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Separation and first structure elucidation of Cremophor® EL-components by hyphenated capillary electrophoresis and delayed extraction-matrix assisted laser desorption/ionization-time of flight-mass spectrometry
The polyethoxylated heterogeneous components of the so far poorly characterized nonionic emulsifier Cremophor® EL (polyoxyl 35 castor oil) (CrEL) were fractionated by cyclodextrin‐modified micellar electrokinetic capillary chromatography (CD‐MEKC). Due to the low UV absorbance of most of the CrEL‐components an indirect UV detection was used with phenobarbital‐sodium as background absorber. For a precise assignment of the resulting peaks to the corresponding components capillary electrophoresis (CE) had to be combined with delayed extraction‐matrix assisted laser desorption/ionization‐time of flight‐mass spectrometry (DE‐MALDI‐TOF‐MS) as detection system. For this purpose, the fractionating robot Probot was employed which enables both the on‐line fractionation of the CE eluate on a MALDI target during the electrophoretic separation and the simultaneous dosage of the MALDI matrix solution. The applied CrEL amount was optimized by varying the CE injection parameters time, pressure and concentration of the sample in order to obtain homologue peak series of sufficient intensity without decreasing the separation efficiency. Evaluation of the mass spectra was performed by comparing the residue masses of the homologue peak series with the calculated residue masses of potential CrEL‐components. However, the high number of polyethoxylated components leads to overlapping of homologue peak series with isobaric residue masses. These isobaric interferences were detected by a high mass accuracy of the measurements (obtained by internal calibration with polyethylene glycol (PEG) 1000 and by means of the residue mass plot, the newly developed evaluation method. The combination of these techniques allowed the first detailed structure analysis of the CrEL‐components showing glycerol polyoxyethylene (POE) monoricinoleate and POE monoricinoleate to be the two main components of the emulsifier. Furthermore, the coupling of CE with DE‐MALDI‐TOF‐MS is generally applicable to the fractionation and identification of polymers.*