ASTM D6584 for Free and Total Glycerin in B-100 Biodiesel Methyl Esters
ASTM D6584 Test Method for Determination of Free and Total Glycerin in B-100 Biodiesel Methyl Esters By Gas Chromatography
10. Procedure
10.1 Set the instrument operating variables to the values specified in Table 1. Weigh to the nearest 0.1 mg approximately 100 mg of sample directly into a 10 mL septa vial. Using microlitre syringes, add exactly 100 µL of each internal standard and MSTFA. Shake the vials, and allow to set for 15 to 20 min at room temperature. Add approximately 8 mL of n-Heptane to the vial and shake.
10.2 Inject 1 µL of the reaction mixture into the cool on-column injection port and start the analysis. Obtain a chromatogram and peak integration report.
10.3 Peak Identification - Identify peaks by comparison of retention times to the standards. For identification of additional peaks, use the relative retention times given in Table 4 and the reference chromatograms given in Fig. 1. The mono-, di, and triglycerides are separated according to carbon numbers (CN).
10.4 Monoglycerides consist of the four overlapping peaks with relative retention times (RRT) of 0.76 and 0.83 to 0.86 with respect to the internal standard tricaprin. A pair of peaks, methyl esters with a carbon number of 24, may appear with RRT of 0.80 to 0.82, and should not be included in the calculation of monoglycerides.
10.5 Diglycerides are also primarily separated according to carbon number, but due to varying double bonds in the molecules, baseline resolution of the peaks does not occur. The grouping of 3 to 4 peaks with RRT of 1.05 to 1.09 (CN 34, 36, and 38) shall be attributed to diglycerides. Carbon number also separates triglycerides. Peaks with RRT of 1.16 to 1.31 (CN 52, 54, 56, and 58) should be included in the calculation.
11. Calculation and Report
11.1 After identifying the peaks, measure the areas of the peaks identified as glycerin, mono, di-, and triglycerides. Using the slope and y-intercept of the calibration functions, calculate the mass of each as follows:
11.1.1 Glycerin:
G = (ag x Ag/Ais1 + bg) x Wis1 x 100/W
where:
G = mass percentage of glycerin in sample,
Ag = peak area of glycerin,
Ais1 = peak area of Internal Standard 1,
Wis1 = weight of Internal Standard 1, mg,
W = weight of sample, mg,
ag = slope of the calibration function,
bg = intercept of the calibration function.
11.1.2 Individual Glycerides:
Gli = (ao x Agli/Ais2 + bo1) x Wis2 x 100/W
where:
Gli = mass percentage of individual glycerides in sample,
Agli = peak area of individual glyceride,
Ais2 = peak area of Internal Standard 2,
Wis2 = weight of Internal Standard 2, mg,
W = weight of sample, mg,
aol = slope of the calibration function for mono, di-, or triolein, and
bo1 = intercept of the calibration function for mono, di, or triolein.
11.1.3 Calculation of Total Glycerin:
total glycerin = free glycerin + bound glycerin
where:
free glycerin= glycerin determined in Eq 7,
bound glycerin = ∑(GlM, GlD, GlT)
where:
GlM = 0.2591 x ∑monoglyceride, mass % determined in Eq 8,
GlD = 0.1488 x ∑diglyceride, mass % determined in Eq 8, and
GlT = 0.1044 x ∑triglyceride, mass % determined in Eq 8.
11.2 Report the free and total glycerin to the nearest 0.001 mass %.
