A Tale of Two Columns (CLPesticides and CLPesticides2)—Part II: Gaining Speed

In my previous blog post, I gave you a little history of the CLPesticides columns.  You’ll remember that I pointed out the 24 minute run times, which were promoted as being fast at the time.  Fortunately, there are ways to attain faster runs on this column pair for standard 8081 pesticides, due to their awesome selectivity and high plate count.

So let’s talk a little about fast GC…what variables can we control to get faster runs?  Here are a few:

1. Increase carrier gas flowrate: Higher linear velocities of carrier gas will reduce analysis time. The catch is that after a certain point, higher flows can sacrifice separation capacity.  If a column has a sufficient number of plates, however, this can be a powerful way to speed up a method and still maintain acceptable separations.
2. Increase temperature ramp rate: Increasing the programming rate will allow for quicker elution of compounds. At the same time, a ramp rate that is too high can lead to co-elutions.  If enough plates are present and the column has good selectivity, the limiting factor may ultimately become the instrument itself and its maximum ramping capability.  In the next two blogs I will offer a solution to push an Agilent GC even further and achieve ramp rates that weren’t previously possible.  (Spoiler alert: It involves a product known as the GC Accelerator Oven Insert Kit, cat# 23849.)  As a final word of caution, increasing the temperature ramp rate can also result in changes in elution order of compounds, so care must be taken.
3. Use hydrogen carrier gas: Because of the higher diffusivity of hydrogen compared to helium or nitrogen, it has a higher optimum linear velocity, providing more separation capacity at higher flow rates comparatively (see point #1 above). The Van Deemter Plot (Figure 1) shows how the optimum linear velocity of hydrogen compares to helium and nitrogen. If switching carrier gases, check out Restek’s EZGC Method Translator, which will allow you to transfer an existing method from one carrier gas to another, without changing your separation.  

Figure 1: The Van Deemter plot shows the height equivalent of a theoretical plate vs the average linear velocity of different carrier gases. A lower plate height leads to more overall plates per length of column, thus allowing higher separation capacity. Hydrogen has the highest optimum linear velocity.

• Shorter columns and reduced I.D.: As intuition would suggest, it takes analytes less time to travel through a shorter column than a long column. If all other things are equal, however, you sacrifice plates and could lose resolving power if you don’t have much to spare in the first place.  The best approach for this to be effective is to actually “scale down” your column; that is, not only make it shorter, but also reduce the inner diameter to increase interactions with the stationary phase.  By doing this and also keeping the phase ratio the same, you can achieve the same separation at a faster speed.  In order to ensure your separation remains the same, use Restek’s EZGC Method Translator, which allows you to translate methods from one column dimension to another.  Note that decreasing length and I.D. will require a higher oven ramp rate to maintain the same separation, which may exceed the limits of your instrument.  (Spoiler Alert #2: This can also be solved with the use of Restek’s GC Accelerator Kit, cat# 23849.)

A few years ago, Restek set out to provide a faster run time on the CLPesticides and CLPesticides2 columns.  Innovations Chemist, Jason Thomas, utilized the first 3 above points to elute all 8081 compounds in under 7 minutes, with near baseline resolution.  Check out this method here: chromatogram GC_EV1325. This method uses hydrogen carrier gas at a high flow rate and a multi-step ramp rate with a fast beginning and ending ramp.  This was performed utilizing 30 meter columns with 0.32mm inner diameters, the most popular column format for this analysis.

 

Due to instrument ramp rate limitations, this method is about as fast as possible using a standard Agilent 120V instrument.  In order to gain any more speed and maintain similar resolution, we must find a way to increase the maximum achievable oven ramp rates.  Enter the GC Accelerator Kit.  Stay tuned for parts 3 and 4 to see how to use this kit to shave a couple more minutes off of the analysis.