1 k2

A value of Rs = 1.5 is normally considered to represent baseline separation for Gaussian shaped peaks. To achieve the maximum peak resolution, both high selectivity and column efficiency (giving narrow bands) are required.

Increased resolution can always be achieved by an increase in column length since the peak separation (AtR) is proportional to the distance of migration down the column, but peak width is only propor where the subscript 2 refers to the second peak.

Conditions for obtaining maximum values of the plate number have already been discussed. The relative retention is mainly governed by the nature of the stationary phase, since in GC at normal pressures, only molecular interactions between the solute molecules and the stationary phase are involved. These interactions are maximized in the concept of 'like has an affinity for like'. Thus, for a sample that contains predominantly nonpolar species a nonpolar stationary phase will optimize the dispersion forces and, since polar interactions will be absent, solutes will elute according to their volatility with the most volatile (lowest boiling point) components eluting first. For polar samples a polar stationary phase is used to maximize both dipole-dipole interactions and dipole-induced dipole interactions. Because the effect of volatility is still present, it is much more difficult to predict elution behaviour in this latter case. Most naturally occurring mixtures contain species spanning a range of polarities, and in this case it is still better to use a polar stationary phase. At least a partial separation can be achieved with a values as low as 1.05, but values in the range 1.5-3.0 are preferable and above a values &5.0 little additional resolution is achieved. Peak resolution increases rapidly with increasing k values, but at values >10 the term k2/(1 + k2) p 1 and the term plays no further part in the resolution. The use of k values < 1 gives very short retention times and poor resolution, so that the optimum range for k is between 1 and 10. The retention equation tR = L/u(1 + k) shows that retention times are a function of both the mobile phase velocity (u) and the retention factor.

In GC, k values are controlled by temperature. The van't Hoff equation describes the change in equilibrium constant with temperature and if the phase ratio (VS/VM) is independent of temperature we can also write for the retention factor:

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