= Extracted-ion chromatogram peak area
An \b[extracted-ion chromatogram peak area] integrates selected ion signal over a stated chromatographic time interval, usually after estimating an <extracted-ion chromatogram baseline>. If $E_W(t)$ is an <extracted ion chromatogram> for mass window $W$ and $b(t)$ is its baseline,
$$
Q_W=\int_{t_1}^{t_2}[E_W(t)-b(t)]\,dt.
$$
Both the <extracted ion mass window> and time boundaries are part of this definition. The selected-ion/time character follows the IUPAC <extracted ion profile> definition.<Extracted ion profile in IUPAC Gold Book>[Entry 12410, selected mass signal through a series of time-dependent spectra.]{ref}
An original nonuniform-sampling example uses times $(0,1,2,4)$ minutes and signals $(1,5,3,1)$. With baseline 1, corrected signals are $(0,4,2,0)$. Straight-line interpolation and the <trapezoidal rule> give
$$
Q_W=\tfrac12(0+4)(1)+\tfrac12(4+2)(1)+\tfrac12(2+0)(2)
=7\,\text{signal units}\cdot\mathrm{min}.
$$
The discrete corrected-signal sum is 6, which ignores the acquisition intervals. Neither number is an amount of analyte without a response calibration.
Baseline error accumulates across the integration interval. Over this four-minute interval, overestimating a constant baseline by 0.5 signal units reduces the assigned area by $0.5(4)=2$ signal-unit minutes, from 7 to 5. A broad weak feature can therefore be especially sensitive to its baseline convention.
Peak area can also include neighboring ions admitted by the mass window and coeluting compounds with the same recorded coordinate. More complete integration does not by itself make the assigned signal chemically specific. State boundaries, baseline, handling of negative corrected values and integration rule. Those choices can alter reported area even when the recorded spectra are unchanged, and differences in area need not mean proportional differences in analyte amount.
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