Define the selectivity factor α and its role in peak separation.

Define the selectivity factor α and its role in peak separation.

• A. α = k2/k1 (>1); larger α improves separation between adjacent peaks.
• B. α = k1/k2 (<1)
• C. α = (tR2 - tR1)/(W1+W2)
• D. α = N/H

Answer: (A)

The selectivity factor measures how differently two solutes partition between the stationary and mobile phases, which directly controls how far apart their peaks will be in the chromatogram. It is defined as the ratio of their retention factors: α = k'2 / k'1, where k'2 is the retention factor of the later-eluting solute and k'1 is that of the earlier-eluting one. Since the second solute is retained longer, α is greater than 1. A larger α means the second peak is held back more strongly relative to the first, increasing the gap between their elution times and reducing overlap. This improves separation between adjacent peaks, assuming the peaks are otherwise well-behaved. The other expressions don’t describe this factor. One is essentially the inverse ratio and would yield a value less than 1, which isn’t how selectivity is defined. Another is a formula for peak resolution, not the ratio of retentions. The last is a relationship involving different chromatographic parameters, not the selectivity factor.

The selectivity factor measures how differently two solutes partition between the stationary and mobile phases, which directly controls how far apart their peaks will be in the chromatogram. It is defined as the ratio of their retention factors: α = k'2 / k'1, where k'2 is the retention factor of the later-eluting solute and k'1 is that of the earlier-eluting one. Since the second solute is retained longer, α is greater than 1.

A larger α means the second peak is held back more strongly relative to the first, increasing the gap between their elution times and reducing overlap. This improves separation between adjacent peaks, assuming the peaks are otherwise well-behaved.

The other expressions don’t describe this factor. One is essentially the inverse ratio and would yield a value less than 1, which isn’t how selectivity is defined. Another is a formula for peak resolution, not the ratio of retentions. The last is a relationship involving different chromatographic parameters, not the selectivity factor.