The present invention relates to chromatography and more  particularly to an improved method of separating the (IR,  1 ’R)-atracurium salt isomers by means ofhigh pressure liquid  chromatography (HPLC).  Cisatracurium besylate possesses an activity that is superior  to atracurium besylate, with significantly less side effects.  Cisatracurium besylate is marketed in the United States  and Europe by Glaxo Wellcome and Abbott Laboratories  under the trade name Nimbexfi, which is a sterile, non› pyrogenic aqueous solution that is adjusted to pH 3.25 to 3.65  with benzenesulfonic acid. The drug is provided in 2.5 ml, 5  ml and 10 ml ampules having a strength of 2 mg/ml cisatra› curium besylate. In addition, a 30 ml vial containing 5 mg/ml  cisatracurium besylate is also available.  Cisatracurium besy late has the chemical name ( 1 R, 1 ’R,2R,  2’R)-2,2’-[  1,5-pentanediylbis[ oxy(3-oxo-3, 1-propanediyl) ]]  bis[l-[ (3,4-dimethoxyphenyl)methyl]-1,2,3,4-tetrahydro-6,  7-dimethoxy-2-methyl-isoquinolinium dibenzenesulfonate  and is represented by the structural formula (I) below:  Cisatracurium besylate slowly loses potency with time at a  rate ofapproximately 5% peryearunder refrigeration (5°  C.).  Nimbex should be refrigerated at 2°  to 8°  C. (36°  to 46°  F.) in  the carton to preserve potency. The rate of loss in potency  increases to approximately 5% per month at 25°  C. (77°  F.).  Atracurium besylate is disclosed in U.S. Pat. No. 4,179,  507 (hereinafter U.S. ’507), which describes a series of bis  (lR, l’R)-ATRACURIUM SALTS SEPARATION  PROCESS  TECHNICAL FIELD  BACKGROUND OF THE INVENTION  1  US 8,461,338 B2  2  OMe  (II)  (III)  1.  benzene, reflux  2. oxalic acid  Cisatracurium besylate is the dibenzenesulfonate salt of  lR-cis,1 ’R-cis isomer of atracurium (i.e., two molecules of  benzenesulfonate per one diammonium molecule of cisatra› curium). Cisatracurium besylate is one of the 10 possible  isomers of atracurium besylate and constitutes approximately  15% of that mixture ( atracurium has four chiral centers, two  on each half of the molecule, one at each of the nitrogen atoms  and the other at position 1 in each of the tetrahydroisoquino› linium rings so theoretically there are expected 16 possible  isomers; however, due to the symmetry of the molecule, the  number of atracurium isomers is reduced to 10).  Cisatracurium besylate is a nondepolarizing neuromuscu› lar blocking agent indicated for inpatients and outpatients as  an adjunct to general anesthesia, to facilitate tracheal intuba› tion, and to provide skeletal muscle relaxation during surgery  or mechanical ventilation in the Intensive Care Unit (ICU).  veratryl isoquinolinium quaternary ammonium salts, includ› ing atracurium besylate. The synthesis of atracurium besy› late, as taught in U.S. ’507, involves the coupling of(–)› tetrahydropapaverine base,  compound (II),  with  1,5- pentamethylene diacrylate, compound Treatment of the  resulting tertiary amine base with oxalic acid results in the  isolation of N,N’-4, 1 0-dioxa-3, 11-dioxotridecylene-l, 13- bis-tetrahydropapaverine dioxalate, compound (IV). The  dioxalate salt ( compound (IV) is converted to the free base,  compound (V), with sodium bicarbonate solution and  extracted into toluene. After evaporation of the toluene, the  residue is dissolved in acetonitrile and treated with methyl  benzenesulfonate. The addition of diethyl ether results in the  precipitation of atracurium besylate, compound (VI), which  is subsequently filtered and dried. Scheme 1 below illustrates  the chemical pathway described above.  (I)  OMe  OMe  OMe  Cisatracurium besylate 