# α – Selectivity Factor

The first thing to notice about this term is that there are diminishing returns to resolution as it is made excessively large.  Table 1 provides values of α and (α – 1)/ α.

Table 1.  Values of α and (α – 1)/ α.

α (α – 1)/α
1 0
2 $$\dfrac{1}{2}$$
3 $$\dfrac{2}{3}$$
4 $$\dfrac{3}{4}$$
5 $$\dfrac{4}{5}$$
6 $$\dfrac{5}{6}$$

Notice that the (α – 1)/α term approaches a value of 1 at high values of α. Optimal α-values for most separations are between 2 and 5.

Remember that the α-value refers to the ratio of the two partition coefficients for the two components.  If we wanted to make a substantive change in α, we need to change the partition coefficient of one component but not the other (or we need to change both appreciably but differently to impact the ratio).  There is really only one way to do this and it involves changing the identity of the stationary phase.  This would involve using a different liquid phase in gas chromatography.  As we will see, there are many available liquid phases for use in gas chromatography, so changing the nature of this phase and hence the α-values is a common procedure.

In liquid chromatography, this would mean switching from a C18 to some other bonded phase.  Since the range of bonded phases is more limited and changes of questionable utility in many instances, this is not done that frequently.  One thing you might wonder about is whether changing the nature of the mobile phase in liquid chromatography would be viewed as constituting a change in α.  This is easily done and is the most common way in liquid chromatography to improve the resolution of two substances that are not fully resolved.  It turns out that changes to the mobile phase are generally regarded as changes to the retention or capacity factor.  Usually this might involve altering some aspect of the mobile phase so that all the compounds exhibit a higher capacity, stay on the column longer, and exhibit better resolution.