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Latest Developments on Characterization of Pure Biodiesel (B100) and Finished Biodiesel Blends by Conventional GC, Ultra Fast GC and CGxCG Daniela Cavagnino 1 , Fausto Munari 1 , Danilo Pierone 2 1. Thermo Fisher Scientific, Milan, Italy 2. Nova Analítica, São Paulo (SP), Brazil (E-mail for contact: [email protected]) Overview Purpose: To describe the characterization of Biodiesel B100 in compliance with GC-based ASTM and EN methods regulating the analysis of free and total glycerin, residual methanol, and total FAME content. All methods can be applied using a single assembly with two GCs automated by a single autosampler for utmost productivity. In addition, the need to speed up the analysis time when high sample throughput is required, led to a deeper investigation into the application of an Ultra Fast GC technique for the determination of Free and Total Glycerin content. Application of GCxGC for the characterization of finished Biodiesel blends is also shown. Results: The reported data demonstrate the suitability of the optimized hardware solutions for the analysis of Biodiesel in full compliance with the official standards. In addition, the Ultra Fast GC approach generated promising results in terms of separation, linearity and repeatability, offering an additional analytical tool for fast sample screening. The advantages of GCxGC for the analysis of finished blends are clearly stated. Introduction Interest in Biodiesel as a clean-burning alternative fuel produced from renewable sources such as vegetable oils has increased tremendously over the last few years, mainly because of its reduced environmental impact in comparison with conventional mineral diesel. In order to be commercialized as pure biofuel or blending stock, biodiesel must meet a set of requirements defined in ASTM D6751 and EN 14214 standards. Gas Chromatography is commonly adopted to determine free and total glycerin (EN 14105/ ASTM D6584), total FAME (EN 14103) and methanol (EN 14110) in pure biodiesel. Free and Total Glycerin EN 14105/ ASTM D6584 The analysis of glycerin, mono, di and triglycerides by GC requires a non-discriminative injection system. The true cold On-column injector used on the TRACE GC Ultra is a permanently cold system, able to prevent discrimination of the heavier fraction and to eliminate any risk of degradation of labile components like triglycerides. The analytical column used is a non-polar Thermo Scientific TRACE TR-BIODIESEL(G), 10 m, 0.32 mm ID, 0.1 µm f.t. This column is designed for this high temperature GC method, featuring enhanced mechanical robustness at high oven temperature, and thus prolonged lifetime. A 1m x 0.53 mm ID pre-column is connected to the column by a high temperature gas-purged metal tee. This low dead volume leak-free metal tee has been specifically conceived for a reliable connection between the guard column and the analytical column at high temperature operation, removing the need for normal glass press-fit unions. This connector remains leak-free even with extremely large and frequent oven temperature variations. Conclusions A single dual-GC system equipped with cold On-Column and PTV- BKF injectors is proposed, which includes the versatility of the Liquid and HS Triplus autosampler for easiest operation and maximum sample throughput. The PTV injector with backflush capabilities reduces the analysis time and extend the polar column lifetime. By using the PTV injector with backflush, it is possible to avoid the use of the Headspace technique for the determination of Methanol and Ethanol, increasing even more the sample throughput. In addition, an Ultra Fast method has been optimized for screening of Glycerides content in B100 in only 5 minutes. GCxGC has proven to be a powerful technique for the characterization of finished biodiesel blends, delivering added-value detailed information in a single analysis. References [1] Comprehensive two-dimensional gas-chromatography applied to diesel and biodiesel analysis – D. Cavagnino, A. Cadoppi, G. Zilioli, F. Pigozzo – Pittcon Conference 2006. [2] Thermo Application Notes 10212, 10215, 10216 - Determination of Total FAME and Linolenic Acid Methyl Ester, Free and Total Glycerin and Methanol Content in Pure Biodiesel (B100). The system is calibrated by analyzing a 4-component calibration mix at four different concentrations, which generates four calibration curves for glycerin, monoolein, diolein, triolein. The linear correlation coefficients (R 2 ) exceed the specification of 0.9 as requested in EN 14105 for each curve, demonstrating very good linearity of the true cold On-column injection technique. Total FAME and Linolenic Acid Methyl Ester EN 14103 A PTV injector with Back Flush option was used, with a polar capillary column Thermo Scientific TRACE TR-BIODIESEL(F), 30 m, 0.25 mm ID, 0.25 µm f.t. When FAMEs are analyzed, the heavier fraction present in biodiesel samples (like di- and tri-glycerides) enters the column, getting stacked onto the polar phase. This means that a few nanograms of heavy compounds will accumulate inside the column, which increases the risk of compromised chromatographic performance after a number of sequences and reduces the column lifetime. Methanol EN 14110 requires a headspace GC method, based on either polar or non-polar columns. In this work, the analysis was performed using an external calibration procedure, which is preferred over internal standard calibrations when automatic headspace equipment is used. A Thermo Scientific TRACE GC Ultra equipped with a split/splitless (SSL) inlet and a flame ionization detector (FID), automated by a TriPlus Autosampler configured for headspace was used. The analytical column is a non-polar Thermo Scientific TRACE TR- BIODIESEL(M), 30 m, 0.32 mm I.D. 3.0 µm f.t. The linearity of the Methanol curve, the repeatability tested on different sample types and the analytical parameters are reported in Figure 4. Biodiesel Blends A very common use of biodiesel is its blending with ultra low sulfur petrodiesel as a lubricity additive. GCxGC has been effectively applied to get comprehensive compositional characterization of biodiesel blends, providing triple information in a single analysis: the petrodiesel composition, the percentage of biodiesel in the blend, the FAMEs detailed composition (Figs. 6 and 7). PTV Back flush (reverse flow device): The glycerides fraction is vented out without entering the column. Glycerin Triolein Additional Information For additional information, please browse: www.analiticaweb.com.br/biodiesel www.thermo.com Instrumentation All the methods are achieved using two Thermo Scientific TRACE GC Ultra and the TriPlus Autosampler, able to uniquely automate sequences on two GCs. The UltraFast GC technique has been accomplished with the Thermo Scientific UltraFast Module, which directly heats the capillary column for a faster temperature programming rate and a much shorter analysis time. The GCxGC analyses were performed with the Thermo Scientific TRACE GCxGC equipped with a cryogenic (CO 2 liquid) Dual Jet modulator and a Fast FID detector. EN 14105/ ASTM D6584 Optimized for Ultra Fast Analysis FIGURE 2. Ultra fast separation of the calibration mixture and typical Biodiesel samples 5 min Glycerin R 2 = 0.9996 Triolein R 2 = 0.9989 Sample Biodiesel #1 Sample Biodiesel #2 By incorporating the Back Flush Option into the PTV, heavy compounds are vented out of the inlet system, preventing column contamination while still allowing efficient transfer of compounds of interest (Figure 3). FIGURE 4. Analytical parameters, linearity and repeatability obtained for EN 14110 As a further possibility, methanol content can be successfully determined by direct liquid injection of Biodiesel diluted in n-Butanol, exploiting the PTV Backflush system to vent out the heavier fraction without letting it enter the capillary column. This solution overcomes the time-consuming sample preparation procedure for Headspace analysis and the possible critical availability of reference FAMEs mixture for calibration (Figure 5). In addition, the same hardware configuration used for the method EN 14103 for FAMEs analysis can be used. FIGURE 5. Methanol determination by direct liquid injection and backflush of heavier components B100 in n-Butanol spiked with Methanol and Ethanol BKF at 3 minutes R 2 = 0.9999 with external calibration Concentration range 0.001% - 0.5% w/w 1.9 2074.5 110133.8 MeOH % RSD % RSD SD SD Avrg Avrg Name Name Peak Area Repeatability (10 runs) Here below the curves obtained for glycerin and triolein are reported For this application, an SE54 phase 0.05 µm thickness on 3 m of capillary 0.32 mm ID has been used and integrated in an UFM. In order to minimize the possible discrimination of heavier glycerides during the injection, a PTV (Programmable Temperature Vaporizing) injector was used. A deactivated glass liner with glass wool showed better performance with a suitable transfer of both volatile and non- volatile compounds (Figure 2). To achieve Ultra Fast GC separation, a narrow bore column is coiled along with a heating element and a properly insulated temperature sensor. It is held in an easy to handle metal cage (UltraFast Module - UFM) installed into a TRACE GC Ultra oven. FIGURE 3. FAMEs chromatogram of an unknown Biodiesel, along with analytical parameters. FIGURE 6. FAMEs chraracterization in petrodiesel at 5% (v/v) (BD5 blend) FIGURE 7. Comparison of typical compositional data (% m/m) in various vegetable oils
