1、Designation: E 305 07Standard Practice forEstablishing and Controlling Atomic EmissionSpectrochemical Analytical Curves1This standard is issued under the fixed designation E 305; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice provides guidance for establishing andcontrolling atomic emission spectrochemical analytical cu
3、rves.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 E 1361 and E 16
4、21 for 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
5、are given 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-bilit
6、y of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related MaterialsE 1329 Practice for Verification and Use of Control Chartsin Spectrochemical AnalysisE 1361 Guide for Correction of Interelemen
7、t Effects inX-Ray Spectrometric AnalysisE 1621 Guide for X-Ray Emission Spectrometric Analysis3. Terminology3.1 For definitions of terms used in this practice, refer toTerminology E 135.4. Summary of Practice4.1 Systematic and random errors that occur in obtainingdata are reviewed. Background correc
8、tions are considered aswell as interferences from other elements. Calibration, stan-dardization, and verification procedures are discussed, includ-ing the use of reference materials and the generation of data. Abasis is given for evaluating second, third, and higher degreeanalytical curves.5. Signif
9、icance and Use5.1 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 describedhere
10、in.6. Precautions6.1 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 concentration orrelative concentration to spectral intensity or intensity ratio.The accuracy o
11、f the calibration may be affected by a number offactors, such as incorrect values for element concentrations,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 datagene
12、rated. It is the users 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
13、. These calibrants may 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).1This practice is under the jurisdiction of ASTM Committee E01 on Analyt
14、icalChemistry for Metals, Ores and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved June 1, 2007. Published June 2007. Originallyapproved in 1966. Discontinued March 2004 and reinstated as E 305 07. Lastprevious edition approv
15、ed in 1994 as E 305 89 (1994)e1.2For 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.1Copyright ASTM International
16、, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.The inclusion of production materials analyzed by independentmethods permits determining whether bias exists because ofdifferences between the metallurgical conditions of the certi-fied reference materials and typi
17、cal samples. In the absence ofcertified reference materials, 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 b
18、e inaccurate. Caution should be exercised inrejecting data that 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 Exp
19、erimental VariationsBias mayarise from experimental variations occurring within the opera-tional procedure (for example, change in optics, source param-eters, and so forth). Such changes may result in bias because ofchanges in sensitivity or background resulting in displacementof the analytical curv
20、e. The analyst may attempt to reduce biasfrom experimental variations during the initial calibrationprocedure by replication and by measuring the referencematerials in random order; but bias may be detected laterduring subsequent operations, as described in 8.3.1.6.2 Random Errors:6.2.1 Measurement
21、ErrorMeasurement repeatability maybe assessed using an estimate of standard deviation of repeatedmeasurements. While the true standard deviation is designateds, an estimate of standard deviation calculated from a limitednumber of values is designated by the symbol s, where:where:s =(xi x!2/ n 1! and
22、 where:xiare individual valuesx = average xi, andn = number of measurements.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
23、number of repli-cate measurements and using the average of the values willreduce the effect of statistical variation and minor specimeninhomogeneity. The use of optimum excitation conditions,including sufficient preburn and exposure times, will alsoreduce statistical variations and increase accuracy
24、.7. Calibration7.1 Spectral BackgroundBackground intensities varythroughout the spectral regions. Correcting for the backgroundin measurements of weak spectral line intensities line (thoseslightly more intense than background) can improve themeasurements. However, the effectiveness of the correction
25、must be evaluated.NOTE 2The need for background correction varies with the 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 correc
26、tion or a shifting ofspectra through exit slits to read background near 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 c
27、onsistent sampling,it can be used to subtract out background. A background 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 consiste
28、nt response from twodetectors, it is rarely used in photomultiplier systems. 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 de
29、tector in rapid succession, not in true simultaneity.Such a design can be 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, prefer
30、ably certified reference materials asdescribed in 6.1.1.1, should span the concentration 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
31、. This includes the curve parameters andany correction coefficients. If the concentration range exceedsone order of magnitude or if several calibrants are close to eachother in concentration, the use of more calibrants is recom-mended, preferable at least three per order of magnitude,spaced as equal
32、ly apart as possible.7.2.2 Standardants and VerifiersAll materials that may beuseful in monitoring and normalizing calibrations should beburned in a random order along with calibrants. Controlsamples, drifts monitors, and recalibration materials shall behomogeneous such that they give repeatable mea
33、surementsover time. The repeatability standard deviation for suitablematerial shall be less than or equal to the interlaboratoryrepeatability goal for the test method. In general, calibrantsshould not be used as standardants or verifiers.7.2.3 Number of Replications for Each ReferenceMaterialThe num
34、ber of replications for each calibrant,standardant and verifier shall at least as great as the numberreplications to be made for each specimen in a determination.7.3 Generating Multiple Linear RegressionAs stated in1.1.1, computer programs can provide the needed multiplelinear regression for develop
35、ing equations of second, third, andhigher order polynomials and incorporate 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. Seesection 7.3.2.2.7.3.1 Typic
36、ally, the data used for calibration are relativeintensities, the ratio of intensity of a spectral line to an internalstandard line. When the scope of an analysis involves signifi-cant change in the concentration of the internal standardE305072element, the relative intensity of the spectral line is p
37、lottedagainst a relative concentration, that is, the known concentra-tion of the calibrant divided by the concentration of the matrixelement, and usually multiplied by 100. The computer programmust be able to convert relative concentrations to actualconcentrations.7.3.1.1 Additive EffectThe addition
38、 of a signal fromanother element. The regression must include an additionalterm that will define the factor needed to subtract this interfer-ence as a function of concentration of the interfering element.In practice, this may sometimes be an addition rather than asubtraction.7.3.1.2 Multiplicative E
39、ffectAn effect on the calibrantsignal that depends on both the analyte signal and the concen-tration 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 concentration of the interferingelement, and x is either the
40、intensity for the analyte or apreliminary estimate of its concentration.7.3.1.3 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 corre
41、ction beingmade, in which case it would be better to opt for defining afamily of calibrations instead of defining a general system. Thedownside of going to a family of calibrations is that such arestriction might require many more calibrants.7.3.2 Precautions in Generating Non-Linear CurvesNon-linea
42、r analytical curves should be plotted to see that theypresent a reasonable looking 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. The
43、se extremes poseno problem if they are not near the useful analytical range. Ifthe concentration, y, is expressed as a quadratic equation:y 5 a01 a1x 1 a2x2(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 w
44、hen the firstorder derivative of the equation is equal to zero, or:dy / dx 5 a11 2a2x 5 0from whichx 5 a1/2a2(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
45、a point of inflection, however, that should beconsidered. The third degree equation can be expressed as:y 5 a01 a1x 1 a2x21 a3x3(3)for whichdy / dx 5 a11 2a2x 1 3a3x25 0 (4)the roots of this equation arex 5 a26 =a223a1a3! /3a3(5)When the expression under the square root sign is negative,the roots ar
46、e imaginary and there is neither a maximum nor aminimum. However, there always is a point 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 2a21 6a3x 5 0for whichx 5 a2/3a3(6)The third degree equation is ca
47、pable of defining a calibrationthat appears to be linear at low concentrations and picking upcurvature at higher concentrations. When it does so, therelikely will be a point of inflection in the apparent linear section.It must be ascertained that, when there is a reversal of bendingin that section,
48、it does not detract from the virtual linearity.7.3.2.3 The use of equation of higher than 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 using to afourth degr
49、ee, or higher, equation, it might be better to restrictthe definition to no more than a third degree by defining twocurves to separately cover a lower and a higher concentrationrange. Typically, this might be a third degree equation for thehigher concentration portion of the curve and a second, or evenfirst degree equation for the lower concentrations. If so, itwould be desirable to have one curve (the higher concentra-tion) become the controlling relationship at a specified concen-tration. The slopes of both curves should virtually be the sameat the point w
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