Revisiting the optimal heating rate for gas chromatography

The primary goal of the GC oven temperature program is to maximize the peak capacity of our chromatogram. Temperature programming, when paired with constant flow (pressure programmed GC) allows us to maintain sharp narrow peaks throughout the entire chromatographic run.

Jack Cochran gave us some of the basic tools for optimizing the temperature program in his blog on optimizing Fast GC over a decade ago, based on Leonid Blumberg’s series of papers: Theory of Fast Capillary Gas Chromatography. These papers gave us the formulas for Speed Optimized Flow (SOF) [capillary column ID (in mm) multiplied by 8 for helium (or 10 for hydrogen or 2.5 for nitrogen)], Efficiency Optimized Flow (EOF) [SOF divided by 1.41], and Optimal Heating Rate (OHR) [10°C divided by the column void time (in minutes)]. Recently, LECO’s Mark Merrick revisited the OHR with Blumberg because the original formula had been determined using constant pressure mode at 50°C. The new OHR for constant flow mode is 12°C divided by the capillary column void time, and was determined at 150°C, a temperature more relevant to modern gas chromatography. This work is especially relevant to LECO because maximizing the peak capacity in the first dimension is a critical part of optimizing GCxGC analysis.

We can see the difference in peak widths using the Pro EZGC to model the 209 PCB congeners on a 60m x 0.25mm x 0.25µm Rxi-XLB column. The old and new OHR values calculate out to 3.4 and 4.5 mL/min. Starting the GC at 100 with SOF and no initial oven hold, the peak widths range (in minutes) from 0.069 to 0.088 for the 3.4°C/min OHR value and from 0.056 to 0.068 for the 4.5°C/min OHR value. The maximum and minimum peak widths had a 24.2 and 19.4 percent difference respectively, indicating that the constant flow derived optimal heating rate better matches the speed optimized flow rate than the heating rate derived under constant pressure conditions.