Transverse Disciplines in Metrology E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 978

Veröffentlichungsjahr: 2013

Beschreibung

Based on The International Metrology Congress meeting, this reference examines the evolution of metrology, and its applications in industry, environment and safety, health and medicine, economy and quality, and new information and communication technologies; details the improvement of measurement procedures to guarantee the quality of products and processes; and discusses the development of metrology linked to innovating technologies. The themes of the Congress (quality and reliability of measurement, measurement uncertainties, calibration, verification, accreditation, sensory metrology, regulations and legal metrology) are developed either in a general way or applied to a specific economic sector or to a specific scientific field.

Leseprobe

Table of Contents

Preface

Chemistry – Statistics/Gas Analysis

Certification of a reference material for herbicides in water by way of inter laboratory comparison

Introduction

Previous studies: feasibility and behavior of RM

Feasibility study

RM behavior during an interlaboratory comparison

Material production and certification

Preparation of materials

Homogenity

Stability

Inter laboratory testing

Results

Statistical data

Certified values

Conclusion

References

Determination of aflatoxin M1 using liquid chromatographic method: comparison between uncertainty obtained from in house validation data and from proficiency test data

Introduction

Experimental

Sample preparation

HPLC analysis

HPLC validation criteria and uncertainty estimation

Internal quality control

Proficiency testing

Results and discussion

Uncertainty estimation

Conclusions

References

Purity analysis in gas metrology

Introduction

Gas Chromatography (GC)

Thermal Conductivity Detector (TCD)

Flame Ionization Detector (FID)

Pure C3H8 analysis

Fourier Transform Infrared Spectrometry (FTIR)

Pure NO analysis

Gas Chromatography – Mass Spectrometry (GC-MS)

Automotive exhaust gas analysis

Conclusions

References

Comparison of two different approaches to evaluate the on line gas chromatography for natural gas

Introduction

Methodology

GUM approach

Control chart approach

Results and discussion

Control chart approach

Conclusion

Acknowledgements

References

Performance characteristics of GF-AAS and some metrological aspects of trace element determination

1. Introduction

2. Performance characteristics of GF-AAS

3. Analytical aspects of metal pollutant determination in water

4. Conclusions

5. References

The influence of water density and conductivity variation in a gravimetric calibration method

Introduction

Conductivity

Density

Discussion of results

Volume determination

Water density tables

Concluding remarks

References

Hydraulic Quantities

Uncertainty estimation of free surface flow rates determined by ADCP or current meter gauging

Introduction

Current meter gauging

Application of standard NF EN ISO 748

Estimation of uncertainty components

Conclusion on the use of NF EN ISO 748 standard

Application of standard NF ENV 13005

Estimation of uncertainty components

Conclusion on the use of NF ENV 13005 standard

Comparison of the NF EN ISO 748 and NF ENV 13005 standards

ADCP gauging

Application to the ADCP of the methods used to determine current meter gauging uncertainty

Experimental determination of uncertainty by comparing ADCP measurement to current meter measurements

Comparative measurements retained

Comparative measurement analysis

Uncertainty estimation of ADCP measurement

Conclusion

References

Influence of the insertion depth on the response of a hot film anemometer in different wind tunnels

Introduction

Experiments

Results

Discussion

Conclusion

References

Recent Development of the ITS

The characterization of relative humidity and temperature probes for use in interlaboratory comparisons

1. Introduction

2. Overview of the work so far

3. Procedures

4. Provisional results

5. Degrees of equivalence

6. Conclusions

Appendix 1 – Sample plot demonstrating repeatability at 23°C (Y Axis range 4%rh)

Appendix 2 – Sample plot demonstrating hysteresis and stability at 23°C (Y Axis Range 4%rh)

Appendix 3 – Sample plot demonstrating hysteresis and stability at 5°C (Y Axis Range 4%rh)

Appendix 4 – Sample plot showing probe hysteresis and stability at 50°C (Y Axis Range 4%rh)

High temperatures: new fixed points and thermodynamic temperature

Introduction

High-temperature fixed points

CCT-WG5 project on high-temperature fixed points

Fourth generation of eutectic cells and new fill method

Thermal optimization of the Vega HTBB 3200 PG furnace

Thermodynamic temperature measurement

The methodology developed at LNE-INM

The current process

Conclusion

REFERENCES

Implementation of a polyvalent measurement set-up for temperature and humidity used to characterize thermal and climate chambers: presentation of results

Development

Equipment requirements

NF X 15 140 standard

Hardware requirements

Software requirements

Selected equipment

Hardware

Software

Actual data

PT100 mapping results

Conclusions

References

The SI, Now and Tomorrow

Determination of the Avogadro constant: a contribution to the new definition of mass

Introduction

Determination of NA

Molar mass

Isotope enrichment

Silicon tetrafluoride

Silane

Polycrystalline silicon

Single crystal growth

Results of the enrichment

Atomic volume

Volume and Density

Conclusion

Acknowledgement

References

Development and application of the femtosecond frequency comb

Introduction

Frequency measurement

Setup

Measurement results

Distance measurements

The effect of noise on the correlation function

Conclusion

References

Recent development in time metrology

National time scales and contribution to coordinated universal time

Atomic clock comparisons by satellite systems

Conclusion and perspectives

References

Original monolithic device of 4-crossed axis with flexure pivots to be used for the French watt balance experiment

Introduction

Watt balance experiment

Flexure pivot system

Conclusion

References

The practical realization of the meter in Portugal

Introduction

Practical realization of the definition of the meter

Primary length standard

Beat-frequency measurement technique at IPQ

Sensitivity coefficients

Output frequency of IPQ3

Traceability

Absolute frequency measurement at IPQ

First results with the OFS at IPQ

Conclusion

Acknowledgments

References

Toward an important evolution of the international system of units: what practical impact?

Introduction

The international system of units (SI)

What perspectives?

Measurement methods

Conclusion

References

Health and Safety

Using proficiency testing results latest alternative method for uncertainty evaluation: application in clinical chemistry

Summary

Typology of evaluation methods for measurement uncertainties

GUM and alternative methods

Justifications for the use of alternative methods

Use of proficiency test results

The approach proposed in clinical chemistry: methods coupling

Application of the evaluation of glycemia measurement uncertainty

GUM REFERENCE METHOD (Chap 8)

ALTERNATIVE METHOD: “Method characteristics” + “Proficiency testing” coupling

Evaluation of trueness and its uncertainty – EEQ operation

Conclusion

References

Facilitating reliability and traceability of measurement results in laboratory medicine by certified reference materials

Introduction

Discussion

Metrological traceability

Certified Reference Materials

Use of Certified Reference Materials

Example

Conclusions

References

Dosimetric standard for continuous x-rays radiation fields of low and medium-energies (< 300 kV)

Introduction

I Materials and method

II Results

III Application

IV Discussion and prospects

Conclusion

References

Metrological Tools and Means

The international vocabulary of metrology, 3rd edition: basic and general concepts and associated terms. Why? How?

The international vocabulary of metrology 3rd edition. Basic and general concepts and associated terms. Why? How?

Current problems of independent laboratories in Europe

1. Past developments

2. Current situation

3. The 2005 amendment of ISO/IEC 17025

4. Proposal of a new long-term strategy

5. Summary

References

The harmonized European gas cubic meter for natural gas as realized by PTB, NMi-VSL and LNE-LADG and its benefit for the user and metrology

The challenge of gas flow measurement

Prerequisites for harmonization

Harmonization process for reference values

European gas cubic meter

The CIPM KCRVs and the european harmonized reference value

Final conclusion

References

Environment

Traceability of environmental chemical analyses: can fundamental metrological principles meet routine practice? Example of chemical monitoring under the Water Framework Directive

Introduction

Main legal requirements

Non-legally binding recommendations

Further binding rules

WFD monitoring and its links with metrology

Conclusions, perspectives

References

Disclaimer

Main standard for refrigerant liquid leak throughputs

Context

Objectives

Measure methods of refrigerant leak throughputs

Preliminary studies

Development of standard volume

Description of the reference

Qualification of the reference

Conclusion and perspectives

References

Quality aspects of determining key rock parameters for the design and performance assessment of a repository for spent nuclear fuel

Introduction

Reliability of data used in analysis

Example no. 1, influence of UCS on the analysis of stress-induced spalling

Example no. 2, Influence of thermal conductivity on the canister spacing in a KBS-3 repositor

Measurement quality assurance

Actual measurements I: Uniaxial compression test

Inter-laboratory experiment [7]

Actual measurements II: Thermal conductivity test

Interlaboratory experiment

Discussion

Estimation of natural variability and uncertainties in testing

Conclusions

References

Environment protection measurements by the technical inspection of vehicles

Introduction

Need for environmental protection measurements of road vehicles

Exhaust gas concentration measurement

Uncertainty of measurement

Proficiency testing as a tool to confirm uncertainty component values

Comparison results

Conclusion

References

Dimensional Metrology and Uncertainty

Behavior of touch trigger dynamic probes: correction and calibration methods

Introduction

Operation principle

Three-dimensional theoretical model

Three-dimensional mixed model

Conclusion

References

Angular measurements at the primary length laboratory in Portugal

Introduction

SI unit

Realization of the Radian Definition

Measurement system and standards of angle at LCO

Autocollimator

Index tables

Polygons

Angle gauges

Calibration of Angle Measurement Systems

Angular gauges, pentaprisms and optical squares

Polygons and index tables

Calibration of an index table

Cross calibration against another index table

Conclusion

Acknowledgments

References

Uncertainty of plane angle unit reproduction by means of a ring laser and holographic optical encoder

Introduction

Concept of reproduction of plane angle unit

Experimental results

Calibration of optical encoders

Conclusion

References

Advanced 2D scanning: the solution for the calibration of thread ring and thread plug gauges

Introduction

The problems with the 2-Ball and 3-Wire measurement

The solution: 2D scanning

The challenge of 2D scanning

Correction of the tips

Fixation and alignment

Intermediate calibration

Sources of measurement uncertainty

Available diameter ranges

Conclusion

References

Geometry and volume measurement of worn cutting tools with an optical surface metrology device

1. Introduction

2. 3D Measurement with InfiniteFocus

3. Registration

4. Form measurement

5. Wear analysis

6. Conclusions

References

Innovation and Knowledge Transfer

The iMERA/EUROMET joint research project for new determinations of the Boltzmann constant

Introduction

Acoustic method

Dielectric-constant gas-thermometer method

Doppler-broadening method

Radiometric method

Conclusions

References

The “Measurement for Innovators” program: stimulating innovation in the UK through improved understanding and appreciation of measurement issues

Introduction

Measurement for Innovators

Integrated support

Secondments

Consultancies

Joint Industry Projects

Key findings

Outcomes and impact

Summary

References

Uncertainty

Variance model components for the analysis of repeated measurements

Introduction

Purpose

The nested or hierarchical design. General model

Model for one factor

Model for two factors

Calibration of a SPRT at the Aluminium Freezing Point

Residual Analysis

Remarks

Conclusion

References

Optimized measurement uncertainty and decision-making

Introduction

Costs, economic and statistical risks in conformity assessment

Consequence, testing and sampling costs

Product and consequence costs

Testing costs

Optimized uncertainty methodology

Consumer risk by variable

Attribute consumer risks

Conclusions

Acknowledgements

References

Giving early ideas to tests: uncertainty project implementation in the field of railway testing

Presentation of Agence d’Essai Ferroviaire (AEF)

Scope of tests

Measurement uncertainties

Organization of the uncertainties project

Methodology for the estimation of measurement uncertainties

Methodology implementation

Products of the methodology output

Advantages

Indicators

Conclusion

Interlaboratory comparisons in Europe: which way to choose?

1. Introduction

2. Current situation in Europe

3. Proposal of a new system

4. Summary

5. References

Common approach to proficiency testing and interlaboratory comparisons

Introduction

Reference versus consensus values

Concluding remarks

References

Sensory Metrology

A European project SysPAQ

Introduction

Project objectives

Approach and methodology

WORKPLAN OF THE PROJECT

Acknowledgement

References

Metrology of appearance

Introduction

Color rendering properties of light sources

Presentation of the project

Conclusion

References

Electricity

Measurement of the reflectivity of radio-wave absorbers

Introduction

Method

Principle 1: Use of complex S-parameters

Principle 2: Motion of the absorbers

Principle 3: Normalization with a metal plate

Principle 4: Assume ideal metal reflections

Measurements

Conclusion

References

Calibration of the spectral density of CISPR (16-1-1) impulse generators

Introduction

CISPR 16-1-1 test method

CISPR 16 definitions

CISPR 16 states that:

Practical realisation of CISPR pulse generators

Frequency domain calibration method

Uncertainty contributions

Time domain calibration method

Uncertainty contributions

Results and comparison

Summary

References

Binary Josephson array power standard

Introduction

Transients and electronics

Array fabrication

Verification of precision waveforms

Sampling and synchronization methods for calibrating waveforms

Combining a Josephson array with a power standard

References

A 100 MΩ step hamon guarded network to improve the traceability level of high resistance up to 1GΩ

Introduction

The 100 MΩ step Hamon resistor

The guarding system

Metrological validation of the realised Hamon standard

Calibration and use of the 100 MΩ step Hamon resistor to extend the high dc resistance traceability

Conclusions

References

Traceability of voltage and resistance measurements in Estonia

Introduction

Voltage standards

Measurement results

Resistance standards

Measurement results

Conclusions

Acknowledgments

References

Set up and characterization of reference measuring systems for on-site live verification of HV instrument transformers

Introduction

Uncertainty budget

Conclusions

References

Testing/calibrating electrical measuring instruments under non-sinusoidal conditions at the national institute of metrology, Romania

Introduction

Testing/calibrating electrical measuring instruments under non-sinusoidal conditions

Conclusions

References

Characterization of high resistance standards in MIKES

Introduction

Traceability of 10 kOhm – 1 GOhm standards

Methods and equipment

Comparison of different methods

Uncertainty estimation

Conclusion

References

Automation to guarantee traceability and improve productivity in the reference laboratory of Mexico’s federal electricity commision

Introduction

Project development

Initial steps of the project execution

UNAM’s proposal

Agreement

Technical development of the project

Conclusions

References

Legal Metrology

An industrial view on global acceptance of measuring instruments in legal metrology

Fiscal metering

Certification

Standards

Vibration and shock

Evaluations

Results

Conclusion

References

Views from a notified body towards global acceptance

Introduction

Who is responsible for global acceptance?

What are the reasons for non-acceptance?

Basic philosophy in acceptance

Conditions for acceptance

References

Monte Carlo

Limits of the uncertainty propagation: Examples and solutions using the Monte Carlo Method

Introduction

Basics of calculating measurement uncertainty

Monte Carlo Method (MCM)

Example 1. Distance of a point to the origin

Example 2. Multiplication of complex valued quantities

Correlated quantities using MCM

Summary

References

High resolution modeling for evaluation of measurement uncertainty

Introduction

From GUF to MCM

High resolution modeling

Basic models

Processes

Calculation

A word about dependencies

Example: Gauge block calibration

Example: modeling a chemical process

Summary

References

Appendix

Evaluation of uncertainty using Monte Carlo simulations

Introduction

Propagation of distributions by the Monte Carlo method

Application in testing

Conclusion

Application in metrology

Conclusions

Acknowledgements

References

Mass

Weighing small samples on laboratory balances

Introduction

Limits of Weighing

Weighing Accuracy

Weighing Shortcomings

Air Drafts

Temperature Difference

Power Dissipation

Heat Radiation

Electrostatic Influence

Weighing Vessels

Pushing the Limits of Weighing

Direct Sample Weighing into Vessels

Weighing Accessories

Conclusion

References

Design and performance of the new Sartorius 1kg-prototype mass comparator for high precision mass determination and research applications

Introduction

Enclosure for airtight and vacuum conditions

The load alternator

The used balance

The control unit with application software

Specifications

Results

Summary

Acknowledgements

References

Series of pressure balances for industrial use

Introduction

Design of the pressure balance

Metrological characteristics

Conclusions

References

Fully automatic mass laboratory from 1 mg up to 50 kg – robots perform high precision mass determination

Initial situation

Introduction

Robot performing high-precision mass determination on weights in the range from 1 kg up to 20 kg

Design

Function

Implementation

Test sequence

Measurements and results

The second robot performs high-precision mass determination on weights in the range from 1 mg up to 10 g

Specifications and objective

Concept

Method of operation

System design

Test procedure

Measurement results

The third robot performs high-precision mass determination on weights in the range from 10 g up to 1 kg

Specifications, objective and concept

Method of operation

System design and test procedure

Measurement results

Interlaboratory comparison of seven standard weights in several Romanian laboratories

Introduction

Circulation Scheme

Measurement instructions

Tasks

Results

Discussion

Conclusions

References

Automatic testing facility for determining the density of liquids and solids; and determining the volume of E1 weights

Summary

Introduction

Initial situation

Specifications and objectives

Concept of the fundamental aperture

Functionality and procedure of the fundamental aperture

Test process of the fundamental aperture

Measurement results with the fundamental aperture

Development of a volume comparator

Requirements and objectives

System design

Measurement procedure

Software

Use of balance calibration certificate to minimize measurement uncertainty in mass determinations performed in physico-chemical laboratories

Introduction

Assumptions

Case report

Dispensable uncertainty sources

Results

Conclusions

References

Optic – Time Frequency

Experimental study of the homogenity of a polychromatic light spectrum

Introduction

Light spectrum characteristics

Spatial stability of the spectral distribution in the rainbow light

Conclusion

References

Statistics

An Innovative Method for the Comparison of Measurement Results

Introduction

Fuzzy and Random-Fuzzy Variables

Mathematical tools

The proposed approach

Discussion

Conclusions

Bibliography

Calibration and recalibration of measuring instruments: a Bayesian perspective

Introduction

The Bayesian approach

Calibrating in a Bayesian perspective

Measurements performed by the calibrated instrument

Conclusions

References

Using the correlation analysis in electrical measurements

The dependence of the correlation coefficient on random deviations

The technique of correlation analysis applied in electrical measurements

Analysis of calculations of dispersion estimations of the measured value

Definition of the relative correction γ2(f1 *) using the autocorrelation coefficient

Example of using correlation analysis in electrical measurements

Conclusions

References

Overview

Automation of testing procedures in accredited metrology laboratories

Introduction and motivations

Benefits and risks of automation in metrological processes

Developed validation procedure

Some practical applications

Conclusions

References

Validation of industrial measurement processes

Introduction

Measurement processes and their metrological characteristics

“Classical approach” and “uncertainty approach” – applied together

Principles of the validation of measurement processes

Conclusions

References

iMERA and the impact of metrology R&D on society

Introduction

Impact measurement

Questionnaire

Literature study

Workshop

Conclusions and recommendations

References

Index of Authors

First published in Great Britain and the United States in 2009 by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK

John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA

www.iste.co.uk

www.wiley.com

© ISTE Ltd 2009

The rights of The French College of Metrology to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

International Metrology Conference (13th : 2007 : French College of Metrology)

Transverse disciplines in metrology : proceedings of the 13th International Metrology Congress, 2007,

Lille, France / French College of Metrology.

p. cm.

Includes bibliographical references.

ISBN 978-1-84821-048-6

ISBN 978-1-84821-142-1

1. Measurement--Congresses. I. Collège Français de métrologie.

T50.I585 2007

620'.0044--dc22

2008035181

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN: 978-1-84821-048-6

Preface

The International Metrology Congress is organized every two years by the French College of Metrology in collaboration with the Laboratoire National de Métrologie et d’Essais (LNE).

In 2007 the congress was organized with the scientific support of Nederlands Meetinstituut (NMI), the national metrology institute of the Netherlands.

The aims of the congress are:

– the evolution of metrology, and its applications in industry, environment and safety, health and medicine, economy and quality, and new information and communication technologies;

– the improvement of measurement procedures to guarantee the quality of products and processes;

– the development of metrology linked to innovating technologies.

The themes of the congress (quality and reliability of measurement, measurement uncertainties, calibration, verification, accreditation, sensory metrology, regulations and legal metrology) are developed either in a general way or applied to a specific economic sector or to a specific scientific field.

An exhibition of the latest technical improvements is also located at the congress: manufacturers of measurement equipments, providers of services and the official organizations of metrology are present.

Technical visits to major firms are possible. Each visit is oriented towards metrological activities connected to product quality or to services provided by the firms.

In this document you could find a selection of conferences presented during the event in June 2007 in Lille.

Certification of a reference material for herbicides in water by way of inter laboratory comparison

B. Lalèrea, V. Le Diourona, M. Désenfanta,K. El Mrabeta,b V. Pichonb, J. Vialb, G. Hervoüeta

a Laboratoire National de Métrologie et d’Essais (LNE), 1 rue Gaston Boissier, 75724 Paris cedex 15, France

b Laboratoire Environnement et Chimie Analytique (LECA) UMR-CNRS 7121, Ecole Supérieure de Physique et Chimie Industrielles, 10 rue Vauquelin, 75231 Paris Cedex 05, France

ABSTRACT: The certified reference materials (CRMs) are among the most appropriate tools for the traceability and validation of analytical methods. Although their number seems to be high 20,000, they only cover a small part of the analytical needs for environmental monitoring. Until now, none were available for analyzing pesticides in water.

After an intralaboratory feasibility study and an evaluation of the behavior of this material during interlaboratory testing, the LNE has produced and certified a CRM for the triazine and phenylurea analysis in water samples.

Introduction

In accordance with the European outline directive on water (DCE), the department of ecology published memorandum service instruction DCE 2006/16 relating to the constitution and implementation of a monitoring program for the different water categories. Because of this, quality of rivers and ground water is regularly monitored. Pesticide content, a micropollutant considered as a priority by the European Union in particular is verified.

In order to achieve this monitoring, several measures are taken daily by different laboratories. It is important to be able to compare results. This objective can only be reached by ensuring traceability of these analyses with the help of different tools such as certified reference material (CRM), the link to national standards through uninterrupted chains of comparison and a participation in interlaboratory testing. CRMs have the advantage of also being able to evaluate accuracy during validation of analytical methods implemented. They can be of two types: those destined to calibrate measurement systems and “matrices” making it possible to consider all steps from the preparation of the sample.

Until now, there were none for the analysis of pesticides in water. That is why LNE in cooperation with LECA decided to develop one. This project started four years ago with a feasibility study of such a material. The behavior of products developed was then evaluated during an interlaboratory comparison.

A group of CRMs was developed in March 2006 and certified in June 2006 for the analysis of triazines and phenylureas in drinking water.

Previous studies: feasibility and behavior of RM

The following components which belong to two pesticide families were selected:

– triazines: deisopropylatrazine (DIA), deethylatrazine (DEA), simazine, atrazine, terbutylazine and terbutryne.

– phenylureas: chlortoluron, diuron, isoproturon and linuron.

Completed in 2002, this choice, except for terbutylazine and DIA, is based on the frequency of detection of these triazines and phenylureas in water [1]. In addition, they are indexed in the list of 50 pesticides sold in larger quantities than 500 tons per year in Europe. Terbutylazine, following the ban on the use of atrazine, is part of the mix used for its replacement in corn cultures and thus was added. Despite its usage ban, atrazine can still be detected in water; because of this, two of its metabolites, DEA and DIA were included in this study.

Feasibility study

In order to achieve the most complete kit possible, two types of reference materials (RM) were retained:

– sealed vials: targeted pesticides are stored:

- in a solution of acetonitrile,

- dried after evaporation of the solvent used for their preparation.

– solid phase extraction (SPE) cartridges called cartridges in this document: a water sample spiked with each compound is percolated on the support in order to retain them with other molecules in the sample. Two polymeric supports were evaluated: a divinylbenzene copolymer functionalized with N-vinylpyrrolidone (Oasis HLB, Waters) groups and a polystyrene divinylbenzene copolymer (ENVI Chrom P, Supelco). Analysis laboratories will then have to carry out the elution of compounds.

These materials were prepared with two levels of concentration, targeted in relation to regulation (0.15 μg/l for drinking water and 0.50 μg/l in surface water) T=-18°C for a year. Every month, the evolution of compound concentration was studied by the analysis of vial and cartridge contents.

The detailed presentation of results for each compound, each type of material and each storage condition was the subject of publication [2]. Between the beginning and ending of the study, the evolution was quantified in percentage of the quantity found in relation to the one initially introduced (rate of recovery). To summarize, compounds are classified into three families of behaviors:

– non-usable family (Figure 1): either the compounds were totally degraded or their recovery level after nine months of study are lower than 10%;

– family with trend (Figure 2): compound concentration evolves over time;

– family without trend (Figure 3): pesticide concentration does not evolve significantly over time.

Figure 1.“Non-usable family” example: Linuron stored in vial after dry evaporation and conservation at ambient temperature

Figure 2.“Family with trend” example DIA stored in vial after dry evaporation and conservation at ambient temperature

Figure 3.“Family without trend” example Atrazine stored in vial after dry evaporation and conservation at T=-18°C

The results have shown that compounds are stable when then are in a solution of acetonitrile when temperature becomes ambient, whereas it is necessary to store them at -20°C when they are dried in a vial. When they are fixed on cartridges, storage temperature must be lower than 0.5°C. Observed behaviors are identical regardless of the level of concentration.

The retained RM was then a kit with:

– vials containing pesticides in acetonitrile;

– cartridges (Oasis HLB, Waters) treated by percolation of drinking water containing selected pesticides.

As a precaution, they will be stored at -20°C.

RM behavior during an interlaboratory comparison

In order to test the conditions of RM usage (sending conditions, reception and analysis with different methods), an interlaboratory test involving about 15 participants was organized [3].

The variation factors of both components of the material tested vary between 14 and 30% depending on compounds, which is very satisfactory for a circuit involving laboratories using different methods compared to other campaigns [4,5].

In view of the results (feasibility and interlaboratory test), a certification campaign was then completed.

Material production and certification

This reference material, in the form of a kit is made up of:

– two vials containing approximately 1.2 ml of a herbicides solution (atrazine, deethylatrazine, deisopropylatrazine, simazine, terbutryne, terbutylazine, chlortoluron, linuron, diuron, and isoproturon) in acetonitrile (concentration for each pesticide ≈ 0.1 mg/l);

– three cartridges on which 250 ml of a drinking water spiked with a solution of herbicides in acetonitrile was percolated, in order to reach a concentration of approximately 0.50 μg/l of each compound in water.

This material is destined to calibrate the measurement devices and/or to validate the analytical methods for the determination of herbicides in water.

Preparation of materials

The preparation of materials required the creation of multi compound solutions by consecutive dilutions of a mother solution obtained with pure compounds.

After preparation, the solution is transferred into vials which are immediately sealed under nitrogen.

For the cartridges, 180 l of tap water were taken the same day. Before transferring to a cartridge according to the protocol described below, 5 l of water are spiked with 5 ml of a pesticide solution.

Cartridges are first conditioned with 3 ml of acetonitrile, 3 ml of methanol, 3 ml of water then 250 ml of spiked water are percolated. Water rinsing and nitrogen flow drying steps are then realized before storing them in specific conditions (protected from light and at a temperature lower than –20°C). All these steps were conducted with the ASPEC XL IV robot (GILSON).

There was a prior verification that vial seals would not alter the solution inside and that the reliability of the robot was sufficient to guarantee good cartridge homogenity.

Vial and cartridge preparations required three days and three weeks respectively.

400 sealed vials and 600 cartridges were produced.

Homogenity

The homogenity was verified on two series of ten vials and ten cartridges (Table 1).

Table 1.Vial and cartridge homogenity

Stability

During the life span of the material, analyses are done each month. Until now and in accordance with the feasibility study, no significant evolution of concentrations was detected as is shown for example in Figures 4 and 5.

Figure 4.Stability of chlortoluron in solution in the vials

Figure 5.DEA stability in cartridge

Inter laboratory testing

16 laboratories with different techniques participated in this test (Table 2).

Samples (3 vials and 3 cartridges) were sent on April 4th 2006 and all results were received on May 20th 2006.

Table 2.Laboratories participing in the certification testing and implemented techniques

Results

Raw results for cartridges and vials are grouped in Tables 3 and 4 respectively.

Table 3.Laboratory results for cartridges expressed in μg/l of water

Table 4.Laboratory results for vials expressed in mg/l

Statistical data

Statistical data analysis

The statistical data analysis was performed based on operation standards for interlaboratory tests NF ISO 5725-2 [6] and NF ISO 5725-5 [7]. The first phase was to detect atypical values from statistical homogenity tests (Grubbs and Cochran). The exclusion of some data was based on statistical and technical conclusions. The second phase, from results obtained, was to quantify the parameters summarizing the RM, average and standard deviation of reproducibility as closely as possible. The results of this process are summarized in Tables 5 and 6.

Table 5.Statistical data for vials

Table 6.Statistical data for cartridges

Certified values

The value assigned to CRM is the average result from laboratories and the standard uncertainty is given by the standard deviation of reproducibility. This uncertainty corresponds to the uncertainty on the result of an analysis of this CRM carried out by a laboratory working under the same conditions as the circuit laboratories.

Tables 7 and 8 group CRM certified values.

Table 7.Certified values for vials

Concentrations in cartridges are expressed in μg of compound by liter of water. In each cartridge, 0.25 l of water was percolated.

Table 8.Certified values for cartridges

Conclusion

Validation for this experimental approach makes it possible to propose a certified reference material which is now available for the analysis of pesticides in water. It is presented in the form of a kit in order to respond to analysis laboratory requirements:

– sealed vials containing compounds in solution in acetonitrile, which can be used for the verification of calibration or for spiking water matrices;

– cartridges on which water containing compounds was percolated representing a real sample.

From the development to certification of this reference material, the study took five years of study. In addition, this CRM is in compliance with the ISO 34 guide specifying the conditions of production, sample conservation and stability.

References

[1] Etudes et Travaux IFEN, “Les pesticides dans les eaux”, September 2000.

[2] J. Deplagne, J. Vial, V. Pichon*, Béatrice Lalere, G. Hervouet and M.-C. Hennion, Feasibility study of a reference material for water chemistry: Long term stability of triazine and phenylurea residues stored in vials or on polymeric sorbents, Journal of Chromatography A, 1123 (2006) 31-37.[3] K. El Mrabet, M. Poitevin, J. Vial*, V. Pichon, S. Amarouche, G. Hervouet, B. Lalere, An Interlaboratory Study to evaluate potential Matrix Reference Materials for pesticides in Water, Journal of Chromatography A, 1134 (2006) 151-161.

[4] S.A. Senseman, T.C. Mueller, M.B. Riley, R.D. Wauchope, C. Clegg, R.W. Young, L.M. Southwick, H.A. Moye, J.A. Dumas, W. Mersie, J.D. Mattice, R.B. Leyy, Interlaboratory comparison of extraction efficiency of pesticides from surface laboratory water using solid-phase extraction disks, J. Agric. Food Chem., 51 (2003) 3748-3752.

[5] M.B. Riley, J.A. Dumas, E.E. Gbur, J.H. Massey, J.D. Mattice, W. Mersie, T.C. Mueller, T. Potter, S.A. Senseman, E. Watson, Pesticide extraction efficiency of two solid phase disk types after shipping, J. Agric. Food Chem., 53 (2005) 5079-5083.

[6] NF ISO 5725-2 Exactitude (justesse et fidélité) des résultats et méthode de mesure partie 2: Méthode de base pour la détermination de la répétabilité et de la reproductibilité d'une méthode de mesure normalisée

[7] NF ISO 5725-5 Exactitude (justesse et fidélité) des résultats et méthode de mesure partie 5: Méthodes alternatives pour la détermination de la fidélité d'une méthode de mesure normalisée (analyses robustes).

Determination of aflatoxin M1 using liquid chromatographic method: comparison between uncertainty obtained from in house validation data and from proficiency test data

C. Focardi, M. Nocentini, S. Ninci, P. Perrella, G.Biancalani

Istituto Zooprofilattico Sperimentale delle Regioni Lazio e Toscana - Sezione di Firenze, Via di Castelpulci 43, 50010 San Martino alla Palma, Florence, Italy. Tel [phone]- Fax [phone] – e-mail: [email protected]

ABSTRACT: An HPLC method with fluorescence detection has been validated for the determination of aflatoxin M1 in milk samples. Certified Reference Materials and Spiked samples have been used for in house validation. These validation data have been applied for the uncertainty estimation with the bottom-up approach. Results obtained with this method have been compared with the expanded uncertainty determined from proficiency testing FAPAS.

Introduction

Aflatoxins are a group of hepatocarcinogen molecules produced by Aspergillus flavus and Aspergillus parasiticus. When aflatoxin B1 (AFB1), present in contaminated feed, is ingested by dairy cattle, it is excreted into milk as aflatoxin M1 (AFM1). Both AFB1 and AFM1 can cause DNA damage, gene mutation, chromosomal anomalies and as a consequence the International Agency for Research on Cancer (IARC) has classified AFB1 and, recently also AFM1, as class 1 (carcinogenic to humans) [1]. Strict regulatory limits for AFM1 are currently in force in the European Community; the Regulation (EC) 1881/2006 [2] set a maximum residue level (MRL) of AFM1 in milk, intended for adults, at 0.050 μg/kg, and at 0.025μg/kg for milk, intended for infants or for baby-food production. The European Commission with the Regulation (EC) 401/2006 [3] fixed the performance criteria for the analytical methods.

The adopted method is a liquid chromatographic one with fluorimetric detection. The sample is purified by using immunoaffinity column. The method is in house fully validated using Certified Reference Materials and spiked samples.

The EN ISO/IEC 17025 [4], as well as the Regulation (EC) 401/2006, requires all the measurements to be accompanied by estimation of expanded uncertainty.

Various methods are available to evaluate measurement uncertainty. One approach, applied by Eurachem [5], is the so called “bottom-up” and consists in separately identifying and quantifying error components that are consider important from in house validation data. Another approach is to use results from proficiency testing, by comparing reproducility variance with the repeteability variance of the laboratory [5].

In our case we use the data obtained with participation of the laboratory at 11 FAPAS during the period 2002-2006.

Experimental

Materials

Aflatoxin M1 Standard Reference Material at a concentration of 500 μg/l in methanol was purchased from Riedel de Haën. The chemical structure of AFM1 is presented in Figure 1.

Figure 1.Chemical structure of Aflatoxin M1

Deionised water obtained by a Milli-Q water (Millipore) and acetonitrile HPLC grade where used throughout. AflaTest® Immunoaffinity columns containing antibodies against AFM1 were purchased from VICAM. AFM1 working standard solutions at five different concentration levels (0.08, 0.2, 0.4, 0.6 and 0.8 ng/ml) were prepared by dilution of the stock solution (5 ng/ml). All the solutions are dissolved in acetonitrile water in the ratio 10/90 (v/v) (Figure 2).

Sample preparation

The reference materials used for HPLC method validation and uncertainty estimation are reported in Table 1.

Table 1.Reference materials used for method validation

Reference Material

Certified Concentration

Certified Uncertainty

Standard Reference Material

500 μg/l

±50 μg/l

CRM 285

0.76 μg/Kg

± 0.05 μg/Kg

FAPAS round 0477

0.44 μg/Kg

± 0.015 μg/Kg

FAPAS round 0445

0.26 μg/Kg

± 0.01 μg/Kg

These milk powder samples were prepared by dilution with water in the ratio 1:10 (w/w), in the way that the final concentration of AFM1 in the samples lays down in the HPLC calibration curve. Besides spiked samples at the nominal concentration of 0.05, 0.04 and 0.03 μg/Kg were prepared adding, under a gently magnetic stirring, respectively 500 μl, 400 μl and 300 μl of AFM1 stock solution (5ng/ml) to 50 g of homogenized milk, previously tested to demonstrate the absence of AFM1 residues, under a gently magnetic stirring.

Reconstituted Certified Materials (FAPAS and CRM), spiked and blank milk samples were extracted and purified with the procedure used by Tuinistra [6] and reported in the flow diagram (Figure 2).

Figure 2.Flow diagram of sample analysis

To ensure the homogenity of samples, they were gently magnetically stirred at room temperature for about 15 minutes. After homogenization, 50 g of samples were centrifuged for 25 minutes at 3500 rpm, to eliminate fat components. Skimmed milk was passed through the immunoaffinity column. The column was washed with 10 ml of water and AFM1 eluted with 5 ml of acetonitrile.

For chromatographic analysis the sample was evaporated under a stream of nitrogen to a volume of approximately 0.3 ml and reconstituted to 5 ml with water.

HPLC analysis

Chromatography was performed with a Perkin Elmer (USA) Series 200 system consisting of a quaternary gradient pump, an autosampler, a fluorescence detector and a degassing system using helium. Chromatographic separation was achieved using a Spherisorb ODS2 (250 × 4.6 mm, 5 μm) reversed phase column, with a guard column of the same type. The mobile phase was constituted by water and acetonitrile in the ratio 75/25 (v/v). An isocratic elution was performed at a flow rate of 1 ml/min for a total run time of 20 minutes. The injection volume was 500 µl. The detector was set at a excitation wavelength of 363 nm with a emission wavelength of 433 nm.

HPLC validation criteria and uncertainty estimation

The method for the determination of Aflatoxin M1 in milk was in house validated by a set of parameters which are in compliance with the Eurachem guide [7] and the Commission Regulation 401/2006 [3].

For evaluating the overall uncertainty, the “bottom up approach” has been adopted, following the statements of Eurachem guide [5].

Internal quality control

The method used for the determination of aflatoxin M1 in milk fulfils the requirements of EN ISO/IEC 17025 [4] and is accredited in the laboratory.

Internal quality control is achieved following the IUPAC harmonized guidelines [8]. A HPLC sequence consists of the analysis of a standard, a blank sample, a Certified Reference Material to check the recovery and finally samples in duplicate.

The Shewart control chart has been applied for the statistical control of the measurement process by using the CRM and an example is reported in Figure 3. The Reference value μ is the concentration level of CRM and σ is the uncertainty associated with the material. The warning limits are considered equal to μ plus and minus 2σ, the action limits are μ plus and minus 3σ below the center line.

Figure 3.Shewart control chart related to FAPAS 0445

Proficiency testing

The performance of the laboratory is periodically checked with the partecipation at Proficiency testing organized by FAPAS®. The results were evaluated in the form of a z-score which is the estimated laboratory bias divided by the target value for the standard deviation [8].

The laboratory has participated in eleven proficiency tests over the years. In Figure 4 we show the value of z-score obtained according to time (omissis); all values are included in the range of ± 1.

Figure 4.z-score profile obtained during the period 2002-2006

Results and discussion

In-house validation of the method

The validation data are obtained by using Certified Reference Materials and spiked samples.

Specificity. The chromatograms of a blank milk sample, CRM and standard solution are previously reported [9]. The blank milk sample is free of interferences at the elution time corresponding to the AFM1 peak, demonstrating a good specificity of the method proposed.

Figure 5.Calibration curve for aflatoxin M1

Evaluation of systematic error. The Bartlett test applied both on Certified Reference Material and spiked samples demonstrated the homogenity between the variance and subsequently the absence of systematic errors.

Accuracy and precision. The precision and accuracy determined are listed in Table 2.

According to the EC Regulation 401/2006, the accuracy for all of the samples fell in the range between –20 % to 10%. The precision of the method is expressed as RSD, Relative Standard Deviation, values for all concentrations. In Table 2 are also reported the values of the RSDmax which is equal to two thirds of the Horwitz equation:

where C is the mass fraction expressed as a power of 10. According to the EC Regulation 401/2006 experimental RSD value is lower than RSDmax.

Table 2.Data of accuracy and precision obtained for CRM, FAPAS and “Spiked” samples, where n is number of replicates and Mean is the average concentration obtained

Limit of detection and limit of quantification. The limit of detection has been calculated from the calibration curve. Taking into account the standard deviation (Sy/x) of the regression analysis, the limit of detection is found to be CLODLOQ

Uncertainty estimation

The equation of the measurand is as follows:

(1)

where M (g) is mass weight of the milk sample, Vext is the final volume of the extraction, Cx (ng/ml) is the concentration obtained by calibration curve and R is the recovery rate obtained on suitable reference sample (CRM or FAPAS sample). Every factor of the equation is shown in the cause and effect diagram (Figure 6).

Figure 6.Cause and effect diagram

The detail of uncertainty estimation, based on bottom-up approach taking into account the in-house validation data are previously reported [9]. For the standard uncertainty associated to the dilution volume and weight, a triangular distribution has been chosen.

In Figure 7 is reported the Error Budget diagram; the contribution to uncertainty of mass weight (M) and dilution (Vext) are negligible compared to the others, for all samples analysed.

Figure 7.Error budget diagram

In Figure 8 are presented the pie charts for the reference materials and for the spiked samples.

If we compare the different components to uncertainty of certified reference materials to those obtain for the spiked samples, it is clear that in the second case increases the contribution of the recovery, due to the homogenity of the sample. At the same time, the contribution due to the concentration Cx decreases. This second effect should be correlated with the value of the chromatographic peak area which is lower in the case of Certified Reference Material, due to the dilution of the sample.

Figure 8.Contributions to combined standard uncertainty. Charts show the relative sizes of uncertainty associated with precision, bias, calibration and other effects

According to the bottom up approach the global uncertainty, is given by the addition of all the uncertainty associated with the component that influences the measurand. Taking into account equation (1) the overall uncertainty, in terms of relative uncertainty, is determined by the formula:

The standard uncertainty is then obtained by the equation

(2)

In Table 3 we show, for the Certified Reference Materials and for the spiked sample, the relative uncertainty ů(C).

Table 3.Relative and standard uncertainty calculated for reference materials and spiked samples

Sample

Concentration (μg/Kg)

ů(C)

CRM 285

0.76

0.08

FAPAS round 0477

0.44

0.07

FAPAS round 0445

0.26

0.08

Spiked

0.05

0.09

Spiked

0.04

0.11

Spiked

0.03

0.13

The relative expanded uncertainty is obtained multiplying the mean value of the relative uncertainty, by a coverage factor, k=2

As an alternative method to measure uncertainty for chemical measurements, the Analytical Method Committee of the Royal Society of Chemistry [10] proposed an approach based on precision data assessed in an interlaboratory study.

The uncertainty associated with a mesurand result y is given by the following formula:

where s2R is the reproducibility variance between laboratories and uref is uncertainty associated with the accepted reference value. A further example of this expression is possible when u2ref is negligible in comparison to the reproducibility variance.

Some applications of interlaboratory data in the estimation of the uncertainty of chromatographic methods have been described [11,12]. Considering the results obtained by the laboratory, applying the proficiency test FAPAS, we can compare laboratory repeatability with the reproducibility of interlaboratory study (Table 4).

Table 4.Results of FAPAS proficiency testing: C is Assigned value and σ is the robust standard deviation, σrel is the relative robust standard deviation

The mean value obtained for the intralaboratory relative repeatability is equal to 0.09, an order of magnitude lower than the interlaboratory reproducibility. This indicates that the method precision of the laboratory is comparable to that of the laboratories which took part in the collaborative trial. It is therefore acceptable to use the reproducibility standard deviation from the collaborative trial in the uncertainty budget of the method. The mean value resulting from the interlaboratory test of relative robust standard deviation is equal to 0.28.

We can calculate the expanded uncertainty at the MRL level by using the relative uncertainty obtained with the two different approaches applying equation (2).

We can conclude that the expanded uncertainty obtained with the two different methods are of the same order of magnitude.

Conclusions

This chromatographic method for the determination of aflatoxin M1 in milk samples meets compliance with the Regulation (EC) 401/2006. Internal quality controls are achieved by applying the Shewart control chart to the data obtained each round with the Certified Reference Materials. Uncertainty has been evaluated following two different methods; the bottom up approach and the interlaboratory data, in the specific case, FAPAS round materials over the period 2002-2006. Expanded uncertainty obtained with the two different approaches calculated at the Maximum Residue Level are comparable.

References

[1]IARC International Agency for Research on Cancer Monograph on evaluation of carcinogenic risks to humans. IARC Lyon France 2002 82, 171

[2]Official Journal of European Commission L 364/5 2006 Commission Regulation No 1881/2006 of 19 December 2006. Brussels Belgium

[3]Official Journal of European Commission L 70/12 2006 Commission Regulation No 401/2006 of 23 February 2006. Brussels Belgium

[4]EN ISO/IEC 17025: 2005. General Requirements for the competence of calibration and testing laboratories. 2005 ISO Geneva

[5] Quantifying uncertainty in analytical measurement Eurachem Guide2000, second edition LGC.

[6]Tuinistra L. et al., J. AOAC Int, 1993, 84, 2. 1248-1254

[7]The Fitness for purpose of analytical methods A laboratory guide to method validation and related topics Eurachem Guide1998, edition 1.0 LGC.

[8] Thomson M., Pure Applied Chem, 1995, 4, 649-666

[9]M. Nocentini, C. Focardi, M. Vonci, F. Palmerini. Proceedings at 11thInternational Metrology Congress, 20-23 October 2003 Toulon- France

[10] AMC (Analytical Method Committee) Analyst, 2005, 130, 721

[11] Dehouck P., et al., J. of Chrom A, 2003, 1010, 63-74

[12] Dehouck P., et al., Anal Chim Acta, 2003, 481, 261-272

Purity analysis in gas metrology

F. Dias and G. Baptista

Laboratório de Gases de Referência, Instituto Português da Qualidade R. António Gião, 2, 2829-513 Caparica, Portugal

ABSTRACT: The Reference Gas Laboratory (LGR) of the Portuguese Institute for Quality (IPQ) is the Primary Laboratory in Portugal in the field of gas metrology. Its main mission is to assure and guarantee the accuracy and traceability [1] of the gas measurements to national and international standards. LGR is also a consumer of pure gases to be used as raw material in the gravimetric preparation of primary gas mixtures. The laboratory needs to measure and control the purity level of the gases supplied by the industry in order to determine the gas composition with higher accuracy. Several methods are used for purity analysis, namely, Gas Chromatography (GC), Fourier Transform Infrared Spectrometry (FTIR) and Mass Spectrometry (MS). These analytical methods will be briefly described and documented with some examples of gas purity analysis made at LGR: Propane (C3H8), Nitrogen Monoxide (NO) and automotive mixtures (CO+CO2+C3H8 in Nitrogen).

Introduction

LGR is a consumer of pure gases to be used as raw material in the preparation of primary gas mixtures. The need for the quantification of impurities is of high interest in the calculation of the measurement uncertainty and in the estimation of the result accuracy.

The choice of the more adequate analytical technique is made according to different properties such as, method specificity, cross-interference and matrix effects and also the method detection limits.

At LGR, several methods are being implemented for purity analysis, namely, Gas Chromatography (GC), Fourier Transform Infrared Spectrometry (FTIR), and Mass Spectrometry (MS). In GC technology, the purity of parent gases is obtained by using the two following techniques: Flame Ionization Detection (FID) and Thermal Conductivity Detection (TCD). The FTIR technique is used to measure gas species, which may be difficult for GC analysis. GC-MS technology is used to detect multi-species by mass signatures and identify the unknown impurities.

In order to obtain a better knowledge of the purity of gases involved in gravimetric preparations and/or in dynamic mixtures, our laboratory is now developing a Purity Analysis service. Although not fully implemented these techniques already provide important information concerning raw material control, fundamental to primary gas standards preparation. The quantification of impurities is of high interest in the calculation and estimation of the results. Indeed, one of the major uncertainty contributions is the purity component of the gas.

The qualitative analysis of impurities in pure gases is also important due to the possibility of having interferents and because of the need to ensure that crossinterferent impurities are small enough not to contribute significantly to the measured results.

The gas mixture preparation method according to ISO 6142 accounts for each component (including impurities) as a fraction amount of the total mixture. It then evaluates the uncertainty of each fraction amount. This approach is totally correct, but it might lead to neglecting certain correlations in the uncertainties when the same source of gas is used more than once in the preparation of a mixture.

Gas Chromatography (GC)

Gas Chromatography (GC) [2] is a common analytical method for gas purity analysis. GC is able to analyze almost all gas components, but requires a variety of columns, specific for certain chemical species, and detectors. This technique has low gas consumption and it is possible to connect it to auto-sampler equipment enabling automatic data acquisition. One limitation is the need for reference standards to validate peak retention time. It is a good tool for quantitative analysis.

In the LGR two different detectors are used, namely, Thermal Conductivity Detector (TCD) and Flame Ionization Detector (FID), which will be briefly described below.

Figure 1.Gas Chromatograph Agilent 5890

Thermal Conductivity Detector (TCD)

Gas Chromatography by TCD is a non-destructive method in which the detector is sensitive to the thermal conductivity of the carrier gas. Each time a component different from the carrier gas passes through the detector, the TC decreases the given origin to a signal proportional to that component concentration. One important advantage is that it is valid for almost all components.

Flame Ionization Detector (FID)

The flame Ionization Detector (FID) is a destructive method on which the carrier gas is mixed with hydrogen and air, being the final mixture burned. The burning produces ions causing an electric current change. The signal generated is proportional to the ion concentration. It is a very sensitive method allowing the detection limit in GC to be decreased. It applies to almost all organic compounds, but, is not sensitive for common inorganic species (CO, NOx, SO2, H2S, H2O). One way to change this is using a Nickel catalyst, which converts the CO and CO2 to CH4 for further detection.

Pure C3H8 analysis

An example is shown of pure Propane (99.95%) analyzes by GC-TCD with different columns.

Equipment: Agilent 5890

Column: Porapack Q, Molecular sieve 5A

Temperature: 160ºC, 150ºC

Figure 2.C3H8 pure sample analysis with Porapak col

Figure 3.C3H8 pure sample analysis with Mol. Sieve col

C3H8 99.95%

Specification: H2O (5 ppm), O2 (10 ppm), CO2 (5 ppm), N2 (40 ppm), C3H6 (200 ppm), CnHm (200 ppm)

Results: H2O (?), O2 (8 ppm), CO2 (2 ppm), N2 (83 ppm), CnHm (42 ppm CH4 + 21 ppm C2H6)

For the C3H8 99.95% sample, the results show that the mixture is under the specification, except for N2 where it was found at a concentration higher than specified. Nevertheless, it was not possible to separate propene from propane, nor to measure H2O with this technique

Fourier Transform Infrared Spectrometry (FTIR)

FTIR [3] is a spectroscopy method where working principle is based on the infrared (IR) absorption by molecules. Any molecular vibrations, which displace an electric charge, will absorb IR radiation. On FTIR the scanning of IR region allows us to detect several components in a sample. However, it applies only to species absorbing an IR radiation, which means that atomic species (He, Ne, Ar), as well as the homonuclear diatomic species without a permanent dipole (H2, N2, O2) cannot be observed.

This limitation does not imply that the majority of environmental and pollutant gases cannot be observed by this technique. According to NIST, approximately 100 of the hazardous air pollutants listed in the US EPA Clean Air Act can be measured.

Figure 4.Fourier Transform Infrared Spectrometer BOMEM MB100

IR signatures are easily recognized and do not change according to the mixture, showing no matrix effects. The main limitation of this method is the difficulty to eliminate residual H2O (1325-1900 cm-1: 3550-3950 cm-1) and CO2 (2295-2385 cm-1) existing in the absorbance spectrum, mainly due to the evolving atmosphere. The large amount of gases consumed during an analysis is also a disadvantage when the sample quantity is a limitation. For all the reasons explained above, FTIR is mainly used to measure gas species difficult to measure in GC. This technique is a useful tool to measure pure NO and its contaminants.

Pure NO analysis

An example of pure NO 99.90% analysis by FTIR is shown.

Equipment: BOMEM MB100

Scanning Region: Mid Infrared (4000-400 cm-1)

Resolution: 8 cm-1

Gas Cell: Graseby Specac glass cell

Optical Path: variable path length (1-8 m)

Figure 6.Absorbance vs wavenumber (cm-1) FTIR analysis

NO 99.90% specification

Impurities: H2O (20 ppm), NO2 (100 ppm), CO2 (100 ppm),

N2O (200 ppm), N2 (500 ppm),

In the example, the NO 99.90% pure sample, is not in accordance with the specification, namely for the NO2 (590 ppm) and N2O (420 ppm) components which are significantly greater. This difference can be due to the reactivity of NO. However, it is an example of the lack of accuracy in analysis made by some gas suppliers not traceable to the SI.

Gas Chromatography – Mass Spectrometry (GC-MS)

GC-MS [4] is a hyphenated technique, which combines the Chromatographic separation with the spectral information. The GC separates mixtures into their components which will then pass through the MS [5]. Here, each component is fragmented into several ions, by the ion source. The mass filter or quadrupole classifies the different ions into the mass-to-charge ratio (m/Z). The detector will further produce a signal proportional to the ion concentration. GC-MS is very useful for identifying unknown compounds, namely trace contaminants that can be found in pure or balance gases. When there are no clues or reference standards to compare with an unexpected signal appearing in a GC analysis, this analytical technique can give a result, by comparing the mass signature obtained with the mass spectrum library, although the accuracy of quantification is poor.

Figure 7.Gas Chromatograph – Mass Spectrometer Agilent 6890

Automotive exhaust gas analysis

An example of an automotive gas analysis of a Certified Reference Material (CRM) provided by a gas supplier is presented. This CRM contained an unexpected impurity.

Equipment: Agilent 6890

Column: HP PlotQ, HP Molesieve

Temperature: Variable (60–150ºC)

Figure 8.Chromatogram for an automotive CRM gas sample

Figure 9.Mass spectrum comparison: above, unknown component sample; below, ethylene oxide

Automotive exhaust gas CRM specification

Composition: CO (0.5%), CO2 (6%), C3H8 (100 ppm), N2 (matrix)

Comparing the mass spectrum of the unknown component sample with the mass spectrum of Ethylene Oxide (C2H4O), it was observed that the contaminant corresponds to this component. Besides not interfering with components in the mixture, this molecule could introduce a considerable source of error if it was, as previewed, used as standard material for calibration of automotive gas analysis equipment, since this apparatus reads total hydrocarbons.

Conclusions

In gas purity analysis there is no universal equipment. The maximum information on a sample characterization will be given by combining different techniques.

To obtain a quantitative analysis there is the need to purchase all the correspondent reference standards, which is not always easy due to price or availability restrictions.

Gas purity analysis is crucial on primary standards preparation enabling us to decrease measurement uncertainty and therefore reach more accurate results.

References

[1]BIPM et al., International Vocabulary of Basic and General Terms in Metrology, 2ª ed, Geneva, ISO, 1993. 59 p. ISBN 92-67-01075-1.

[2]Agilent 6890 Series Gas Chromatograph Operating Manual, USA, 2000.

[3]The Michelson Series FT-IR Spectrometer – BOMEM User’s Guide, Version 1.0, Canada, 1994.

[4]Agilent GC-MSD Chemstation and Instrument Operation “basique” H 872090000. USA, 2005.

[5 Agilent Technologies. 5973 Inert Mass Selective Detector Hardware Manual. USA, 2003.

Comparison of two different approaches to evaluate the on line gas chromatography for natural gas

Elcio Cruz de Oliveira

Petrobras Transporte S.A., Av. Presidente Vargas 328, 7o andar – Rio de Janeiro – RJ, ZIP 20021-060, Brazil; Tel + 55 21 3211 9223; Fax + 55 21 3211 9300 Email: [email protected]

ABSTRACT: Recently, several approaches to evaluate the uncertainty in measurement have been developed. Within these, we may highlight the following: the guide to the expression of uncertainty in measurement (GUM), which evaluates the uncertainty of a measured result through the combination of each source of uncertainty in the measuring process and the approach derived from control chart techniques. The objective of this article is to determine if these two approaches are equivalent, or if in the case of gas chromatography of natural gas, there are differences between them.

KEYWORDS: Measurement uncertainty; GUM approach; Control charts; Natural gas.

Introduction

The evaluation of uncertainty associated with an analytic result is an essential part of the measurement process. The uncertainty of a measurement is defined as “a parameter associated with the result of a measurement, which characterizes the dispersion of values that can be fundamentally attributed to a measurand” [1]. The result of a measurement is considered as the best estimate of the value of measuring accompanied with all the sources of uncertainty that contribute to its propagation [2]. Consequently, the result of a measurement cannot be correctly interpreted without knowledge of the uncertainty of the result [2].

Several concepts have been developed for the evaluation of uncertainty related to the result of a measurement. The approach most used is the one proposed by GUM [3] for the expression of uncertainty in measurements, which combines the diverse sources of uncertainty, by expansion of the Taylor Series. In the beginning of the 1990s, EURACHEM [5] adopted GUM for analytical chemistry.

The control chart as a graphical means of applying the statistical principles of significance to the control of a production process was first proposed by Dr. Walter Shewhart in 1924 [4,5]. Control chart theory recognizes two types of variability. The first type is random variability due to “chance causes” (also known as “common causes”). This is due to the wide variety of causes that are consistently present and not readily identifiable, each of which constitutes a very small component of the total variability but none of which contributes any significant amount.