ARTICLE pubs.acs.org/EF
Toward the Development of Certified Reference Materials for Effective Biodiesel Testing. Part 2: Characterization and Value Assignment Manuela Ulberth-Buchgraber,* Monica Potalivo, and Andrea Held Institute for Reference Materials and Measurements (IRMM), Joint Research Centre (JRC), European Commission (EC), Retieseweg 111, 2440 Geel, Belgium ABSTRACT: A comprehensive feasibility study for the production of a biodiesel reference material for relevant specification parameters as provided in EN 142141 was launched because of insufficient knowledge available for most of the steps needed for the production of new biodiesel certified reference materials (CRMs). This paper summarizes the findings of this study for the characterization and final value assignment of a biodiesel reference material. Assigned values were calculated as the unweighted mean of the laboratory means of the accepted sets of results for each parameter. Uncertainties were estimated in compliance with the “Guide to the Expression of Uncertainty in Measurement” (GUM, ISO/IEC Guide 98-3:2008),2 including contributions from characterization, homogeneity, and stability studies. Of the 22 parameters considered overall, it could be shown that it would be feasible to prepare and certify reference material for 7 of them (ester, linolenic acid methyl ester, triacylglycerol, total glycerol, iodine value, oxidation stability, and acid value) with the approach taken thus far. For the remaining parameters, different problems were encountered. Although raising a considerable number of challenges, the development of a future biodiesel CRM for many parameters seems feasible. The provision of such a material would favor the building of a harmonized measurement system and, consequently, comparable analytical results.
1. INTRODUCTION There is an increasing demand to accurately measure the quality of biofuel products, particularly in view of the European directives promoting renewable energies3 and setting out fuel quality requirements.4 Using ISO/IEC 17025 “General Requirements for the Competence of Testing and Calibration Laboratories” as the measure for good analytical practice, the important role of reference materials in chemical measurements becomes apparent.5 The creation of certified reference materials (CRMs) requires the reference material producer to implement a highly integrated process planning and quality assurance scheme to realize the physical production of and value assignment to the candidate materials. In principle, CRM production consists of four components: (i) careful preparation and bottling of the material, (ii) homogeneity testing using a design optimized to give positive proof of homogeneity, (iii) stability testing using a design optimized to give positive proof of stability, and (iv) value assignment to the batch. All four components are equally important, and putting less emphasis on any one of them will result in a significant decrease of the quality of the material.6 To make the whole process transparent, ISO issued a number of guidelines, in particular, ISO Guides 347 and 35.8 Using a CRM adds value to all applications of materials for analytical quality control. CRMs guarantee beforehand a certain between unit homogeneity. Distribution conditions have already been defined, and stability during storage has been tested. Apart from these practical considerations, the assigned values, together with the uncertainty statement, would allow for a laboratory to check the performance of their methods. This is also true for standardized methods, the use of which does not per se guarantee proper implementation in each laboratory and reliable results r 2011 American Chemical Society
thereof. To provide the analytical laboratories with the necessary tools for adequate quality assurance and quality control during the analysis of biodiesel, suitable CRMs are necessary. Mostly, the production of a new CRM requires preliminary research. Feasibility studies are organized to test the planned processes for their suitability for the production of the envisaged CRM. A comprehensive feasibility study for the production of a biodiesel reference material for relevant specification parameters as provided in EN 142141 was launched because of insufficient knowledge available for most of the steps mentioned above. Preparation and bottling, homogeneity, and stability of the biodiesel material were investigated with dedicated studies, and the results are described in detail in ref 9. This paper summarizes the findings of this feasibility study related to the characterization and final value assignment for various parameters in a biodiesel material. Characterization and value assignment are sophisticated and resource-intensive processes. It is mandatory that every property value assigned to a reference material is accompanied by a statement on its established metrological traceability. In principle, the concept of traceability rests on two anchor points, namely, identity and quantity value. The process of characterization is strongly dependent upon the reference to which the certified value should be traceable. In ideal cases, the certified values of a reference material are traceable to the international system of units (SI). This is the best option, because the values are independent of any method and/or artifact and are universally valid. This means that the analyte is structurally clearly defined, and all analytical methods that measure this Received: June 20, 2011 Revised: September 12, 2011 Published: September 15, 2011 4630
dx.doi.org/10.1021/ef200896c | Energy Fuels 2011, 25, 4630–4637
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Table 1. Summary of Technical Evaluation laboratory
ester
linolenic acid methyl ester monoacylglycerol diacylglycerol triacylglycerol
total glycerol
methanol
sulfur
L1
OK
OK
OK
OK
OK
OK
>R;a >RSDRb OK
L2
OK
OK
OK
OK
OK
OK
OK
d
npc
L3
OK
OK
OK
OK
OK
>r
OK
OK
L4
OK
OK
OK
OK
OK
OK
OK
OK
L5
OK
OK
OK
OK
OK
OK
>RSDR
np
L6
>r
OK
>R; >RSDR
value = 0
value = 0
see monoacylglycerol, etc. >R; >RSDR
OK
L7
OK
OK
OK
OK
OK
OK
>R; >RSDR
np
L8 L9
OK OK
OK OK
OK >R
OK >r
value = 0 OK
see triacylglycerol >r
value = 0 >R; >RSDR
reported < 3 OK
L10
OK
OK
OK
OK
OK
OK
OK
>R; >RSDR
L11
OK
OK
OK
>r
OK
see diacylglycerol
OK
OK
L12
value > 100%
OK
OK
OK
OK
OK
>R; >RSDR
OK
L13
OK
OK
OK
OK
OK
>r
OK
OK
L14
OK
OK
np
np
np
np
np
np
flash point
oxidation stability
OK OK
OK np
OK OK
>RSDR
OK
OK
OK
OK
OK
L5
np
OK
L6
OK
L7 L8 L9 L10
laboratory
water
L1 L2
OK >RSDR
L3 L4
iodine value
acid value
viscosity
density
>RSDR OK
>R; >RSDR OK
OK OK
OK
OK
OK
OK
OK
only 4 values
OK
OK
np
OK
OK
np
np
OK
OK
OK
OK
>R; >RSDR
np
OK
OK
np
np
OK
OK
OK
OK
>R; >RSDR
OK
>r
OK
OK
OK
OK OK
>R >R; >RSDR
np np
OK OK
>R; >RSDR OK
>R; >RSDR OK
OK OK
L11
>RSDR
OK
OK
OK
OK
OK
OK
L12
OK
OK
np
OK
OK
>R; >RSDR
OK
L13
OK
OK
OK
OK
OK
>R; >RSDR
OK
L14
np
np
np
np
np
np
np
a
>R = absolute difference between two independent test results from two different vials exceeded the reproducibility limit (R) stated in the standard test method. b >RSDR = relative standard deviation of the measurement results exceeded the relative reproducibility standard deviation (RSDR, standard method) stated in the standard test method. c np = laboratory did not participate. d >r = absolute difference between two independent test results from the same vial exceeded the repeatability limit (r) stated in the standard test method.
clearly defined analyte should find the same result. Characterization measurements may be carried out by one or more laboratories using either a primary method for which measurement results are traceable without further calibration steps to the SI, a method where all possible sources of bias are known, or several independent methods. However, the situation could be different for applications, for which such structurally defined analytes are still not identified or agreed upon. In these cases, a definitive analyte (or group thereof) may not be known for the, mostly, functional properties of relevance for the user. Hence, methoddefined (also called operationally defined) properties could be assigned that are only valid when a specific measurement protocol is strictly followed. Because many of the parameters described in EN 142141 are operationally defined, certified values could only be obtained if the specific analytical methods were used. In this case, the identity of the measurand would be defined by the applied standard method. This approach is extremely important for the production of CRMs, but it does raise a number of issues. These are, for example, the selection of appropriate procedures for assigning certified values and uncertainties or the identification of suitable participating laboratories; as such,
CRMs are commonly produced by characterization through multiple expert laboratories, with all of them using the same method. Because there is a non-negligible chance of having statistically as well as technically invalid results, the minimum number of laboratories should be at least 10 and preferably 15. This minimum number allows for data to be scrutinized with the aid of outlier treatment techniques and allows for the achievement of an adequate level of uncertainty for the property values thus established.8 In this study, the campaign for material characterization was based on an intercomparison among several experienced laboratories using standard test methods as provided in EN 14214.1 The objective of this intercomparison study was (i) to establish the performance of routine laboratories in the determination of various parameters in biodiesel using standard methods of EN 14214 and (ii) to determine the degree of conformity of test results obtained to ascertain the feasibility of the production of a new biodiesel CRM. The characterization process provides information on some uncertainty components in the certification model but usually lacks components from material homogeneity and stability. For completeness, this paper also describes the 4631
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Table 2. Results of Characterization Study analyte number of accepted laboratories
12
number of discarded laboratories
2
linolenic acid methyl ester [% (m/m)] 14
monoacylglycerol [% (m/m)] 11 2
mean of means
97.9
8.89
0.75
RSDa of average (%)
0.90
1.20
30.19
RSEb of average (%)
0.26
0.32
9.10
diacylglycerol [% (m/m)]
triacylglycerol [% (m/m)]
total glycerol [% (m/m)]
analyte number of accepted laboratories
10
11
7
number of discarded laboratories mean of means
3 0.145
2 0.067
6 0.207
RSD of average (%)
13.01
60.10
2.48
RSE of average (%)
4.11
18.12
0.94
methanol [% (m/m)]
sulfur (mg/kg)
water (mg/kg)
analyte number of accepted laboratories
6
8
9
number of discarded laboratories
7
1
3
mean of means
0.020
1.3
127
RSD of average (%) RSE of average (%)
3.02 1.23
41.60 14.71
22.00 7.33
iodine (g of iodine/100 g)
flashpoint (°C)
oxidation (h)
analyte number of accepted laboratories
10
number of discarded laboratories
3
mean of means
112.9
174
16.9
RSD of average (%)
0.61
3.68
5.54
RSE of average (%)
0.19
1.39
1.67
acid value (mg of KOH/g)
viscosity (mm2/s)
density (kg/m3)
analyte
a
ester [% (m/m)]
7
11 1
number of accepted laboratories
10
7
number of discarded laboratories
3
5
11
mean of means
0.145
4.49
RSD of average (%)
13.64
0.18
0.01
RSE of average (%)
4.31
0.07
0.004
883.37
RSD = relative standard deviation. b RSE = relative standard error.
integration of the work carried out in homogeneity and stability testing9 and in characterization, leading to the final value assignment of the material.
2. INTERCOMPARISON 2.1. Selection of Participants. Laboratories were selected on the basis of their expertise in biodiesel analysis. In principle, laboratories had to prove their measurement competence and to demonstrate experience in the biodiesel analytical field by providing documentary evidence, e.g., through successful participation in relevant interlaboratory programs or proficiency testing schemes. Accreditation in the specific analysis to be performed was considered an asset. Thus, databases of accreditation bodies were searched for laboratories accredited for the determination of various parameters in biodiesel, and these laboratories were contacted directly. In total, 14 laboratories expressed interest in the study, and most of them were accredited to ISO/IEC 17025.5 2.2. Samples. A rapeseed oil fatty acid methyl ester material was selected for the feasibility study as a candidate material. Samples for the characterization study covered the whole batch produced as described in ‘Toward the Development of Certified Reference Materials for Effective Biodiesel Testing. Part 1: Processing, Homogeneity, and Stability”
(10.1021/ef200895x)9 and were selected using a random stratified sampling scheme. For practical reasons, the target analytes were grouped into 8 lots. For each lot, 3 ampules were provided, and for each ampule, two independent measurements had to be performed (in total, six independent measurement results). A larger volume of the sample was needed for the measurement of viscosity and acid value. Therefore, 6 ampules were provided. A single analysis had to be performed from each. All test samples were packed into insulated boxes, along with cooling bags, and sent by courier mail to the participants. Upon receipt of the samples (
