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Figure 1:

Get the same separation in nearly half the time by

using Restek’s

GC® software to properly convert instrument

conditions when switching from helium to hydrogen carrier gas.

By Jack Cochran and Jaap de Zeeuw

When discussing the conversion of GCmethods from helium to hydro-

gen 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

GC® method translator/

flow calculator is an easy-to-use tool that ensures proper conversion

from helium to hydrogen for either speed-optimized or matched

retention time scenarios.

Increase Sample Throughput with Faster Separations

Obtaining faster GC run times so more samples can be analyzed 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

GC® method translator and the software

returned a translated method. This translated method uses a faster

flow rate and oven ramp rate. As shown in Figure 1, the translated

method yielded a very comparable chromatographic separation with

no elution order changes in nearly half the time.

Maintain the Original Retention Times for Easier

Calibration Updates and Method Revalidation

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

•

Improve throughput by translating your GC method from slower helium to faster hydrogen carrier gas.

•

Substitute expensive helium GC carrier gas with hydrogen and get the same chromatogram with translation.

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Improve MS detectability by using hydrogen at a lower flow rate without sacrificing separations.

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

GC® method trans-

lator is used in custommode and the holdup time (and/or linear veloc-

ity) for hydrogen is set to match that of helium (Figure 2). This means

the GC column is operating below the optimum flow rate for hydro-

gen carrier gas, but an advantage is gained in being able to use exactly

the same GC oven program from the original heliummethod. Figure 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 to Hydrogen:

Optimize for Speed or Match Your Original Compound

Retention Times with Restek's

Method Translator