Determining your GC column length is important so that electronic pneumatic control of carrier gas flow is accurate, whether during initial installation of the column or after maintenance column trimming. Otherwise, you can have more flow going into the detector than you think (especially problematic in MS as you might lose sensitivity) or even see elution order changes in your chromatography. Calculation of GC column length is easy (or EZ!) with the EZGC Method Translator and Flow Calculator, just by performing a holdup time determination. Holdup time, the amount of time it takes an unretained compound to traverse the GC column from inlet to detector, can be determined by split injecting air (GC-MS), methane (GC-FID), or methylene chloride headspace (GC-ECD) and noting the “retention time”. In the last case, GC-ECD, make sure the oven temperature is at least 250° since methylene chloride will show some retention at lower column temperatures. Don’t overload the column with the compound, i.e., the peak should be symmetrical for an accurate determination. Start your work by entering the nominal value (e.g. 30m) for length in your GC control software, enter a flow, and then do the holdup time determination. I usually take the average “retention time” of three analyses.
I’ve outlined the rest of the procedure in the figures below. Note that I’ve used the Download-ed version, which has the “spinner” for easy adjustment of column length, but it will work for the web-based version, too, just by entering column length values instead of “spinning”. Shoot me an e-mail if you have any questions.