L. Mchenry, XOS, East Greenbush, New York
According to the European Committee for Standardization (EN), both ultraviolet fluorescence (ISO 208461, sulfur by UVF) and wavelength dispersive x-ray fluorescence (ISO 208842, sulfur by WDXRF) are applicable test methods for fatty acid methyl ester (FAME), also known as biodiesel (EN 142143), and both are listed as referee methods. However, the situation is different at ASTM International (formerly the American Society for Testing and Materials).
According to ASTM D6751 “Standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels,”4 several options exist for testing sulfur in biodiesel. ASTM D5453 (sulfur by UVF) is listed as the referee method, but D7039 (sulfur by MWDXRF) may also be used. Additionally, the D6751 specification also lists other sulfur test methods, but states that these methods—D2622, in particular—may give falsely high results due to the presence of oxygen in biodiesel (typically 10 wt%–12 wt% oxygen).
This is a curious statement, as Section 6.2 of ASTM D2622 “Standard test method for sulfur in petroleum products by wavelength dispersive X-ray fluorescence spectrometry,”5 states the opposite:
“6.2 Fuels containing large quantities of FAME, ethanol, or methanol (see TABLE 1) have a high oxygen content leading to significant absorption of sulfur Kα radiation and low sulfur results. Such fuels can, however, be analyzed using this test method provided either that correction factors are applied to the results (when calibrating with white oils) or that the calibration standards are prepared to match the matrix of the sample. …”
* The concentrations of ethanol and methanol were calculated assuming a theoretical mixture of hydrocarbons and di-butyl sulfide to which ethanol (or methanol) was added until the sum of the mass coefficients times mass fractions increased by 5%. In other words, the amount of ethanol (or methanol) that caused a negative 5% error in the sulfur measurement was calculated. This information is included in TABLE 1 to inform those who wish to use Test Method D2622 to determine sulfur in FAME blends (biodiesel), gasohol, M-85 or M-100 of the nature of the error involved.
How then has this discrepancy between the D6751 specification and D2622 occurred?
D6751 RR:D02-1480 research report setup. Turning back to the D6751 B100 specification, Appendix X1 for “Significance of properties specified for biodiesel fuel,” Section X1.5.1 on Sulfur has a Note X1.1 that states:
“NOTE X1.1—Test Method D5453 should be used with biodiesel. Use of other test methods may provide falsely high results when analyzing B100 with extremely low sulfur levels (less than 5 ppm). Biodiesel sulfur analysis from RR:D02-1480, Biodiesel Fuel Cetane Number Testing Program, January-April 1999, using Test Method D2622 yielded falsely high results due to the presence of the oxygen in the biodiesel. Sulfur results using Test Method D2622 were more accurate with B20 than with B100 due to the lower oxygen content of B20. Potential improvements to Test Method D2622 may provide more accurate values in the future.”
In 1999, an interlaboratory study (ILS) was initiated by the Biodiesel Fuel Cetane Number Testing Program sponsored by ASTM subcommittee D02.01 on combustion characteristics. For this study, a diesel sample (sample ID 902) and three biodiesel samples (903B, 904B, 905B) were used both as test samples and blend stock for additional B20 biodiesel blends. Sample compositions were:
Multiple sample properties besides cetane number were tested, including sulfur. Detailed directions for cetane testing were provided to participants, but directions for the rest of the fuel inspection tests (including sulfur) were lacking.
Also included in the study directions were expected test results, or “inspection data,” for all sample properties, excluding cetane number. It was not reported who conducted the initial inspection data testing, or what ASTM methods were used in that testing. Note: It is unusual to provide expected test results to study participants before the testing occurs. See TABLE 2 for a summary of the sulfur data.
D6751 RR:D02-1480 research report results. TABLE 2 lists the sample ID, sample matrix, number of participating laboratories, the sulfur inspection values (values given to the participants prior to testing), the ILS results (labeled D02-1480) and the standard deviation of the ILS data.
The D02-1480 study results in TABLE 2 are the final sulfur test data generated from the study using “D2622 (primarily), D4294, D5453 and CGSB” sulfur test methods. ASTM D4294 is EDXRF, D5453 is UVF and CGSB is not defined in the research report. The total number of laboratories testing each sample is identified in TABLE 2, but a breakdown of the number of laboratories using each test method was not provided in the research report.
It is interesting to note that while the sulfur results are reported in this research report as indicated in TABLE 2, there are no comments, conclusions or discussion of these sulfur results in this research report.
DISCUSSION
It is unclear how the B100 D6751 specification arrived at the Note X1.1 conclusion, which indicates: “…Test Method D2622 yielded falsely high results due to the presence of the oxygen in the biodiesel. Sulfur results using Test Method D2622 were more accurate with B20 than with B100 due to the lower oxygen content of B20 …” Several inconsistencies with this conclusion will be discussed in the following section.
Falsely high due to the presence of oxygen. The D02-1480 research report does not say the B100 results were falsely high due to oxygen content—this conclusion originates elsewhere. There is no concluding statement in the research report where the study values are compared to the inspection values, much less any statement that the B100 samples are falsely high due to oxygen content. Additionally, no investigation was detailed in this research report of why the B20 samples were in closer agreement with their inspection values than the B100 test samples were with their inspection values.
Inspection values. For the biodiesel and biodiesel blend samples, there is no detail in the research report of who conducted this testing, where these values originate (single laboratory results, round robin testing, etc.) or what test methods were used to determine these values. These samples do not appear to be certified reference materials, as there is no associated uncertainty or traceability provided with the inspection values (at least as far as the research report is concerned).
The report does indicate that the 902 conventional diesel sample is a round robin sample provided to NEG members for testing in January 1999 to provide comparative data for the biodiesel blends, so it is possible that this sample may be a reference material.
However, it is unclear where the biodiesel and B20 biodiesel blend inspection values originate. Added to the fact that it is unusual to provide inspection test values to study participants prior to testing, it seems unwise to use the inspection values as the true value(s) for the samples. Then, using these values to further make an accuracy statement about the research report data seems speculative at best.
D2622 (primarily), D4294, D5453 and CGSB sulfur test methods. The D02-1480 study results were generated using D2622 (primarily), D4294, D5453 and CGSB sulfur test methods. The total number of laboratories testing each sample is identified in TABLE 2, but a breakdown of the number of laboratories using each test method was not provided in the research report. This is disturbing—what does “primarily” mean? This is not reported in the research report. When dealing with statistics from multiple elemental analysis methods, best practices today evaluate each method separately and combine the data if statistically feasible. Due to the low number of participants (10–16 labs), there may not be enough degrees of freedom to separate by method. What then determines whether a specific method is fit (or not fit) for use?
Research report data. Closer investigation of the D02-1480 ILS sulfur data in TABLE 2 indicates that the participating laboratories had trouble analyzing the ultra-low sulfur biodiesel samples (903B and 905B highlighted in TABLE 2), because the standard deviation of the sulfur results are greater than the sulfur values themselves. This is likely not an issue with only one or two participants because the data was subject to the generalized extreme Studentized deviate many-outlier (GESD) procedure, which would remove gross outlying values/participants. This problem was not observed with biodiesel sample 904B, which had an average measurement of 31 ppm sulfur with a standard deviation of 16 ppm, which agrees with its inspection value of 28 ppm.
Due to bad statistics, samples 903B and 905B should be discarded from the analysis statement made in the D6751 specification. If these two data points are thrown out, it can be observed that the remaining biodiesel sample is in close agreement with the inspection value, as it is well within D2622 reproducibility (6.2 ppm). However, a statement based on one data point is weak, so a good compromise would be removing the statement from the D6751 specification altogether.
ASTM B100 PTP program. Perhaps the best way to put this issue to rest is to obtain some impartial data comparing the D5453 and D2622 test methods. ASTM runs a B100 Proficiency Testing Program (PTP)6 wherein B100 samples are sent out three times per year to laboratory participants for analysis of approximately 24 different properties, including sulfur. Unfortunately, D2622 is not included in this PTP program, likely due to the statement regarding D2622 in the biodiesel specification.
Prior to the publication of this paper, a request was made to include D2622 in the B100 PTP program. Unfortunately, it was denied as ASTM stated that the method was inappropriate for biodiesel due to the samples being low in sulfur and high in oxygen content.
However, it is interesting to note that D7039 (another WDXRF method, specifically for MWDXRF) is included in this PTP; in a separate paper7 written by the author, the low bias of D7039 PTP data (in orange, FIG. 1) to D5453 PTP data (in blue, FIG. 1) is discussed, and it is proposed that this bias is due to the lack of oxygen correction by participants.
Note: This low bias is the same expectation that exists for high oxygen-containing samples in Section 6.2 of D2622 but contradicts the expectation of Note X1.1 in D6751.
Takeaways. A contradiction exists between the ASTM D6751 biodiesel specification and the D2622 sulfur by WDXRF test method. The B100 specification states that the high oxygen of B100 samples leads to high bias by D2622, but the D2622 method states that high-oxygen samples lead to sulfur Kα radiation absorption and a low sulfur bias (if not corrected for). There is an existing research report D02-1480 detailing the D6751 specification references; however, when investigated, a number of issues include:
Although D2622 is not in the ASTM B100 proficiency testing program, comparison of another WDXRF method (D7039 sulfur by MWDXRF) with D5453 (sulfur by UVF) indicates that the D2622 method is correct, and that high-oxygen content causes low rather than high bias.
Considering these discoveries, an investigation by ASTM should be undertaken to address this discrepancy. At a minimum, it is recommended that the high bias statement should be immediately balloted for removal from the D6751 B100 specification and D2622 be added to the ASTM B100 PTP program to collect impartial method comparison data. HP
LITERATURE CITED
LESLIE MCHENRY is an Applications Supervisor for XOS. With more than two decades of experience in petroleum applications—first as a chemist specializing in downstream product analysis at a North American petroleum refinery, then as an Applications Scientist and Supervisor at XOS—McHenry is recognized as an industry expert. She is the author of numerous white papers and ASTM XRF method revisions, including D7039, D7536, D7757 and the addition of Procedure C for XRF to ASTM D4929 “Determination of chloride content in crude oil.” As a former XOS customer, McHenry enjoys assisting others with their elemental analysis testing challenges.