dead time
/ DED TYME /
Drop a rubber duck and a sticky leaf into a river at the same spot. The duck, never catching on anything, simply rides the current and marks the fastest a floating thing can travel. The leaf lags because it keeps snagging. Dead time is the duck's trip time — how long it takes for something that does not interact at all to pass straight through the column.
Formally, dead time is the time taken by the mobile phase, or by an unretained substance riding with it, to travel from injection to detector. It represents the baseline transit time everything must spend just being carried through, before any interaction with the stationary phase adds further delay.
It matters because dead time is the reference point that turns raw retention times into meaningful comparisons: subtracting it isolates the part of a peak's delay that is actually due to retention, which is what the retention factor needs. The honest caveat is that it must be measured with a truly unretained marker — pick a marker that secretly interacts even a little and every downstream calculation skews.
To find the dead time, an analyst injects a tiny amount of a compound known not to stick to the column; the small peak it makes at 1.5 minutes marks how long it takes to simply ride the mobile phase through.
How long an unretained 'rubber duck' takes to cross the column.
Dead time is also called void time, and it is closely tied to the column's void volume — the empty, mobile-phase-filled space inside the packed column.