Most likely a retention time shift is due to dimensional differences in the old and new columns. The previous column was likely trimmed during inlet maintenance and so the new column is likely longer than the old one. Also there is column-to-column variation due to dimensional tolerances range for diameter and length. The diameter of a column can be +/- 6 mm than the nominal diameter. The length can be up to 1 meter longer than the nominal length so that if a 30 meter column is ordered, it is guaranteed that 30m ≤ column length ≤ 31m.
Usually these differences are too small to cause a problem. However, if the old and new columns happen to be at opposite extremes, the difference can be large enough to cause an issue. Here are some suggestions to minimize this effect:
1. If the GC has electronic pressure control (EPC), the best way is to enter the actual column diameter into the software. This actual measured value for the diameter is reported on the Performance Summary Sheet that came with the column. By inputting these values, the ECP will make adjustments to the flow to compensate for any differences in diameter.
2. If you ever trim the column you will want to report the new length to the software as well so that the EPC can compensate.
The main lesson here is that flows and velocities are all mathematically derived based on the column dimensions input in the software. The only way to accurately set the velocity the same is to make a direct measurement using a non-retained analyte. For more information, please refer to the attached document.
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