Yang C, El Rassi Z
Department of Chemistry, Oklahoma State University, Stillwater 74078-3071, USA.
Electrophoresis. 1999 Sep;20(12):2337-42. doi: 10.1002/(SICI)1522-2683(19990801)20:12<2337::AID-ELPS2337>3.0.CO;2-O.
Capillary electrochromatography (CEC) was introduced to the separation of nine important urea herbicides using octadecyl-silica (ODS) capillary columns that were specially designed to allow the realization of a relatively strong electroosmotic flow (EOF) and, in turn, fast separations. The ODS stationary phase was intentionally prepared to have a low surface coverage in octadecyl ligands in order to ensure a strong EOF. This ODS stationary phase of low surface coverage exhibited the usual reversed-phase chromatographic behavior as was manifested by the linearity of plots of log kappa versus the percent organic modifier in the mobile phase. The nature of the organic modifier of the mobile phase influenced the order of elution as well as the separation efficiency of the nine urea herbicides. Mobile phases containing acetonitrile yielded higher separation efficiency (by a factor of 1.5) than methanol-containing mobile phases. This was attributed to the higher mass transfer resistances of the solute in and out of the pores in the presence of the more viscous methanol-containing mobile phases. Due to the relatively strong affinity of the urea herbicides to the ODS stationary phase, on-line preconcentration consisting of prolonged injections allowed the determination of 10(-5) M urea herbicide samples using a UV detector without sacrificing separation efficiency. This was further decreased to 10(-7) M when the prolonged injection was preceded by the injection of a plug of water. The plug of water (the more retentive mobile phase) brought about an enhanced accumulation of the dilute samples into a narrow band at the inlet of the CEC column. When this on-column sample enrichment approach was combined with an off-line sample preconcentration step, which consisted of a solid-phase extraction process, ultra dilute samples of 10(-10) M (0.1 ppb) could be detected.
毛细管电色谱（CEC）被用于分离九种重要的尿素类除草剂，使用的是专门设计的十八烷基硅胶（ODS）毛细管柱，该柱能够实现相对较强的电渗流（EOF），进而实现快速分离。故意制备的ODS固定相在十八烷基配体中的表面覆盖率较低，以确保有较强的EOF。这种低表面覆盖率的ODS固定相表现出通常的反相色谱行为，这通过log kappa与流动相中有机改性剂百分比的线性关系图得以体现。流动相有机改性剂的性质影响了九种尿素类除草剂的洗脱顺序以及分离效率。含乙腈的流动相产生的分离效率比含甲醇的流动相高（高1.5倍）。这归因于在含甲醇的更粘稠流动相存在下，溶质进出孔隙的传质阻力更高。由于尿素类除草剂对ODS固定相具有相对较强的亲和力，由长时间进样组成的在线预浓缩使得使用紫外检测器能够测定10⁻⁵ M的尿素类除草剂样品，而不牺牲分离效率。当在长时间进样之前先注入一段水时，检测限进一步降低到10⁻⁷ M。这段水（保留性更强的流动相）使稀释样品在CEC柱入口处增强积累成窄带。当这种柱上样品富集方法与离线样品预浓缩步骤（由固相萃取过程组成）相结合时，能够检测到10⁻¹⁰ M（0.1 ppb）的超稀样品。
