The stop depth or seating depth is the distance between the end of a ferrule and the end of a section of tubing in the inlet port of the column. If the stop depth is too short, there will be a gap between the tubing and the column inlet. On the other hand, if the distance is too long, the ferrule will not seat properly in the inlet port. Each of these two scenarios is shown below, with the extra volume contribution from the improper fit shown in red. This will result in band broadening and/or leaking.
Knowing the right stop depth is important because once a ferrule is swaged onto a piece of SS tubing, the distance cannot be altered. If it needs to be changed, you will have to cut off that section and start over with a new ferrule.
On Cogent™ HPLC columns, the stop depth is approximately 2.3–2.4mm. Note that this distance is not the same for columns of all manufacturers. When you use another column, you may need to use a different tubing/ferrule to accommodate the different stop depth for that column.