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Verify your GC packed/micropacked column carrier gas flow to obtain reproducible results

28 Sep 2014

Working in technical service has taught me many things, but one of the most important has been “don’t assume anything”. This is especially relevant when it comes to verifying the carrier gas flow through GC columns.  To obtain reproducible results from column to column, verifying the carrier gas flow rate through the column should always be done after installing a packed/micropacked column into your GC oven.

 
Packed Column

Setting and verifying the carrier gas flow for packed/micropacked columns is commonly done using an electronic or bubble flow meter.  This should absolutely be done with every column installation because each packed/micropacked column has a unique pressure drop. Remember that these columns contain packings which are not of a uniform particle size, but rather contain particles which fall within a specified range.  In addition, particle size distribution within this specified range can vary.   As a result, do not rely on simply setting the head-pressure.  

As far as I know, there is no instrument software that can automatically set the desired flow rate. Even for those GC’s which have inlets controlled by a mass flow controller, it is still a good idea to measure/verify the flow rate exiting the column.

To set and/or verify the desired carrier gas flow rate when using a packed or micropacked column, follow these steps.

1.  Cool all heated zones and then turn off all GC gases.  Allow the current packed/micropacked column (if one is installed) to depressurize so that its removal from the GC oven (more specifically, the inlet) doesn’t cause a pressure surge which can expel packing from the column.

2.  Install the new column into the injection port.  Do not connect to the detector at this time.   Do not turn on any heated zones.   Slowly increase the head-pressure just until carrier gas starts exiting from the column (holding a thin strip of tissue paper at the column outlet and observing when it moves is a good indicator).

3.  Attach an electronic flow meter (or soap-bubble flow meter) to the outlet of the column and once again begin slowly increasing the inlet’s head-pressure.   When the desired column flow rate has been obtained, continue to monitor the exiting carrier gas flow rate for five minutes to make sure it is stable.

Very long packed columns, packings with irregular shaped particles, very small mesh sizes, and/or very small internal diameter micropacked columns, require high head pressures to obtain proper carrier gas flow.  As a result, many of these columns are commonly used with valve (switching) systems because obtaining reproducible results using a syringe injection can be difficult (sample loss through a punctured injection port septum is common).

By verifying the column’s carrier gas flow rate when you install a different packed/micropacked column, you should obtain reproducible results every time. Thanks for reading.