Extracted-ion chromatogram baseline

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.