Fused silica columns traditionally have been used for GC biodiesel analysis, but metal columns offer significant performance advantages. How can analysts determine which column is best for their lab? Here we compare fused silica and metal column performance for total glycerin analysis of biodiesel and offer guidelines for column selection.
Excellent chromatography can be obtained using Rtx®-Biodiesel TG fused silica columns. However, for high temperature work (>380°C) metal columns are much more rugged because the polyimide resin used to make fused silica hardens at high temperatures, making columns brittle and producing active sites in the column. To maximize column lifetime, tubing choice should be based on the maximum temperature setting in the GC temperature program. If the temperature program will be 400°C or lower, high temperature fused silica tubing is an acceptable choice; for GC temperatures that will exceed 400°C, metal tubing should be used.
Two fused silica GC column dimensions are available for the analysis of total glycerin: 10m x 0.32mm ID or 15m x 0.32mm ID, both of which are connected to a 2m x 0.53mm ID retention gap for cool on-column injection. The retention gap is factory coupled using Restek's unique Alumaseal™ connector (Figure 1). This innovative connector is leak-tight and low dead volume, making it advantageous for high temperature work.
The primary advantage of using metal MXT® columns is that they are more stable at high temperatures than fused silica columns. This means they will exhibit lower bleed, improving analytical performance, and have longer lifetimes, making them a cost-effective option. High temperature tolerance also means these columns can be brought to high temperatures (430°C) allowing nonvolatile material to be baked off of the column. MXT®-Biodiesel TG columns are available in the same dimensions as their fused silica counterparts: 10 x 0.32mm ID and 15m x 0.32mm ID, both of which are factory coupled to a 2m x 0.53mm retention gap using an Alumaseal™ connector. Excellent resolution of all glycerides is achieved, as shown in Figure 2.
Restek has also developed an innovative column where the analytical column includes a built-in retention gap in a continuous section of tubing, requiring no connectors. This 0.53mm ID MXT®-Biodiesel TG column, is 14m long, and features a 2m x 0.53mm ID Integra-Gap™ integrated retention gap (Figure 3). This product eliminates any need for connections because the column and retention gap are in the same piece of tubing. Target analytes resolve exceptionally well, and the solvent and triglyceride peaks show excellent symmetry (Figure 4). Peak shape for butanetriol is very good, demonstrating inertness, and the resolution and response of the glycerides is also excellent.
There are several column options available for analyzing total glycerin in biodiesel fuels. The best chromatographic solution for this analysis is the 14m x 0.53mm ID MXT®-Biodiesel TG column with the 2m x 0.53mm ID Integra-Gap™ integrated retention gap. This column eliminates the column connection and can be used to 430°C allowing for faster analysis times and higher sample throughput.
| Figure 1: The Alumaseal™ connector | |
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The Alumaseal™ connector is the best column connector for coupling fused silica and metal columns, even columns of different internal diameters. Made of aluminum, it is designed for high temperature performance. These connectors have been factory-coupled and tested using temperature programmed mass spectrometry and have shown no signs of leaks, even at 430°C. |
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The Alumaseal™ connector offers:
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| Figure 2: Derivatized B100 samples resolve well on the 15m x 0.32mm MXT®-Biodiesel TG column, which is factory coupled to a 0.53mm retention gap using an Alumaseal™ connector. | |||||||||||||||||||||||||||||||||||||
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