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Do you measure the length of your GC column?

15 Aug 2013

When purchasing a GC column the length that you buy isn’t necessarily the exact length that you get.  In fact, it is typical that you will receive 0.5 to 1.0 meter more than what you bought!  Knowing the exact length of your column and inputting the correct value into the GC instrument software is important so that the electronic pressure control (EPC) can accurately deliver your desired carrier gas flow.   When you need to trim your GC column for maintenance, the new column length needs to be determined to properly translate the current method and maintain analyte resolution.

So how do you measure the length of your GC column? Well, one way to NOT measure your column is by unraveling the column from the cage and measure using a measuring tape.  That would be pretty messy! There are actually two ways to get an accurate column length measurement.

The first way is to calculate the column length by using the following equation:

Column length = π * diameter of column cage * number of loops on the cage

For example:   π *0.175m * 56 loops = 30.8m

Accuratly determine column length by measuring the diameter of the column cage and counting how many times the column loops around the cage

The second way to determine accurate column length is by first measuring a holdup time (aka dead time, or dead volume) of a non-retained peak.  Then increase or decrease your GC column length in a column pressure/flow calculator until the calculated holdup time matches the experimental holdup time.

Calculating column length using the GC pressure/flow calculator

So do you determine the accurate length of your GC column?  If so, what method do you use?