Velocity compressible mobile phases

For an understanding of band broadening in chromatographic systems, the linear velocity of the mobile phase is more important than the column volumetric flow rate. Complications in identifying a suitable velocity arise for compressible mobile phases... 

Liquids have relatively low compressibility compared with gases and, thus, the mobile phase velocity is sensibly constant throughout the column. As a consequence, elution volumes measured at the column exit can be used to obtain retention volume data and, unless extreme accuracy is required for special applications, there is no need for the retention volume to be corrected for pressure effects. 

FIGURE 2.12 Pressure drop as a function of liquid fiow through a packed bed of Superose 6 prep grade packed in a HR 10/30 column. At high velocities of the mobile phase the beads are compressed and the void channels reduced, which leads to a high pressure drop. If this happens, the material can be resuspended and packed at a lower flow rate. [Reproduced from Hagel and Andersson (1984), with permission.]... 

Equation (I.IS) Is valid for open tubular columns under all normal conditions and for packed columns at low mobile phase velocities. The average carrier gas velocity is calculated from the outlet velocity by correcting the latter for the pressure drop across the column, and is simply given by u - ju, where j is the gas compressibility correction factor, defined In equation (1.2). 

One of the primary benefits of pressure/denslty programming is peak compression that results in later eluting peaks having the same width, or an even narrower width, than the earliest peaks in the chromatogram. Qualitatively, this can be ascribed to either positional variations in mobile phase density or velocity along... 

When soft compressible particles arc used, the pressure drop increases substantially with the velocity of the mobile phase above a certain velocity because of compression of the particles, thus limiting the allowable velocity. Such compression of particles becomes significant with increases in the column diameter. 

The calculation of retention time tr is simple when the flow velocity in the column is uniform throughout the column length and has a known value v which can be substituted into Eq. 10.2. However in gas chromatography, where the mobile phase is highly compressible, v varies substantially with the level of compression, which changes through the column. This requires special consideration. 

When the rate equation is applied to GC, allowance must be made for the fact that the mobile phase (a gas) is compressible and the linear velocity u is not a constant but increases throughout the column. This effect will be discussed further in Chapter 5. 

The mobile phase can be allowed to flow under the force of gravity, a low pressure pump can be used, or compressed gas can be used to pressurize a solvent reservoir. The linear velocity should be about one-third of that used in analytical columns. The sample can be applied to the top of the column with a microsyringe or pipet using the stop-flow method, or an inexpensive, low-pressure valve can be used. The eluent is usually collected in separate tubes using an automated fraction collector. Inexpensive UV detectors with large solvent volumes are available, or flow cells can be fitted to conventional UV/visible instruments. 

A. D. Jerkovich, J. S. Mellors, J. W. Thompson, and J. W. Jorgenson, Linear velocity surge caused by mobile-phase compression as a source of band broadening in iso-cratic ultrahigh-pressure liquid chromatography, AnaZ. Chem. 77 (2005), 6292-6299. 

As stated above, we can neglect the compressibility of the mobile phase. Thus, its density along the column and its velocity remain constant and are independent of the pressure. 

In liquid chromatography, the compressibility of the mobile phase is small, and in practice it is often negligible, unless high accuracy measurements are made for a specific purpose [24]. Under steady-state conditions, the equation of continuity of the mobile phase can be integrated to give u = uq = constant, where mq is the outlet velocity. When the viscosity of the sample and that of the mobile phase do not differ significantly, Eq. 5.86 can be integrated, and the total pressure drop can be calculated as... 

Jacob et al. used the method of characteristics to discuss the general properties of the system of mass balance equations in multicomponent preparative gas chromatography (GC) [34-36], assuming either a linear or a nonlinear isotherm. The GC problem is more complicated than the HPLC one because the gas mobile phase is much more compressible than a solution and the mobile phase velocity is very different inside and outside a high concentration band because the partial molar volumes of compounds are much larger in the gas mobile phase than in the condensed stationary phase (the sorption effect). They showed that the method of characteristics appHes to multicomponent systems as well as to single component... 

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