ASTM 6584 Free and Total Glycerin in BioDiesel Analysis
This application guide describes the configuration and operation of an SRI Gas Chromatograph with PeakSimple software version 3.67 for performing ASTM 6584 free and total glycerin analysis in BioDiesel samples.
Column Configuration
The ASTM method suggests a 12 meter .32mm id narrow-bore column coupled with a 2.5 meter guard column but permits the use of any column which exhibits acceptable resolution of the glyceride analytes. For ease of use, a 15 meter fused silica lined metal capillary column commonly called a wide-bore MXT column is preferred. The ideal column has a thin film (.16 micron thick) and a temperature rating of 380°C or higher.
This type of column is unbreakable (unlike plain fused silica columns) and allows the injection syringe to deposit the sample directly into the bore of the column itself. This is important because heated or split/splitless injectors can discriminate against high boiling analytes like triglycerides. The ASTM 6584 method specifies cool-on-column injection to avoid boiling point discrimination.
Alternative column configuration: Chrompack HT5 .32mm id fused silica coupled with 2.5 meter .53mm id guard column. A 1/16" stainless steel union with graphite ferrules connects the guard column to the analytical column. The 26 ga. syringe needle fits inside the .53mm column to accomplish a cool on-column injection as specified in the method.
Detector and Carrier Gas
Helium is recommended as carrier gas. Hydrogen and air are required for the FID (flame ionization detector) which is used to detect the sample molecules.
Required Materials and Standards
Calibration Standards (diluted in Pyridine)
- Glycerin (500ppm) - Restek# 33020
- Butanetriol (1000ppm) - Restek# 33024
- Monoolien (10000ppm) - Restek# 33021
- Tricaprin (8000ppm) - Restek# 33025
- Diolien (5000ppm) - Restek# 33022
- Triolien (5000ppm) - Restek# 33023
Reagents and Supplies
- MSTFA (Derivatization Reagent) - Restek# 35601
- Heptane (a common solvent) - Sigma# 34873-1L
- 8ml vials - Grace# 98862 or Cole-Parmer# WU-08919-86
- 100ul syringe - Grace# 85012
- Disposable pipets - Sigma# Z350613-400EA
Calibration Standard Preparation
The ASTM 6584 method specifies that for the highest calibration level (level 5), 100ul of each material (in pyridine) is added to an 8 ml vial along with 100ul of MSTFA. Allow 20 minutes for the reaction to occur, then add heptane to bring the final volume to 8ml.
The ASTM 6584 method describes making the calibration standards at 5 different levels. The level 1 calibration standard is prepared using 10ul of each starting material instead of 100ul, but the procedure is otherwise the same.
Calibration Level Amounts
| Level |
Glycerin (ppm) |
Mono (ppm) |
Di (ppm) |
Tri (ppm) |
| 1 |
50 |
1000 |
500 |
500 |
| 2 |
150 |
2500 |
1000 |
1000 |
| 3 |
250 |
5000 |
2000 |
2000 |
| 4 |
350 |
7500 |
3500 |
3500 |
| 5 |
500 |
10000 |
5000 |
5000 |
Temperature Program
Set up a temperature program starting at 40 degrees, holding there for 2 minutes, then ramping at 20 degrees per minute to 380 degrees, and holding there for 11 minutes. The ASTM 6584 method does not specifically recommend a temperature program so long as the peaks are well separated from each other and from any interfering peaks. The temperature program can be modified if particular raw material (soybean oil, chicken fat etc.) works better with a faster or slower ramp rate.
Sample Preparation
Prepare BioDiesel samples by placing 100ul of BioDiesel, 100ul of Tricaprin in pyridine, 100ul of Butanetriol in pyridine, and 100ul of MSTFA in a vial. Allow 20 minutes for the reaction then dilute to 8ml with heptane. Inject 1ul to generate a chromatogram.
Analysis Components
The method identifies and quantifies six primary components:
- Butanetriol (internal standard, 1000ppm)
- Glycerol/Glycerin (free glycerin)
- Monoolien
- Tricaprin (internal standard, 8000ppm)
- Diolien
- Triolien
Three additional retention windows are created for total calculations:
- Total MonoGlycerides (multiplication factor: 0.2591)
- Total DiGlycerides (multiplication factor: 0.1488)
- Total Triglycerides (multiplication factor: 0.1044)
These multiplication factors account for the percentage of the molecule which is bound glycerin as specified in the ASTM 6584 method.
Internal Standards
Butanetriol is used as the internal standard for Glycerol (free glycerin). Tricaprin is used as the internal standard for monoolien, diolien, triolien, and the Total Mono, Total Di, and Total Tri calculations.
Results Interpretation
The Results screen displays the calculated results for the BioDiesel sample. The result for Glycerol is the free glycerin and the sum of the Total Mono, Total Di and Total Tri values are the bound glycerin.
Units Conversion Reference
Unit conversions for calibration:
- 1,000,000 ppm = 100%
- 100,000 ppm = 10%
- 10,000 ppm = 1%
- 1,000 ppm = 0.1%
- 100 ppm = 0.01%
- 10 ppm = 0.001%
- 1 ppm = 0.0001%
