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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.

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 mass window is the set of mass-to-charge ratio coordinates included when reconstructing an extracted ion profile or extracted ion chromatogram. A common choice is an interval centered at $m_0$ with half-width $w$: $|m-m_0|\le w$. The profile definition permits one value or a set of values; specifying the window makes that selection reproducible.^[ref]

A relative half-width can be stated in parts per million:where $p$ is the stipulated ppm half-tolerance and $m,m_0$ use the same displayed mass-coordinate scale. At $m_0=500$, a5ppm half-tolerance gives $w=0.0025$ and interval $499.9975\le m\le 500.0025$. At $m_0=1000$, the same relative tolerance gives $w=0.005$. Conversely, fixed $w=0.005$ corresponds to10ppm at500 and5ppm at1000. State whether a quoted width means half-width or full width.

Finite peak width makes window choice a signal-selection operation. A target whose measured center shifts from500.000 to500.003 moves outside the first window, although part of its broadened signal may remain inside. Widening the window can recover more target signal while admitting a neighboring ion. The extracted-ion window capture fraction depends on the peak shape, width and coordinate offset, not merely on the nominal target formula.

Mass resolving power describes separation of nearby ion peaks; coordinate accuracy and the chosen extraction tolerance are different properties. A narrow software window does not improve instrumental resolving power or recreate information that was not acquired. Nor does fitting the expected window establish a unique ion identity. The window should be interpreted together with acquisition conditions, reference measurements and the other evidence used for assignment.

An extracted ion profile plots selected mass-to-charge ratio signal across a time-ordered series of mass spectra. The selection can be one value or a set of values. When time follows a chromatographic separation, the result is an extracted ion chromatogram; the broader profile term also covers other time-dependent sampling.^[ref]

For recorded spectra $I_k(t_j)$, an explicit extracted ion mass window $W$ gives the reconstructed traceThis selects information already present in the spectra. It does not create missing mass measurements or turn the acquisition into a different instrumental measurement. Unlike a total ion current profile, it excludes signal outside the stated window.

An original three-scan example shows how the window changes the result. At mass coordinates $499.98$, $500.00$ and $500.03$, let the intensity vectors be $(0,1,0)$, $(3,8,5)$ and $(0,2,10)$ at successive times. A window $500.00\pm 0.01$ selects only the middle coordinate and gives $(1,8,2)$. Widening to $500.00\pm 0.04$ includes all three coordinates and gives $(1,16,12)$. The larger late signal in the broad trace comes from added spectral content; it is not evidence that the middle-coordinate signal remained high.

Choosing a window requires considering mass resolving power, mass-coordinate accuracy and what the instrument actually recorded. A narrow window can exclude a shifted target peak; a wide window can include neighbors. Even an isolated mass coordinate need not identify a unique chemical species, because different ions can share it. Preserve the raw spectra and state the window, summation rule and time axis so the profile can be reproduced and interpreted.