1、Designation: E305 13Standard Practice forEstablishing and Controlling Atomic EmissionSpectrochemical Analytical Curves1This standard is issued under the fixed designation E305; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers guidance for establishing andcontrolling atomic emission spectrochemical analytical curves.
3、The generation of analytical curves and their routine controlare considered as separate although interrelated operations.This practice is applicable to atomic emission spectrometers.NOTE 1X-ray emission spectrometric applications are no longercovered by this practice. See Guides E1361 and E1621 for
4、discussion ofthis technique.1.1.1 Since computer programs are readily available to runmultiple linear regressions that can be used to generateanalytical curves and since most instruments include thisfeature, this practice does not go into detail on the procedure.However, some recommendations are giv
5、en on evaluating theequations that are generated.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of re
6、gulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related MaterialsE1329 Practice for Verification and Use of Control Charts inSpectrochemical AnalysisE1361 Guide for Correction of Interelement Effects
7、inX-Ray Spectrometric AnalysisE1621 Guide for X-Ray Emission Spectrometric Analysis3. Terminology3.1 For definitions of terms used in this practice, refer toTerminology E135.4. Summary of Practice4.1 Systematic and random errors that occur in obtainingdata are reviewed. Background corrections are co
8、nsidered aswell as interferences from other elements. Calibration,standardization, and verification procedures are discussed,including the use of reference materials and the generation ofdata. A basis is given for evaluating second, third, and higherdegree analytical curves.5. Significance and Use5.
9、1 This practice is intended as a fundamental guide for thecalibration, standardization, and daily control of the analyticalcurves for atomic emission spectrometers.5.2 It is assumed that this practice will be used by trainedoperators capable of performing the procedures describedherein.6. Precaution
10、s6.1 Potential Errors:6.1.1 Bias Because of Incorrect CalibrationIn the proce-dure for quantitative spectrochemical analysis, the initial gen-eration of the analytical curve relates element composition orrelative composition to spectral intensity or intensity ratio. Theaccuracy of the calibration ma
11、y be affected by a number offactors, such as incorrect values for element compositions,heterogeneity of the reference materials, spectral interferences,and matrix effects. These factors may cause a shift in theanalytical curve, thereby leading to bias in the analytical datagenerated. It is the users
12、 responsibility to apply calibrationmodels designed to evaluate the effect of, and mathematicallycorrect for, spectral interferences and matrix effects.6.1.1.1 Calibration bias because of incorrect element con-centrations are minimized by the use of certified referencematerials. These calibrants may
13、 be augmented with one ormore other reference materials for which the chemical compo-sitions have been carefully determined by approved methods ofanalysis, such as ASTM or BSI (British Standards Institute).The inclusion of production materials analyzed by independent1This practice is under the juris
14、diction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores, and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved June 1, 2013. Published July 2013. Originally approvedin 1966. Last previous edition approved in 2007
15、as E305 07. DOI: 10.1520/E0305-13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM Internationa
16、l, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1methods permits determining whether bias exists because ofdifferences between the metallurgical conditions of the certi-fied reference materials and typical samples. In the absence ofcertified reference materials,
17、 it is helpful to use severalreference materials from a variety of sources to detect bias inthese materials.6.1.1.2 In general, the use of a large number of referencematerials will aid in the detection and rejection of those thatappear to be inaccurate. Caution should be exercised inrejecting data t
18、hat appears to be inaccurate as it may bereflecting complicated matrix effects or the impact of unknownvariables.6.1.1.3 It is advisable that analyzed materials used ascalibrants be tested initially for homogeneity.6.1.2 Bias Because of Experimental VariationsBias mayarise from experimental variatio
19、ns occurring within the opera-tional procedure (for example, change in optics, sourceparameters, and so forth). Such changes may result in biasbecause of changes in sensitivity or background resulting indisplacement of the analytical curve. The analyst may attemptto reduce bias from experimental var
20、iations during the initialcalibration procedure by replication and by measuring thereference materials in random order; but bias may be detectedlater during subsequent operations, as described in 8.3.1.6.2 Random Errors:6.2.1 Measurement ErrorMeasurement repeatability maybe assessed using an estimat
21、e of standard deviation of repeatedmeasurements. While the true standard deviation is designated, an estimate of standard deviation calculated from a limitednumber of values is designated by the symbol s,where:s =(xi2x!2/n21!and where:xi= are individual valuesx = average xi, andn = number of measure
22、ments.6.2.1.1 Errors in determining the average signal intensity orintensity ratio from reference materials occur because ofstatistical variation, less than optimum excitation parameters,and specimen inhomogeneity. Increasing the number of repli-cate measurements and using the average of the values
23、willreduce the effect of statistical variation and minor specimeninhomogeneity. The use of optimum excitation conditions,including sufficient preburn and integration times, will alsoreduce statistical variations and increase accuracy.7. Calibration7.1 Spectral BackgroundBackground intensities varyth
24、roughout the spectral regions. Correcting for the backgroundin measurements of weak spectral line intensities (thoseslightly more intense than background) can improve themeasurements. However, the effectiveness of the correctionmust be evaluated.NOTE 2The need for background correction varies with t
25、he type ofmaterial being analyzed. Ensure that background correction is necessaryand can be accomplished consistently before proceeding.7.1.1 Background CorrectionMethods of background cor-rection may use either a dynamic correction or a shifting ofspectra through exit slits to read background near
26、a line.7.1.1.1 In a dynamic background correction, a selectedportion of the background of a spectrum is integrated simulta-neously with analytical signals. When this integrated measure-ment is strong and broad enough to give a consistent sampling,it can be used to subtract out background. A backgrou
27、nd areamay be made to have a strong signal by using a wide exit slitor by using an extra-sensitive detector, or by a combination ofthese. Because the dynamic approach is difficult to control andmay depend on maintaining consistent response from twodetectors, it is rarely used in photomultiplier syst
28、ems. It can beused more effectively with solid-state detector systems.NOTE 3Measurement of spectral intensity may not be truly simulta-neous even with solid-state detectors. Some spectrometer designs readmultiple regions of a detector in rapid succession, not in true simultaneity.Such a design can b
29、e subject to instrument drift.7.1.1.2 Shifting to read background has validity only if thegeneration of background intensity shows little variation fromburn to burn.7.2 Generation of the Analytical Curve:7.2.1 Calibrants, preferably certified reference materials asdescribed in 6.1.1.1, should span t
30、he composition ranges andtypes of materials expected. Extrapolation should be avoided.It is recommended that the number of calibrants to be used foreach curve be twice the number of coefficients to be deter-mined by regression. This includes the curve parameters andany correction coefficients. If th
31、e composition range exceedsone order of magnitude or if several calibrants are close to eachother in composition, the use of more calibrants isrecommended, preferable at least three per order of magnitude,spaced as equally apart as possible.7.2.2 Drift Correction Samples and VerifiersAll materialsth
32、at may be useful in monitoring and normalizing calibrationsshould be burned in a random order along with calibrants.Control and drift correction samples shall be homogeneoussuch that they give repeatable measurements over time. Therepeatability standard deviation for suitable material shall beless t
33、han or equal to the interlaboratory repeatability goal forthe test method. In general, calibrants should not be used asdrift correction samples or verifiers.7.2.3 Number of Replications for Each ReferenceMaterialThe number of replications for each calibrant, driftcorrection sample and verifier shall
34、 be at least as great as thenumber replications to be made for each specimen in adetermination.7.3 Generating Multiple Linear RegressionAs stated in1.1.1, computer programs can provide the needed multiplelinear regression for developing equations of second, third, andhigher order polynomials and inc
35、orporate corrections forinterferences from other elements. When using higher orderpolynomials, the useable portion of a curve must not be near toa maximum or a minimum nor include a point of inflection. See7.3.2.2.7.3.1 Typically, the data used for calibration are relativeintensities, the ratio of i
36、ntensity of a spectral line to an internalE305 132standard line. When the scope of an analysis involves signifi-cant change in the composition of the internal standardelement, the relative intensity of the spectral line is plottedagainst a relative mass fraction, that is, the known massfraction of t
37、he calibrant divided by the mass fraction of thematrix element, and usually multiplied by 100. The computerprogram must be able to convert relative mass fractions toactual mass fractions.7.3.1.1 Additive EffectThe addition of a signal from an-other element. The regression must include an additional
38、termthat will define the factor needed to subtract this interference asa function of mass fraction of the interfering element. Inpractice, this may sometimes be an addition rather than asubtraction.7.3.1.2 Multiplicative EffectAn effect on the calibrantsignal that depends on both the analyte signal
39、and the massfraction of the interfering element. The regression must includean additional term that will define a factor such as k in(1 6 kc)x, where c is the mass fraction of the interferingelement, and x is either the intensity for the analyte or apreliminary estimate of its mass fraction.7.3.1.3
40、Introducing corrections for elemental interferencesmay pose a problem. Even if the interference seems wellsupported by calibrants, the increased variability from addi-tional factors may be greater than the level of correction beingmade, in which case it would be better to opt for defining afamily of
41、 calibrations instead of defining a general system. Thedownside of utilizing a family of calibrations is that such arestriction might require many more calibrants.7.3.2 Precautions in Generating Non-Linear CurvesNon-linear analytical curves should be plotted to see that theypresent a reasonable look
42、ing relationship. Mathematical checkscan also be used to calculate where any maxima, minima, orpoints of inflection occur.7.3.2.1 By their nature, quadratic equations (second degree)always have a maximum or a minimum. These extremes poseno problem if they are not near the useful analytical range. If
43、the mass fraction, y, is expressed as a quadratic equation:y 5 a01a1x1a2x2(1)where:a0,a1, .an= the coefficients of the polynomial, andx = the reading obtained in a determination.Eq 1 will reach a maximum or a minimum when the firstorder derivative of the equation is equal to zero, or:dy/dx 5 a112a2x
44、 5 0from which:x 52a1/2a2(2)7.3.2.2 A third degree equation is commonly used. Since itsfirst order derivative has two roots it may have both amaximum and a minimum, unless the roots are imaginary. Itwill always have a point of inflection, however, that should beconsidered. The third degree equation
45、can be expressed as:y 5 a01a1x1a2x21a3x3(3)for which:dy/dx 5 a112a2x13a3x25 0 (4)the roots of this equation are:x 5 2a26=a222 3a1a3!/3a3(5)When the expression under the square root sign is negative,the roots are imaginary and there is neither a maximum nor aminimum. However, there always is a point
46、of inflection thatmight be missed in evaluating a calibration. It is defined whenthe second derivative of Eq 3 is made equal to zero:d2y/dx25 2a216a3x 5 0for which:x 52a2/3a3(6)The third degree equation is capable of defining a calibrationthat appears to be linear at low mass fractions and picking u
47、pcurvature at higher mass fractions. When it does so, there likelywill be a point of inflection in the apparent linear section. Itmust be ascertained that, when there is a reversal of bending inthat section, it does not detract from the virtual linearity.7.3.2.3 The use of an equation of higher than
48、 third degree isdiscouraged. Lower residuals obtained through the use of suchequations is deceptive and the use of these equations does notrepresent reality in instrumental analysis. Rather than using afourth degree, or higher, equation, it might be better to restrictthe definition to no more than a
49、 third degree by defining twocurves to separately cover a lower and a higher mass fractionrange. Typically, this might be a third degree equation for thehigher mass fraction portion of the curve and a second, or evenfirst degree equation for the lower mass fractions. If so, itwould be desirable to have one curve (the higher mass fraction)become the controlling relationship at a specified mass frac-tion. The slopes of both curves should virtually be the same atthe point where the transition is made.7.3.2.4 Number of Data Points RequiredAlthough itmight appear,
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