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Production of Two Certified Reference Materials for the Determination of SY124 (Euromarker) in Gas Oil Thomas Linsinger,*,† Ger Koomen,‡ Håkan Emteborg,† Gert Roebben,† Gerard Kramer,† and A. Lamberty† European Commission, JRC, Institute for Reference Materials and Measurements, Retieseweg 111, 2440 Geel, Belgium, and Dutch Customs Laboratory, Kingsfordweg 1, 1043GN Amsterdam, The Netherlands Received May 17, 2005. Revised Manuscript Received September 12, 2005
Two reference materials with certified mass fractions of SY124 in gas oil have been prepared. Samples were prepared by the spiking of blank gas oil with pure SY124. Homogeneity and stability were confirmed, and maximum heterogeneity and degradation were estimated. The purity of the SY124 used for spiking was determined using thermogravimetric analysis, high-performance liquid chromatography with UV detection, gas chromatography with flame ionization and mass spectrometric detection, nuclear magnetic resonance, and Karl Fischer titration. Full uncertainty budgets comprising all potential uncertainty sources were established. The following mass fractions were derived: ERM-EF317, 0.141 ( 0.018 mg kg-1; ERM-EF318, 7.0 ( 0.4 mg kg-1.
1. Introduction Gas oils used for transport and heating purposes are taxed differently in most European countries, resulting in substantial price differences. In 1995, the European Commission decided to introduce a common marker to facilitate harmonization of excise duties on mineral oils1. Solvent Yellow 124 (SY124), or N-ethyl-N-[2-(1-isobutoxyethoxy)ethyl]-4-(phenylazo)aniline, CAS Registry Number 34432-92-3 (Figure 1), was selected as the most suitable candidate for such a common marker and was introduced as common marker in the European Union on August 1, 2002,2 with a lower marking level of at least 6 mg L-1. In 2003,3 the upper marking level was set to 9 mg L-1. A common reference method was developed and validated as described elsewhere.4 Enforcement of the relevant directive does not only require validated methods but also requires quality assurance tools to prove that laboratories are able to apply the methods correctly. ISO 17025 5 explicitly mentions the use of certified reference materials (CRMs) to test method performance, and they are recognized as particularly useful for this purpose. It was, therefore, decided to produce two CRMs with well-defined concentrations of SY124 in gas oil to allow laboratories worldwide to check for any significant bias of their methods. The intended use of the materials was defined * Corresponding author. E-mail:
[email protected]. † European Commission, JRC, Institute for Reference Materials and Measurements. ‡ Dutch Customs Laboratory. (1) European Council Directive 95/60/EC. (2) European Commission Decision 2001/574/EC. (3) European Commission Decision 2003/900/EC. (4) Linsinger, T. P. J.; Koomen, G.; Emteborg, H.; Roebben, G.; Kramer, G.; Lamberty, A. Energy Fuels 2004, 18, 1851-1854. (5) ISO 17025: General Requirements for the Competence of Testing and Calibration Laboratories; International Organization for Standardization (ISO): Geneva, Switzerland, 2000.
Figure 1. Chemical formula of SY124.
as a tool for the estimation of measurement bias rather than calibration. 2. Experimental Section The production of CRMs comprises the complete process of project planning, sample preparation, assessment of homogeneity and stability, characterization and value assignment, and issuing of the certificate. 2.1. Sample Preparation. A total of 120 L of commercially available gas oil was delivered to the Institute for Reference Materials and Measurements. An analysis by high-performance liquid chromatography coupled with the detection of absorption of ultraviolet light (HPLC-UV) confirmed that the material did not contain SY124 (limit of detection: 0.020 mg L-1 as reported elsewhere4). Characteristics of the material are shown in Table 1. A stock solution of SY124 was prepared by dissolving 0.1920 g of SY124 in 422.9 g of blank gas oil at a temperature of 22 ( 2 °C. This solution was diluted with gas oil to 26 500 g (31.2 L) in a 40 L polypropylene drum to obtain a concentration of 7.2 mg kg-1 (about 6 mg L-1) (ERM-EF318) and thoroughly stirred using a polytetrafluoroethylene (PTFE) paddle. A total of 476.6 g (0.575 L) of this solution was diluted further to 23 805 g (28.7 L) with blank fuel in another 40 L drum and thoroughly stirred using a PTFE paddle to give a concentration of 0.15 mg kg-1 (about 0.12 mg L-1) (ERMEF317). Twenty milliliter portions of the solutions were filled into amber glass ampules on a ROTA R911/MA-G-32 automatic ampuling machine (Wehr/Baden, Germany). A total of 1401 ampules of ERM-EF317 and 1298 ampules of ERM-EF318 was
10.1021/ef0501492 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/22/2005
Determination of SY124 in Gas Oil
Energy & Fuels, Vol. 19, No. 6, 2005 2461 Table 1. Characteristics of the Gas Oil Useda
density (kg m-3)
percentage distilling at 250 °C
percentage distilling at 350 °C
SY124 (mg L-1)
sulfur content (mg kg-1)
color
829.4
33
>85
0.00
45.0
colorless
a Methods: density, ASTM D4052 at 15 °C; distillation, ASTM D86; SY124, reference method (HPLC-UV); sulfur content, ISO/DIS 20486. Pure SY124 was obtained from John Hogg Technical Solutions Ltd., Manchester, U. K.
produced. The headspace of the ampules was filled with a He/ Ar mixture before flame sealing. The mass of each ampule was recorded, and tightness was confirmed by putting the ampules into a vacuum oven and checking for traces of He, indicating potential leaks. Correctness of the gravimetric preparation was confirmed by the results of the method validation study reported elsewhere.4 2.2. Homogeneity. Fifteen units of both materials were selected randomly stratified over the whole batch and analyzed in duplicate using HPLC-UV. The results were plotted against the filling sequence to check for any significant trends. It was checked whether the individual data and ampule averages follow a normal distribution, using normal probability plots, and whether the individual data are unimodally distributed, using histograms. Standard deviations within units and between units were calculated using ANOVA. Furthermore, ubb*, the maximum heterogeneity that could be hidden by method repeatability, was calculated as described by Linsinger et al.6 2.3. Stability. Stability testing of reference materials must provide information regarding conditions under which the material can be shipped (short-term study) and which storage conditions must be chosen to ensure stability during storage (long-term study). Isochronous stability studies allow all measurements to be performed under repeatability conditions.7 For this type of study, samples are stored at a test temperature for a certain period of time and are then moved to a reference temperature, at which any degradation is known to be negligible. The stability status of the material is, therefore, “frozen” until the complete study is finished. A temperature of +18 °C was chosen as the reference temperature, as it was known from preliminary tests that the materials are stable at this temperature for at least the duration of the study. Twenty-six units of each material were selected randomly stratified for the short-term study. Two units of each material were stored for 1, 2, 3, and 4 weeks at -20, +4, and +60 °C. The t ) 0 points were formed by two units that stayed the entire time at +18 °C. All units were analyzed in duplicate by HPLC-UV in one analytical run under repeatability conditions, and control standards were injected after every 10th run to check for instrument drift. Averages and standard deviations for each series were calculated to check whether the data quality was good enough to allow meaningful statements, which was the case for all series. The slopes of the linear regression lines were calculated for all temperatures and materials, were tested for significance, and were all found nonsignificant at a 95% confidence level. Neither freezing nor exposure to 60 °C for 4 weeks affects the analyte concentrations. They can, therefore, be shipped at ambient temperature. The stability of the SY124 in solution during storage was assessed from samples from a test batch stored at room temperature in the dark. Gas oil with SY124 concentrations of 5.4 and 0.10 mg L-1 was tested on 24 occasions over a period of 8 months, with 69 measurement points in total for each material. Stability was evaluated using linear regression, and an uncertainty of stability for a given shelf life was estimated. (6) Linsinger, T. P. J.; Pauwels, J.; van der Veen, A. M. H.; Schimmel, H.; Lamberty, A. Accredit. Qual. Assur. 2001, 6, 20-25. (7) Lamberty, A.; Schimmel, H.; Pauwels, J. Fresenius’ J. Anal. Chem. 1998, 360, 359-361.
2.4. Characterization. Characterization refers to the process of determining the values to be certified. In this case, characterization mainly had to focus on two issues, namely, establishing the identity of the substance used for spiking and that for determining its purity. 2.4.1. Identity. Confirmation of the assumption that the substance used was indeed SY124 was derived from several sources. John Hogg Technical Solutions supplied a certificate of analysis stating that the material supplied was SY124 and was solvent-free. The material from John Hogg was tested with gas chromatography with mass spectrometric detection (GCMS), and the obtained mass spectrum of the main constituent can be explained very well by the structure proposed for SY124 (Figure 1). 1H and 13C nuclear magnetic resonance (NMR) measurements were performed, and the spectra agree with the structure proposed for SY124. HPLC-UV and GC-MS measurements were performed in which the material from John Hogg Technical solutions was compared to independently produced material from BASF. The agreement of both retention times and mass spectra showed that both materials were the same. This also confirms identity, as it is extremely unlikely that two independently prepared substances are the same but different from the intended substance. It was concluded that identity was confirmed with negligible uncertainty. 2.4.2. Purity. Commercial SY124 is a mixture of the main compound and reaction byproducts of a similar structure. Most likely, impurities, therefore, are byproducts of a similar structure and residual solvents. Inorganic residues are not expected to be present in high amounts. The various potential impurities require a purity assessment by several methods performed independently at LGC (LGC Ltd., Teddington, U. K.) and the Dutch Customs Laboratory (DCL; Amsterdam, NL). The purity of the material was independently assessed by thermogravimetric analysis (TGA), 1H NMR and 13C NMR, HPLC-UV, GC with mass spectrometric (MS) and flame ionization detection (FID), and Karl Fischer titration.
3. Results and Discussion 3.1. Results of the Purity Determination. 3.1.1. TGA. LGC reported two weight loss regions, indicating the presence of more than one compound. The first weight loss step (96.7 ( 0.43%) is complete in the temperature region of 360-380 °C, and the second (3.17 ( 0.38%) is complete by 650 °C. Total weight loss was determined as 99.90 ( 0.09%. DCL reported only a total mass loss of 100.04% with an uncertainty of (0.22% (95% confidence level), thus confirming the results of LGC. 3.1.2. NMR. DCL determined the purity of SY124 using a solution in CDCl3, and a frequency of 200 MHz was used (Bruker AC200). The estimated amount of each organic impurity was below 1%. LGC used 1H and 13C NMR spectroscopy. The sample was dissolved in d 6 dimethylsulfoxide (DMSO) and analyzed using an AM500 NMR spectrometer (Bruker) equipped with a 5 mm 1-H/ 13-C dual probehead. The 1H and 13C spectra were consistent with the structure proposed for SY124. There was evidence of additional resonance signals at trace
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Table 2. Masses, Nominal Values, and Uncertainties for the Preparation of ERM-EF318a
a
step
value
uncertainty
relative uncertainty
m1 (mass SY124) m2 (final mass gas oil) final concentration: m1/m2
0.192 00 g 26 500.0 g 7.2453 mg kg-1
0.000 15 g 0.5 g 0.0056 mg kg-1
0.078% 0.0019% 0.078%
The uncertainties of each weighing step are taken from the calibration certificates of the balances used. Table 3. Masses, Nominal Values, and Uncertainties for the Preparation of ERM-EF317a step m1 (mass SY124) m2 (final mass gas oil for ERM-EF318) m3 (mass of ERM-EF318 taken) m4 (final mass gas oil for ERM-EF317) final concentration: m1m3/m2m4
aThe
value
uncertainty
relative uncertainty
0.192 00 g 26 500.0 g
0.000 15 g 0.5 g
0.078% 0.0019%
476.60 g 23 805.0 g
0.15 g 0.5 g
0.0031% 0.0021%
0.1451 mg kg-1
0.000 11 mg kg-1
0.078%
uncertainties of each weighing step are taken from the calibration certificates of the balances used.
levels in the 1H spectrum at δH 6.5 and 7.35 ppm. The structure of this impurity could not be characterized, as a result of the low intensity of the spurious peaks. 3.1.3. GC-MS. Ten analyses were performed in one analytical run using an Agilent 5973i GC-MS and a HP5MS column (30 m × 0.35 mm × 0.1 µm) by DCL. The impurities in the materials could not be identified via the available reference mass spectra libraries. The mass spectra of the two most abundant impurities (total 0.33%) resemble the mass spectrum of SY124. They are probably reaction intermediates or reaction byproducts. Also, small amounts (< 0.10%) of toluene and xylene are present. The other impurities (all < 0.2%) could not be identified via the available reference mass spectra libraries. A mean purity of 98.32% with a standard deviation of 0.13% was obtained. 3.1.4. GC-FID. Purity determinations by GC-FID were performed by LGC on a Thermo Trace 2000 GC-FID using a DB1 column (60 m × 0.32 mm × 0.25 µm). A sample of 0.05% (mass per mass) of SY124 in toluene was used. Thirteen replicates were performed. Five major impurities with levels at or greater than 0.1% were detected, and a mean purity of 95.96% with a standard deviation of 0.15% was obtained. The presence of five major impurities was confirmed by GC-MS using a split/ splitless injector on a GC-MS column. The agreement of results using different injection systems indicates that no significant thermal decomposition of the sample within the injector occurred. The standard uncertainty of the result was estimated to be 0.4% on the basis of previous data, comprising contributions from injector repeatability, linearity of the detector, and an analyte and impurity recovery from the column. 3.1.5. HPLC. HPLC determinations were performed by LGC. A Waters 2695 Separations module with a Waters 996 diode array detector and Millenium232 V3.0 processing software was employed. No separation of coeluting compounds could be achieved using several C18, modified C18, and polymer-based columns employing both isocratic and gradient elutions on acetonitrile/ water/methanol mixtures. However, the presence of other compounds was confirmed by a shoulder on the main peak. Moreover, six to seven impurities were found (none at the level found with GC) in addition to the shoulder at the main peak. The laboratory concluded that the main impurities (seen on GC-FID) are struc-
turally similar to SY124 and were coeluting with the main peak. 3.1.6. Water Content. Coulometric Karl Fischer titration was employed by LGC on a Mitsubishi CA-06 moisture meter. Background levels were assessed by exposing the cell to the atmosphere for a brief period to imitate the addition of the sample through the weighing boat. These background readings were then subtracted from the results for water. The performance of the instrument was checked using the CRM SRM 2890 (water-saturated octanol) from NIST. Two replicate determinations on approximately 85 mg samples were performed. Samples were introduced directly into the coulometer. A run time of 5 min was used to ensure a robust measurement. The measured water content was 0.062% (mass per mass) with a standard uncertainty of 0.003% (mass per mass) comprising weighing of the sample, purity of the calibration standards, purity of the reagents, and the electrode response. 3.2. Uncertainty Evaluation. 3.2.1. Uncertainty of the Gravimetric Preparation. The uncertainty from the gravimetric preparation depends on the accuracy of the balances used for weighing the pure SY124 and the gas oil used for dilution. The individual uncertainty contributions are given in Tables 2 and 3. 3.2.2. Uncertainty of Homogeneity. A regression analysis of concentration versus filling sequence () ampule number) was performed to elucidate whether there had been any concentration gradient over the filling. The slopes of the regression lines over the filling sequence were highly nonsignificant (P > 0.54), proving the absence of significant evaporation losses or contamination. Neither the individual data for ERM-EF317 nor the ones for ERM-EF318 followed a normal distribution: one and five results, respectively, deviated from the straight line of all data for ERM-EF317 and ERMEF318. These deviating results were spread over various units (and, in the case of ERM-EF318, over the whole batch) and, therefore, do not constitute outlying averages. However, the individual data for both materials followed unimodal distributions, thus allowing evaluation by ANOVA. Between-unit heterogeneity could not be quantified because it was lower than could be determined with the method used. Method repeatability was, therefore, the limiting factor for any statement regarding heterogeneity. In such cases, an upper limit for between-unit
Determination of SY124 in Gas Oil
Energy & Fuels, Vol. 19, No. 6, 2005 2463
Table 4. Results of the Long-Term Stability Study for Test Batches of 0.10 and 5.4 mg L-1 SY124 in Gas Oila L-1]
average [mg RSD [%] slope of the regression line ( s [mg L-1/month] ults [% per month] ults for 24 months
0.10 mg L-1
5.4 mg L-1
0.10 4.65 (0.002 ( 0.268) × 10-3
5.39 1.70 (-0.713 ( 5.037) × 10-3
0.254% 6.11%
0.093% 2.23%
a Average: average of all results in the stability study. RSD: relative standard deviation of all results in the stability study. u : lts uncertainty of long-term stability.
Table 5. Summary of the Results for the Purity Assessmenta data source inorganic residues (TGA) water purity by NMR purity by GC-MS purity by GC-FID purity by TGA purity by HPLC-UV a
result DCL LGC LGC DCL LGC DCL LGC LGC LGC
uncertaintyb
definition of the uncertainty
unit
0 0.23 0 0.3 0.062 0.003 no impurity > 1 n.a. one impurity n.a. 98.3 0.13 96.0 0.4 96.7 0.4 no quantification possible
mass % mass % mass % mol %
standard deviation combined uncertainty combined uncertainty
area % area % mol %
standard deviation combined uncertainty standard deviation
All results in mass %. b n.a. ) not applicable.
heterogeneity (ubb*) that can be hidden by method repeatability can be estimated.6 ubb* values were calculated to be 0.65% for ERM-EF317 and 0.25% for ERMEF318, respectively. The theoretically possible higher heterogeneity for the lower concentration level indicates that this value is solely due to method repeatability. 3.2.3. Uncertainty of Stability. Uncertainty of degradation during transport is negligible, as shown by the short-term stability study. Regression lines were calculated from the results of the long-term stability, and none of the slopes was found significant on a 95% confidence level. Uncertainties of stability (ults) were estimated as chosen shelf life (24 months) multiplied by the standard deviation of the slope of the regression line, as described elsewhere.8 The results of these calculations are shown in Table 4. The results for the uncertainty of stability are conservative estimates, as the measurements were not performed under repeatability conditions. Furthermore, as a result of the higher effort in processing (sealed ampules flushed with inert gas rather than glass bottles), ERM-EF317 and ERM-EF318 are most likely more stable than the test batches. 3.2.4. Uncertainty of Characterization. All results for the determination of purity are summarized in Table 5. The GC measurements showed the presence of significant impurities. As the amount of each of them was rather small, they did not all appear on the NMR spectra. The negative finding of the NMR was, therefore, not in contradiction with the finding of the GC. The GC results were corroborated by the findings of the TGA. The level of impurity found with TGA (second weight loss step: 3.17 ( 0.38%) agreed with the GC measurements of both laboratories. The rather poor limit of detection of the NMR ruled out the use of the results of this technique for the final purity assessment. Similarly, as TGA was not validated
for quantitative measurements of mixtures of compounds, these results were used for confirmation only. The final purity assessment was, therefore, based on the results of the GC measurements, corrected for the amounts of inorganic residues and water. The mean of the two GC results was estimated as the unweighted mean of the two GC results. The combined uncertainty of the GC determinations was modeled to consist of the following: (i) Uncertainties of the individual GC measurements (uGC1 and uGC2). These uncertainties were assumed to be completely independent. The contribution of both of them is, therefore, reduced by x2, as two contributions are combined. (ii) An uncertainty contribution arising from the difference in the two GC results. This contribution was estimated from a rectangular distribution between the two averages (xjGC1, xjGC2) and was, hence, estimated as the halfwidth of the difference divided by x3, as described in ref 9. The combined uncertainty of the GC measurements (uc,GC) was then estimated as
(8) Linsinger, T. P. J.; Pauwels, J.; Lamberty, A.; Schimmel, H.; van der Veen, A. M. H.; Siekmann, L. Fresenius’ J. Anal. Chem 2001, 370, 183-188.
(9) 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.
uc,GC )
x
(
uGC12 uGC22 xjGC1 - xjGC2 + + 2 2 2x3
)
2
Using this equation, a combined uncertainty of 0.73% was estimated. The final uncertainty of purity (uc,pur) was estimated as the square root of the sum of the squared uncertainty of the water determination (uwater), the determination of the inorganic impurities (uinorg.impur), and the uncertainty of the GC measurements.
uc,pur ) xuwater2 + uinorg.impur2 + uc,GC2 Uncertainty of the determination of the inorganic impurities was assumed to be 0.3%. This conservative
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Table 6. Final Calculation of the Uncertainty of Puritya data source
result
uncertainty
inorganic residues (TGA) water purity by GC total purity
0% 0.062% 97.15% 97.09%
0.3% 0.003% 0.73% 0.79%
a
All results in mass %. Table 7. Uncertainty Budget of ERM-EF317 and ERM-EF318a ERM-EF317
certified mass fraction corrected for purity (mg kg-1) ubb (%) ults (%) uc,prep (%) uc,pur (%) uCRM (%) UCRM (k ) 2) (%) UCRM (k ) 2) (mg kg-1) a
0.141 negligible 6.11 0.078 0.81 6.16 12.32 0.0174
ERM-EF318 7.034 2.23 0.078 0.81 2.38 4.76 0.335
Certified mass fraction. All uncertainties are relative %.
estimate allows for the fact that the same method was used by both laboratories. All relevant data are summarized in Table 6. Purity, including inorganic residues and water, is 97.09 ( 0.79% mass per mass (combined uncertainty). This uncertainty corresponds to 0.81% relative uncertainty. 3.3. Certified Values and Their Uncertainties. The certified concentrations were calculated as the nominal concentrations (Tables 2 and 3) multiplied by the purity (Table 6) of the material. Mass fractions of 0.141 mg kg-1 and 7.034 mg kg-1, respectively, were obtained. The uncertainties of the certified concentrations were calculated as
uCRM ) xubb2 + ults2 + uc,prep2 + uc,pur2 Certified values and all uncertainty contributions are summarized in Table 7. A coverage factor (k) of 2 was chosen, as the main contribution of the combined uncertainty comes from ults, which has a sufficiently high number of degrees of freedom to warrant the assumption of a normal distribution. The final uncertainties were rounded to 0.018 mg kg-1 and 0.4 mg kg-1 for ERM-EF317 and ERM-EF318, respectively. This rounding follows the principles stated in guidelines10 on the production of reference materials. Its two principles are that the error introduced by rounding should be between 3 and 33% and that uncertainties should always be rounded up to avoid the underestimation of uncertainties. Therefore, the following certified mass fractions were derived: ERM-EF317: 0.141 ( 0.018 mg kg-1 ERM-EF318: 7.0 ( 0.4 mg kg-1 The uncertainties are expanded uncertainties according to the Guide to the Expression of Uncertainty in (10) Guidelines for the Production of Certification of BCR Reference Materials, BCR/01/97; European Commission: Brussels, Belgium, 1997.
Measurement,11 with a coverage factor of 2, corresponding, approximately, to a confidence level of 95%. The following mass concentrations (not certified) were obtained using a density of 829.4 kg m-3 at 15 °C (specification of the supplier). The uncertainty contribution of the density determination was estimated as the reproducibility standard deviation, as specified in ASTM D405212 (0.5 kg m-3 ) 0.06%). This contribution is negligible compared with the other contributions: ERM-EF317: 0.117 ( 0.015 mg L-1 ERM-EF318: 5.84 ( 0.28 mg L-1 An initial period of validity of 24 was assigned and is stated on the certificate. This period will be extended on the basis of the results of the ongoing stability monitoring. Inorganic impurities and water were determined by primary methods. Organic impurities were determined by gas chromatography in two different laboratories using different columns, temperature programs, injection techniques, and detectors. The results were confirmed by measurements based on different measurement principles. The assigned purity is, therefore, traceable to the International System of Units (SI). The blank material has been tested and was found not to contain SY124. All dilutions were performed gravimetrically on calibrated balances, making the gravimetric value traceable to the SI. The assigned mass fraction is SI-traceable, as both purity and preparation are SI-traceable. The assigned mass concentration values are SItraceable, as purity, preparation, and density determination are SI-traceable. Gas oils containing SY124 are synthetic products, and there is no a priori reason to assume that ERM-EF317 and ERM-EF318 would behave differently from commercially available gas oils containing SY124. This assumption was confirmed by the method validation study,4 where no difference in method performance between the synthetic samples from IRMM and a commercially available gas oil containing SY124 was found. ERM-EF317 and ERM-EF318 are, therefore, commutable with (i.e., behave in the same way as) commercial gas oils according to the European Union reference method. 4. Conclusions Two reference materials have been produced with certified mass fractions of SY124 in full compliance with ISO Guide 34 13 and the new version, ISO Guide 35.14 The materials are primarily intended to be used to assess method performance, that is, to check for a (11) Guide to the Expression of Uncertainty in Measurement (GUM); International Organization for Standardization (ISO): Geneva, Switzerland, 1995. (12) ASTM International D4052: Standard test method for density and relative density by digital density meter. ASTM International: West Conshohocken, PA, 1996 (13) ISO Guide 34: General Requirements for the Competence of Reference Material Producers; International Organization for Standardization (ISO): Geneva, Switzerland, 2000. (14) ISO Guide 35: Certification of Reference MaterialssGeneral and Statistical Principles, accepted final draft 3rd edition; International Organization for Standardization (ISO): Geneva, Switzerland, 2004.
Determination of SY124 in Gas Oil
significant measurement bias. Being homogeneous, they can also be used to establish quality control charts. The certified uncertainties are sufficiently small to allow laboratories assessment of their performance and will, therefore, be an indispensable tool for method validation and for combatting fraud.
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The materials are not intended for calibration. If they are, nevertheless, used for this purpose, it must be understood that measurement bias must be estimated using other materials. The materials can be ordered from IRMM (www.irmm.jrc.be; www.erm-crm.org). EF0501492
