CHAPTER 3
PARAMETERS AFFECTING SELECTIVITY Before starting any optimization process, we should ask ourselves what exactly it is we want to optimize. In general terms our goal is to obtain the best possible chromatogram for a particular purpose. In the next chapter we will discuss criteria by which to judge the quality of a chromatogram. In the present chapter, we will describe the parameters that influence retention and selectivity, and see which parameters we might consider (or exploit) during the optimization process. Where possible, we will derive simple relationships between retention and the relevant parameters. For reasons of clarity, we will express all equations in terms of the capacity factor (k).Obviously, the simplest possible equations will be most useful for optimization purposes. Ideally, we will be looking for linear relationships, since straight lines allow straightforward interpolation. At the end of the chapter (section 3.5) we will summarize the relationships that are recommended for the various parameters in different kinds of chromatography. 3.1 GAS CHROMATOGRAPHY 3.1.1 Gas-liquid chromatography (GLC) In this form of chromatography retention can readily be expressed in thermodynamic terms. The definition equation for the capacity factor ( k ) is
where q i is the total quantity of the solute in either phase, Ci the average solute concentration and Y the total volume of the indicated phase in the column. If we assume very dilute solutions (as is usually the case in chromatography), we can write for the concentration in the stationary phase
where xi,sis the mole fraction of the solute in the stationary phase, and p, and M, are the density and the molecular weight of this phase, respectively. For the mobile (gaseous) phase we can write = l/vi,m =
Pi ~
~
R
T
’
(3.3)
where vi,,, is the partial molar volume of the solute in the gaseous phase, p i its partial pressure, and (= p i vi,,, / R T ) the compressibility coefficient. Since we are dealing with gases at low pressures, we may usually (and definitely for the present purpose) assume the ideal gas law to be valid. Hence, we may assume to be equal to one. 37