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

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.

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

If this is important to me, I would consider to buy a Restek column as they specify column length. I remember that a paper was presented where it was shown that several suppliers supply a “minimum” length. A few meters “extra” , the user would not mind as it could be perceived as “extra”. If users however rely on calculations by instruments, to set their parameters, we have to be mor careful. Thast why Restek made a decision 5 years ago, to supply exact length: A 30 m = a 30m. This is also important if cusatomer works with fixed retention times , like the Retention time locking systems Agilent offers.

If counting loops is not preferred, another way to measure length is to weigh the column. Balances are accurate, the cage has same weight that can be deducted, so if we know the mass/length we can measure it..

Heh, heh, Restek is making you purchase columns and you work for them, Jaap?! Hopefully you’re getting a discount anyway…

Actually, our 30m columns are NOT 30m exactly. They are 30.5m. But that’s why it’s important to use the technique that Michelle describes to get an ACCURATE column length, at the beginning of installation, and then throughout the life of the column.

Jack

This technique is also important because it gives you the effective length seen by the instrument, which takes internal diameter variation into account. A 30.5 meter column with an average internal diameter of 0.24 mm will have the holdup time of a 31.8 meter column with a 0.25 mm ID tested under the same conditions. That is an average internal diameter difference of 0.01 mm (or 10 µm).

[…] time and using the EZGC Flow calculator I determined the true length of the installed column (see Do you measure the length of your GC column? For details), set the He flow rate appropriately to obtain the EZGC recommended 40 cm/sec linear […]