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Comparison reveals the better GC column for active semi-volatile analytes

By Ken Lynam and Doris Smith
Agilent Application Chemists

Environmental labs that analyze semi-volatile organic compounds know that some active analytes such as acidic and basic compounds can be difficult to chromatograph. The peaks for these compounds tend to tail when there are active sites along the GC flow path, which makes it hard to determine accurate peak areas and compromises limits of detection. These difficulties can necessitate costly reruns of samples. To overcome this problem, Agilent J&W Ultra Inert Capillary GC columns are individually QC tested with challenging active compounds as test probes. A recent comparison of Agilent’s J&W HP-5ms Ultra Inert GC column and Restek’s Rxi-5ms column showed the advantage of the Ultra Inert column.

Figure 1. The Agilent J&W HP-5ms Ultra Inert column delivers excellent peak shapes for acids and bases, while the Restek Rxi-5ms column shows poor peak shapes for some active compounds. (Enlarge image.)

Rigorous preshipment evaluation ensures performance

Semi-volatile analyses using methods similar to the United States Environmental Protection Agency (US EPA) Method 8270[1] are important in environmental laboratories worldwide. A number of very active analytes in this sample set present significant challenges. Acidic compounds such as benzoic acid or 2,4-dinitrophenol and bases such as aniline or benzidine are particularly susceptible to adsorption onto active surfaces in the sample flow path, including the column itself.

To ensure that you can accurately analyze active compounds at trace levels, each Agilent J&W Ultra Inert Capillary GC column is individually tested with aggressive active probes at low concentration and low temperature.[2-4] This ensures consistent inertness performance, resulting in better peak shapes for active compounds and lower detection limits for your application.

Analysis of active compounds highlights the benefit of inertness testing

To illustrate, Agilent scientists analyzed a subset of the most active semi-volatile analytes included in EPA Method 8270 (an “EPA 8270 short mix”) with an Agilent J&W HP-5ms Ultra Inert column and a Restek Rxi-5ms column, both 20 m × 0.18 mm × 0.18 µm. They chose this column dimension because it can deliver the same resolution as the standard 30 m × 0.25 mm × 0.25 µm columns, but typically with a 30 to 40 percent improvement in analysis speed, allowing you to conduct large numbers of semi-volatile analyses quickly.

The experiments used an Agilent 6890N network GC system equipped with an Agilent 5975B Series MSD and an Agilent 7683B automatic liquid sampler. The experimental conditions were the same for both Agilent and Restek columns, using the same GC, inlet liner, test solutions, and chromatographic conditions. In each case, the researchers installed a new column and conditioned it according to the manufacturer’s instructions. They then used it immediately without exposure to other samples or extraneous factors that could lead to column damage. The goal was to keep all factors constant with the exception of the column, to provide a valid comparison.

Figure 1 shows total ion chromatograms (TICs) of the analytes and internal standards in the “EPA 8270 short mix.” At this low level of 0.5 ng on-column, compound-specific tailing is easy to recognize. The TIC in Figure 1A was produced on an Agilent J&W HP-5ms Ultra Inert column, while the TIC in Figure 1B was produced on a comparable column from Restek. Acidic analytes are highlighted in both chromatograms. You can easily see that the peak shape on the Agilent column is superior for these compounds.

Relative response ratios (RRRs) of 2,4-dinitrophenol to the internal standard acenaphthylene-d10  were measured on two of the Agilent J&W HP-5ms Ultra Inert columns and two of the comparable Restek columns. As shown in Table 1, the RRRs for the Restek Rxi-5ms column were lower across the entire concentration range studied, especially at the low end of the calibration curve.

Table 1. The Agilent columns show superior RRRs for 2,4-dinitrophenol (2,4-DNP). Values are average RRRs from four measurements on each column.

According to EPA Method 8270, the RRR for 2,4-dinitrophenol can be as low as 0.05. At 1 µg/mL, the average RRRs for the two Restek columns were 0.053 and 0.074, respectively, compared to those for the Agilent columns at 0.136 and 0.158, respectively. Having the RRR values near the 0.05 limit will impact reportable detection limits for 2,4-dinitrophenol, potentially resulting in costly reanalysis.

Get better peak shapes and quantification

When you analyze active compounds, consistent column inertness is essential for accurate results. Our preshipment QC testing with demanding active probes is the best means to verify that each column provides excellent performance. When you use columns that are validated at the factory, you can be sure of good peak shapes and reliable quantification for your active analytes. This comparison clearly showed that the Agilent J&W HP-5ms Ultra Inert Capillary GC columns provided better peak shapes for active acidic compounds than the comparable Restek columns. For more information and results, read the complete Application Note (5990-4041EN) about this study.

References

1. US EPA Method 8270D, “Semivolatile Organic Compounds by Gas Chromatography/Mass Spectrometry (GC/MS),” Revision 4, February 2007.
2. Mitch Hastings, Allen K. Vickers, and Cameron George, “Inertness Comparison of Sample of 5% Phenyldimethylpolysiloxane Columns,” Poster Presentation, 54th Annual Pittsburg Conference, Orlando, FL, March 2003.
3. Jim Luong, Ronda Gras, and Walter Jennings, “An Advanced Solventless Column Test for Capillary GC Columns,” J. Sep. Sci., 30, 2480-2492, 2007.
4. “Agilent J&W Ultra Inert GC Columns: A New Tool to Battle Challenging Active Analytes,” Technical Overview, 5989-8665EN, 2008.

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