Microparticulate high efficiency

Analytical hplc these days is nearly always done with microparticulate column packings, which are small porous particles, usually spherical or irregular silica, with nominal diameters of 3,5 or 10 fxm. They combine the best features of the other two types, having high efficiency as well as a large surface area. In bulk, the appearance of a microparticulate silica resembles that of a fine talcum powder. With microparticulates, dry packing methods result in column beds that are unstable under pressure, so they are packed into columns using a slurry of the material in a suitable solvent and under considerable pressure. 

McCalley, D.V. (2002). Comparison of conventional microparticulate and a monolithic reversed-phase column for high-efficiency fast liquid chromatography of basic compounds. J. Chromatogr. A 965, 51-64. 

Currently, almost all the available SEC columns are packed with the high efficiency, microparticulate packings (<10u). Recent state-of-the-art developments on column packings have been described by Majors (10). A listing of such type of packing materials is shown in Table 1. 

One can see three routes to an increase in the value of resolution First, one can increase the plate number N which results in an increase in resolution proportional to the square root of N The use of microparticulate packings was a major breakthrough in this respect If we assume, however, that the chromatographer already has at his disposal a high efficiency column, then he must go to great... 

Fresh solutions of the standards are prepared in the solvent used as chromatographic eluent. Calibration solutions should be as dilute as possible, in order to avoid any concentration dependence of sample retention volumes. The concentration effect causes an increase of retention volumes with increased sample concentration. As a rule of thumb, when high efficiency microparticulate packings are used, the concentration of narrow standards should be < 0.025% (w/v) for MW over 10 , < 0.05% for MW between 10 and 2 x 10 , and < 0.1% for MW down to lOl With a lower MW and in the oligomer range, the sample concentration can be higher than the previously suggested values. 

The sample introduction system can simply be re placed by a high-efficiency one, such as an ultrasonic nebulizer coupled to a desolvation system. Analyte transport efficiencies of 10-20% are typically achieved using such a system. Other high performance nebulization systems include direct injection nebulization, which introduces 100% of the sample directly into the plasma (i.e. no spray chamber is needed), thermospray, hydraulic high-pressure nebulization and monodisperse dried microparticulate injection. 

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