Mobile phase handling

Solvents used as the mobile phase are contained in solvent reservoirs. In their simplest forms, the reservoirs are glass bottles or flasks into which feed Lines to the pump are inserted. To remove particles from solvents, inline filters are placed on the inlets of the feed lines. Sophisticated mobile phase handling systems available commercially contain specially designed bottles with internal, conically shaped bottoms that allow small solvent volumes to be used. These handling systems also feature three or four valve caps that permit the filtration, storage, and delivery of solvents, and a stopcock for vacuum degassing. 

With vacuum degassing, the mobile phase is stirred in a side-armed flask or mobile phase handling system under... 

Liquid chromatography is complementary to gas chromatography because samples that cannot be easily handled in the gas phase, such as nonvolatile compounds or thermally unstable ones, eg, many natural products, pharmaceuticals, and biomacromolecules, are separable by partitioning between a Hquid mobile phase and a stationary phase, often at ambient temperature. Developments in the technology of Ic have led to many separations, done by gc in the past, to be carried out by Hquid chromatography. 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

Emphasis has to be on choice and proper handling of the mobile phase. In Chapter 4 different approaches for mobile phase selection are discussed. General hints for selection are the avoidance of the following ... 

Developing solvents should be composed of as few solvents as possible however, they mostly consist of 2 to 4 solvent components. It is recommended that developing solvents be prepared fresh every time and to use them only once. Some clues for handling of mobile phase rnixmres are as follows ... 

Reagents were purchased from commercial sources and used without further purification. Product identities were confirmed by comparison of HPLC retention times with authentic samples. The Pd/C catalyst was purchased as a dry, edge-coated, unreduced catalyst with 5 wt% in Pd. HPLC analyses were performed on a HP 1090 (stationary phase C18, 25 cm x 0.46 cm mobile phase CH3CN/H20 1% H3P04, 90% H20 to 100% CH3CN gradient over 25 min, 1 ml/min flow, UV-Vis detection). For safety aspects of handling Pd/C, see ref. 7.7... 

As the mobile phase in this system is normally a sodium hydroxide solution there is no need to handle or dispose of organic solvents. This is a particular bonus to some smaller sites that are not set up to use organic solvents. 

Equipment for HPLC is slightly more complicated (see Figure 2.18), due to the need to handle a liquid mobile phase. This requires the presence of one or... 

Eluent consumption, since this determines the cost for mobile phase in terms of preparation and handling (tanks, water preparing systems, pumps)... 

The moving wire device has a number of major shortcomings. Due to the small surface area of the stainless-steel wire, such as available from a 0.1 mm diameter wire, the device can only accommodate about 10 pL/min eluent which results in poor sensitivity. The system is difficult to operate in a continuous mode. Modification of the moving wire approach has led to the invention of a continuous moving belt, which offers improved transfer efficiency and therefore higher sensitivity. The moving belt interface is capable of handling up to 1 mL/min of mobile phase. 

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