= Total ion current profile
A \b[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.<Total ion current in IUPAC Gold Book>[Related entry12600: total ion current profile and examples of time-dependent sampling.]{ref}
With fixed included <mass-to-charge ratio> bins $K$, a reconstructed profile is
$$
S(t_j)=\sum_{k\in K}I_k(t_j),
$$
where $t_j$ 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 area
$$
\frac{1+5}{2}(1\,\mathrm{s})+\frac{5+5}{2}(1\,\mathrm{s})
=8\,\text{signal units}\cdot\mathrm{s}.
$$
The 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.
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