= Total ion current
\b[Total ion current] is a sum of ion currents in <mass spectrometry>, with an important distinction about where it is measured. After mass analysis, it is the sum of separate ion currents contributing to the <mass spectrum>, sometimes called reconstructed ion current. Before mass analysis, it can mean the sum of all same-sign ion currents. A reconstructed total over a selected analyzer range need not include every ion initially produced.<Total ion current in IUPAC Gold Book>[T06410, definitions1–2, after versus before mass analysis.]{ref}
For discretely recorded spectral intensities $I_k$, a corresponding reconstructed signal is $S=\sum_{k\in K}I_k$, where $K$ specifies the included <mass-to-charge ratio> bins. Instruments may store intensities in arbitrary response units or counts rather than amperes, so the reported total needs its acquisition convention. Equal-bin sums and integrals of intensity density also require different bin-width bookkeeping.
For an invented three-bin example, spectra $(4,6,0)$ and $(1,1,8)$ both have total $10$. The total alone cannot distinguish their different ionic compositions. If acquisition excludes the third bin, the two reconstructed totals become $10$ and $2$ without any change in the stipulated source outputs. A changing measurement range can thus change the reported total.
Plotting reconstructed totals from successive spectra against time gives a <total ion current profile>; in coupled <liquid chromatography>, this can display when substantial summed signal reaches the detector.<Total ion current in IUPAC Gold Book>[Related entry12600, definition of total ion current profile.]{ref} Individual chemical signals can overlap or move in opposite directions while their sum remains unchanged. <Total ion current normalization> uses such a sum to rescale a spectrum, but the arithmetic does not independently establish chemical identity, detector linearity or equal response per molecule.
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