resolution
/ rez-uh-LOO-shun /
Picture two ripples spreading on a pond. If their crests are far apart and each ripple is narrow, you clearly see two; if they sit close and each is broad and fuzzy, they merge into one blur. Resolution measures exactly this for chromatography peaks: how cleanly two neighbouring peaks are pulled apart from each other.
Formally, resolution compares how far apart two peaks' centres sit against how wide the peaks are; it rewards a big gap between peaks and punishes broad peaks. A resolution near 1.5 means the two peaks are essentially fully separated down to the baseline, while values much below 1 mean they overlap so badly you cannot measure either one cleanly.
It matters because the whole point of a separation is to tell components apart, and resolution is the single number that says whether you succeeded. The honest caveat is that you can improve it in two very different ways — either by spreading the peaks further apart, or by making them sharper — and chasing one without watching the other (for instance, a longer run that also broadens peaks) can leave resolution barely changed.
Two drug metabolites elute very close together and partly overlap; switching to a longer, more efficient column sharpens both peaks, raising the resolution above 1.5 so each can be measured on its own.
One number telling you whether two peaks are truly separated.
Resolution depends jointly on three things — column efficiency (theoretical plates), the selectivity factor, and the retention factor — so improving any one of them can rescue a marginal separation.