# Speed Optimized Flow and Optimal Heating Rate in Gas Chromatography

21 Nov 2010Leon Blumberg and Matt Klee have written some excellent articles on optimization of gas chromatography for flow and oven temperature programming that seem to have flown largely under the radar of many (most?) practicing gas chromatographers. I’ve put two significant references below on Speed Optimized Flow (SOF) and Optimal Heating Rate (OHR) that I think you’ll want to check out. SOF and OHR are aimed at maximizing peak capacity under relatively fast analysis conditions. Until you get these articles (you will, won’t you?), let me show you how I used these concepts to improve throughput and sensitivity for polybrominated diphenyl ether (PBDE) analysis using GC.

I used a 15m x 0.25mm x 0.10µm Rtx-1614 column with various helium carrier linear velocities (determined at starting oven temperature) or SOF. We think of helium being optimum in the 20-40 cm/sec range. SOF in mL/min for helium, by Blumberg’s definition, is 8 x Column ID (mm), or in this case, 8 x 0.25 to give 2 mL/min SOF. I also used the OHR for GC oven temperature program, no matter what the flow was. And OHR, by definition, is 10 / void time (holdup time in min). Remember that you can either determine the void time by injecting an unretained component, or get it by using the HP Column Pressure/Flow Calculator (free as a download from Agilent). I’ll let you work out the math and get right to the figures to make my points. By the way, click on the thumbnails to see the figures well and then use the back button of your browser to get back to my wonderfully written text!

A couple of other details: After setting the linear velocity where I wanted it at starting oven temperature in the figures below, I used Constant Flow. And the work was done with electron capture detector (ECD).

Can you tell any differences in the first two figures, the ones where I varied the linear velocity, finally ending with SOF? I’ll give you the answer. There is essentially no difference in the separation efficiency, but the analysis time drops from 25 min to about 9 min. Amazing, huh?! Don’t believe it? Well review the next figure, the one with four chromatograms on it showing resolution between two important BDEs, 49 and 71. The resolution for all practical purposes is the same. Notice anything else? Since those chromatograms are all plotted on the same scale, we are improving our sensitivity as we move towards SOF. That is because we are keeping peaks narrow by having optimized flow and optimal column heating rate. We get a five-fold increase in sensitivity from 20 cm/sec to SOF with no tradeoff in separation!

Finally, if you really want to set a speed record, go for hydrogen. SOF for hydrogen carrier is 10 x Column ID (mm), or in this case 2.5 mL/min. Matching that with OHR, we now have a 6.6 min analysis time for this set of PBDEs. I call this Chromatographic Magic. But it’s really just good science. Thanks Leon and Matt!

Helium carrier at ~ 20 and 30 cm/sec with optimal heating rate conditions for PBDE analysis.

Helium carrier at ~ 40 cm/sec and Speed Optimized Flow with optimal heating rates for GC-ECD of PBDEs.

Resolution between critical BDEs is similar, but sensitivity improves with narrower peaks from Speed Optimized Flow and Optimal Heating Rate GC-ECD conditions.

Opitmized Fast GC with Hydrogen Carrier!