Two for the Price of One? Using CPP to Simultaneously Clean Pesticide and PCB Extracts

In a recently published blog, we introduced the advantages of cleaning EPA Method 8081 extracts using CarboPrep Plus (CPP) as a replacement.  The sample preparation for PCBs by EPA Method 8082 is commonly combined with the pesticides EPA Method 8081 for the extraction step then split into two portions for sample cleanup.  The logical question is: can I run the pesticide/PCB extract through CPP prior to splitting for separate analysis, thereby avoiding separate clean up procedures?

Before I answer, I should explain that the reason the single extract is sometimes split for post extraction processing is the sulfuric acid procedure cannot be applied to the pesticide portion, it will degrade the acid sensitive analytes.  Sulfuric acid cleanup has been used with PCB extracts since it can be effective at removing much of the colored material that Florisil leaves behind, so it is possible that CPP may be used instead of splitting the sample which results in additional extraction steps.

 Why clean the PCBs extracts at all?  If pesticide extracts are run on the same instrument regular maintenance would be required to meet the stringent inlet inertness requirements for endrin and DDT.  PCB methods typically do not have the same stringent requirements, however samples typically have high concentrations of non-volatile matrix compounds that build up in the inlet effecting chromatography, which effects quantitation.

An effective cleanup is also important since chromatographic interferences make it difficult to perform Aroclor pattern identifications.  Other challenges include weathering which can significantly change the Aroclor pattern combined with different Aroclor mixtures.

One characteristic to the CPP cartridge based upon experience with GCB, lies in its strong affinity for certain compounds.  In this case, the non-ortho PCB congeners have varying levels of interaction to GCB.  The logical question is how the pattern of various Aroclors will be affected by this preferential adsorption of some congeners relative to others when using CPP. Jason Thomas set off on a crusade to answer these questions. First, he started by running Aroclors through the cartridge using the standard elution method of 10 mL hexane/acetone 90:10.  Second, he ran the individual congeners through in a series of mixes, collecting fractions in 5 mL increments and identified congeners that required an additional amount of solvent for elution.  Finally, Jason compared our concentrations with the various Aroclor ratios of congeners reported in a paper by Frame & Cochran1.

First, all PCB congeners eluted in the first 5 mL elution volume except for the following PCBs in Table 1.  The congeners that exhibited retention beyond the first 5 mL of elution were those possessing no chlorine substitution at the otho positions (known as non-ortho PCB congeners) as illustrated in the chart below:

Figure 1: Recoveries of selected PCBs with different chlorine substitutions measured against amount of elution solvent necessary to remove the analyte from the carbon.

The non-ortho PCB congeners have a longer retention on the carbon, but for quantitation of Aroclors, these congeners may not be relevant as it is recognized that not all 209 are actually present in the Aroclor mixtures, which is where the Frame-Cochran paper comes in handy.  Ratios of these congeners in each specific Aroclor were reported.  Adding this information to the above table, it becomes apparent that only five of these non-ortho congeners (highlighted in orange) are expected to appear in any meaningful quantity as defined here with a cutoff of 0.3%.

As an example, let us compare the effect of CPP on the elution of Aroclor 1221 using the standard pesticide Florisil methodology, 10 mL 10% acetone/hexane elution solvent.  We will specifically focus on PCB-15 in Aroclor 1221 as it represents the congener with the highest % representation in a given Aroclor, (4.2% in 1221).

Chromatogram 1: Demonstration of the elution of Aroclor 1221 using standard pesticide Florisil methodology. Control standard is shown in blue and the extraction standard is shown in red.

There is a notable response missing from the CPP processed standard (red trace) versus the control standard (blue trace), roughly half, as expected with a 10 mL elution-based data in Table 1 above.  As indicated, PCB-15 is a minor component of the Aroclor 1221 pattern, the absence of which will not affect quantitation.

This does not constitute a significant change in the overall appearance of 1221.  In the event of quantitation however, choosing this as one of the quant peaks will provide some degree of error, but there are clearly several more prominent and representative peaks to choose from in order to establish the 3 to 5 peak calibration required by EPA 8082.

None of the heavily retained congeners are present in the heavier Aroclor mixtures, 1254 – 1268.  This may also be corroborated by comparing a 1254 standard processed with CPP and one without.  No discernable difference exists between the two suggesting that none of the components of this Aroclor is retained on the SPE cartridge.

Chromatogram 2: Demonstration of 1254 recoveries using CPP where blue is the standard and red is the extract. Little to no differences were observed.

In summary, CarboPrep Plus may be used to clean EPA 8082 extracts for Aroclor quantitation, provided caution is exercised in choosing the quant peaks, setting up the possibility of processing a combined pesticide/PCB extract without the sulfuric acid clean up step.

  1. Frame, G. M., Cochran, J. W., Bowadt, S. S. (1996), Taming Complete PCB Congener Distributions for 17 Aroclor Mixtures Determined by 3 HRGC Systems Optimized for Comprehensive, Quantitative, Congener-Specific Analysis. Journal of High Resolution Chromatography, 19: 657-668.

 

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