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.

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.