Elcio Cruz de Oliveira

Internal standard versus external standard calibration: an uncertainty case study of a liquid chromatography analysis

Traditionally, in the cigarettes industry, the determination of ammonium ion in the mainstream smoke is performed by ion chromatography. This work studies this determination and compares the results of this technique with the use of external and internal standard calibration. A reference cigarette sample presented measurement uncertainty of 2.0 μg/cigarette and 1.5 μg/cigarette, with external and internal standard, respectively. It is observed that the greatest source of uncertainty is the bias correction factor and that it is even more significant when using external standard, confirming thus the importance of internal standardization for this correction.

VALIDAÇÃO DA METODOLOGIA DA AVALIAÇÃO DE INCERTEZA EM CURVAS DE CALIBRAÇÃO MELHOR AJUSTADAS POR POLINÔMIOS DE SEGUNDO GRAU

The most widespread literature for the evaluation of uncertainty - GUM and Eurachem - does not describe explicitly how to deal with uncertainty of the concentration coming from non-linear calibration curves. This work had the objective of describing and validating a methodology, as recommended by the recent GUM Supplement approach, to evaluate the uncertainty through polynomial models of the second order. In the uncertainty determination of the concentration of benzatone (C) by chromatography, it is observed that the uncertainty of measurement between the methodology proposed and Monte Carlo Simulation, does not diverge by more than 0.0005 unit, thus validating the model proposed for one significant digit.

Comparação de diferentes abordagens para avaliação da incerteza na cromatografia gasosa do gás natural

The evaluation of uncertainty associated with an analytic result is an essential part of the measurement process. Recently, several approaches to evaluate the uncertainty in measurement have been developed. Here, the gas chromatography assay uncertainty for natural gas is compared by some of these approaches: the guide to the expression of uncertainty in measurement (GUM) approach, top-down approach (reproducibility estimate from an inter-laboratory study), Barwick & Ellison (data from validation), study of variability and fuzzy approach. The comparison shows that GUM, Barwick & Ellison and fuzzy approaches lead to comparable uncertainty evaluations, which does not happen with the top-down approach and study of variability by the absence of data normality.

Least squares regression with errors in both variables: case studies

Analytical curves are normally obtained from discrete data by least squares regression. The least squares regression of data involving significant error in both x and y values should not be implemented by ordinary least squares (OLS). In this work, the use of orthogonal distance regression (ODR) is discussed as an alternative approach in order to take into account the error in the x variable. Four examples are presented to illustrate deviation between the results from both regression methods. The examples studied show that, in some situations, ODR coefficients must substitute for those of OLS, and, in other situations, the difference is not significant.

AVALIAÇÃO METROLÓGICA DA CURVA ANALÍTICA PARA DETERMINAÇÃO DO TEOR DE ENXOFRE EM BAIXÍSSIMAS CONCENTRAÇÕES EM ÓLEO DIESEL S-10

The increasing search for a reduction in air emissions has provided a challenging analytical scenario. This paper discusses metrologically, the best practices relating to analytical curves for the determination of very low concentrations of sulfur content in diesel S-10 by monochromatic wavelength dispersive X-ray fluorescence spectrometry (MWDXRF). For weighted linear regression, a new strategy to correlate the weights with the output variable is proposed, which is lacking in the literature. The analytical curve 0-500 mg kg-1, built in compliance with international standards, featured heteroscedastic behavior. The results showed that weighted linear regression is the most suitable choice for assessing concentrations at the beginning of an analytical curve, such as 10 mg kg-1 (product specification limit) rather than simple linear regression which holds that the measured results must be closest to the centroid of the regression line. Finally, it was observed that, when a shorter analytical curve is built, it loses both in accuracy and in precision for very low concentrations, as compared to t simple and weighted linear regression.

Validation of the performance of process stream analyzer systems with nonparametric behavior

Literature describes several procedures based on statistical principles to validate whether the degree of agreement between the results produced by a stream analyzer system—systems for determining chemical and physical characteristics by continual sampling, automatic analysis, and recording or otherwise signaling of output data—and those produced by an independent test measurement procedure, which purports to measure the same property, meets user-specified requirements. However, these documents always consider that the data gathered during validation procedure are normally distributed, and this supposition it is not always true. The aim of this manuscript is to develop four different procedures applied to data with nonparametric behavior. The most representative validation procedure compares measures of position and dispersion, using typical process samples, while the less representative one compares only measures of position, using a reference sample. These nonparametric data are also treated using parametric statistics for comparison. The results show that different conclusions can be reached when the nonparametric data are treated as parametric. A new Brazilian standard is being carried out based on this subject, in order to complement the international standards.

EVALUATION OF THE UNCERTAINTY IN MEASUREMENT VERSUS THE LIMIT OF SPECIFICATION: QUALITATIVE AND QUANTITATIVE ASPECTS OF COMPLIANCE

In many cases, the declaration of compliance of a result of measurement is not clear. This is observed when there is a partial superposition of the expanded uncertainty of a quantity with its limit of specification. In these cases, a judgment is required based in pre-established criteria between the client and the supplier. The consequences of an inadequate decision may be disastrous. Either the supplier will have financial losses or it will lose credibility with its client. This work demonstrates how to calculate the probability, for an appropriate confidence level, that the uncertainty associated with the measurement is within the limits that were established in a contract, declaring the compliance or non-compliance relative to the specification, and study two methodologies for helping the claimer to take the decision of compliance or non-compliance, using as example the heat capacity of natural gas, calculated from its gas chromatography. This approach encourages the client/supplier relationship. This occurs because the results are reported to both parts with a probability of compliance instead of a single value. The first methodology is based on the increase in the size of the sample. The second proposal is based in the calibration of the chromatograph against certificate reference materials with more exact and precise uncertainty values.Copyright

The state of the art of the impact of sampling uncertainty on measurement uncertainty

The measurement uncertainty is a parameter that marks the reliability and can be divided into two large groups: sampling and analytical variations. Analytical uncertainty is a controlled process, performed in the laboratory. The same does not occur with the sampling uncertainty, which, because it faces several obstacles and there is no clarity on how to perform the procedures, has been neglected, although it is admittedly indispensable to the measurement process. This paper aims at describing the state of the art of sampling uncertainty and at assessing its relevance to measurement uncertainty.

Pressure Influence in LPG Measurements by Uncertainty Evaluation

In Brazil, National Metrology Institute, INMETRO, elaborated an N° 64 Ordinance, on April 11th 2003 [1], based on OIML [2]. This document approves the Metrological Technical Regulation, establishing the metrological and technical requirements applicable to measurement systems supplied with flow meters used in the measurement of oil, its derivatives liquid, alcohol anhydrous and hydrous alcohol fuel. For custody transfer purposes, liquefied petroleum gas (LPG) volumes are generally stated at a fixed base temperature and saturation pressure. As most volume transfers occur at temperatures and pressures other than standard conditions, these volumes are adjusted to standard conditions through the use of correction factors. The measurement uncertainty of these custody transfer systems must be controlled. Based in norms ASTM, IP, ISO and API – MPMS, the mathematical model for the calculation of the volume, it basically depends on quantities as: K factor, density, pulse number, turbine meter factor, temperature and pressure corrections. This work develops a methodology to calculate the LPG mass uncertainty that is completely different form the other liquid oil derivatives. This work details this calculation showing the complex temperature and pressure algorithms, including the equilibrium bubble point pressure. Based on this methodology, it is noticed that the pressure uncertainty contribution is not less than 43% overall the process and the expanded uncertainty is less than 1%, as required by INMETRO.Copyright

Uncertainty Evaluation of the Natural Gas Molar Mass and Its Flow Rate Impact

In Brazil, the National Oil Agency — ANP and the National Metrology Institute elaborated Regulation No 1, on June 19th 2000. This government decree approves the Regulation of the Measurement Technique of Oil and Natural Gas, which establishes the minimum conditions and requirements for the oil and natural gas measurement systems, in order to guarantee accurate and complete results. The natural gas measurement fiscal systems must be projected, calibrated and operated so that the measurement uncertainty does not exceed 1.5%. Based on the norms AGA and ISO, the mathematical model for the calculation of the mass flow rate, depends on quantities that have well known uncertainty such as: orifice plate diameter, pipeline internal diameter, compressibility factor, discharge coefficient, differential pressure, static pressure and flow temperature. However, for the molar mass standard uncertainty fixed values are utilized in Brazil (mainly by IPT and PUC-RJ), around 0.30%, independent of the natural gas composition. The objective of this work is to develop a methodology to calculate the molar mass uncertainty of the natural gas derived from its chemical composition, analyzed by gaseous chromatography and to comparing it with the value currently practiced, evaluating the impact proceeding from this difference in the mass flow rate of the natural gas. Based on this methodology, the molar mass uncertainty is around 0.05% and the fiscal system uncertainty decreases in more than 10% when it is compared with the mass molar fixed value uncertainty.Copyright