Extracted-ion chromatogram peak area

An 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,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.^[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 giveThe 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.

An extracted-ion chromatogram baseline is a model of the signal underlying a selected chromatographic peak in an extracted ion chromatogram. Subtracting it helps assign an extracted-ion chromatogram peak area to the feature of interest. The baseline need not be zero or constant, and it is not automatically the signal from a separate reagent blank.

Let $E_W(t)$ be the signal from an extracted ion mass window $W$ and let $b(t)$ be the baseline over $[t_1,t_2]$. The assigned extracted-ion chromatogram peak area isThe time-ordered mass selection follows the IUPAC extracted ion profile definition; the baseline is an additional integration-model choice.^[ref]

An original example takes $0\le t\le 4\min$ and a baseline rising linearly from 1 to 3 signal units. Its integrated area is $4(1+3)/2=8$ signal-unit minutes. If the recorded trace has total area 20, the corrected area is 12. Using a constant baseline 1 would subtract only 4 and assign area 16: a 33% overestimate relative to 12. No recorded ion signal changed; only its allocation between baseline and peak changed.

If a fitted baseline is $b(t)=a+kt$ over a duration $L$ starting at $t=0$, then the subtracted area is $B=aL+k{L}^2/2$. Chromatographic baseline-area uncertainty therefore includes the joint uncertainty of the fitted intercept and slope:This is a direct application of correlated-input uncertainty propagation.^[ref]

Candidate baseline regions should be inspected for neighboring peaks, broad tails and drifting background. Two overlapping compounds can generate one smooth trace; subtracting a convenient baseline cannot establish chemical specificity. Record the integration boundaries, baseline model and treatment of negative corrected values. Those choices are necessary to interpret or reproduce a reported extracted-ion chromatogram peak area.

Chromatographic baseline-area uncertainty quantifies uncertainty in the integrated baseline subtracted when assigning a chromatographic extracted-ion chromatogram peak area. For an extracted-ion chromatogram baseline $b(t)$ over fixed limits $t_1,t_2$, let $B={\int }_{t_1}^{t_2}b(t)dt$. The baseline's uncertainty is shared across time whenever fitted parameters or other common inputs are shared.

For sampled times with integration weights $w_i$ and baseline estimates $b_i$,The weights include the actual sampling intervals and chosen integration rule. This is correlated-input uncertainty propagation, not an assumption that each drawn baseline point is independently uncertain.^[ref]

An original constant-baseline example uses 100 equal intervals of $0.1\min$ and one common fitted baseline with standard uncertainty0.2 signal units. Its integrated uncertainty is $10(0.2)=2$ signal-unit minutes. Treating those 100 appearances of the same fitted quantity as independent would instead give $\sqrt{100}(0.1)(0.2)=0.2$, underestimating uncertainty tenfold. More sampling points do not average away a shared offset.

For total recorded area $P$ and assigned peak area $Q=P-B$,With stipulated $u_P=0.5$, $u_B=0.4$ and correlation coefficient0.6, the result is $u_Q=\sqrt{0.17}\approx 0.412$ area units, compared with 0.640 under independence. The appropriate covariance depends on how the trace and its baseline were estimated; it cannot be selected to obtain a preferred uncertainty.

Changing integration limits or choosing a different baseline form can create additional effects beyond parameter uncertainty within one fixed model. Inspect neighboring peaks and baseline regions, and record the fitted model and boundary choices. A precisely computed extracted-ion chromatogram peak area can still depend on an unsuitable baseline model.