 Extracted-ion window capture fraction

An extracted-ion window capture fraction is the fraction of a particular ion peak's integrated mass-coordinate signal retained by an extracted ion mass window. For a nonnegative signal-density model $s(m)$ and window $W$,assuming the denominator is finite and positive. This is a fraction of the specified peak model, not necessarily a fraction of analyte molecules or of the entire total ion current. The extraction window selects which mass-coordinate signal enters a profile.^[ref]

An original Gaussian function model makes window-capture sensitivity to mass offset explicit. Let the peak center be displaced by $\delta$ from the nominal window center, with standard deviation $\sigma$ and symmetric half-width $w$. If $\Phi$ is the standard normal distribution's cumulative distribution function,At zero offset and $w=2\sigma$, the retained fraction is approximately $0.9545$. Shifting the peak center to the upper window boundary, $\delta =2\sigma$, reduces it to approximately $0.5000$. The model's total peak area has not changed; the selected fraction has.

A wider window admits more of this peak but can also admit another ion. The measured window sum can therefore increase for two different reasons: greater target capture or added interference. They cannot generally be distinguished from the summed trace alone.

The Gaussian shape is a stipulated teaching model, not a universal instrument response. Real peak asymmetry, finite recorded ranges, background and overlapping signals can change the fraction. Mass resolving power and mass-coordinate accuracy remain instrumental properties; a software window only selects from the information available.