almost no degradation (0.2%) was observed after 26 hours in  a solvent mixture at 4 °  C. using a nitrate buffer ( see Example  6, Table 12).  [0045]  A desired counter anion may be introduced, e.g., by  an ion exchange process, which can include, e.g., adding  desired counter anions to the system to exchange with other  anions utilized in the separation process, which may associate  with one or more of the (IR, 1 ’R)-atracurium isomers. Any  suitable ion exchange methods may be utilized as well, e.g., a  suitable ion exchange resin.  [0046]  The isolation of the cisatracurium from the elution  liquid mixture can be carried out by any suitable method such  as Solid Phase Extraction (SPE). A non-limiting example of a  method for isolating the cisatracurium solution from the elu› tion liquid is by SPE. For example, the isolation can be  effected by successively transferring a cisatracurium besylate  sample solution ( containing the aqueous phase and the  organic solvent) and water through a C18 sorbent, which  causes the separation of the cisatracurium besylate from the  buffer species. The cisatracurium besylate retained by the  sorbent can be then removed, e.g., by eluting from the sorbent  with methanol. The evaluation of the anions is preferably  carried out by HPLC, and the cations are preferably evaluated  indirectly (since the cations and the anions are removed at  about the same rate). Recovery of the isolated material and the  anions can be checked after performing each step of the SPE  method.  [0047]  Thus, the present invention provides a SPE method  for isolating a product (e.g., an isomer of (lR,l’R)-actracu› rium besylate) from an HPLC elution liquid mixture, which  method preferably includes:  [0048]  successively transferring the sample solution and  water through a sorbent;  washing the sorbent with an organic solvent, e.g.,  [0049]  eluting the product from the sorbent with an organic  solvent, e.g., methanol; and  [0050]  methanol.  [0051]  The SPE method of the present invention can be  utilized for isolating the 1 R-cis, 1 ’R-cis isomer from a HPLC  elution liquid mixture thereof. In one embodiment, the  method includes:  [0052]  optionally evaporating at least a portion of the  organic solvent from an aqueous HPLC elution liquid con› taining an organic solvent;  [0053]  adding an organic solvent and separating the phases  and optionally washing the organic layer;  [0054]  optionally changing the anion using a suitable ion  exchange method; and  [0055]  isolating the product from the aqueous phase by  spray-drying or freeze-drying.  [0056]  The organic solvent added in the SPE method of the  present invention can include, for example, ethyl acetate,  n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl  acetate, toluene, chloroform, dichloromethane, and the like,  and mixtures thereof. A preferred solvent for the SPE method  of the present invention is dichloromethane.  [0057]  The present invention further provides a pharma› ceutical composition comprising substantially pure cisatra› curium besylate, which can be obtained in accordance with  the present invention, and can be employed, e.g., in surgery as  a neuromuscular blocking agent as a formulation for admin› istration, e.g., by injection or infusion.  [0058]  FIG.  3  depicts the chromatogram of a reference  sample obtained by diluting a lyophilized atracurium besylate  sample, containing, inter alia, approximately 54% lR-cis,  1 ’R-cis, 34% lR-cis,1 ’R-trans and 6% IR-trans,! ’R-trans iso› mers (hereinafter the "reference sample"), and a small  amount of benzenesulfonic acid in the corresponding aque› ous phase, and injecting the mixture into the HPLC system,  equipped with a C18 stationary phase according to a gradient  method in which the eluent contains a mixture of an aqueous  phase and at least one solvent. FIG.  4  depicts the chromato› gram ofan exemplary starting sample of (IR, 1 ’R)-atracurium  besylate isomer mixture (hereinafter "sample 1 "), and FIG.  5  depicts the chromatogram of a cisatracurium besylate (NIM› BEXfi) buffer solution (hereinafter "sample 2").  [0059]  The following examples further illustrate the inven› tion but, of course, should not be construed as in any way  limiting its scope.  [0060]  Cisatracurium besylate was analyzed by High Per› formance Liquid Chromatography (HPLC). Exemplary  HPLC separations were performed, e.g., using the following  conditions: Colunm and packing-Hypersil Hyperprep HS  C18, 250x21.2 mm, 15µ, P.N. 37115-125; UV detection› UV operated at 280 nm; flow rate: 13 ml/min; Mobile phase:  Eluent A: 20 mM NaNO$_{3}$, pH adjusted to 2.0 with HNO$_{3}$ .  Eluent B: methanol. The gradient elution is as detailed in  Table 2:  Another example ofHPLC separation using the same colunm  as in the previous example is: UV detection at 280 nm; flow  rate: 13 ml/min; Mobile phase: Eluent A: 40 mM buffer  solution containing diethylamine (DEA), pH adjusted to 3.5  with formic acid Eluent B: methanol. The gradient elution is  as detailed in Table 3:  The chromatogram corresponding to the separation of (IR,  1 ’R)-atracurium besylate, according to the method using the  gradient detailed in Table 3, is depicted in FIG.  6.  [0061]  This example demonstrates a Reverse Phase proce› dure for isolating cisatracurium besylate on C18 stationary  phase using different aqueous phases.  [0062]  Atracurium besylate reference sample (10 mg/ml)  was analyzed on a C18 stationary phase by gradient elution,  using different aqueous phases in the solvent mixtures with  methanol. Colunm and packing: Alltech, Altima C18, 250x  4.6x5µ,  Cat. No. 88056; UV detection: 280 nm; flow rate: 1  ml/min; The results, including the gradient elutions, are  detailed in Table 4:  EXAMPLES  Example 1  US 2010/0087650 Al  5  Apr. 8, 2010  TABLE3  TABLE2  Time, minutes  0  65  67  90  o/oeluentA  60  60  55  55  % eluent B  40  40  45  45  Time, minutes  0  70  72  90  o/oeluentA  65  65  55  55  % eluent B  35  35  45  45 