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How Important is it to Know the Correct GC Column Length?

All gas flows in a GC system are controlled by the electronic pneumatic control (EPC).  The EPC calculates the flow from the applied pressure, nature of the carrier gas, and the column dimensions.  Variations in the carrier gas flow rate will result in retention time issues; therefore, it is important to input the correct method conditions into the system. 

However, how important is it to know the exact length of the column?

Columns are regularly cut by users, with varying unspecified lengths being removed.  It would be impractical to expect users to record the exact length of the small pieces of column that are generally removed, and it is totally impractical to measure the length of the existing column - the standard length being 30 m.  Having to unwind and rewind 30 m would likely damage the column.

In general removal of these small portions has negligible effect on the performance of the column (e.g. removal of a typical length of 3 cm from a 30 m column equates to 0.1% of total length).  Columns may not even be manufactured to length tolerances with this level of precision. 

The assessment of exact column length is of no benefit whatsoever for the majority of applications and is better served by applying meaningful system suitability criteria to critical parameters within the method that are affected by reducing the column length - this would be absolute retention time, resolution, and possibly some effect on peak area due to slight changes in the split ratio (Figure 1).  The criticality of these parameters has to be assessed on a method by method basis and adequate provisions made for system suitability criteria that are relevant, meaningful, and have sensible limits applied.  In general these would be tests such as retention time limits, resolution limits, and linearity.

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Figure 1: Effect of column length on GC separations.


There are certain cases where reduction in column length is more significant - either with short columns (10 m or less) or columns which are exposed to excessive contamination and need larger lengths removed more regularly.  Again this should be captured by system suitability or by the use of retention gaps.  If the column is too short, retention times will be too short and vice versa.

The act of trimming the column will in itself require a change in the column length setting in the instrument (most instruments will calculate column length based on the elution time of an unretained peak and the nominal internal diameter); although the absolute retention time will change, the retention factor value should remain constant (Figure 2).

A high k value indicates that the sample is highly retained and has spent a significant amount of time interacting with the stationary phase.

Figure 2: Calculation of retention factor.

If you do want to measure the length of the column (without unwinding it and possibly breaking it) there are two methods that can be used.1

1) Calculate the length using the following equation:

Column length = π x column cage diameter x number of loops in the cage

For example:
Column length = π x 0.175 m x 56 loops = 30.8 m
 

2) Measure the hold-up time of a non-retained peak, then increase or decrease your GC column length in a column pressure/flow calculator until the hold-up time matches the experimental hold-up time (//www.restek.com/ezgc-mtfc).

 

References

  1. //blog.restek.com/?p=9597

 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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