetic  acid, citric acid, formic acid, camphoric acid, adamanta› neacetic acid and the like, and combinations thereof. Pre› ferred inorganic acids include, for example, hydrochloric  acid, hydrobromic acid, phosphoric acid, boric acid, nitric  acid, and the like, and combinations thereof.  [0041]  Suitable organic solvents, which may be included in  the aqueous mobile phase in accordance with the present  invention, include, for example, acetonitrile, methanol, etha› nol, isopropyl alcohol, tetrahydrofuran (THF), and the like,  and mixtures thereof.  [0042]  In one embodiment, the (lR,l’R)-atracurium besy› late isomers are separated by eluting with an aqueous phase  that contains a salt of an acid, or an acid and a conjugate salt  of an acid ( e.g., nitric acid and sodium nitrate), at a pH of from  about 1.0 to about 5.5, and more preferably at a pH of from  about 3.0 to about 3.5. Suitable acid salts can include, for  example, ammonium formate, sodium formate, ammonium  acetate, sodium acetate, sodium nitrate, sodium chloride,  potassium chloride, barium chloride, sodium bromide, cal› cium bromide, monopotassium dihydrogenphosphate, mono› sodium dihydrogenphosphate, and the like, and combinations  thereof.  [0043]  Preferred buffer concentrations, for isolating the  1 R-cis, 1 ’R-cis isomer, range from about 20 mM to about 40  mM. In some instances, a buffer concentration in the higher  range increases the retention time of the lR-cis,l’R-cis iso› mer, as depicted in FIG.  1,  and also improves the isomer’s  separation, as depicted in FIG.  2.  [0044]  The cisatracurium salt is substantially stable under  the separation conditions of the present invention. For  instance, the extent of cisatracurium degradation is only  slightly (less than 2%) after 26 hours at room temperature in  a solvent mixture, e.g., at pH 2, using nitrate buffer, and  US 2010/0087650 Al  4  Apr. 8, 2010  [0039]  According to a preferred embodiment of the present  invention, the eluent includes an aqueous phase that contains  at least one organic solvent. The aqueous phase preferably  comprises an aqueous mixture of an acid and optionally also  an inorganic salt ( e.g. NaCl) or an amine ( e.g., triethylamine ).  described above, using strong acids (having pKa value of 0  and lower) such as benzenesulfonic acid.  [0027]  The term "buffered" mobile phase, as described  herein, refers to a mobile phases containing an aqueous  phase, which includes a weak organic acid and its conjugate  salt, such as formic acid and ammonium formate  [0028]  As used herein the term "substantially free of other  geometrical and optical isomers" means that no other geo› metrical and optical can be detected within the limits of the  HPLC method.  [0029]  The term "isomeric purity" as defined herein, refers  to the area percent of the peak corresponding to the lR-cis,  1 ’R-cis isomer relative to the area percent of the (lR-cis,1 ’R› cis isomer), (lR-cis,1 ’R-trans) and (lR-trans,1 ’R-trans) iso› mers. Isomeric purity may be expressed in terms of the  following equation:  ing the lR-cis-l’R-cis isomer from the (lR,l’R)-atracurium  besylate isomer mixture on different stationary phases are  provided in Table 1 below.  A  Isomeric purity=  A  +  B  +  C x 100  Stationary phase  C18  C4  Cl  CN  Phenyl  Result  Complete separation of all the isomers  Complete separation of all the isomers  Complete separation of all the isomers  Poor separation of the lR-cis-1 ’R-cis isomer  from the lR-cis,1 ’R-trans isomer, good  separation ofthe lR-trans,l’R-trans isomer  No separation of the lR-cis,1 ’R-cis isomer  from the lR-cis,1 ’R-trans isomer, good  separation ofthe lR-trans,l’R-trans isomer  TABLE 1 