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High Temp. Stability Problem Solved with New Metal Columns

Analysis of Total Glycerides in Biodiesel Oils by ASTM D-6584 Using New MXT-Biodiesel TG Capillary Columns

One of the biggest challenges in biodiesel fuel analysis is the accurate determination of residual triglyceride content. Triglycerides are present at low levels and elute at high chromatographic temperatures. Any suitable column must be operated at temperatures up to 380°C, which considerably strains conventional capillary tubing and stationary phases. Restek has developed new 0.32mm ID and 0.53mm ID MXT stainless steel capillary columns—the MXT-Biodiesel TG line—specifically for high temperature biodiesel analysis. Here we demonstrate the analytical advantages of full metal columns: unsurpassed stability at high temperatures, excellent peak symmetry for triglycerides, highly reproducible retention times, and unsurpassed column lifetimes.

Introduction

"Biodiesel", "B100", "B20", "B10", and "transesterification" are fast becoming everyday terminology. Biodiesel oil is biodegradable, nontoxic, does not contain aromatics, and the absence of sulfur from B100 precludes sulfur dioxide emissions. The "B" number designates the percentage of biodiesel in a biodiesel/petroleum diesel blend (e.g., B20 is 20% biodiesel / 80% petroleum diesel). Biodiesel is recognized as a desirable alternative energy source to petroleum-based products. However, excessive amounts of glycerides in biodiesel oil products can foul engine injectors and form deposits on valves, pistons, and injector nozzles. Also, separation of the glycerin during storage or in vehicle fuel tanks can reduce the shelf-life of the product. Clearly, accurate, efficient methods for quantifying glycerin and glycerides are critical to the biodiesel industry.

The American Society for Testing and Materials (ASTM) and the European method Deutsches Institut fur Normung (DIN) describe several physical and chemical testing methods for biodiesel oil. Gas chromatography (GC) is ideal for measuring important parameters such as total glycerin, fatty acid methyl esters (FAMEs) and methanol levels in biodiesel fuel. Methods like ASTM D-6584 and EN14105 set the industry standards for testing total glycerin and glycerides in biodiesel oil. The gas chromatographic column recommended for the analysis is a 10m x 0.32mm ID column with a 0.1µm film of 5% diphenyl/95% dimethyl polysiloxane, connected to a 0.53mm retention gap. The high temperatures required by these methods to elute triglycerides are a significant challenge to column stability and restrict column material choice to fused silica or metal.

Full Metal Advantage

Typically the fused silica column is the first choice for GC analysis, however at higher oven temperatures (>380°C) the polyamide coating on the fused silica tubing deteriorates, reducing column lifetimes (Figure 1). Even fused silica columns designed for high temperature applications (HT equivalents) become unpredictable and break down relatively quickly. In response, Restek has developed the MXT-Biodiesel TG column line, a line of metal columns designed with stainless steel tubing and our patented Siltek deactivation technology, ensuring maximum heat tolerance. The metal MXT tubing does not degrade, even under temperatures up to 430°C, which allows analysts to "bake out" any residue eluting out after the triglycerides without damaging the column. This "bake out" process keeps the analytical system clean so subsequent injections do not have carry-over from previous samples.

Figure 1a & 1b  MXT-Biodiesel TG columns (Figure 1a) are undamaged by high temperature heat cycles compared to the HT fused silica columns (Figure 1b) which break down under the same conditions (100 temperature cycles to 430°C totaling 500 minutes at maximum temperature.) Note extensive pitting on the fused silica column.
Figure 1a  MXT-Biodiesel TG column Figure 1b  ZB-5HT Inferno column

Stability Solutions

The MXT Biodiesel TG columns are deactivated using Siltek technology, which creates a unique intermediate layer that stabilizes the stationary phase and provides unsurpassed inertness. Due to Siltek deactivation, the stationary phase is extremely stable, exhibiting virtually no bleed even at temperatures as high as 430°C. Column inertness is demonstrated by evaluating peak shape and retention time stability.

Peak shape can be affected by active sites in the analytical column; higher asymmetry values indicate greater exposure to active sites, meaning the column is less inert. Peak symmetry of butanetriol on a commercial HT equivalent fused silica column deteriorates after just 20 injections, compared to the excellent symmetry that is maintained on the MXT-Biodiesel column (Figure 2). While the HT equivalent fused silica column was specified to be stable up to 430°C the metal MXT-Biodiesel TG column shows no sign of activity and is clearly more stable and inert.

In addition to peak shape, consistent analyte retention times can be used to demonstrate column stability. The data in Figure 3 show the retention time of tricaprin over 100 injections for the HT equivalent fused silica and the MXT-Biodiesel TG columns. The decrease in retention time of tricaprin on the HT equivalent fused silica column indicates liquid phase is being lost from the column. In contrast, the retention time for tricaprin on the MXT-Biodiesel TG column stays consistent, indicating no phase loss due to cycling the column at high temperature. Practically, this translates into reliable performance and longer column lifetimes.

Figure 2  Metal MXT-Biodiesel TG columns are more stable and inert than the commercially available HT equivalent fused silica columns as evidenced by symmetric and consistent peak shape for the internal standard butanetriol.

Figure 3  Retention time is stable on metal MXT-Biodiesel TG columns, even under high temperature cycling.

Unique Solutions that Simplify Practical Operation

Factory connected 0.32mm MXT-Biodiesel TG columns & 0.53mm retention gaps

For accurate analysis of heavy triglycerides, on-column or PTV injection is required. Analytical methods ASTM D-6584 and EN-14105 describe the use of a 0.32mm analytical column coupled with a 0.53mm retention gap. The 0.53mm ID retention gap allows the on-column technique to be used, but care must be taken to minimize dead volume and to establish a leak-tight connection. Restek’s 0.32mm MXT-Biodiesel TG columns are factory-coupled to a 0.53mm MXT retention gap with an Alumaseal connector, ensuring a leak-tight connection. Target analytes resolve well and the solvent and triglyceride peaks show excellent symmetry (Figure 4).

0.53mm MXT-Biodiesel TG columns with Integra-Gap technology

The 0.53mm MXT-Biodiesel TG columns are a simpler alternative to using a 0.32mm column coupled to a 0.53mm retention gap. Restek applied Integra-Gap technology to the 0.53mm MXT Biodiesel TG columns, eliminating the column coupling. These 100% leak-proof columns feature a built-in retention gap, reducing the risk of peak broadening and tailing, and guaranteeing the user many analyses without downtime. Chromatography from the 0.53mm MXT-Biodiesel TG with Integra-Gap technology (Figure 5) is excellent and comparable to that obtained on the 0.32mm ID column in Figure 4.

Figure 4  Glycerin and glycerides in derivatized B100 samples resolve well and show excellent peak symmetry on the 0.32mm MXT-Biodiesel TG column, which is factory-coupled to a 0.53mm MXT retention gap.

GC_PC00935 Column MXT-Biodiesel TG w/2 m x 0.53 mm retention gap, 10 m, 0.32 mm ID, 0.10 µm (cat.# 70290) Sample B100 + IS butanetriol & tricaprin derivatized with MSTFA as per ASTM D6584 Injection Inj. Vol.: 1.0 µL cold on-column Temp. Program: Oven track Oven Oven Temp.: 50 °C (hold 1 min) to 180 °C at 15 °C/min to 230 °C at 7 °C/min to 430 °C at 30 °C/min (hold 5 min) Carrier Gas H2, constant flow Flow Rate: 4 mL/min Detector FID @ 430 °C

Figure 5  Equivalent chromatographic quality can be obtained on the 0.53mm MXT-Biodiesel TG analytical column. The built-in retention gap eliminates the need for a connector, thus preventing peak tailing from dead volume.

GC_PC00934 Column MXT-Biodiesel TG 13m, 0.53mm ID, 0.16µm with built-in 2m Integra-Gap (total column length 15 m) Sample B100 + IS Butanetriol & Tricaprin derivatized with MSTFA as per ASTM D-6584 Injection Inj. Vol.: 1.0 µL cold on-column Temp. Program: oven track Oven Oven Temp.: 50 °C (hold 1 min) to 180 °C at 15 °C/min to 230 °C at 7 °C/min to 430 °C at 30 °C/min (hold 5 min) Carrier Gas H2, constant flow Flow Rate: 4 mL/min Detector FID @ 430 °C Instrument Shimadzu 2010 GC Notes (Data acquired on prototype column, similar column to cat# 70290)

Conclusion

As demonstrated, for high-temperature GC analysis, the metal MXT-Biodiesel TG columns are rugged and withstand the harsh temperatures required for total residual glycerin analysis. The columns have the resolution needed for accurate, reliable results and are more stable at high temperatures than competitive fused silica columns. This high temperature stability leads to longer column lifetimes and less downtime for maintenance and/or column change outs.

For additional information about the MXT-Biodiesel TG capillary column line or other analytical needs for biodiesel analysis contact your nearest Restek Sales Representative or Distributor.