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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

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ESTIMATES OF UNCERTAINTY IN OPTICAL EMISSIONSPECTROMETRIC METHOD WITH ARC AND SPARKS EXCITEMENT

(SEOASE).

Anghelina F. V. 1 , Bratu V. 1, Popescu I. N. 1, Ungureanu D.N. 1

1Universitatea Valahia Targoviste, Facultatea de Ingineria Materialelor, Mecatronica si Robotica,Bd-ul Unirii 18-20,130022, Targoviste, Romania, [email protected]

Abstract: The practice of technologically advanced countries and economic default and the current rules of the European Union(EU), all measurements have undergone a significant process of standards to ensure quality undisputed of the test results. In the

spectrometric field there are standards of competence of laboratories, as well as of analytical method. SR EN ISO / IEC 17025, EN 13005 and European standards 04-16 EA, ILAC, etc. 2000. clearly specified scientific substantiation of the measurand to bemodeled properly and to prepare the budget of uncertainty in knowledge. In this context, to estimate correctly the measurand

spectrometric test is necessary to specify the current state of theoretical modeling of radiation emission by atoms. It is clear thatattempts spectrochimice are affected by a relatively large number of influencing factors, which generates errors and thuscontribute to the uncertainty budget method, in a way difficult to estimate. Accuracy and reliability spectrometric tests and / orcalibrations performed by a laboratory are determined by many factors. These factors will be estimated and analyzed in this

paper.

Keywords : Uncertainty budget,SEOASE, quality assurance, factors of influence, measurand

1. INTRODUCTION.

The practice of technologically advanced countriesand economic default and the current rules of theEuropean Union (EU), all measurements haveundergone a significant process of standards toensure quality undisputed of the test results. The

promoter of this policy was the U.S. in this regard seethe set of "Standard Practice for ..." published byASTM [1,2]. This policy proved extremely profitable

production practice, especially the "equipment"technologically advanced. Policy was taken, amongmany other states and the European Union (EU) andimplemented by European standards becomenormative reference, default and Romania. In mostcases the qualitative analysis of minerals or metalsspectrometry is used the electric arc excitation,

because the electric arc will transfer more substancein the discharge space than electric sparks, whichfavors the appearance of lines of analysis for dosedelements.

2.THEORETICAL ASPECTS OF QUALITYASSURANCE OF TESTS RESULTS FROMANALYSIS METHODS.

In the spectrometric field there are standards ofcompetence of laboratories, as well as of analyticalmethod. SR EN ISO / IEC 17025, EN 13005 and

European standards 04-16 EA, ILAC, etc. 2000.clearly specified scientific substantiation of the

measurand to be modeled properly and to prepare the budget of uncertainty in knowledge. In this context,to estimate correctly the measurand spectrometric testis necessary to specify the current state of theoreticalmodeling of radiation emission by atoms. It is clearthat spectrometric tests are affected by a relativelylarge number of influencing factors, which generateserrors and thus contribute to the uncertainty budget method, in a way difficult to estimate. Accuracy andreliability spectrometric tests and / or calibrations

performed by a laboratory are determined by manyfactors.These factors include contributions from

-human factors;-working and environmental conditions;-test and calibration methods and method validation;-equipment;-measurement traceability;-sampling;-handling test and calibration items.

The extent to which these factors contribute to thetotal uncertainty of measurement differs considerablyon the type of tests and method, and the type ofcalibration. The laboratory shall take account of thesefactors in developing test methods and procedures

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and calibration, in training and qualification, andselection and calibration of equipment they use.

2.1. THE ELECTIONS AND VALIDATION OFTEST METHODS AND ANALYSIS.

The laboratory shall use test methods and / orcalibration, sampling methods, which meet customerneeds and are appropriate testing and / or calibrationthat will do. Published methods in national standards,regional or international should be used preferably.The laboratory must ensure that utilizes the latestedition of the standard existing, unless it is notappropriate or possible to do so. When necessary, thestandard must be supplemented with additionaldetails to ensure proper application.

Validation is confirmation by examination and provision of objective evidence, that some specificrequirements for the intended use are fulfilled. Thelaboratory must validate non-standard methods,methods designed / developed by the laboratory,standard methods used outside their scope, andextending and modifying standard methods toconfirm that the methods are suitable for intendeduses. Validation should be as comprehensive asnecessary to meet the needs of the application orgiven scope. The laboratory shall record the resultsobtained, the procedure used for validation and adeclaration that the method is suitable for use

intended. Techniques used to determine the performance of a method should be one or acombination of the following:-calibration using reference standards or referencematerials;-comparing the results obtained by other methods;-interlaboratory comparisons;-Systematic evaluation of factors influences theresult;-evaluation of the uncertainty of results based onscientific understanding of theoretical principles ofthe method and practical experience.When, in non-standardized validated methods, changes are made,

their influence should be documented and ifnecessary, a new validation should be performed.Range and accuracy values that can be obtained byvalidated methods must be relevant to customerneeds.Validation includes specification requirements,characterization methods, verification that therequirements can be met using the method, and adeclaration on the validity.

2.2. ESTIMATION THE UNCERTAINTY OFMEASUREMENT.

Testing laboratories shall have and apply procedures for estimating uncertainty ofmeasurement. In some cases the nature of the test

method may not allow a rigorous calculation, and thatis at the same time, metrologically and statisticallyvalid for uncertainty of measurement. In such casesthe laboratory shall at least attempt to identify allcomponents of uncertainty and make a reasonableestimate and should ensure that the form of reportingwill not fumiza a false impression of theuncertainty.Reasonable estimate should be based onknowledge of performance measurement method andthe field and use, for example, previous experienceand validation data.The degree of rigor required inestimating the uncertainty of measurement dependson factors such as:-requirements test methods;-customer requirements;-the existence of tight limits that are based thedecision to accordance with an specification.When estimating the uncertainty of measurementshould be taken into account all uncertaintycomponents which are important in the givensituation, using appropriate analytical methods.

2.3. ASSESSMENT OF THE EFFECTS OFSOME FACTORS OF INFLUENCE ONUNCERTAINTY BUDGET FROMSPECTROMETRY TESTS (SEOASE).

In the case of testing optical emission spectrometryatomic or ionic excited by electric discharge in thespark or electric arc, significant influencing factorsmust be analyzed carefully because they can generaterelatively large uncertainties.Thus:1. Errors dueto sample: metal sample condition, testedrespectively structural homogeneity, chemicalhomogeneity, the content of non-metallic inclusions , the content of compoundsand precipitated, homogeneity surface processing bymechanical polishing, the state ofcontamination after processing, the content of O,

N and H dissolved, the state of

magnetization , thermal conductivity of the sample,type the discharge to thermal load, respectivelythe type of cooling determined by size and shape ofthe sample (sample, thick or thin washer etc).

2. Errors due to excitation of optical emission:vaporization and excitation are two factors coupled,which in turn depend on the nature and condition ofthe sample and the electrical parameters of discharge,respectively the size and shape space discharge(discharge gap), the power dissipation during adischarge (sparks), the frequency of spark or arcdischarge, alternatively, the amount of material

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vaporized during a discharge, the distribution ofindividual discharges, respectively discharge craterson the surface of the sample (a patch discharge ), the

number of atoms suffering ionisation into dischargespace and emitting ionic lines, the number of excitedatoms emitting characteristic atomic lines,respectively line of analysis.

Thus, during on sparking, comprising all the sparks produced when making the measurement "theintensity" spectral lines, vaporize about 10 -4 g ofsample (eg Fe), this mass is composed of about 10 16 atoms of Fe. [3] If the detection limit of Al in Fe is10 ppm then the number of Al atoms on which mustcontain "the plasma" of discharge is:

NAl=10 10-6 NFeMFe / M Al 10

-5101656/27 2 1011

atomi, unde:atoms, where:

NAl - number of aluminum atoms NFe - number of iron atomsMAl= 27 g/mol molar mass of aluminumMFe = 56 g / mol molar mass of iron

Although NAl is evaluated approximatelylarge enough, it is clear that in order to be detectedaluminum as an alloying element of Fe is necessarythat "the plasma" of discharge to have arepresentative number of about 1011 atoms, atomsrespectively issue.

Note: was noted "plasma" because the sparkdischarge is not a plasma, but plasma is called inmany works, because discharge emits light like a

plasma arc excited.From the above it follows that the number of atomsof the analyte in the "plasma" is relatively high andcan vary widely because of local factors whichintervened in each initiation sparks. According tostatistical theory, the error of "counting" of atoms isof the order

AI N = 5 . 105 which shows the relative

error is:Er 5 . 10

5/10 11 = 5 . 10 -6 =5 ppm3. Errors due to optical spectrometer:

Errors generated by the optical measurementare due to:

different processing of radiation flow linedata analysis and the reference line;

optical fibers in different opacitydepending on the wavelength (differentspectral opacity);

diffraction aging network and theemergence of "ghosts" that overlap analyteline;

profile modification on source of radiation because of mechanical factors (size

spectrum download, profile stand,magnetized samples, etc.).

Errors introduced by the optical measurement are

insignificant when the device is operating underoptimum conditions and the operator complies withthe test procedure. Evaluation of uncertainty due tooptical spectrometers can be done only by specialistsusing special equipment.

4. Errors due to fluctuation electronics, measuringthe intensity of spectral lines, and electrical signal

processing (CCDs and+ PC)Electronics errors of measurement are negligibleunder normal conditions of operation of the device.[3,4] These can be evaluated by testing ofrepeatability with chemically and structurallyhomogeneous samples, respectively certifiedstandards.5. Errors due to contamination state of argon inwhich the electric discharge:Contamination of argon with oxygen, nitrogen, watervapor and hydrogen, etc.., But particularly withoxygen and water vapor, introduces errors as large

because it favors the oxidation of the discharge areaof surface sample and reduces vaporizationefficiency. This factor of influence is reduced toinsignificant level by using a spectral pure argon(purity 99, 998) or by using argon purification filtercartridges, filter installations of "BOC rare gas

purifier", "KAS purifier for Argon", etc. [3 ]. Note: The condition of argon used ,affect (in mostcases) the accuracy of measurements that needspecial attention would acquiring the special type ofAr spectral.Evaluation of uncertainty due to Ar can be

performed using the Ar standard spectrometrycertified for use. The uncertainty due to the Arshould be reduced at minimum level by using asuitable type of Ar.Influence factors specified above, place a standarddeviation of the form:

25

24

23

22

21

2 s s s s s s ++++= (1)For evaluation (calculation) of s2 should beevaluated standard dispersions s12 .s 52 which, inturn, consist of dispersions factors that influencethem. For example, , s 21 is evaluated usingthe relation:

2222222221 t mcont pmepiocos s s s s s s s s s +++++++=

(2)where:s2os - standard dispersion associated with structuralhomogeneity,s2oc - standard dispersion associated chemicalhomogeneity,

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s2i - standard dispersion associated contentinclusions,s2cp - standard dispersion associated content

compounds and precipitates,s2 pm - standard dispersion associated metal processing,s2cont - standard dispersion associated contaminations2m - standard dispersion associated with the samplemagnetization,s2t - discharge dispersion associated technicaldischarge of sample.

3. DISCUSSION.

For those in the previous section, it is clear thatattempts spectrometric are affected by a relativelylarge number of influencing factors, which generateserrors and thus contribute to the uncertainty existingin a difficult to estimate.For ensure a reduction of the effects factors listedabove influence is frequently used in spectroscopy, inaddition to certified reference materials (CRMs) andother calibration standards, ie Setting Up Sample(UP).Spectral standards (MRC) for ferrous materials mustmeet a number of conditions to be used in theanalytical chemical characterization ofa) the condition representativity is expressed

primarily by the fact that these standards mustcontain the same chemical elements, and range ofconcentrations similar to the types of materials to beanalyzed.

b) chemical and structural homogeneity of thespectral standards are based on their condition in thedirection to obtain accurate results.Standards (MRC) for ferrous materials are used tocalibrate spectral instruments and standards SUS areused for periodic recalibration of instruments.Certified reference materials can be obtained fromvarious manufacturers. They provide materialscertified by official analysis, or released by aninstitute. Rules governing this procedure is ISOGuide 35. Reference materials (SUS) does not satisfy

fully the requirements of this standard, but mostmanufacturers of spectrometers using MRC-SUSsuccessful combination for drawing the calibrationcurves.Due to the special needs that require such materials(MRC and SUS) their costs are high.

4.CONCLUSIONS.ESTIMATES OF UNCERTAINTY IN

OPTICAL EMISSION SPECTROMETRICMETHOD

According to Deming concept the most importantroute to optimize the process is to improveknowledge about the process. From studies on

electrical discharge spark, used as a spectral source,revealed that it can be given a budget of uncertainty,composed of the following factors significantinfluence:

1. instability of the electronic parameters ofdischarge stand,2. reactivity of the environment discharge ,the

debt by Ar contaminated ,3. the distance electrode-sample variation due to

poor of sample preparation,4. "Attack" selective scintillation due of the

effects to combined source--environment discharge-sample,

5. Understand enough, i.e. inconsistenttheoretical modeling of the process of discharge[1.5,6.7]Factor 1 , the instability of electronic parameters,

belonging to spark stand, can not be approached bythe operator but only the manufacturing company. At

present, current and voltage stability in a SEOSEspectrometer is 99, 999. Otherwise,the fault of the operator would not provide adequatequality. Also, the presence of oxygen in the dischargespace, one can see by the look of the sparked areamacrostructure. An operator with the necessaryexperience can estimate, sufficiently good the Arquality and its impact on quality discharge asmacroscopic aspect of the area sparkle.Factor 3 , electrode-sample the distance variation dueto improper of sample preparation, can be controlled

by the operator, by correct preparation of sample, and by analyzing samples with proper geometry, ie platesor cylinders with flat surfaces, where can enroll animaginary circle with diameter> 2 cm. Exceptions tothis chapter, leading to large uncertainty analysis.

The operator should periodically check the electrode-sample distance, with specific resources, delivered bythe supplier, along with equipment.Factor 4 , "attack" selective sparkle, due to combinedeffects of source--enviroment discharge-sample, isfor the specialist physician in this case, the party,which may bring theoretical and experimentalcontributions, and also to respond to Demingconcept, transformed in influence factor and acontributor to the budget of uncertainty [7].

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5. REFERENCES

[1]. Standards SR EN ISO/CEI 17 025, SR EN 13005[2]. .European rules EA 04-16, ILAC 2000.[3] . I-I. Popescu, E. Toader, Optica , Ed. t.Enciclopedica, Buc.1989[4]. www.energienucleara.go.ro [5]. I. Ioni , Metode Spectrale pentru Analize

Medicale Ed. Univ. Bucure ti 2002.[6]. K. Slickers, Spectrochemical analysis in themetalurgical industry, Pure and appl. Chemistry,vol 65, No 12, pp2443-2452, 1993.[7]. Anghelina Florina Violeta, Analiza structurala

si compozitionala a matricilor metalice cu metodeatomice si nucleare -Universitatea din Bucuresti,Facultatea de Fizica, Oct. 2010.

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