Can You Use a Polyethylene Glycol (PEG) Phase in an MS?

One of the most common stationary phases for gas chromatography (GC) is the polyethylene glycol (PEG) or “wax” columns. They are highly polar when compared to polydimethylsiloxane (PDMS) phases, such as a 5-type column
(5% diphenyl/95% dimethyl polysiloxane). The carbon-carbon–oxygen backbone is highly selective for alcohols, glycols, aldehydes, and ethers, offering unique selectivity unavailable in other phases. While many analysts avoid using a mass spectrometer (MS) with a PEG phase, this article will explore the differences in sensitivity, selectivity, reproducibility,
and bleed compared with a traditional PDMS-based column.

In gas chromatography (GC), the most important factor influencing resolution is selectivity. More than 300 different stationary phases for packed columns covering the entire selectivity spectrum from nonpolar (squalane) to highly polar (1,2,3,4-tetrakis(2-cyanoethoxy)butane) have been evaluated. Because coating material onto diatomaceous earth is less challenging than fused-silica surfaces, only a limited number of highly polar GC capillary column phases are available, with few choices for analyzing complex mixtures of polar analytes. One option is polyethylene glycol (PEG), represented by [-C-C-O]_n and available in a range of molecular weights, most commonly between 20,000–40,000. Cross-linking has been successfully performed to increase lifetime and modestly decrease bleed. For many laboratories, the mass spectrometer (MS) has become the detector of choice, since it can provide high sensitivity, quantitative retention time data, and reliable identification. The problem with using a PEG column on an MS has as much to do with high bleed as the fragmentation of the bleed, where a PDMS phase produces primarily m/z = 73, 207, 281, and 355, the PEG produces hundreds of ions that range from m/z -40 to 295. Understanding how bleed impacts sensitivity and reproducibility is the key to successfully using this phase on an MS.

Experimental

The “wax phase” was compared to a “5-type” column: PEG, “wax phase”; and a 5% diphenyl/95% polydimethylsiloxane phase, “5-type phase”. Column dimensions were the same for the phases evaluated (30 m x 0.25 mm, 0.50-µm d_f columns; Restek). Compounds were selected to cover polar (glycerol, 2-butoxyethanol), high-molecular-weight polar (tetraethyleneglycol monomethyl ether and nootkatone), and nonpolar (2-fluorobiphenyl and 2-methylnaphthalene). The MS fragmentation was also considered, since tetraethyleneglycol monomethyl ether (tetra-EGME), 2-butoxyethanol, and, to a lesser extent, glycerol share the same ions as those observed in PEG bleed fragmentation. Glycerol is a useful candidate because it will elute early on the “5-type” and late on the PEG phase. Nootkatone is an intermediate-polar compound with more than 100 fragmentation ions from m/z = 26 to 220 and elutes at 250 °C, where the column bleed is the highest. Both 2-methylnaphthalene and 2-fluorobiphenyl (internal standard) are nonpolar compounds with an intense molecular weight ion and can be used to monitor instrument performance. Ethanol was chosen as a solvent since, at the concentrations evaluated, all compounds were stable in solution. Oven conditions started at 40 °C with a hold time of 1 min, followed by a 20 °C/min to 250 °C, with a final hold of 20 min. The final hold time was set to 20 min to ensure the baseline would be clean from siloxanes or other material condensing onto the column over the course of the study. Three split ratios were used to adjust the on-column concentrations using a 100 ppm standard: 10:1, 100:1, and 200:1 for on-column concentrations of 10, 1.0, and 0.5 ng, respectively. The instrument was tuned after each column installation, and area counts for perfluorotributylamine (PFTBA) m/z = 69 were within 3%, indicating instrument response and sensitivity were equivalent.

Discussion

After conditioning the columns at 250 °C for 1 h, bleed comparisons were performed; while the bleed would be expected to be significantly higher, the differences were still surprising. In Figure 1, the two chromatograms were overlaid, and the peak height for 2-butoxyethanol was set equal for both traces. 2-Fluorobiphenyl was used as an internal standard to measure response factors. A comparison with external response factors showed a difference of less than 2%, confirming that the instrument remained stable throughout the study, even after multiple column changes. Extracted ion chromatograms (EICs) were compared to total ion chromatograms (TICs) for percentage relative standard deviations (%RSDs) with similar results. The data and figures discussed here focus on TICs. The response factor for 2-butoxyethanol averaged 0.32 compared to 0.35 for the 5-type column, with the %RSD being 1% higher for the PEG phase. This compound elutes early in the chromatogram on both columns (Figure 1), and phase bleed did not impact response. Although 2-methylnaphthalene on the PEG column eluted where there was significant bleed, the responses and %RSDs were nearly identical to the 5-type column. This is understandable given the intense molecular ion at m/z = 142, and the low abundance of m/z = 142 as part of the PEG bleed spectrum. A lower concentration standard for this compound would be necessary to show differences. Figure 2 highlights the greatest differences observed during the study, focusing on late-eluting polar compounds, which share many of the same fragmentation ions as those observed with the PEG stationary phase. The tetra-EGME and glycerol responses on the PEG phase were inconsistent, and not surprisingly, the %RSDs increased as concentration decreased. Although glycerol was detected at 0.5 ng on‑column on the PEG phase, the signal‑to-noise ratio was 5:1, which helps explain the high %RSDs for this compound.

Conclusion

Overall, the analysis of polar compounds using a standard PEG phase is possible; however, the differences in bleed between a 5-type and PEG column are significant. Surprisingly, it was possible to detect nootkatone and tetra-EGME at 0.5 ng on‑column concentration, even though those compounds share similar fragmentation ions to those found in the PEG stationary phase.