2P0 dDy cos

The validity of this equation is now well established. As early as 1968, Stahl plotted z2 versus t and obtained a straight line. Stewart had already demonstrated the proportionality between kv and y/q. These early experiments were carried out either on paper or on thin layers made with a rather large particle size distribution. More precise data can be gathered from measurements carried out with layers of a narrow particle size distribution (Figure 3).

From the slope of the line the value of P0 may be derived. Table 1 lists some values obtained with plain silica assuming perfect wettability by solvent so that cos 6' = 1.

These data from either commercial or homemade plates are in excellent agreement. The coating procedure does not play a significant role and the binding agent has apparently no influence on the hydrodynamics of the plate. In the same way intraparticular k _2bPo dp y cos 0 — zf

Table 1 Experimental values of P0

This assumes that r = dp/6, which is rather large (but the pores of different particles do not connect directly) and it is found that b = 1.

Table 1 Experimental values of P0

Origin of data

Values ofP0

Stahl

6.7 x 10~3

Guiochon and Siouffi

8.15 x 10~3

Ripphahn and Halpaap

6.7 x 10~3

pore dimensions have no effect as there is no marked difference between Si 60 and Si 100 silica of the same particle size. An experiment was performed on very wide pore silica (Si 50 000) and the velocity constant was the same as the speed was very rapid. The average P0 value (8 x 10~3) is rather large when compared to column chromatography (1 x 10~3).

The proportionality of the velocity constant to the particle diameter has been checked with carefully sorted silica particles and excellent agreement is obtained.

It is convenient to introduce a coefficient of velocity 6, is given by:

kv dp

pressure P (if the gas phase is homogeneous, otherwise calculations would be nearly impossible).

The flow of solvent leaving the solvent source, is proportional to both the cross-sectional area of the layer, el (where e is the thickness of the layer bed), and the aspiration speed, u.

kvel 2zf

The actual flow rate is therefore:

This gives a parameter independent of dp, which is easier to take into account when calculating the velocity of the eluent in beds made with particles of different diameter. The utility of 6 will become apparent in the following sections.

The main drawback of TLC is the presence of a gas phase in contact with the eluent, which yields deviations from the ideal flow velocity. How to get reproducible retention data has given rise to a huge volume of literature. Many practical methods of operations are possible: some workers use saturated tanks in which blotter pads are placed and a certain amount of developing solution is poured over the pads; some workers use nonequilibrated tanks where the plate is placed inside the tank and developed immediately, in contrast to equilibrated tanks where plates are placed in the tank an hour prior to development. Whatever method is used, the presence of the gas phase will influence the solvent front rise through adsorption or vaporization.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment