Rebecca Stevens and Chris English explore how replacing a 624-type column with an Rxi-1301Sil MS column reduces GC-MS analysis time while maintaining good separation of glycol ethers
Glycol ethers are high production volume industrial chemicals that often occur as complex mixtures of isomers. Their unique amphiphilic structure gives them favourable properties such as low volatility, strong solvent strength, high water solubility and the ability to serve as coupling agents that promote the miscibility of aqueous and organic phases. These properties make glycol ethers a popular alternative to traditional oxygenated solvents such as ketones, ethers and alcohols.
Because toxicological studies indicate that some glycol ethers pose a serious health hazard, the EU and Canada have both adopted regulations limiting their use in consumer products. In the EU, several glycol ethers are banned entirely while others are regulated to a specific concentration level. In 2010, Environment Canada (EC) published a reference method [1] for analysis of glycol ethers in consumer products including cleaners, degreasers and coatings. The EC method addresses the most commonly used glycol ethers and describes an analytical approach for achieving reporting limits.
Column choice is key to faster analysis
The EC method recommends a standard 624-type column in a 60m x 0.32mm x 1.80 µm format and a temperature program that spans 30 minutes for glycol ether analysis. To reduce analysis time, we used an Rxi-1301Sil MS column instead of a 624 column. The Rxi-1301Sil MS column has very similar selectivity to that of 624-type columns as both are based on cyanopropylphenyl stationary phases; however, the thinner film and higher thermal stability of the Rxi-1301Sil MS column provide good separations in a fraction of the time.
The Rxi-1301Sil MS column in a 30m x 0.25mm x 0.25 µm format with optimised run conditions results in an analysis time of just 8 minutes (Fig. 1). Using a fast temperature program, baseline resolution of the DPGME isomer group is possible and is comparable to - if not better than - the original method. The 0.25 µm film results in higher efficiency, narrow peaks, and, in turn, improved resolution with less retention. The Rxi-1301Sil MS column also has a temperature limit of 320°C, allowing for a high-temperature isothermal hold to remove any low-volatility sample components from the column. If these low-volatility contaminants are not removed they may elute in subsequent analyses as broad ‘ghost’ peaks, possibly interfering with the analytes of interest.
Faster run times can also be achieved using narrow-bore 624-type columns with the same phase ratio (e.g. 30m x 0.25mm x 1.4 µm). However, as shown in Fig. 2, chromatographic resolution is reduced in exchange for speed in this situation, and coelution of the DPGME III isomer with DEGEE occurs. Poor resolution of the TPGME isomer group is also apparent. In contrast to 624-type columns, the Rxi-1301Sil MS column provides better resolution of critical compounds for glycol ether analysis in a much faster analysis time.
Although column bleed can be a concern with some stationary phases, the Rxi-1301Sil MS column can be used with confidence. At 300°C (the maximum temperature used in the EC method) column bleed is minimal and does not interfere with mass spectral compound identification. Low column bleed is critical to the operation of sensitive mass selective detectors and allows full scan spectra to be collected without interference from bleed ions.
Summarising the merits of this approach
The new Rxi-1301Sil MS column from Restek provides a significant opportunity to improve glycol ether analysis. Although 624-type columns are often used for this work, the Rxi-1301Sil MS column allow separations to be achieved in much faster analysis times and with very low bleed. As shown here, glycol ethers of regulatory concern can be analysed in just eight minutes while still achieving good separations of the key isomers. Labs interested in reducing analysis times for current methods based on 624-type columns should consider adoption of the Rxi-1301Sil MS column into their testing programmes.
References: [1] Environment Canada, En14-33/2010E-PDF, Reference method for the analysis of 2-butoxyethanol (2-BE) and other glycol ethers (GEs) in selected products (automotive and household cleaners, paints, paint strippers and solvents), 2010.
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Rebecca Stevens is product development chemist and Chris English is applications manager at Restek.
