Separator column maintenance

The column is arguably the most important component in HPLC separations. The availability of a stable, high-performance column is essential for developing a rugged, reproducible analytical method. Performance of columns from different vendors can vary widely. Separation selectivity, resolution, and efficiency depend on the type and quality of the column. Proper column maintenance is the key to ensure optimum column performance as well as an extended column lifetime. It ensures stability of column plate number, band symmetry, retention, and resolution. The major issues related to column performance and maintenance are discussed here. 

Both methods require minimal if any sample preparation, and extensively automated systems are available. The highly corrosive chemicals and the harsh conditions used in the Kjeldahl digestion call for appropriate fume hoods and exhaust systems, and standardization of the digestion itself may sometimes be difficult. The relatively low sensitivity and the fairly large amount of sample required are usually no problem in the food industry. As for chromatographic methods, controlled oxidative pyrolysis of food releases a number of volatile compounds that may foul the separation columns. This requires careful maintenance of the equipment, and in particular of the precolumn that guards the separation apparatus. 

Monoliths Low backpressure, suited for conventional HPLC Higher separation efficiency by column coupling Rugged against delay volume and extra-column band broadening fast column re-equilibration Reduced maintenance on pumps and injector seals Reduced need for sample pre-treatment... 

The addition of buffering salts to the mobile phase often improves chromatographic separation, provides a stable pH during separation, and reduces problems associated with column disturbances produced by highly variable samples. These salts are usually volatile (examples are ammonium formate, ammonium acetate, and i-ethylammonium hydroxide) and the concentrations used are usually less than 10 mM. With the advent of orthogonal interfaces for ESI and APCI, the absolute requirement for volatile salts has disappeared. However, the prolonged use of nonvolatile salts is not recommended as the accumulation of salts in the spray chamber of the MS reduces sensitivity and increases maintenance requirements. 

Other electrode configurations, such as the radial arrangement consisting of four thin wires placed perpendicularly around the circumference of the separation capillary column, have found less application due to more complicated construction and restriction in space and diameter of the separation capillary [56]. Due to its low cost, robustness, minimal maintenance demands, possibility to be freely moved along the capillary [57], or combined with either UV-absorbance [58] or fluorescence [59] detection, the capacitively coupled contactless conductivity detector has recently gained wide acceptance not only for the determination of inorganic ions but also for biomolecules and organic ions, as it has been recently comprehensively reviewed by Kuban and Hauser [1]. 

A rapid, sensitive, reversed-phase HPLC method for the separation of BHA isomers was described by Berridge et al. (117). Using a column packed with Hypersil ODS of 3-/nn particle size and with a mobile phase consisting of an acetonitrile-water mixture, it was possible to detect less than 0.5 ng of the isomers injected. The procedure is reliable and robust and compared to another HPLC method, claimed to have a more stable, longer-lasting column requiring only occasional maintenance (117). 

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