CHROMATOGRAPHY 47
pattern for the compounds of interest. For example, when translating a GC-MS pesticides analysis from helium to hydrogen, the conditions for the original method using helium were simply entered into the EZGC method translator and the software returned a translated method. Tis translated method uses a faster flow rate and oven ramp rate. As shown in Fig. 1 on the previous page, the translated method yielded a very comparable chromatographic separation with no elution order changes in nearly half the time.
Maintain original retention times for easier calibration updates In the second scenario, where the goal is to maintain not just the same peak elution order, but also the same retention times as closely as possible, the method conversion is based on using approximately the same linear velocity for both gases, which is best done by matching the holdup time of the new hydrogen carrier method with the helium holdup time from the original method. Here, the EZGC method translator is used in custom mode and the holdup time (and/ or linear velocity) for hydrogen is set to match that of helium (see Fig. 2).
Tis means the GC column is operating below the optimum flow rate for hydrogen carrier gas, but an advantage is gained in being able to use exactly the same GC oven program from the original helium method.
Fig. 3 demonstrates that this approach gives essentially the same retention times as were obtained when using helium, with no noticeable loss in separation even though hydrogen is used at a sub- optimum flow.
Tis technique of matching the linear velocities and holdup times for helium and hydrogen when switching carrier gases can be used to some advantage with GC-MS, where hydrogen is not easily pumped and a higher (optimum) flow would lead to a more drastic detectability loss.
In addition, confirmation of method performance is simpler as the oven program and retention time windows do not change.
Tis approach should allow easier entry for labs making the switch from helium to hydrogen carrier gas for GC.
Summarising the merits of this approach In summary, the EZGC method translator has been built specifically for GC method development. It has a number of practical uses, including increasing speed of analysis through decreasing column length and/or decreasing inner diameter and/or switching to a faster carrier gas.
Te method translator is also suitable for updating the oven temperature program through translation after column trimming for maintenance so peak elution orders do not change.
Another usage is for improving original methods in separation and/or speed of analysis by solving for efficiency or speed in translation.
In addition, the EZGC method translator can also be used for translating methods from GC- FID (or other atmospheric outlet detector) to GC-MS (vacuum outlet) or vice versa.
To conclude, if you are looking to develop a new GC method or to reliably optimise an existingapplication, Restek’s latest EZGC method development tool can save you hours of calculations, guesswork, and trial-and-error. Te free, web-based application is easily accessible and Windows users can download it for offline use.
For more information ✔ at
www.scientistlive.com/eurola
Fig. 3. Get the advantage of switching to hydrogen, without having to reset retention time windows. Use the EZGC Method Translator/flow calculator to establish conditions that give the same retention times as your original method.
Jack Cochran is with Restek.
www.restek.com
www.scientistlive.com
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