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A graph depicting the stability of the lR-cis, lR’-trans isomer at different pH values is provided in FIG. 7, which demon› strates that at pH 3, after 20 hours the% area of the lR-cis, 1 ’R-cis isomer is only slightly reduced while at pH 5.5 the% area of the 1 R-cis, 1 ’R-cis isomer is significantly reduced. 

[0076] The degradation (D) according to the data presented in Table 12 was calculated as follows: 

wherein, Xa=% of cisatracurium at Ta, and X$_{21 }$=% of cisa› tracurium at after 24 hours. 

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*The buffer used was the Na•CH 3 COO-/CH 3 COOH buffer at 3 different pH values, that is pH values of3.0, 4.6 and 5.5. The values in the table are repre› sented as % of cisatracurium besylate. 

A sample solution of (lR,l’R)-atracurium besylate (10 mg/ml) was prepared using two buffer solutions at pH values of 1.0 and 2.0 and analyzed on the C18 stationary phase by gradient elution [20 mM KN0$_{3 }$ buffer (at pH corresponding to sample preparation)-methanol]. The stability of the sample solution at the mentioned pH values was demon› strated at room temperature and at 4 ° C., as depicted in Table 13. 

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RT= room temperature, D = degradation 

wherein, Xa=% of cisatracurium at Ta, and X$_{6126}$ =% of cisa› tracurium at after 6 or 26 hours. 

[0077] This example demonstrates a method for purifica› tion of the cisatracurium solution from the buffer’s mixture by Solid Phase Extraction (SPE). 

[0078] A series of the sample solutions of (lR,1 ’R)-atracu› rium besylate isomer mixture (55% cis-cis; 35% cis-trans and 6% trans-trans isomer) was prepared in diluents containing different buffers (varying by the nature of the cation and the anion). The diluents consisted ofa mixture of90% buffer and 10% methanol. The sample solutions were purified using SPE C18 cartridge. 

[0079] The evaluation of the buffer anions was carried out by HPLC. The cations were evaluated indirectly. The recov› ery of the isolate (IR cis,1 ’R-cis isomer) and anions was checked after each step of the SPE method, which comprises the steps of: 

[0080] 1) successive transferring of the sample solution and water through the sorbent; 

[0081] 2) elution of the sample with methanol; and 

[0082] 3) washing the sorbent with methanol. The results of this study are summarized in the Table 14. 

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## Example 7 

