[0001]  The present invention relates to chromatography  and more particularly to an improved method of separating  the (lR,1 ’R)-atracurium salts isomers by means of high pres› sure liquid chromatography (HPLC).  [0002]  Neuromuscular blocking agents (e.g., atracurium  besylate, pancuronium bromide, rocuronium bromide, vecu› ronium bromide) are known to have muscle paralyzing activ› ity that is similar to the alkaloid curare or d-tubocurarine.  Neuromuscular blocking agents interrupt transmission of  nerve impulses at the skeletal neuromuscular junction and are  typically divided into two types: competitive, stabilizing  blockers (non-depolarizing neuromuscular agents) and non› competitive, depolarizing agents ( depolarizing neuromuscu› lar agents). Both types prevent acetylcholine from triggering  the muscle contraction and are typically used as anesthesia  adjuvants in the operating theatre for aiding intubation i.e.  relaxation of vocal cords, trachea, jaw muscles etc and also  for surgery i.e. providing generalized muscle relaxation, as  relaxants during electroshock, in convulsive states, etc. Typi› cally, therapy is performed by i.v. administration of a suitable  dosage form.  [0005]  Because the atracurium compound has 4 chiral cen› ters, there are theoretically 16 possible isomers. Due to sym› metry of the molecule, the number ofisomers is reduced to 10  (optical and geometrical isomers). This is discussed in detail  by J. B. Stenlake et al. in "Biodegradable neuromuscular  blocking agents,"  Eur. J. Med. Chem.-Chem. Ther.,  19, No. 5,  pages 441-450 (1984 ).  US 2010/0087650 Al  1  Apr. 8, 2010  (lR,l’R)-ATRACURIUM SALTS SEPARATION  PROCESS  TECHNICAL FIELD  BACKGROUND OF THE INVENTION  [0006]  The preparation of cisatracurium besylate is  described in U.S. Pat. Nos. 5,453,510, and 5,556,987 and is  depicted in Scheme 1 below. The process involves reacting  1,5-pentanediol with 3-bromopropionic acid in toluene with a  trace of p-toluenesulfonic acid to afford 1,5-pentamethylene  diacrylate.  (–)-terahydropapaverine  hydrochloride  1s  resolved with N-acetyl-L-leucine to obtain (R)-terahydropa› paverine N-acetyl-L-leucinate, which is converted to the free  base and reacted with the 1,5-pentamethylene diacrylate in  hot glacial acetic acid, purified by colunm chromatography,  and treated with oxalic acid to afford (lR,l’R)-2,2’-(3,11- dioxo-4, 10-dioxatridecylene )-bis-(1,2,3,4-tetrahydro-6, 7- dimethoxy-2-methy 1-1-veratryliso-quinolinium  dioxalate  (II), which is treated with methyl benzenesulfonate to obtain  the (lR,l’R)-atracurium besylate isomer mixture, i.e., (lR› cis-l’R-cis), (lR-cis-1 ’R-trans) and (lR-trans-1 ’R-trans) iso› mers in a ratio of 58:34:6 respectively. The mixture is sepa› rated by means of liquid chromatography, using either  irregular or spherical silica colunm in a mixture of dichlo› romethane and a strong acid, e.g., methanesulfonic acid, or in  a mixture of dichloromethane, methanol and a strong acid,  e.g., benzenesulfonic acid.  [0003]  Atracurium besylate [2,2’-[1,5-pentanediylbis[ oxy  (3-oxo-3, 1-propanediyl) ]]bis[ 1-[ (3,4-dimethoxyphenyl)me› thyl]-1,2,3,4-tetrahydro-6, 7-dimethoxy-2-methyl-isoquino› linium dibenzenesulfonate] was first approved for human  medical use in 1982. The isomer lR-cis-l’R-cis-2,2’2,2’-[1,  5-pentanediylbis[ oxy(3-oxo-3, 1-propanediyl) ]]bis[ 1-[ (3,4- dimethoxyphenyl)methyl]-1,2,3,4-tetrahydro-6, 7- dimethoxy-2-methy 1-isoquinolinium dibenzenesulfonate is  named cisatracurium besylate and it is 3-4 times more potent  than atracurium besylate itself. The product was launched by  GSK and Abbott Laboratories in 1996 under the trade name  NIMBEXfi. NIMBEXfi is a sterile, non-pyrogenic aqueous  solution that is adjusted to pH 3.25 to 3.65 with benzene› sulfonic acid. The drug is provided in 2.5 ml, 5 ml and 10 ml  ampules having a strength of2 mg/ml cisatracurium besylate.  In addition, a 30 ml vial containing 5 mg/ml cisatracurium  besylate is also available.  [0004]  Cisatracurium besylate has the structural formula  below.  cisatracurium besylate  (I)  OMe  I  N/  0  0  ’-...N  MeO  #  ~  + •,,,,,~  ~ ~  +  OMe  (YPhS03- O  O  MeO~  OMe  OMe  OMe 