peak tailing
/ PEEK TAY-ling /
Imagine a crowd leaving a stadium through a doorway where a few people keep stopping to tie their shoes. The bulk pours out quickly, but a thin straggling trail of latecomers drags on long afterward. A tailing peak looks just like that: it rises and falls fairly sharply, then trails off in a long, lopsided tail on the back side.
Formally, peak tailing is an asymmetric peak shape in which the trailing (later) edge falls off much more gradually than the leading edge rises. It usually arises when a small fraction of molecules are held by unusually strong sticking spots on the stationary phase and let go only reluctantly, so they emerge late and smeared out.
It matters because tailing peaks are harder to measure accurately, overlap their neighbours more, and lower resolution — a heavily tailing peak can hide a small one lurking just behind it. The honest caveat is that some tailing is almost always present; the practical question is whether it is small enough (measured by a peak-shape or asymmetry factor) for the result to be trusted, not whether it is perfectly absent.
A basic drug shows a peak with a long trailing tail because it sticks to leftover acidic sites on the silica; the analyst adds a small amount of an amine to the mobile phase to block those sites and restore a symmetric peak.
A long straggling tail on the back of a peak — a sign of too-strong sticking spots.
Tailing's mirror image is fronting, where the leading edge is the gradual one; both are forms of peak asymmetry, often caused by overloading the column or by uneven interaction with the stationary phase.