Mobile Phase Delivery

Liquid carbon dioxide, which normally is the main constituent of the mobile phase, is delivered by a dip tube from the bottom of a pressurized tank. The dip tube is needed to get the liquid CO2, which is delivered to a large-volume cooled piston 

Mixtures with methanol can be obtained as premixed fluids in tanks or can be made by mixing streams of CO2 and methanol from two different pumps. 

Tanks with premixed fluid suffer from the disadvantage that the concentration of modifier increases in the liquid at the lower part of the tank, particularly when there is little liquid left. The reason for this is that the gas phase above the liquid in the tank contains a disproportionately higher amount of CO2 due to the higher partial pressure of CO2. As a result, the liquid phase is enriched with modifier. 

premixed fluids in tanks are little used. Pumping each constituent separately requires two pumps and a mixing device, but allows flexibility in changing the concentration of modifier, and is the preferred way to introduce modifier. 

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The basic SFC system comprises a mobile phase delivery system, an injector (as in HPLC), oven, restrictor, detector and a control/data system. In SFC the mobile phase is supplied to the LC pump where the pressure of the fluid is raised above the critical pressure. Pressure control is the primary variable in SFC. In SFC temperature is also important, but more as a supplementary parameter to pressure programming. Samples are introduced into the fluid stream via an LC injection valve and separated on a column placed in a GC oven thermostatted above the critical temperature of the mobile phase. A postcolumn restrictor ensures that the fluid is maintained above its critical pressure throughout the separation process. Detectors positioned either before or after the postcolumn restrictor monitor analytes eluting from the column. The key feature differentiating SFC from conventional techniques is the use of the significantly elevated pressure at the column outlet. This allows not only to use mobile phases that are either impossible or impractical under conventional LC and GC conditions but also to use more ordinary... 

Tilting glass plate used to control mobile phase delivery... 

For GC the mobile phase delivery box could consist of a gas cylinder, a reducing valve and a flow controller. For LC a high pressure pump will be required. In this book the instrumentation required for chromatography will not be discussed. Only where the equipment used is relevant to the cause of optimization of selectivity will it feature in the present text (e.g. sections 5.6 and 7.4). 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

Solvent (mobile phase) delivery is achieved using high pressure pumps. There are several types of pumps... 

R. L. Stevenson, Mobile-phase delivery systems for HPLC, in Handbook of HPLC (E. Katz, R. Eksteen, P Schoenmakers, and N. Miller, eds.). Chromatography Science Series Vol. 78, Marcel Dekker, Inc., New York, 1998. 

The disadvantages of HPLC centre around the detection systems available. Ultraviolet spectrometers are most commonly used detectors but they require that the compound has a UV absorbing chromophore. Variable wavelength UV spectrophotometers offer reasonable versatility, but some steroids and other drugs must be derivatized before UV detection is possible. With the introduction of electrochemical and spectrophotometric detection in region other than the ultraviolet, and the rationalization of mobile phase delivery systems by the use of microprocessor control, capabilities of HPLC have significantly increased. 

The basic components of a high performance liquid chromatograph are a high pressure mobile phase delivery system, a metal colunm packed with fine parficles confaining the stationary phase, a sample injector, and a detector. A high pressure pump is used to force the mobile phase solvent or solvent mixture through the packed column. 

Pumping system and flow rate The mobile-phase delivery should be pulseless, and the most economical way to do this is to use a less expensive... 

A column alone cannot give the information the analyst requires. A complete instrument is needed, one with a mobile-phase delivery system, an injection device, a detector, and a data station. At all stages the requirements and properties of the modules force the analyst to accept compromises and lose resolution. Contributions of the injector and detector to band broadening should be of special concern. The lucid analysis of these problems with respect to GC by Sternberg [53] has become a classic. Most of the conclusions are easily extended to LC. 

The separation column contains the stationary phase, while the mobile phase (frequently referred to as the carrier gas) is permitted to flow through this column from a pressurized gas cylinder (source of the mobile phase). The rate of mobile-phase delivery is controlled by a pressure and/or flow-regulating unit. An exclusive separation mode for the analytical GC is elution chromatography, in which the sample (a mixture of chemicals to be separated) is introduced at once, as a sharp concentration impulse (band), into the mobile-phase stream. The unit where sample introduction is performed is called the injector. The unfrac-tionated sample is transferred from the injector into the chromatographic column, where it is subjected to a continuous redistribution between the mobile phase and the stationary phase. Due to their different affinities for the... 

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