Peak Capacity in Capillary GC

Peak Capacity in capillary gas chromatography is different than sample loading capacity, which is something I’ve posted on multiple times recently in ChromaBLOGraphy.  Peak capacity is simply the number of theoretical peaks that can “fit” inside a chromatogram under some definition of how much they should be separated (e.g. baseline resolved or some other criterion).  While peak capacity is something we like to maximize in GC, it always comes at the cost of speed of analysis.

I did some experiments to define peak capacities on a 30m x 0.25mm x 0.25µm Rxi-5ms using hydrogen carrier gas under efficiency-optimized flow (EOF), speed-optimized flow (SOF), and optimal heating rate conditions, and then filled in spaces between and outside-of those starting points.  I analyzed SV Calibration Mix #5, a polycyclic aromatic hydrocarbon (PAH) standard, with split injection – GC-FID.  This standard includes 16 PAHs across a wide volatility/elution range from naphthalene to benzo[ghi]perylene.  Split injection via a Precision split liner with wool minimizes injection band widths, which is critical to estimating peak capacity based on the column conditions.

As you can see from the graph below, peak capacity plateaus in the range of 1.3 to 2.5 mL/min hydrogen carrier gas under the imposed criterion of using an optimal heating rate (OHR) of 10°C divided by the holdup time in min.   Importantly, using SOF with an OHR drops the analysis time substantially without a huge loss in peak capacity.

The Performance Measurements table shows that resolution between PAH isomers benzo[b]fluoranthene and benzo[k]fluoranthene holds up well for EOF and SOF conditions.  Not surprisingly, as faster column flow and heating conditions are used, signal-to-noise is better for analyzed components.

In summary, if you are looking to maximize the number of peaks you can put in a chromatogram, while simultaneously paying attention to speed of analysis and detectability, give EOF and SOF and OHR a try.  These are great concepts as method development starting points.  Use the EZGC™ Method Translator and Flow Calculator to help with holdup time and other considerations.

Finally, don’t forget stationary phase selectivity, which is the most important parameter for separating specific components.  But we’ll get back to that in a later post…

Theory of Fast Capillary Gas Chromatography – Part 3: Column Performance vs. Gas Flow Rate
Leonid M. Blumberg
Journal of High Resolution Chromatography  – 1999, 22, (7) 403-413

Optimal Heating Rate in Gas Chromatography
L.M. Blumberg and M.S. Klee
Journal of Microcolumn Separations – 2000, 12 (9), 508-514

Plate Height Formula Widely Accepted in GC is Not Correct
Leonid M. Blumberg
Journal of Chromatography A – 2011, 1218, 8722-8723

Temperature-Programmed Gas Chromatography
Leonid M. Blumberg
Wiley-VCH Verlag & Co. – 2010

Peak Capacity Graph 2

Peak Capacity Table

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