When using a dual column TCD must both columns be the same? I would prefer to balance the bridge with one empty and one packed (working) column. As long as the bridge is balanced and the flow rates match the TCD won't know the difference?

Steve,

Yes, this is generally true as long as both flow sources are using flow controllers that keep the column flow constant as the oven temperature changes (during a temperature program). You may see minor upsets due to a difference in the hysteresis characteristics of the flow controllers when presented with an essentially zero pressure drop for the empty tube versus the higher pressures on the packed column.

Be aware that any column bleed from the analytical column will not be matched by a corresponding amount of bleed from the (empty) reference "column". This isn't usually a problem with the silicone phases in use today, but was more of a problem with more bleed-prone phases that were predominant 10 or more years ago.

John

Thanks!