Hi Sarah_Woodland ,
If there are no data about the dwell times for a mass spec file, iolite will try to guess by looking at the jumps between values for each channel. For example, if your channel data look quantised like this:
iolite will guess that the difference between the levels is one additional ion arriving at the detector during each dwell time. Because ion counts are divided by the dwell time to get counts per second, we just convert this jump level to time. For example, if each jump is 100 CPS apart, we know that each count is being multiplied by 100, so the dwell time must be 10 ms.
Of course, this is completely stumped by data that doesn't show these discrete levels, so it won't always get it right. That's why iolite shows the total sweep time and the sum of dwell times, which should roughly match if the calculation worked. If you have channels with really high count rates (e.g. anything that doesn't remain below a few 10s of CPS for any length of time) this guess will be way off, but that's why you can edit the guesses in the Dwell Times tool in the Tools menu.
I hope that helps. If you have any other questions, please let us know.
Kind regards,
-Bence