# ASTM E1329-10

Designation: E1329 − 10Standard Practice forVerification and Use of Control Charts in SpectrochemicalAnalysis1This standard is issued under the fixed designation E1329; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year 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 procedures for determining if aspectrochemical analysis is under statistical control.1.2 Criteria are presented for determining when correctiveaction is required.1.3 Control will be effected by using verifiers to testinstrument response. It is recommended, although not required,that this be accompanied by the plotting of control charts.1.4 The preparation of control charts is described.1.5 Limitations—The procedures that are described do notapply to analyses that require a calibration each time a set ofanalyses is run. Reference is made specifically to atomicemission spectrometry, but the practice has a more generalapplication.1.6 This practice does not apply to validation proceduresthat monitor the correctness of calibration.2. Referenced Documents2.1 ASTM Standards:2E135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related MaterialsE158 Practice for Fundamental Calculations to ConvertIntensities into Concentrations in Optical Emission Spec-trochemical Analysis (Withdrawn 2004)3E305 Practice for Establishing and Controlling AtomicEmission Spectrochemical Analytical CurvesE456 Terminology Relating to Quality and StatisticsE876 Practice for Use of Statistics in the Evaluation ofSpectrometric Data (Withdrawn 2003)32.2 Other ASTM Documents:MNL 7A Manual on Presentation of Data and Control ChartAnalysis43. Terminology3.1 Definitions—For definitions of terms used in thispractice, refer to Terminologies E135 and E456 and PracticeE876. Refer also to the glossary of terms and symbolsappearing in MNL 7A.3.2 Definitions of Terms Specific to This Standard:3.2.1 control limits—in control charts, the upper and lowerlimits of a statistic that are not expected to be exceeded,designated as UCL and LCL respectively in this practice. Forthe statistic that is the average of more than one reading ordetermination, the upper and lower limits will be equidistantfrom a central line (CL) representing the expected average. Forthe statistic of either standard deviation or range, the upperlimit will be farther from the central line if the lower limit iszero.3.2.2 normalization—a procedure for correcting readings toa common basis. A special case of normalization is standard-ization in which readings are made to conform to an existingcalibration. Normalization permits gathering data in differentperiods of time and correcting for drift in a way that may beindependent of standardization routines.3.2.3 variation—difference in an observed value from anaccepted value.3.2.3.1 assignable cause—variation which can be identifiedand corrected. It may be the result of a condition of aninstrument or a method of operation. For example, signalintensities may be affected because a spectrometer is notprofiled properly.3.2.3.2 chance or common cause—random variation whichconsistently affects a system, contributing to the imprecision ina predictable way. In the application of control charts, theassumption is made that chance causes of variation arenormally distributed.1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores, and Related Materials and is the direct responsibility ofSubcommittee E01.22 on Laboratory Quality.Current edition approved Oct. 1, 2010. Published December 2010. Originallyapproved in 1990. Last previous edition approved in 2003 as E1329 – 00 (2003).DOI: 10.1520/E1329-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.4ASTM Manual Series, ASTM, 7th edition, 2002.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14. Significance and Use4.1 Consistency in analysis depends on being aware of asignificant change in instrumental response, such as that causedby drift or changes in analytical precision, or both, and takingcorrective action. The usual corrective action for drift isstandardization. Standardization, however, when there is noreal need, can only broaden the spread of subsequent analyses.One purpose of this practice is to set guidelines that will avoid“unnecessary standardization.”4.2 To control manufacturing processes, there must beconfidence that a consistent material is being produced and thatthe analysis of the material is reliable. For assurance that thematerial meets specification, a purchaser may require thesupporting record of control charts to assess that properanalytical control has been maintained.4.3 Ideally, variations in analytical results may be held tochance causes. The concept of a confidence interval or limitson a control chart is based on what can be expected when allnormal precautions are exercised. When results appear to goout of control, the analyst should consider and correct whatmight be an assignable cause. As experience is accumulated,however, it may not seem unusual for readings to drift withtime as optics degrade, detector response changes, or excita-tions conditions change, for example, when deposits build upon a counter electrode (a correctable assignable cause), or thelonger range effects as an X-ray tube deteriorates.5. Problems in Applying Control5.1 A complication in effecting verification control or inusing control charts with spectrochemical analyses is that themeasurements being taken are not absolute. Determinationsdepend upon comparisons of one measurement to another: therelative intensity of an analytical line to the relative intensity ofan internal standard line in atomic emission spectrometry; theinterrelationship of counts in X-ray spectrometry under somespecified condition of maintaining a fixed intensity from anirradiating source and holding to a consistent response from adetector with or without pulse height analyzers and with orwithout an external monitor; and the relative response inintegrating for fixed times with ostensibly constant radiationsources. Added to these is the complication of backgroundsignal in all techniques.5.2 It is important to recognize that there are several sourcesof random variation, including variations from the measuringmethod as well as inhomogeneity in the specimens. The devicebeing used to test analytical response is the analytical systemitself. This differs from normal statistical process control wherean independent and usually more accurate measuring device isused to verify the process variability.6. Verifiers6.1 It is recommended that readings for all potential verifi-ers as well as standardants be established by measuring themalong with the calibrants.6.1.1 Ideally, the full set of potential standardants andverifiers should be run before and after a series of calibrants topermit normalizing all calibration data to a common basis. Toachieve the best normalization of data, readings should berecorded for all elements of interest on every standardant andverifier, even if there is no knowledge of expected concentra-tions. Unless there is a marked change in the before and aftermeasurements, the averages of a set of before and afterreadings will be used for normalization.NOTE 1—If there appears to be a drift between readings of standardantsobtained before and after a set of calibrants has been run, an instrumentproblem may need to be investigated and corrected or the operationalenvironment improved. Reliable calibration data can be obtained only ifan instrument shows a stable operation. Practice E876 describes ways totest for drift.6.1.1.1 Unless a curve fitting routine is being used thatrequires “standardizing” before running a set of referencematerials it is recommended that no normalization be doneuntil all calibration data has been recorded. Strictly speakingstandardization, as defined in Terminology E135, only can bedone after a calibration has been established. If a normalizationto some prescribed set of readings is done as if it were astandardization before each time a set of reference materials isrun, the resulting record of readings can be treated as if nostandardization had been done.6.1.2 Choose one set of averages of before and afterreadings of 6.1.1 as the norm. A grand overall average of thesets may be used if that seems like a reasonable median of allsets. Exclude any readings for an element in a referencematerial that does not show comparable repeatability to whatwas observed for that element in other materials. For higher-level readings, the comparison should be made to observedrelative repeatabilities.6.1.2.1 For an ideal normalization of readings, determinethe regression fit of a set of observed readings, x, to expectedreadings, y. This linear regression, which is also supported byPractice E305, commonly is done on electronic calculators orcomputers by the following equations to determine a slope, m,and a constant, k, which can be used to correct observedreadings to an established norm:m 5n(xy 2(x~(y!n(~x2! 2 ~(x!2(1)andk 5 ~(y 2 m(x!/n (2)where the summations of functions ofx = the observed average readings of an element in acalibration set,y = the expected normal readings for that element, andn = the number of pairs of x and y readings.6.1.2.2 Apply the appropriate m and k corrections to theaverages of the verifiers and standardants, as well as to thecalibrants in each calibration set, as follows:RN5 mRO1k (3)where:RN= normalized reading, andRO= observed average reading.E1329 − 102The grand averages of the normalized readings of thestandardants and verifiers will become the values used forstandardizing.6.1.3 If the analytical system only can support the “two-point” standardization, and if the only permissible normaliza-tion is a quasi-standardization, before collecting calibrationdata it is still advisable to record all readings for all elementsin all reference materials to establish a full record of what canbe expected for all the reference materials (see 8.6). The initialset of “normal” readings are reasonable starting points. Neitherthe preferred method of using a regression fit nor the recom-mendation of waiting until all data have been logged beforeassigning normal values are infallible. Modification of thesevalues always should be an option as more experience isgained. It is expected, however, that the preferred methods willarrive at the ideal normal values earlier.6.1.3.1 If the operating system is based on two-pointstandardization, Eq 3 still would be used to normalize orstandardize readings. The generation of slope and constantcorrections, however, would be as follows:m 5 ~HR2 LR!/~HO2 LO! (4)andk 5 HR2 m~HO! (5)where:HR= reference or normal reading of the high standardant,LR= reference or normal reading of the low standardant,HO= observed reading of the high standardant, andLO= observed reading of the low standardant.6.1.4 If data are later transformed by a slope and intercept togive a different scaling for the calibration, the same transfor-mation must be applied to the readings of standardants andverifiers.6.2 If a verifier (or a new standardant) is established after acalibration has been defined, the expected reading can beestablished as follows:6.2.1 Shortly after a standardization, run the verifier inreplicate and keep a record of its average reading. Average aminimum of ten such observations made after new standard-izations to obtain a good representation of the expectedreading.6.2.1.1 Normalization coefficients are determined by mak-ing a linear regression fit of normal readings as a function ofobserved readings, such as is done in Practice E305 inestablishing a straight line relationship by the method of leastsquares. The “normal” set of readings can be either overallaverages or a set that appears to be a median of all sets. The“slope” of this regression becomes the proportional factor, m,and the “intercept” the constant, k.6.2.2 If a verifier must be established in a shorter time thanthe requirements of 6.2.1, a set of standardants and selectedcalibrants can be run with the verifier. The data may then beanalyzed as described in 6.2.1.1, with the expected readings ofstandardants and calibrants used as the normal readings. Thisshould be repeated at least two more times. Average thecorrected verifier readings to obtain a good estimate of theexpected reading.6.2.2.1 The estimate of standard deviation for the verifiercan be improved by pooling with readings that are similar or itcan be defined by an overall pattern of deviation with intensity.6.3 Final statements of performance of a verifier should bein terms of concentration. Standard deviations in terms ofintensity / intensity ratio reading can be converted to anequivalent standard deviation in terms of concentration bymultiplying by the slope of the of the calibration equation at thepoint of the verifier reading. Details are given in Annex A1.6.3.1 If a deliberate change is made in the slope of acalibration curve after the collection of data, such as might bedone in the transformation in 6.1.4, the effective standarddeviation of the reading will be the previous observed standarddeviation divided by the factor used to change the slope of thecurve. Thus, if a standard deviation has been calculated asbeing 0.6 when a curve slope (change of concentration dividedby change in reading) at some point was 0.4, it would become0.3 if the curve was made twice as steep, that is, when the slopeat the same point was changed to 0.8.7. Use of Confidence Interval to Control SpectrochemicalAnalysis7.1 Practice E876 uses Student’s t-table to establish therange of reading or concentration around an average that willinclude the true reading or concentration at some confidencelevel. The calculation includes the standard deviation of themeasurement. To be effective, the standard deviation should beestimated with at least 16 df. The interval straddling theaverage will be 6ts/=n, where t is a factor from the t-table forsome probability level, s is the estimate of standard deviation,and n is the number of readings taken for one observation. Ifcontrol of a method depends upon observing an intensityreading, the confidence interval may be in terms of an intensityreading. If a method uses a computer to display concentration,the confidence interval should be in terms of concentration.7.1.1 If the confidence interval is used to judge when drifthas occurred, it will be appropriate to use a confidence level of95 % t