Is separation of benzo[b]fluoranthene and benzo[k]fluoranthene on 5% phenyl-type GC columns really that important for environmental analyses?

Some gas chromatographers might say so because of the following statement in US EPA Method 8270D (Semivolatile Organic Compounds by Gas Chromatography – Mass Spectrometry (GC-MS):

“ Structural isomers that produce very similar mass spectra should be identified as individual isomers if they have sufficiently different GC retention times.  Sufficient GC resolution is achieved if the height of the valley between two isomer peaks is less than 50% of the average of the two peak heights.  Otherwise, structural isomers are identified as isomeric pairs.  The resolution should be verified on the mid-point concentration of the initial calibration as well as the laboratory designated continuing calibration verification level if closely eluting isomers are to be reported (e.g., benzo[b]fluoranthene and benzo[k]fluoranthene).”

The language above ignores the fact that benzo[j]fluoranthene is a polycyclic aromatic hydrocarbon (PAH) isomer consistently found at significant levels in environmental (and food) samples, and more importantly, that it coelutes with either, or both, benzo[b]fluoranthene and benzo[k]fluoranthene on 5% phenyl-type GC columns. In this blog post, I will define the extent of the coelution on 30m x 0.25mm x 0.25µm Rxi-5Sil MS (5% phenyl as silphenylene, similar to DB-5MS) and 30m x 0.25mm x 0.25µm Rxi-5ms (5% phenyl as diphenyl, similar to DB-5).

PAHs were first analyzed using hydrogen efficiency-optimized flow (EOF), and optimal heating rate (OHR), with an Agilent 6890 GC-FID. Injections of PAH standards (prepared from SV Calibration Mix #5 / 610 PAH Mix and a custom benzo[j]fluoranthene standard) at a split ratio of 100:1 into a 4mm Precision split liner with wool served to minimize injection band widths to keep overall system efficiency high.  In addition to OHR, chromatograms for each column were generated at 1.5 x OHR and 0.5 x OHR to force different elution temperatures for the benzofluoranthenes to see what effect that had on their separation.

As you can see in Figure 1, the Rxi-5Sil MS shows an intractable coelution for benzo[b]fluoranthene and benzo[j]fluoranthene under all experimental conditions, with elution temperatures around 278, 293, and 257°C for OHR, 1.5 x OHR, and 0.5 x OHR, respectively.  The Rxi-5ms has a different coelution, that of benzo[j]fluoranthene and benzo[k]fluoranthene for OHR and 1.5 x OHR, but interestingly is able to partially resolve the benzofluoranthenes under 0.5 x OHR conditions (Figure 2).  Elution temperatures in this case were approximately 280, 295, and 258°C (OHR, 1.5 x OHR, 0.5 x OHR).

What does this coelution situation mean for environmental analysts? First, all quantitative values reported for benzo[b]fluoranthene and benzo[k]fluoranthene are likely inaccurate for at least one of the isomers depending on the GC column choice if using a “5-type” stationary phase.  Second, technically speaking, analysts should not be reporting the benzofluoranthenes as “individual isomers” according to the language of EPA Method 8270D when using a “5-type” GC column.

If an analyst needs to report benzofluoranthenes as “individual isomers”, a 30m x 0.25mm x 0.25µm Rxi-17Sil MS (or even a 15m version) will work well.  If the challenge is to separate the benzofluoranthenes and, triphenylene and chrysene (228 m/z coelution on 5-type columns), consider the Rxi-PAH GC column.

BF Fig 1


BF Fig 2

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