Introduction

It is the role of the column to achieve separation of the components of injected mixtures. In many cases, it is also possible to accomplish extra-column 'separation', i.e. 'separation' by distinguishing between merged or co-eluting solutes. Selective detectors, such as the flame photometric detector, are useful in this regard, but our focus here will be directed toward separation as accomplished in the column.

The resolution equation is usually presented in one of two popular forms:

Equation [1] is used to estimate the number of theoretical plates that will be required (Nreq) to separate any two solutes to some specified degree of resolution (Rs), as functions of the retention factor (k) of the second of those two solutes, and of their separation factor (a). These relationships will be utilized later.

Equation [2] emphasizes that resolution (i.e. the degree to which solutes are separated) is affected by only three parameters: (1) the number of theoretical plates (N); (2) solute retention factors (k); and (3) solute separation factors (a).

The number of theoretical plates is a function of the 'sharpness' of a peak, relative to the time that the solute spends in the column. The measurement is affected by the length of the solute band introduced into the column (it is assumed that this is infinitesimal-ly small, which is never the case), the length (L) and radius (rc) of the column, the retention factor (k) of the solute, and the average linear velocity (u) of the carrier gas. It can also be affected by the thickness of the stationary phase film (df), and by solute diffusivity in the stationary phase (DS). Small values of k yield disproportionately large values of N. This anomaly essentially disappears with values of k > 5.

From this, it is evident that k = Kc x 1/ft, or (using the definitions in the Glossary):

For a given solute, k varies directly with solubility of that solute in the stationary phase (e.g. the k value of pentane would, under similar conditions, be higher in a polydimethylsiloxane stationary phase than in a polyethylene glycol stationary phase). In a given stationary phase, k varies indirectly with temperature: as the temperature increases, cS decreases and cM increases, and k decreases in a manner that is essentially exponential.

Finally (as evidenced in eqns [3] and [4]), k varies inversely with the column phase ratio, ft (or directly with 1/ft). In other words, solute retention factors (k) increase as the volume of column occupied by stationary phase (VS) increases and/or the volume occupied by mobile phase (VM) decreases. In packed columns, ft is usually controlled by the stationary phase 'loading'; in the open-tubular column, ft is controlled primarily through df, and to a smaller extent, through rc. There are some practical limitations: where df < 0.1 |im, columns can exhibit excessive activity, as evidenced by 'tailing' (reversible and irreversible adsorption) of 'active' solutes. Where df > 1.0 |im (or even less if DM < 10~7cm2 s_1), the mass transport term of the van Deemter equation (CM) becomes limiting, and column efficiency (as reflected by N) decreases. Solute retention factors interrelate with determinations of N. Unless the gas velocity is so high that solutes can no longer undergo equilibrium partitioning (this would require extremely high velocities), k is independent of u.

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Solar Panel Basics

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