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Energy & Fuels 2007, 21, 2240-2244
Production of Three Certified Reference Materials for the Sulfur Content in Gasoline (Petrol) T. Linsinger,*,† W. Andrzejuk,‡ A. Bau′,† J. Charoud-Got,† P. De Vos,† H. Emteborg,† R. Hearn,§ A. Lamberty,† A. Oostra,† W. Pritzkow,| C. Que´tel,† G. Roebben,† I. Tresˇl,† J. Vogl,| and S. Wood§ European Commission, Joint Research Centre, Institute for Reference Materials and Measurements (IRMM), Retieseweg 111, 2440 Geel, Belgium, Referat Laboratorium Celne, Terespol, Poland, LGC Ltd., Queens Road, Teddington, United Kingdom, and Bundesanstalt fu¨r Materialforschung und Pru¨fung (BAM), Berlin, Germany ReceiVed March 27, 2007. ReVised Manuscript ReceiVed May 11, 2007
Directive 2003/17/EC of the European Parliament and the European Council stipulates that petrol (gasoline) with a total sulfur content below 10 mg kg-1 must be available in all European Union member states by 2009. Three certified reference materials were produced in support of this directive in a joint effort of the members of the European Reference Materials Initiative (ERM). Two of the materials were made from commercial petrol, while the third one was prepared from a blend of commercial petrols. Relative between-ampule heterogeneity of the materials was quantified and found to be below 2.5%. Potential degradation during storage and dispatch was quantified, and shelf lives based on these values were set. The three materials were characterized by three institutes using different variants of isotope-dilution mass spectrometry. The results from the three institutes were combined, and the final uncertainties of the respective sulfur mass fractions were estimated including contributions from heterogeneity, stability, and characterization. The following mass fractions were derived: ERM-EF211, 48.8 ( 1.7 mg kg-1; ERM-EF212, 20.2 ( 1.1 mg kg-1; and ERMEF213, 9.1 ( 0.8 mg kg-1.
1. Introduction Atmospheric sulfur oxides are the main source of acid rain, which causes significant damage to buildings as well as to the environment. Therefore, such emissions have been regulated in many countries. Sulfur in petrol (gasoline) for road traffic is a significant source of sulfur oxides in the air. To reduce this load, Directive 2003/17/EC of the European Parliament and the European Council1 demanded subsequent phasing-in of fuels (both petrol and diesel) containing less than 10 mg kg-1 of sulfur by January 1, 2005 in the European Union (EU). By 2009, such petrol must be available in all EU member states. Enforcement of such legislation requires reliable control measurements. Laboratories need to be able to check the performance of their methods for the determination of S in petrol. This is also true for standardized methods, the use of which does not per se guarantee accurate results. It is widely accepted that laboratories need to demonstrate their proficiency in the applicability of standard methods, for example, by using certified reference materials (CRMs). While there are several CRMs available certified for the S content in diesel (gas oil), only three CRMs were available for sulfur content in petrol, all available from the National Institute of Standards and Techology (NIST; Gaithersburg, MD). Only one of them, SRM 2299, consists of * Corresponding author. Fax: +32 14 571 548. E-mail:
[email protected]. † IRMM. ‡ Referat Laboratorium Celne. § LGC Ltd. | BAM. (1) Directive 2003/17/EC of the European Parliament and of the European Council.
commercial petrol, whereas the other two (SRM 2294 and SRM 2298) are synthetic gasolines, which contain only 25 different organic compounds in contrast to commercially available petrol, which contains many hundreds of different organic compounds. Therefore, a need for CRMs resembling commercial petrol exists. To address this lack of CRMs, the partners of the European Reference Materials Initiative (ERM), the Bundesanstalt fu¨r Materialforschung und Pru¨fung (BAM), Berlin, Germany; LGC Ltd., Teddington, United Kingdom; and the Reference Materials and Measurements (IRMM), European Commission, Joint Research Centre, Geel, Belgium, embarked on a project aimed at the certification of the sulfur content in commercially available petrol with a lower uncertainty for the certified values than presently available. 2. Experimental Section The term “production” of CRMs comprises the complete process from project planning to after-sales service. In particular, it includes the steps of processing of material, homogeneity testing, stability testing, characterization, value assignment, and preparation of the certificate. This project was aimed at producing three reference materials with S contents from 10-50 mg kg-1, to cover the complete current and future range of legislation related to S mass fractions in petrol. 2.1. Processing of Material. Commercial petrol with a S mass fraction of about 10 mg kg-1 for material ERM-EF213 was provided by ESSO Deutschland GmbH, Ingolstadt, Germany. Petrol with a S mass fraction of about 50 mg kg-1 for ERM-EF211 was provided by Motor Oil Ltd., Corinth Refineries, Corinth, Greece. A blend of petrols with a mass fraction of about 20 mg kg-1 for ERM-
10.1021/ef070155t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
Sulfur Content in Gasoline EF212 was provided by BP CTC, The Manorway, Stanford-leHope, Essex, U.K. The petrol was ampouled to minimize the possibility of evaporation and oxidation. Special borosilicate glass ampoules of 1 mm wall thickness were used. Borosilicate glass had to be used as only this type of glass withstands the thermal stress of flame-sealing of ampoules containing precooled petrol at a temperature of below -50 °C. Break-down of the organic substances by UV was prevented by storing the ampoules in the dark. Ampoules were opened by flame in an ampouling machine, rinsed with ultraclean water to remove dust, and dried before filling. This rinsing step reduced the sulfur blank to 1-3 µg L-1, which is negligible compared to the sulfur content of the petrols. The ampoules were filled with approximately 19 mL of the petrol and cooled in liquid nitrogen to about -50 °C while they were flame sealed one by one. Approximately 3000 ampoules of each material were prepared. The processing is described in greater detail in ref 2. 2.2. Homogeneity Study. The homogeneity study aimed to quantify any between-ampule heterogeneity of the total S mass fraction. A total of 28-35 ampoules of each material were selected using random stratified sampling of the whole batch. Three subsamples per ampule were analyzed for their total S mass fraction five times each by combustion-UV fluorescence. Care was taken to ensure that the order of measurements did not correspond to the filling sequence of the ampoules, which enabled a differentiation between a potential trend in the filling sequence and analytical drift. The three data sets were screened for outliers using the Grubbs procedure, and regression analyses were performed to check for potential trends in the filling sequence, which might have been caused by evaporation during the filling process. The distribution of results was checked using normal probability plots and histograms. Finally, an analysis of variance (ANOVA) was performed to quantify the within-ampule variation (swb; repeatability) and between-ampule variation (sbb). Method repeatability sets a limit to the heterogeneity that can be detected by a particular method with a given study setup (number of repetitions per sample; number of samples). The maximum heterogeneity that could be hidden by method repeatability (u*bb) was calculated as described by Linsinger et al.3 2.3. Stability Study. While petrol is sensitive to light, storage in closed ampoules in the dark makes time and temperature the only relevant parameters having an effect on degradation. Stability of the total S mass fraction of the materials was tested at 60 °C using isochronous designs.4 In such designs, samples are moved to a reference temperature after having been kept for some time at a certain test temperature. By moving the sample to a reference temperature, the degradation incurred at this reference temperature is “frozen”. Therefore, at the end of the study, all samples from the complete study can be analyzed in one analytical run, which greatly reduces analytical variation from between-day variation and increases the significance of the result. The reference temperature was set at 4 °C for all samples. Two isochronous schemes were conducted for all three materials. The short-term study consisted of storage for 0, 1, 2, and 4 months at 60 °C. The long-term study consisted of storage for 0, 4, 8, and 12 months at 60 °C for EF211 and EF212 and for 0, 6, and 8 months at 60 °C and 12 months at 18 °C for EF213. Four ampoules per time point were used for all materials. After completion of the studies, the samples were analyzed in duplicate for their total S mass fraction using the method ASTM D5453-04 (combustion with subsequent determination of SO2 by UV fluorescence). It was (2) Emteborg, H.; Oostra, A.; Charoud-Got, J.; de Vos, P.; Bau′, A. Private Communication (Processing of the European Reference Materials EF211, EF212 and EF213sSulfur in Petrol, RM Unit Internal Report GE/ RM Unit/06/2005/August12, IRMM, Geel, 2005). (3) Linsinger, T. P. J.; Pauwels, J.; van der Veen, A. M. H.; Schimmel, H.; Lamberty, A. Accredit. Qual. Assur. 2005, 6, 20-25. (4) Lamberty, A.; Schimmel, H.; Pauwels, J. Fres. J. Anal. Chem. 1998, 360, 359-361.
Energy & Fuels, Vol. 21, No. 4, 2007 2241 ensured that the analytical sequence did not coincide with the sequence in time. After testing for outliers, the data for each study (short- and longterm, for each material) were evaluated individually. Linear regression analyses of the result versus the time were performed, and slopes were tested for significance using t-tests. Uncertainty of long- and short-term stability was estimated as the uncertainty of the regression line with a slope of zero multiplied by a fixed shelf life as described by Linsinger et al.5 Uncertainties of stability were estimated for t ) 1 week to estimate potential degradation during dispatch. Furthermore, the shelf life for a chosen uncertainty of stability of 1% was calculated using the same approach. 2.4. Characterization. Characterization refers to the process of determining the value to be certified. Characterization of the S mass fraction of the three materials was based on isotope-dilution mass spectrometry (ID-MS) performed as the primary method of measurement independently by the three ERM partners, who have demonstrated their technical expertise for the determination of S in diesel (gas oil) by successful participation in the CCQM key comparison K35.6 All individual results of the three institutes are traceable to the International System of Units (SI) as the measurement methods were applied as primary methods of measurement. The value of the spikes used is traceable to the SI via back-spiking with a CRM whose value is traceable to the SI, as it has been characterized by two primary methods of measurement. 2.4.1. Method Employed by BAM. The method employed by BAM was a two-way isotope dilution thermal ionization mass spectrometry (ID-TIMS) method applied as a primary method of measurement, as described in detail in ref 7. The spike was characterized by back-spiking with SRM3154 (NIST, Gaithersburg, MD). An approximately 0.3 g sample was, after the addition of a solution of the isotopic spike, digested in a high-pressure asher (4 h; maximum temperature of 320 °C) using a HNO3/H2O2 mixture. The SO42- formed was reduced to H2S using an HI/HCl/H3PO2 mixture. The H2S was absorbed in an ammoniacal As(III) oxide solution and precipitated as arsenic (III) sulfide by adding HCl. The precipitate was washed and dissolved in ammonia. Rhenium filaments were loaded from these solutions for conducting the thermal ionization mass spectrometry measurements. Mass spectrometric measurements were performed using the multicollector TIMS (MC-TIMS) instrument Sector54 (Micromass/GV Instruments, Manchester, U.K.). A full uncertainty budget was established in line with the ISO Guide to the Expression of Uncertainty in Measurement (GUM).8 2.4.2. Method Employed by LGC. The method employed by LGC was a two-way isotope dilution mass spectrometry method described in detail in ref 9. Oak Ridge high-purity sulfur isotopically enriched to 94 ( 0.06% 34S was used to prepare the spike solution. About 0.2 g of the sample was weighed and digested together with the isotopic spike with 6 mL of HNO3 using closed-vessel microwave digestion (maximum temperature 260 °C; 80 bar). The solutions were measured using an Element magnetic sector inductively coupled plasma mass spectrometer (ThermoFinnigan Element 1, Bremen, Germany) at a medium resolution of 4000, and a full uncertainty budget was established in line with the GUM. 2.4.3. Method Employed by IRMM. The method employed by IRMM was two-way ID-ICP-MS under as close as possible to (5) Linsinger, T. P. J.; Pauwels, J.; Lamberty, A.; Schimmel, H.; van der Veen, A. M. H.; Siekmann, L. Fres. J. Anal. Chem. 2001, 370, 183188. (6) The BIPM Key Comparison Database. http://kcdb.bipm.org/AppendixD/default.asp (report in preparation). Results of the laboratories can be found at www.chemsoc.se/sidor/KK/anadag/Christophe%20Quetel.pdf (accessed May 2007). (7) Pritzkow, W.; Vogl, J.; Ko¨ppen, R.; Ostermann, M. Int. J. Mass. Spectrom. 2005, 242, 309-318. (8) ISO Guide to the Expression of Uncertainty in Measurements; International Organization for Standardization (ISO): Geneva, Switzerland, 1995. (9) Holland, J. G.; Tanner, S. D. Plasma Source Mass Spectrometry: Applications and Emerging Technologies; Royal Society of Chemistry: Cambridge, U.K., 2003; pp 185-192.
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Linsinger et al.
Table 1. Results of the Homogeneity Tests as Calculated from ANOVAa
ERM-EF211 ERM-EF212 ERM-EF213
swb [%]
sbb [%]
u*bb [%]
1.598 1.707 1.254
n.c. 2.328 1.714
0.538 0.425 0.315
a n.c.) Cannot be calculated, as mean squares among groups are smaller than the means squares within groups. swb: Between-ampule standard deviation (repeatability); sbb: between-ampule standard deviation; u*bb: maximum heterogeneity that can be hidden by method repeatability.3
“exact matching” conditions (ratios of blend ratio values ranged 0.8-1.2), as described in detail elsewhere.10 Samples of 0.400.60 g were gravimetrically prepared (substitution weighing) directly in digestion vessels and blended with aliquots of the 34S enriched material IRMM-646, which had been characterized using SRM3154 (NIST, Gaithersburg, MD). A modified version of a program for the decomposition of smaller-size diesel samples by high-pressure ashing11 was employed [i.e., using 8 mL of HNO3 (70%) instead of 5 mL]. ICP-MS measurements were carried out with an Element 2 (Thermo) at a medium mass resolution of 4000. The validation scheme included calculation of a full uncertainty budget in line with the GUM and the measurement of the S content in NIST gasoline SRM-2299 and SRM-2298. 2.5. Commutability. A commutability study was organized to demonstrate that the values obtained by ID-MS are not different from those obtained by routine methods. A total of 11 European laboratories received one sample each of EF211 and EF213 and analyzed them using their routine methods. Laboratories were encouraged, but not obliged, to indicate their measurement uncertainty. The results were tested for outliers using the Grubbs procedure, and the means were compared with the certified values.
3. Results and Discussion 3.1. Results of the Homogeneity Evaluation. Only one outlying mean value and one outlying individual result were detected within all three materials. These results were retained, as they were outliers at a 95%, but not a 99%, confidence level, and this fraction of outliers is expected on statistical grounds only. Furthermore, retaining the data led to conservative estimates of homogeneity. No trend in the filling sequence was detected for any of the materials at a 95% level. The distributions of all ampule averages are unimodal, thus allowing evaluation by ANOVA. The results of the ANOVA, that is, the estimation of between-unit heterogeneity, are shown in Table 1. Homogeneity could not be quantified for EF211, but the setup of the study guarantees that any heterogeneity above 0.538% (u*bb) would have been detected. u*bb was therefore used as an estimate of heterogeneity for EF211. Heterogeneity could be quantified for EF212 and EF213 due to the good repeatability of the method used. The between-ampule variations are with 2.3% (EF212) and 1.7% (EF213), small enough to ensure the usefulness of the materials. 3.2. Results of the Stability Evaluation. The slopes of the stability data regression lines for all materials were not significantly different from zero. While EF211 particularly shows significant darkening when exposed to light, no such darkening was observed even after storage for 1 year in the dark at +60 °C. However, there was no evidence that this darkening influences the total sulfur content. Stability During Dispatch. Potential degradation during dispatch was estimated from the 4-month studies. As it is highly (10) Tresˇl, I.; Que´tel, C. R. J. Am. Soc. Mass Spectrom. 2005, 16, 708716. (11) Ostermann, M.; Berglund, M.; Taylor, P. D. P. J. Anal. At. Spectrom. 2002, 17, 1368-1372.
Table 2. Summary of Results for ERM-EF211, ERM-EF212, and ERM-EF213a
BAM LGC IRMM
ERM-EF211
ERM-EF212
ERM-EF213
49.76 ( 0.54 47.96 ( 0.96 48.6 ( 1.3
20.40 ( 0.32 19.81 ( 0.49 20.50 ( 0.56
9.60 ( 0.64 8.62 ( 0.22 8.99 ( 0.25
a Given are mean S mass fractions measured by each institute and their expanded uncertainties (k ) 2) in mg kg-1.
unlikely that transport takes longer than 1 week () 0.25 months), the potential extent of degradation during a 1-week transport was quantified and found to be between 0.02 and 0.06%. This uncertainty contribution is negligible compared to uncertainties from homogeneity and characterization. The material can therefore be dispatched under ambient conditions. Stability During Storage. Potential degradation during storage was assessed from the 12-month (EF211 and EF212) and 8-month (EF213) studies at 60 °C. The slopes of the stability data regression lines were not significantly different from zero at a 95% confidence level. Shelf lives calculated for an uncertainty of stability of 1% were 9 months (EF213), 30 months (EF211), and 39 months (EF212). The shorter shelf life for ERM-BF213 is most likely caused by the higher analytical variation at this low concentration as well as the shorter duration of the study (only 8 months at 60 °C rather than 12 months). It can be therefore expected that the material shows stability similar to that of the two other materials. When assessing stability, the storage temperature must be taken into account. Stability was tested at 60 °C, while the recommended storage temperature is 20 ( 5 °C. This lower temperature is expected to improve stability. When the rule of thumb that organic reaction rates are decreased by a factor of 2 with a decrease of 10 °C is applied, stability of the materials for more than 3 years is ensured. Additional studies to assess the stability over longer periods of time are ongoing. 3.3. Characterization. Control of the blank was a key issue of the sophisticated analytical method applied by BAM, dominating the overall uncertainty. This contribution was less important for the methods used by IRMM and LGC. As shown in Table 2, formal differences between laboratories exist, because of the very small uncertainties of the participating laboratories which are not routinely achievable. It was concluded that there was no reason to favor one set of results over the others, and all sets of results were accepted on technical grounds. 3.4. Uncertainty Evaluation. Expanded uncertainties of the certified value of an individual ampule (UCRM) consist of contributions for homogeneity (ubb), stability during storage (ults) and dispatch (usts), and characterization (uchar). These are combined to the final uncertainties as given below:12
UCRM ) kxubb2 + ults2 + usts2 + uchar2 ubb was estimated as between-ampule variation (sbb) or the maximum heterogeneity hidden by method repeatability (u*bb) as shown in Table 1. ults was chosen as 1%, which, as described above, ensures validity of the certificate for at least 3 years. usts values for the three materials range from 0.02 to 0.06%, which is far smaller than 1/3 of the largest uncertainty component for each material and is therefore negligible. (12) Pauwels, J.; van der Veen, A. M. H.; Lamberty, A.; Schimmel, H. Accredit. Qual. Assur. 2000, 5, 95-90.
Sulfur Content in Gasoline
Energy & Fuels, Vol. 21, No. 4, 2007 2243
Table 3. Uncertainty Budgets for the Certified Values of ERM-EF211, ERM-EF212, and ERM-EF213a
u(I) [%] u(R) [%] uchar [%] ubb [%] ults [%] usts [%] UCRM (k ) 2) [%] average sulfur content [mg kg-1] UCRM (k ) 2) [mg kg-1] a
ERM-EF211
ERM-EF212
ERM-EF213
0.58 1.07 1.21 0.54 1.00 negligible 3.32 48.77 1.62
0.67 not necessary 0.67 2.33 1.00 negligible 5.24 20.24 1.06
1.33 3.12 3.39 1.71 1.00 negligible 7.86 9.07 0.71
Relative uncertainties are based on the unweighted mean of means as shown in Table 2.
uncorrelated, as different sample preparation and quantification methods were used. Furthermore, the uncertainty budgets for each set of results differ significantly, supporting the assumption of independence. u(II) and u(III) were therefore considered as negligible and set to zero. The additional uncertainty component u(R) has to be added for EF211 and EF213, as not all results agree with one another within the respective uncertainties. u(R) was modeled as a rectangular distribution between the highest and the lowest value as suggested by Levenson et al.14 u(R) was therefore estimated as
u(R) )
Figure 1. Results of the characterization campaign (solid diamonds) and the commutability study (open diamonds). Error bars correspond to expanded uncertainties (k ) 2) as reported by the laboratories. The shaded area is the certified interval. No commutability study was performed for ERM-EF212.
Uncertainty of characterization was estimated using an approach described by Pauwels et al.13 In this approach, the uncertainty of characterization is separated into exclusively laboratory-dependent uncertainties u(I), uncertainties common to all laboratories u(II), uncertainties common to groups of laboratories u(III), and an uncertainty component attributed to the disagreement between measurement results u(R). u(I) was therefore estimated as
u(I) )
x∑ui2 n
with ui being the combined standard uncertainty for the mean value quoted by each laboratory and n the number of laboratories (3). It can be assumed that the three sets of results are completely (13) Pauwels, J.; Lamberty, A.; Schimmel, H. Accredit. Qual. Assur. 1998, 3, 180-181.
max - min 1 2 x3
with min and max being the lowest and highest value for the particular material, respectively. The complete uncertainty budget for the three materials is shown in Table 3. When the principle that uncertainties are always rounded up is used, the certified values become ERM-EF211, 48.8 ( 1.7 mg kg-1; ERM-EF212, 20.2 ( 1.1 mg kg-1; and ERM-EF213, 9.1 ( 0.8 mg kg-1. The certified values are derived from measurement results traceable to the SI reported by the three participating laboratories. All three laboratories have used ID-MS as a primary method of measurement. Agreement of the results confirms the absence of any gross errors. As the certified values are combinations of SI traceable individual results, they are themselves traceable to the SI. The results also agree with the results from the commutability study, as shown in Figure 1, within their uncertainties. Figure 1 also demonstrates that the agreement of results of the laboratories participating in the characterization study is very good in comparison to what can be achieved under routine conditions. 4. Conclusion A set of three reference materials has been certified for the materials’ respective sulfur mass fraction in full compliance with ISO Guide 3415 and ISO Guide 35.16 These materials are primarily intended to give laboratories the possibility to demonstrate their method proficiency. These materials have comparable (ERM-EF211) or lower (ERM-EF212 and -EF213) (14) Levenson, M. S.; Banks, D. L.; Eberhardt, K. R.; Gill, L. M.; Guthrie, W. F.; Liu, H. K.; Vangel, M. G.; Yen, J. H.; Zhang, N. F. J. Res. Natl. Inst. Stand. Technol. 2000, 105, 571-579. (15) ISO Guide 34: General Requirements for the Competence of Reference Material Producers; International Organization for Standardization (ISO): Geneva, Switzerland, 2000. (16) ISO Guide 35: Certification of Reference Materials General and Statistical Principles, 3rd ed.; International Organization for Standardization (ISO): Geneva, Switzerland, 2006.
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expanded uncertainties than previously existing materials and are the first set of CRMs available made from commercial petrol. Acknowledgment. The work described in this paper was in part supported under contract with the U.K. Department of Trade and Industry as part of the National Measurement System’s Valid Analytical Measurement (VAM) Programme. The authors want to
Linsinger et al. thank A. Held and R. Zeleny (IRMM, BE) and the experts of the Certification Advisory Panel “Elements and Inorganic Ions”, O. Donard (CNRS/Universite´ de Pau et des pays de l’Adour, FR), L. Jorhem (Livsmedelsverket, SE), and H. Muntau (Ranco, IT) for their review of the project. EF070155T
