Column-switching valves, requirements

Often more expensive than individual module.s No flexibility for changing requirements, e.g. is there space and the option to incorporate a second detector and a column switching valve ... 

Direct-injection techniques into either GC or HPLC instruments bypass the extraction step and can offer a very rapid analytical process. In GC, solid-phase microextraction (SPE) can be used, while HPLC tends to require use of pre-columns that are backflushed with the use of column-switching valves. 

Independent Temperature Control—Haas test method requires the temperature control of five columns, column switching valves and sample lines. The columns consist of polar, non-polar, Tenax, platinum, and molecular sieve columns. The specifications for these columns are listed in Table 1. The polar column, non-polar column, column switching valves, and sample lines require isothermal operation at a temperature equivalent to e temperature of the gas chromatograph oven. These components may be located in the gas chromatograph oven. The Tenax column, platinum column, and molecular sieve column require operation at temperatures other than the gas chromatograph oven temperature. These columns may be temperature controlled by any means that will meet the following specifications ... 

Valve Actuation Gas—This test method permits the use of any type of valve switching or valve actuation. When pneumatic vtdves are used, air of any grade that will result in no water condensation or will not introduce oil or other contaminates in the switching valves may be used. Air from a piston operated compressor equipped with a water and oil separator has been found suitable. Column switching valves that do not require air to operate do not have this air requirement. 

Multidimensional liquid chromatography encompasses a variety of techniques used for seunple separation, cleanup and trace enrichment [12,279-289]. A characteristic feature of these methods is the use of two or more columns for the separation with either manual or automatic switching by a valve interface of fractions between columns. These techniques require only minor modification to existing equipment, and of equal importance, enable the sample preparation and separation procedures to be completely automated. 

Experimentally, two column switching can be demonstrated using apparatus no more elaborate than a conventional six-port valve. Column switching can create tremendous separating power, but it is a requirement that each one in the sequence of selectivity mechanisms not be redundant. On this basis it was calculated that a trillion compounds could be separated with available column switching technology (19). 

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