Sample loading capacity for PAHs on a 30m x 0.25mm x 0.25µm Rxi-5ms GC column20 Oct 2014
Sample loading capacity for a GC column (also known as “column capacity” and “sample capacity”) is essentially the amount of non-active compound that can be put on a GC column and chromatographed at some set of conditions where the peak shape is symmetrical. Conversely, if a GC column is overloaded with a component amount, the peak will exhibit the classic “shark fin” shape. Most vendors estimate the sample loading capacity for a 0.25mm x 0.25µm GC column to be around 50-100 ng per analyte. But is this even close to accurate? Perhaps, but it depends on the compound of interest.
Environmental analysts already know that polycyclic aromatic hydrocarbons (PAHs) are prone to overload on GC columns, especially the typical “five” type columns used for semivolatile organic compound analysis in EPA methods, e.g., 8270 and CLP. Although the CLP semivolatiles method states that a GC column should be able to “accept up to 160 ng of each compound listed in Exhibit C (Semivolatiles), without becoming overloaded”, I could find no quantitative data indicating when overload occurs for Exhibit C PAHs on the commonly used 0.25mm x 0.25µm GC columns. That’s my lead-in to say, “I’ll test it myself!” With the follow-up… “And post about it on ChromaBLOGraphy”.
All work was done with a 30m x 0.25mm x 0.25µm Rxi-5ms utilizing hydrogen efficiency-optimized flow, and optimal heating rate, with an Agilent 6890 GC-FID. Injections of PAH standards (prepared from SV Calibration Mix #5 / 610 PAH Mix) at a split ratio of 10:1 into a 4mm Precision split liner with wool served to minimize injection band widths to keep any peak deformations associated with the column, and not the inlet. As you can see in Figure 1 by the “shark fin” shape of the peaks and the unacceptable resolution between benzo[b]fluoranthene and benzo[k]fluoranthene, and indeno[123-cd]pyrene and dibenzo[ah]anthracene, 200 ng of each of these PAHs greatly exceeds the sample loading capacity of this column. Figure 2, at 50 ng each compound, also shows gross overloading of PAHs, and although the separation starts to improve between critical pairs, the retention times are still shifted later based on peak deformation. Finally, at 12.5 ng (Figure 3), the chromatogram looks good in separations and retention times, comparing relatively nicely to Figure 4 at 3.13 ng, where we would not expect overload.
Even at 25 ng of later eluting PAHs (Figure 5), we have overload, which puts the maximum sample loading capacity of the 30m x 0.25mm x 0.25µm Rxi-5ms GC column under these operational conditions between 6.25 and 25 ng each PAH, an order-of-magnitude less than what is suggested to be possible by the CLP method. Can we tolerate overload and still get our work done? That depends on your data quality objectives, which might include:
Separation between critical, often isobaric, pairs.
Overall required peak capacity for the chromatogram (the 200 ng overload cut the peak capacity by almost 3).
Capacity of the detector to handle overload (MS systems are very sensitive these days).
I prefer the peaks look more like those seen in Figure 7, and given the sensitivity of today’s MS systems, we can inject less (maybe via “Shoot-and-Dilute”), see almost as much, and probably keep our systems up longer due to less involatile “dirt” put on the GC column.