Recent applications in nanoliquid chromatography

Since its first introduction by Karlsson and Novotny in 1988 nano-LC has emerged as a complementary and/or competitive separation method to conventional HPLC, offering several advantages such as higher efficiency, ability to work with minute sample sizes and lower consumption of mobile phases, and better compatibility with MS, etc. Although its use was not so extended initially, in the last years new and interesting applications have appeared which deserve to be carefully considered. The aim of this review is therefore to provide an updated and critical survey of different nano-LC applications in analytical chemistry.

Capillary columns in liquid chromatography: between conventional columns and microchips

Liquid chromatography on columns with small internal diameters has been reviewed as the intermediate technique between conventional liquid chromatography and microchip separations. The development of micro column separations in the early years has been described, starting with the papers of Horváth and co-workers and Ishii and co-workers, continuing into the first part of the eighties, then making a leap in time to recent innovations with small-bore columns. Based on internal diameters a classification of the different analytical HPLC columns has been suggested. The advantages of small-bore columns have been discussed, with particular emphasis on the advantage of coupling to concentration sensitive detectors when the sample amount is limited. Open tubular columns are treated as a part of the historic background. The recent developments include a brief look into the current status of monolithic columns, the use of packed nano columns and micro columns with electrospray mass spectrometry, and the potential of two-dimensional comprehensive liquid chromatography. Finally, the coupling of sample preparation to analytical columns and the future applications of the novel technological improvements to the microchip separation methods have been discussed.

Electrochromatography in chemically modified etched fused-silica capillaries

A new method of electrochromatography is described in which a 50-microns capillary is etched with ammonium hydrogen difluoride, followed by modification of the new surface via a silation reaction with triethoxysilane to produce a hydride intermediate, and then subsequently subjected to hydrosilation using 1-octadecene in the presence of a platinum complex catalyst. The C18 bonded phase is then compared with bare capillaries, etched bare capillaries and the hydride etched capillary to determine if any solute-bonded phase interactions are present. With bradykinin as a test solute, peak efficiencies are quite similar for all capillaries without C18 but become noticeably broader when the organic moiety is attached to the etched capillary wall. A test mixture of peptides and proteins displays shorter retention for some of the components when methanol is added to the mobile phase which is typical of reversed-phase behavior. The same result is also obtained when a mixture of tetracyclines is separated on the C18 capillary with and without methanol as part of the mobile phase. The reproducibility of retention times for two proteins is +/- 1.5%. A few results for several neutral compounds indicate small but measurable k' values.