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An extracted ion chromatogram is an extracted ion profile whose time axis follows a chromatographic separation. A mass spectrometer records successive mass spectra as separated material reaches the ion source; software follows a chosen mass-to-charge ratio interval through those spectra. It is a selected view of the recorded data rather than a direct picture of a single chemical species.^[ref]

The apparatus supplies two kinds of discrimination: chromatography changes when material reaches the detector, while mass analysis distinguishes ion coordinates at each time. An extracted ion mass window specifies which recorded coordinates contribute. A narrow window cannot separate two coeluting species whose relevant ions occupy the same mass coordinate.

An extracted-ion chromatogram peak area integrates selected signal over an explicitly chosen time range. For original sample points $(t,E)=(0,0),(1,4),(3,0)$, in minutes and arbitrary signal units, straight-line interpolation givesThe unweighted sum of ordinates is4, which ignores the unequal time intervals. These stipulated data illustrate numerical integration, not an experimental chromatogram.

Peak height and area also answer different questions. Triangular model peaks with height5 and full base widths2 and4 minutes have areas5 and10 signal-unit minutes. The same peak height does not imply the same integrated response. Converting area to amount requires an appropriate calibration and response model.

State the mass window, acquisition time spacing, baseline and time boundaries. Changes in baseline or overlap can change assigned area without changing analyte amount. Comparing retention behavior and other chemical evidence helps interpret a selected trace; its apparent isolation alone does not establish identity or freedom from analytical matrix effects.

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.

A total ion current profile plots total ion current from successive mass spectra against time. In liquid chromatography–mass spectrometry, it can show the passage of summed ion signal through the measurement system. IUPAC uses the broader profile term for time-dependent sampling beyond chromatography as well.^[ref]

With fixed included mass-to-charge ratio bins $K$, a reconstructed profile iswhere $t_j$ labels a spectral acquisition. An extracted ion profile instead follows a selected mass interval or feature. Changing $K$, scan conditions or detector response can change the total profile even if the underlying sample composition is unchanged.

Consider three scans at $t=(0,1,2)\mathrm{s}$. An invented ion A contributes $(0,4,0)$, ion B contributes $(0,0,4)$, and constant background contributes $(1,1,1)$. Their total profile is $(1,5,5)$. Its last two points form a plateau even though the individual ion signals have changed completely. The total alone cannot distinguish that exchange from one ion remaining constant.

If one deliberately chooses straight-line interpolation between these three total-signal measurements, the trapezoidal rule gives areaThe discrete sum $1+5+5=11$ is a different quantity. Neither calculation identifies a chemical species, and unsampled signal changes remain unknown. State the time spacing, mass range, baseline and interpolation or integration rule when comparing profile areas. A total profile is useful for acquisition overview; the underlying mass spectra preserve the information needed to investigate its apparent peaks and plateaus.