 Total ion current

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.^[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.^[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.