Robust Solutions for High-Temperature Biodiesel Analysis: Combining Deactivated Metal Columns, Leak-Tight Coupling, and Built-In Retention Gap Technology

There has been a huge increase in the use of biodiesel fuels. Biodiesel oil is biodegradable, nontoxic, does not contain aromatics, and does not contain sulfur, which avoids sulfur emissions. Basically, biodiesel is made by the transesterification of vegetable/animal waste cooking oils. The pure product, called B100, is added to normal diesel fuel in varying amounts. The “B” number designates the percentage of biodiesel in a biodiesel/petroleum diesel blend (e.g., B20 is 20% biodiesel/80% petroleum diesel).

Glyceride residues in biodiesel and blends can foul engine injectors and form deposits on essential parts in the diesel engine. Also, the shelf life of fuels depends on the level of glycerin. It is essential that accurate, efficient methods for quantifying glycerin and glycerides are available to the biodiesel industry.

The American Society for Testing and Materials (ASTM) and the European Method Deutsches Institutfür Normung (DIN) describe the primary physical and chemical testing methods for biodiesel oil. Gas chromatography is ideal for measuring important parameters such as total glycerin, fatty acid methyl esters (FAMEs), and methanol levels in biodiesel fuel. Methods such as ASTM D-6584 and EN14105 are industry standards for testing total glycerin and glycerides in biodiesel oil. In these methods, the column recommendation is listed as a 10 m × 0.32 mm i.d. column with a 0.1-μm film of 5% diphenyl/95% dimethyl polysiloxane. This column is connected with a retention gap to make the on-column injection feasible. The methods allow the use of fused silica as well as metal as tubing material. The 0.53-mm MXT®-Biodiesel TG solution with integrated Integra-Gap™ retention gap (Restek Corp., Bellefonte, PA), as discussed here, will hopefully trigger a timely review of these methods, to make triglycerides analysis more practical.

Unbreakable column solutions

Figure 1 - Result of high-temperature application to polyimide-coated capillary: oxidation of the protective coating.

Fused-silica columns are generally very temperature stable and are usually the first choice for GC analysis. However, when operating at temperatures exceeding 350 °C, fused-silica columns rapidly become black due to oxidation of the protective polyimide coating. This coating is in place to protect the fused-silica column from mechanical damage. Any damage to the outside coating will make the lifetime of the column very unpredictable.

Figure 2 - MXT metal columns can be made to fit in any dimension.

Even fused-silica columns with special high-temperature (HT) polyimide coating (HT equivalents) become unpredictable and break down relatively quickly. Figure 1 shows the impact of long-term exposure at 430 °C: The polyimide hardens, becomes black, and eventually disappears completely. Even in an early stage of this process, the risk of column breakage increases. To overcome this problem, a stabilized metal column line was developed: MXT®-Biodiesel TG columns are deactivated using Siltek® gap deactivation technology (Restek Corp.). This technology not only deactivates but also stabilizes the stationary phase, making the column extremely stable. The columns virtually do not degrade, even under temperatures up to 430 °C. Thus, it is possible to “bake out” any residue eluting out after the triglycerides without damaging the column. This process keeps the analytical system clean, so subsequent injections do not have carryover from previous samples. Metal columns are virtually unbreakable and can therefore be used in difficult environments such as high-temperature GC, process applications, space missions, and miniaturized solutions (Figure 2).

Injection techniques for biodiesel

To measure the high-boiling triglycerides at low levels, it is essential to have a good injection setup that allows quantitative measurement of triglycerides. The best technique is cold on-column, where the sample is introduced as a liquid into the capillary column.Other techniques, such as splitless and programmed temperature vaporization (PTV), will show a significant degree of discrimination.

In order to obtain a good on-column injection, it is necessary to use a retention gap1,2 to make sure there is a band-focusing mechanism in place. The retention gap is usually 2–3 m nonpolar deactivated capillary tubing with an inside diameter of 0.53 mm to allow most on-column injections.

The mechanism of the retention gap is described below and illustrated in Figure 3.

Figure 3 - Mechanism of a retention gap (see text).

  1. When applying on-column injection, the solvent with analytes will be distributed over a section of the retention gap. The length of this section depends on the injection speed, the carrier gas flow, the type of solvent, and the temperature. The solvent must be distributed evenly over the retention gap. This will only happen if the wettability of the surface of the retention gap allows it. Practically, for nonpolar solvents such as heptane and cyclohexane, nonpolar (polydimethyl siloxane) deactivated retention gaps are used.
  2. During and after injection, the solvent will evaporate and the analytes will deposit on the retention gap surface. This is the initial bandwidth, which can be significant depending on the injection parameters.
  3. Starting the temperature program will move the analytes toward the column, where they will concentrate in the stationary phase, focusing the broad initial injection band.
  4. The result is correct chromatographic peaks: no peak splitting or peak broadening.

If a retention gap is not used and the sample is injected straight onto a “coated” capillary, the initial bandwidth, as shown in Figure 3 (line 2), will not be focused, resulting in unpredictable peak broadening and peak splitting. The retention gap “fixes” the injection error that is always made when performing the liquid on-column injection. Retention gaps are commercially available with different deactivations. If not offered as a “retention gap,” one can also purchase 0.53-mm-i.d. deactivated fused-silica or MXT tubing and use that as a retention gap.

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