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Chemical Standards and Capillary Columns for US EPA Methods 608.1 and 608.2

By: Katherine Stenerson, Reporter US Volume 26.5


Katherine Stenerson

Katherine.stenerson@sial.com

Introduction

US EPA Methods 608.1 and 608.2 are extensions of Method 608, and describe the extraction and analysis of certain organochlorine pesticides in municipal and industrial wastewater. This method requires the use of an electron capture detector (ECD) and two GC packed columns of different chemistries to confirm the presence of pesticides found in unknown samples. EPA, however, allows the use of open-tubular capillary columns, as described in the Gas Chromatography sections of both 608.1 and 608.2. Considered an improvement over packed columns for this application, capillary columns will provide superior resolution and peak shape, and often have longer usable lifetimes.

Recently, Supelco introduced two new analytical standards for Methods 608.1 and 608.2. The standards were formulated to contain all pesticides listed in the methods, and are made in isooctane, making them compatible with further dilution into hexane. This article will discuss the analysis of these new standards on three different capillary GC columns. Since many laboratories utilizing these methods may choose to cover both pesticide lists in a single GC analysis, we will show analysis of the combined two standards on each column.

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Non-Polar Capillary GC Column: SLB-5ms

A 5% phenyl-type column is a common choice as the primary GC analysis column for organochlorine pesticide methodologies. This type of column incorporates phenyl groups on a dimethylpolysiloxane polymer backbone. Selectivity is based mainly on dispersive interaction (London dispersion forces), resulting in separation primarily by boiling point. The phenyl content of the phase also provides some selectivity for molecules in which a dipole can be induced. The Supelco SLB-5ms capillary column is in this class.

In many 5% phenyl phases, the phenyl group is attached as a side or “pendant” group on the dimethylpolysiloxane polymer. The polymeric structure of the phase used in the SLB-5ms is such that the phenyl groups are incorporated into the polymer backbone itself. This type of polymeric structure is termed “silphenylene”. The structure of a silphenylene polymer makes it more stable and less susceptible to the degradation reaction that produces column bleed. This is a desirable characteristic for columns used with highly sensitive detectors such as an ECD.

In Figure 1, the analysis of the 608.1 and 608.2 pesticides is illustrated on this column. The pesticides eluted generally from lowest to highest boiling. One coelution was noted, chlorobenzilate and chloropropylate. These two pesticides are very similar in structure, differing by a single methyl group. In this case, the use of a more polar confirmation column is necessary to resolve the pair, by differentiating between the subtle differences in dipole moments between the compounds.


Figure 1. Method 608.1 and 608.2 Pesticides on the SLB-5ms, 100 ppb (28471-U)(24103-U)(28372-U )(40351-U)(40352-U)

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Intermediate Polar Capillary Columns: SPB™-608 and Equity®-1701

As stated earlier, the confirmation column chosen for Methods 608.1 and 608.2 must be of a different chemistry than the primary column, and thus result in different retention times for each of the analytes. It should also resolve any coelutions observed on the primary column.

The Supelco SPB-608 has higher polarity than the SLB-5ms, and consequently, provides different selectivity. The exact composition of the phase is proprietary, however it is known that it displays selectivity for pesticides based on pi-pi and dipole-induced dipole, in addition to dispersive interaction. The mixture of 608.1 and 608.2 pesticides shown previously on the SLB-5ms was analyzed on the SPB-608 (Figure 2). Overall, the pesticides were more strongly retained by the SPB-608 than the SLB-5ms. The selectivity of the SPB-608 provided resolution of the chloropropylate/chlorobenzilate pair, however dicloran and pentachloronitrobenzene coeluted.


Figure 2. Method 608.1 and 608.2 Pesticides on the SPB-608, 100 ppb



The Supelco Equity-1701 column is also of a higher polarity than the SLB-5ms. It is made using a cyanopropylphenyl dimethylsiloxane stationary phase. In addition to dispersive, the Equity-1701 also exhibits strong dipole and moderate basic interactions. Its selectivity for pesticides is different from the SLB-5ms, thus it has often been used as a confirmation column. The 608.1/608.2 pesticide mixture was analyzed on this column (Figure 3), and as expected, retention times were different by >2 minutes from the SLB-5ms. However, compared to the SPB-608, retention times were similar (<0.5 min.) Despite this, a difference in selectivity was noted between the Equity-1701 and SPB-608, as dicloran was resolved from pentachloronitrobenzene. The chloropropylate/chlorobenzilate pair also resolved, although not to baseline as on the SPB-608.


Figure 3. Method 608.1 and 608.2 Pesticides on the Equity-1701, 100 ppb


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Conclusion

If the Supelco SLB-5ms is chosen as the primary column for the analysis of EPA Methods 608.1/608.2 pesticides, a confirmation column must be chosen that will resolve any coelutions, and exhibit different retention times for the analytes. Both Supelco SPB-608 and Equity-1701 columns meet these criteria, and either could be used as a confirmation column for this application. In addition, these columns have utility for other applications such as the analysis of pesticides by EPA Method 8081 or PCBs by EPA Method 8082.

The analytical standards used for this illustration are now available as separate standard solutions for Methods 608.1 and 608.2. They can be easily diluted into hexane for final use, and used separately or combined into a single working standard. As always, each chemical and solvent used in the preparation of these standards has been screened for identity and purity. The mixtures are gravimetrically prepared and quantitatively analyzed by gas chromatography. A certificate of analysis accompanies each standard.

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