Poly(ethylene oxide) helical conformation and alkali metal cation selectivity studied using electrospray ionization mass spectrometry.

RATIONALE: The poly(ethylene oxide) (PEO)-alkali metal cation interaction is widely used in many areas. The conformation of the PEO-alkali metal cation complex has been studied extensively, but the conformational mechanism is still unclear. Simulations have been used to explain the mechanism, but there is a lack of experimental data from long PEO chains to verify the simulation results. METHODS: The relative peak abundance of PEO (iso-C10 H21 (OC2 H4 )n OH (naverage = 7, where n denotes the number of ethylene oxide (EO) units) oligomers complexed to five alkali metal cations (Li+ , Na+ , K+ , Rb+ and Cs+ ) was studied using positive electrospray ionization mass spectrometry (ESI-MS). The ion selectivity of PEO oligomers to alkali metal cations corresponded to the peak abundance in competitive ESI-MS. RESULTS: PEO formed its first helix when the number of EO units reached six and the helix played an important role in the ion selectivity of PEO. For larger PEO oligomers with a helix, the ion selectivity of PEO depended on the degree of host-guest matching of the cations and the helix. The highest selectivity of PEO to K+ was due to K+ providing the best shape matching with the helical cavity. For smaller PEO oligomers without a helix, the selectivity was mainly determined by the surface charge density of the cations. CONCLUSIONS: The formational mechanism of the PEO-alkali metal cation complex was predicted. The results gave straightforward evidence to explain the conformational mechanism of the PEO-alkali metal cation complex and provided experimental data for further simulation studies.

Characterization of polyethermethylsiloxanes using ultra-high performance liquid chromatography-electrospray ionization and time-of-flight mass spectrometry.

Polyethermethylsiloxanes (PEMSs) are silicone based polymers formed by attaching one or more ethylene oxide (EO)/propylene oxide (PO) chains to a siloxane chain. As the siloxane chain is lengthened, the polarity of the PEMSs are reduced. Little research has been conducted on the use of ultra-high performance liquid chromatography (UHPLC)-mass spectrometry (MS) to analyze PEMS oligomers with more than 4 siloxane groups because of their low polarity and poor solubility in water and acetonitrile (ACN). In this study, we developed a high chromatographic resolution method for PEMS and polyether oligomers using the water-ACN-isopropanol (IPA) tertiary solvents gradient. PEMSs oligomers containing one EO pendant chain and 3-13 silicone groups were analyzed. More than 102 PEMS oligomers and 21 polyether oligomers were separated within 45 min and identified by accurate molecular masses. During this gradient elution process, different chromatographic modes were used: both precipitation-redissolution and adsorption mode for PEMSs, adsorption mode for EO chains in polyethers, and exclusion mode for EO chains in PEMSs. This efficient separation method for PEMSs would broaden the characterization of silicone surfactants. Also, it is beneficial to establish the relationship between surfactant structure, synthetic route and performance optimization.

Comparison of microemulsion electrokinetic chromatography with high-performance liquid chromatography for fingerprint analysis of resina draconis.

Microemulsion electrokinetic chromatography (MEEKC) has been developed for fingerprint analysis of resina draconis, a substitute for sanguis draconis in the Chinese market. The microemulsion as the running buffer was made up of 3.3% (w/v) sodium dodecyl sulfate (SDS), 6.6% (w/v) n-butanol, 0.8% (w/v) n-octane, and 10 mmol/L sodium tetraborate buffer (pH 9.2), which was also used as the solvent for ultrasonic extraction of both water- and fat-soluble compounds in the traditional Chinese medicine samples. Four batches of resina draconis obtained from different pharmaceutical factories located in different geographic regions were used to establish the electrophoretic fingerprint. MEEKC was performed using a Beckman PACE/MDQ system equipped with a diode-array detector and with monitoring at 280 nm. The fingerprint of resina draconis comprised 27 common peaks within 100 min. The relative standard deviations of the relative migration time of these common peaks were less than 2.1%. Through repetitive injection of the sample solution six times in 24 h, all relative standard deviations of the migration time and peak area of loureirin A and loureirin B were less than 2.5 and 3.8%, which demonstrated that the method had good stability and reproducibility. The relative peak areas of these common peaks in the electropherograms of four batches of resina draconis were processed with two mathematical methods, the correlation coefficient and the interangle cosine, to valuate the similarity. The values of the similarity degree of all samples were more than 0.91, which showed resina draconis samples from different origins were consistent. On the other hand, high-performance liquid chromatography (HPLC) coupled with photodiode-array detection was also applied to establish the fingerprint of resina draconis. The samples were separated with a LiChrospher C(18) column using acetonitrile (solvent A) and water containing 0.1% H(3)PO(4) (solvent B) as the mobile phase in linear gradient elution mode at a flow rate of 0.6 mL/min and detection was at 280 nm. There were only 20 common peaks in the HPLC fingerprint, and the values of the similarity degree of all samples were also more than 0.91. Though the similarity results of fingerprint analysis seemed to be the same, MEEKC resulted in more common peaks and higher separation efficiency for a variety of polarities of the components than HPLC. So, MEEKC was more suitable for development of the fingerprint of resina draconis.

Rapid determination of water- and fat-soluble vitamins with microemulsion electrokinetic chromatography.

A rapid, reliable and reproducible method based on microemulsion electrokinetic chromatography (MEEKC) for simultaneous determination of 13 kinds of water- and fat-soluble vitamins has been developed in this work. A novel microemulsion system consisting of 1.2% (w/w) sodium lauryl sulphate (SDS), 21% (v/v) 1-butanol, 18% (v/v) acetonitrile, 0.8% (w/w) n-hexane, 20mM borax buffer (pH 8.7) was applied to improve selectivity and efficiency, as well as shorten analysis time. The composition of microemulsion used as the MEEKC running buffer was investigated thoroughly to obtain stable separation medium, as well as the optimum determination conditions. Acetonitrile as the organic solvent modifier, pH of the running buffer and 1-butanol as the co-surfactant played the most important roles for the separation of the fat-soluble vitamins, water-soluble vitamins and stabilization of system, respectively. The 13 water- and fat-soluble vitamins were baseline separated within 30 min. The system was applied to determine water- and fat-soluble vitamins in commercial multivitamin pharmaceutical formulation, good accuracy and precision were obtained with recoveries between 97% and 105%, relative standard derivations (RSDs) less than 1.8% except vitamin C, and acceptable quantitative results corresponding to label claim.