Reliable Analysis of Glycerin in Biodiesel
January 17, 2008
BY Ngoc Nguyen, Kory Kelly and Sky Countryman
Note: This article originally appeared in the November 2007 issue of Laboratory Equipment, a publication of Advantage Business Media.
In the past decade, biodiesel has emerged as a leading alternative fuel source because it is easily derived from common feedstocks and can be used in unmodified diesel engines. The relative ease of biodiesel production can mask the importance of maintaining high quality diesel fuel standards. To support the growth of the biodiesel industry, the United States' American Society for Testing and Materials and the European Deutsches Institut fur Normung recently outlined physical and chemical tests and specified the minimum quality standard for biodiesel fuel used in modern diesel engines. These are outlined in "Biodiesel Analytical Methods," published by the National Renewable Energy Laboratory in 2004.
Arguably the most critical test for biodiesel is the measure of glycerin content. Glycerin is the major byproduct of the biodiesel production process, called transesterification, where oils and fats are reacted with an alcohol to produce fatty acid methyl esters. High glycerin content can lead to a number of fuel problems, such as clogged fuel filters and fuel pressure drops, and its presence must be minimized.
Figure 1. Lifetime comparison of the DB-5ht and ZB-5HT Inferno. The lifetime and bleed profile comparisons were performed on new, unused DB-5ht and the ZB-5HT gas chromatography columns. Careful measures were taken to ensure that all conditions were similar for both columns.
ASTM D 6584-00, "Test Method for Determination of Free and Total Glycerin in B100 Biodiesel Methyl Esters by Gas Chromatography," outlines testing methods measuring the total amount of glycerin in biodiesel. Although gas chromatography is the standard analysis technique for this method, it has several inherent challenges. The tests run at very high temperatures and standard fused silica columns are not engineered to withstand temperatures above 380 degrees Celsius (716 degrees Fahrenheit). In fact, at temperatures above 380 degrees Celsius, the polyimide coating of most fused silica columns starts to degrade, eventually becoming brittle and inflexible.
The alternative, metal columns, present other challenges. While metal columns can withstand higher oven temperatures, they are inflexible, difficult to use and require special tubing cutters. In addition, they often develop leaks due to the expansion and contraction that occurs during oven heating cycles and are highly active to acids and bases. Thus, using metal columns might compromise the accuracy of the biodiesel analysis.
Phenomenex Inc. has recently developed unique fused silica columns designed specifically for high-temperature analysis. These columns, called the Zebron ZB-1HT and ZB-5HT Inferno, are specially processed to be thermally stable up to 430 degrees Celsius (806 degrees Fahrenheit). As a result, their stationary phases and a polyimide coating are more rugged and can withstand higher temperatures than other conventional columns. This article compares the lifetime and stability of the Zebron Inferno column with the leading fused-silica columns and presents analysis results on the Zebron column, using ASTM methods.
Lifetime Comparison
For the high-temperature lifetime comparison, three columns were compared: Agilent's (J&W) DB-5ht, Varian's VF-5HT and Phenomenex's Zebron ZB-5HT. The columns were held at 400 degrees Celsius (752 degrees Fahrenheit) for two hours. After lowering the oven temperature to 120 degrees Celsius (248 degrees Fahrenheit), 1 liter (0.26 gallons) of pentadecane was injected and its retention time was measured. This process was repeated 50 times, totaling 100 hours at 400 degrees Celsius (752 degrees Fahrenheit) for each column tested.
Figure 2. Bleed comparison of the DB-5ht and ZB-5HT Inferno. The shaded area depicts the bleed criteria for mass spectrometry certified columns on a mass spectrometry detector. Mass spectrometry bleed certification values are typically read at 320 degrees Celsius (608 degrees Fahrenheit). The ZB-5HT demonstrated lower bleed than the DB-5ht. It meets mass spectrometry certification limits, even at 360 degrees Celsius (680 degrees Fahrenheit).
Figure 3. The standards and biodiesel sample were both run under the same conditions using a 15 meter (49 feet) by 0.32 millimeter (0.13 inch) by 0.10 micron Zebron ZB-5HT Inferno column (7EM-G015-02) with a 2 meter (3.3 feet) by 0.53 millimeter (0.02 inch) high-temperature guard column. One liter (0.26 gallons) of the reaction mixture was injected onto a cool-on-column injector connected to a HP 6890 Gas Chromatograph (provided by Agilent Technologies, Palo Alto, Calif.) and detected by flame ionization. The gas chromatography oven is as follows: 50 degrees Celsius (122 degrees Fahrenheit) for 1 minute to 180 degrees Celsius (356 degrees Fahrenheit) at 15 degrees Celsius (59 degrees Fahrenheit) per minute to 230 degrees Celsius (446 degrees Fahrenheit) at 7 degrees Celsius (44.6 degrees Fahrenheit) per minute to 380 degrees Celsius (716 degrees Fahrenheit) at 30 degrees Celsius (86 degrees Fahrenheit per minute for 10 minutes. Samples: 1) Glycerol, 2) Butanetriol (ISTD1), 3) Esters, 4) Monoglycerides, 5) 1-Monooleoyl-rac-glycerol, 6) Tricarpin (ISTD2), 7) Diglycerides, 8) 1,3-Diolein, 9) Triglycerides and 10) Triolein.
Bleed Profile
Bleed (pA) was measured using a flame ionization detector as the gas chromatography oven program increased from 120 degrees Celsius (248 degrees Fahrenheit) to 400 degrees Celsius (752 degrees Fahrenheit). The gas chromatography oven was held at 120 degrees Celsius (248 degrees Fahrenheit) for three minutes then increased to 320 degrees Celsius (608 degrees Fahrenheit) at 30 degrees Celsius (86 degrees Fahrenheit) per minute. A null injection was made at 250 degrees Celsius (482 degrees Fahrenheit).
Biodiesel Analysis
Calibration standards, sample preparation and gas chromatography analysis were performed per ASTM method D 6584 (reference ASTM method). In brief, the samples were derivatized with N-Methyl-N-trimethylsilyltrifluoroacetamide (MSTFA).
Results and Discussions
A good indicator of a column's stability is its consistency in retention time for hydrocarbons, such as pentadecane. The pentadecane retention time comparison revealed that the Zebron ZB-5HT has higher thermal stability than the DB-5ht and VF-5HT. After 40 hours at 400 degrees Celsius (752 degrees Fahrenheit), the VF-5HT column broke (data not shown). Pentadecane has the same retention time on the DB-5ht after 40 hours at 400 degrees Celsius (752 degrees Fahrenheit) as on the Zebron ZB-5HT after 80 hours at that temperature. This suggests that the ZB-5HT is two times more stable and has twice the column lifetime as the DB-5ht.
Bleed can also be an indicator of the stability and lifetime of a gas chromatography column. In the bleed comparison test, the DB-5ht showed significantly higher bleed than ZB-5HT at 380 degrees Celsius (716 degrees Fahrenheit) (Figure 2). This difference is amplified at 400 degrees Celsius (752 degrees Fahrenheit).
After these initial comparative tests, we conducted the ASTM D 6584's free and total glycerin analysis on the Zebron ZB-5HT column. The calibration curve for each reference components had a correlation coefficient greater than 0.99 (data not shown).
Good peak shape is also indicative of a column's suitability for a designated method. ASTM D 6584 specifies that Butanetriol and Tricarpin be used as internal standards because they have similar properties as many components in a biodiesel sample. Because we observed good peak shape in the biodiesel gas chromatography run, it indicates that the Zebron ZB-5HT is well suited for the analysis and exhibits very low activity for the analytes (Figure 3).
Conclusions
To ensure the quality of the biodiesel product, the U.S. ASTM D 6584 specifies a high temperature analysis of free and total glycerin in biodiesel products. However at temperatures above 380 degrees Celsius (716 degrees Fahrenheit), most fused silica columns become brittle and spontaneously break. Our studies suggest that Phenomenex's Zebron ZB-5HT is well suited for high temperature analyses.
Having a rugged and durable high-temperature gas chromatography column will help manufacturers develop methods that will produce high-quality, alternative fuel products. In addition, a column with a longer lifetime will help lower the cost per sample of the analysis, thus driving down the cost of production.
Ngoc Nguyen is the gas chromatography and solid phase extraction marketing brand manager for Phenomenex Inc. Reach her at ngocn@Phenomenex.com or (310) 212-0555. Kory Kelly is a research specialist and Sky Countryman is the product manager for gas chromatography with Phenomenex Inc.
In the past decade, biodiesel has emerged as a leading alternative fuel source because it is easily derived from common feedstocks and can be used in unmodified diesel engines. The relative ease of biodiesel production can mask the importance of maintaining high quality diesel fuel standards. To support the growth of the biodiesel industry, the United States' American Society for Testing and Materials and the European Deutsches Institut fur Normung recently outlined physical and chemical tests and specified the minimum quality standard for biodiesel fuel used in modern diesel engines. These are outlined in "Biodiesel Analytical Methods," published by the National Renewable Energy Laboratory in 2004.
Arguably the most critical test for biodiesel is the measure of glycerin content. Glycerin is the major byproduct of the biodiesel production process, called transesterification, where oils and fats are reacted with an alcohol to produce fatty acid methyl esters. High glycerin content can lead to a number of fuel problems, such as clogged fuel filters and fuel pressure drops, and its presence must be minimized.
Figure 1. Lifetime comparison of the DB-5ht and ZB-5HT Inferno. The lifetime and bleed profile comparisons were performed on new, unused DB-5ht and the ZB-5HT gas chromatography columns. Careful measures were taken to ensure that all conditions were similar for both columns.
ASTM D 6584-00, "Test Method for Determination of Free and Total Glycerin in B100 Biodiesel Methyl Esters by Gas Chromatography," outlines testing methods measuring the total amount of glycerin in biodiesel. Although gas chromatography is the standard analysis technique for this method, it has several inherent challenges. The tests run at very high temperatures and standard fused silica columns are not engineered to withstand temperatures above 380 degrees Celsius (716 degrees Fahrenheit). In fact, at temperatures above 380 degrees Celsius, the polyimide coating of most fused silica columns starts to degrade, eventually becoming brittle and inflexible.
The alternative, metal columns, present other challenges. While metal columns can withstand higher oven temperatures, they are inflexible, difficult to use and require special tubing cutters. In addition, they often develop leaks due to the expansion and contraction that occurs during oven heating cycles and are highly active to acids and bases. Thus, using metal columns might compromise the accuracy of the biodiesel analysis.
Phenomenex Inc. has recently developed unique fused silica columns designed specifically for high-temperature analysis. These columns, called the Zebron ZB-1HT and ZB-5HT Inferno, are specially processed to be thermally stable up to 430 degrees Celsius (806 degrees Fahrenheit). As a result, their stationary phases and a polyimide coating are more rugged and can withstand higher temperatures than other conventional columns. This article compares the lifetime and stability of the Zebron Inferno column with the leading fused-silica columns and presents analysis results on the Zebron column, using ASTM methods.
Lifetime Comparison
For the high-temperature lifetime comparison, three columns were compared: Agilent's (J&W) DB-5ht, Varian's VF-5HT and Phenomenex's Zebron ZB-5HT. The columns were held at 400 degrees Celsius (752 degrees Fahrenheit) for two hours. After lowering the oven temperature to 120 degrees Celsius (248 degrees Fahrenheit), 1 liter (0.26 gallons) of pentadecane was injected and its retention time was measured. This process was repeated 50 times, totaling 100 hours at 400 degrees Celsius (752 degrees Fahrenheit) for each column tested.
Figure 2. Bleed comparison of the DB-5ht and ZB-5HT Inferno. The shaded area depicts the bleed criteria for mass spectrometry certified columns on a mass spectrometry detector. Mass spectrometry bleed certification values are typically read at 320 degrees Celsius (608 degrees Fahrenheit). The ZB-5HT demonstrated lower bleed than the DB-5ht. It meets mass spectrometry certification limits, even at 360 degrees Celsius (680 degrees Fahrenheit).
Bleed Profile
Bleed (pA) was measured using a flame ionization detector as the gas chromatography oven program increased from 120 degrees Celsius (248 degrees Fahrenheit) to 400 degrees Celsius (752 degrees Fahrenheit). The gas chromatography oven was held at 120 degrees Celsius (248 degrees Fahrenheit) for three minutes then increased to 320 degrees Celsius (608 degrees Fahrenheit) at 30 degrees Celsius (86 degrees Fahrenheit) per minute. A null injection was made at 250 degrees Celsius (482 degrees Fahrenheit).
Biodiesel Analysis
Calibration standards, sample preparation and gas chromatography analysis were performed per ASTM method D 6584 (reference ASTM method). In brief, the samples were derivatized with N-Methyl-N-trimethylsilyltrifluoroacetamide (MSTFA).
Results and Discussions
A good indicator of a column's stability is its consistency in retention time for hydrocarbons, such as pentadecane. The pentadecane retention time comparison revealed that the Zebron ZB-5HT has higher thermal stability than the DB-5ht and VF-5HT. After 40 hours at 400 degrees Celsius (752 degrees Fahrenheit), the VF-5HT column broke (data not shown). Pentadecane has the same retention time on the DB-5ht after 40 hours at 400 degrees Celsius (752 degrees Fahrenheit) as on the Zebron ZB-5HT after 80 hours at that temperature. This suggests that the ZB-5HT is two times more stable and has twice the column lifetime as the DB-5ht.
Bleed can also be an indicator of the stability and lifetime of a gas chromatography column. In the bleed comparison test, the DB-5ht showed significantly higher bleed than ZB-5HT at 380 degrees Celsius (716 degrees Fahrenheit) (Figure 2). This difference is amplified at 400 degrees Celsius (752 degrees Fahrenheit).
After these initial comparative tests, we conducted the ASTM D 6584's free and total glycerin analysis on the Zebron ZB-5HT column. The calibration curve for each reference components had a correlation coefficient greater than 0.99 (data not shown).
Good peak shape is also indicative of a column's suitability for a designated method. ASTM D 6584 specifies that Butanetriol and Tricarpin be used as internal standards because they have similar properties as many components in a biodiesel sample. Because we observed good peak shape in the biodiesel gas chromatography run, it indicates that the Zebron ZB-5HT is well suited for the analysis and exhibits very low activity for the analytes (Figure 3).
Conclusions
To ensure the quality of the biodiesel product, the U.S. ASTM D 6584 specifies a high temperature analysis of free and total glycerin in biodiesel products. However at temperatures above 380 degrees Celsius (716 degrees Fahrenheit), most fused silica columns become brittle and spontaneously break. Our studies suggest that Phenomenex's Zebron ZB-5HT is well suited for high temperature analyses.
Having a rugged and durable high-temperature gas chromatography column will help manufacturers develop methods that will produce high-quality, alternative fuel products. In addition, a column with a longer lifetime will help lower the cost per sample of the analysis, thus driving down the cost of production.
Ngoc Nguyen is the gas chromatography and solid phase extraction marketing brand manager for Phenomenex Inc. Reach her at ngocn@Phenomenex.com or (310) 212-0555. Kory Kelly is a research specialist and Sky Countryman is the product manager for gas chromatography with Phenomenex Inc.
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