Total ion current profile

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.