Jack Cochran explains how to optimise for speed or match your original compound retention times via the use of a novel translator
When discussing the conversion of GC methods from helium to hydrogen carrier gas, generally the focus is on speed as hydrogen has a higher optimal flow rate than helium and can be used to achieve faster run times without sacrificing separation efficiency.
While speedier analysis times offer the attraction of improved productivity, there are times when matching the original compound retention times is more important, for example, to make calibration updates or new method validation easier.
Regardless of whether the goal is faster analyses or maintaining the original compound retention times, proper method translation is critical for success. The new EZGC method translator/flow calculator from Restek is an easy-to-use tool that ensures proper conversion from helium to hydrogen for either speed optimised or matched retention time scenarios.
Increase sample throughput with faster separations
Obtaining faster GC run times so more samples can be analysed in a day is often the driving force behind converting from helium carrier gas to hydrogen. With proper method translation, this can be an easy way to improve productivity and reduce dependence on expensive and increasingly scarce helium.
The conversion requires a faster GC oven program rate for hydrogen versus helium to maintain the same chromatographic elution 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. This 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).
This 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.
This 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.
This 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.
The 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. The free, web-based application is easily accessible and Windows users can download it for offline use.
For more information at www.scientistlive.com/eurolab
Jack Cochran is with Restek.