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RESEARCH PROFILES Perfect particles from the monomers up In an effort to analyze every component in complex mixtures, such as those typically found in cell lysates or whole tissue, many researchers are hanging their hopes on capillary-based separation techniques. Capillary electrochromatography (CEC) offers the potential of separating large numbers of compounds into very narrow bands. However, realizing CEC’s full theoretical separation power has been elusive because of problems in combining it with detection techniques such as optical absorption or MS. Nanoparticles running in a CEC separation. In the September 15 issue of Analytical Chemmass spectrometer and are collected at istry (pp 4595–4601), Staffan Nilsson and colleagues at Lund University (Swe- a plate inside the interface. Less-massive den) describe how they successfully cou- analyte ions are sufficiently susceptible to attractive forces of the mass spectromepled CEC with MS by introducing taiter entrance lens to be deflected from lor-made nanoparticles into the mobile the effluent stream and into the mass phase. These particles act as a selective, analyzer. “So far, we have not seen any tunable stationary phase for CEC and particles inside the mass spectrometer enable a seamless connection with an using the ‘continuous full filling’ strateelectrospray ionization (ESI) interface gy,” says Nilsson. and a mass spectrometer. Nilsson’s group makes their spherical The nanoparticles serve as a freely particles with an average diameter of flowing “pseudostationary phase”, says 160 nm, using a controlled precipitaNilsson. During analysis, they move tion polymerization of selected monothrough and out of the capillary, driven mers. Once cleaned and dried, these by electroosmotic forces. The particle nanoparticles can be suspended in CEC bed is therefore replaced after each inelectrolyte solution without the use jection. “This is ideal for dirty samof surfactants. The nanoparticles are ples—you can’t contaminate the colsynthesized to contain acidic groups, umn, because you use a fresh column which leaves them with a surface ionic every time,” explains Nilsson. Morecharge. over, the free flow eliminates the need The researchers use a pressure differfor sintered silica frits, which can cause ential to “pump” the suspended nanoparband broadening. ticles into the capillary and then to inject The nanoparticles carry enough mothe sample; an applied voltage drives the mentum to continue in a straight path separation. In the article, the researchers once they leave the capillary. When an demonstrated that analytes, such as the orthogonal ESI interface is used, the antihistamine diphenhydramine, the antiparticles stream past the entrance to the
depressant nortriptyline, and the bronchodilator salbutamol, are separated by ionic interactions with the moving nanoparticle bed. By adjusting the conditions of the polymerization, different levels of interaction can be designed into the particles. The polymerization flexibility makes for a very “tunable” interaction phase, says Nilsson. By comparison, separations that use micelles as the pseudostationary phase must be adjusted after each change in organic content to ensure that the conditions are still above the critical micelle concentration, says Nilsson. Otherwise, the micelles will not form. “Micelles are not really solid objects, and you can’t trust them,” Nilsson says. However, with the polymeric nanoparticles, a wider range of mobilephase adjustments can be made to optimize separations. As long as they remain suspended, “the nanoparticles will behave themselves,” says Nilsson. In addition, the micelles must be separated from analytes before detection, because the surfactants from which micelles are made cause large reductions in the ESI ionization efficiency and contaminate the mass spectrometer. Nilsson’s group has refined the polymerization process to the point where they can now control the degree of hydrophilic or hydrophobic character that the particles exhibit, in addition to their ionic character. Moreover, by using partial filling strategies, several different types of particles can be loaded onto a capillary at once. One section can be used as a precolumn to perform complex separations of dirty samples directly on the capillary, says Nilsson. a —Zelda Ziegler
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