ASTM_E_168_-_06.pdf

Designation:E 168-06INTERNATIONALStandard Practices forGeneral Techniques of Infrared Quantitative AnalysisThis standard is issued under the fixed designation E 168;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(e)indicates an editorial change since the last revision or reapproval.1.Scope3.Terminology1.1 These practices cover the techniques most often used in3.1 For definitions of terms and symbols,refer to Terminol-infrared quantitative analysis.Practices associated with theogy E 131.collection and analysis of data on a computer are included aswell as practices that do not use a computer.4.Significance and Use1.2 This practice does not purport to address all of the4.1 These practices are intended for all infrared spectrosco-concerns associated with developing a new quantitativepists.For novices,these practices will serve as an overview ofmethod.It is the responsibility of the developer to ensure thatpreparation,operation,and calculation techniques.For experi-the results of the method fall in the desired range of precisionenced persons,these practices will serve as a review whenand bias.seldom-used techniques are needed.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is the5.Apparatusresponsibiliry of the user of this standard to establish appro-5.1 The infrared techniques described here assume that thepriate safety and health practices and determine the applica-equipment is of at least the usual commercial quality and meetsbility of regulatory limitations prior to use.Specific hazardthe standard specifications of the manufacturer.For dispersivestatements appear in Section 6 and Note A4.7,Note A4.11,andinstruments,also refer to Practice E 932.For Fourier Trans-Note A5.6.form and dispersive instruments,also refer to Practices E 1421and E932 respectively,and for microanalysis with these2.Referenced Documentsinstruments see Practice E 334.2.1 ASTM Standards:25.2 In developing a spectroscopic method,it is the respon-E 131 Terminology Relating to Molecular Spectroscopysibility of the originator to describe the instrumentation and theE 334 Practice for General Techniques of Infrared Mi-performance required to duplicate the precision and bias of acroanalysismethod.It is necessary to specify this performance in termsE 932 Practice for Describing and Measuring Performancethat can be used by others in applications of the method.of Dispersive Infrared SpectrometersE 1252 Practice for General Techniques for Obtaining In-6.Hazardsfrared Spectra for Qualitative Analysis6.1 Users of these practices must be aware that there areE 1421 Practice for Describing and Measuring Performanceinherent dangers associated with the use of electrical instru-of Fourier Transform Mid-Infrared(FT-MIR)Spectrom-mentation,infrared cells,solvents,and other chemicals,andeters:Level Zero and Level One Teststhat these practices cannot and will not substitute for a practicalE 1655 Practices for Infrared Multivariate Quantitativeknowledge of the instrument,cells,and chemicals used in aAnalysisparticular analysis.7.Considerations for Quantitative InfraredMeasurementsThese practices are under the jurisdiction of ASTM Committee E13 on7.1 Quantitative infrared analysis is commonly done withMolecular Spectroscopy and are the direct responsibility of Subcommittee E13.03on Infrared Spectroscopy.grating,filter,prism,or interferometer instruments.The fol-Current edition approved March 1,2006.Published April 2006.Originallylowing guidelines for setting up an analytical procedure areapproved in 1964.Last previous edition approved in 2004 as E 168-99(2004).appropriate:For referenced ASTM standards,visit the ASTM website,www.astm.org,or7.1.1 Always operate the instrument in the most stable andcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onreproducible conditions attainable.This includes instrumentthe ASTM website.warm-up time,sample temperature equilibration,and exactCopyright ASTM Intermational,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959,United States.E168-06reproduction of instrument performance tests for both stan-to absorbance.If spectra cannot be obtained in absorbance.dards and samples.After calibration,use equivalent settings forthen Eq A12.1 and A12.2 in Annex A12 can be used to convertanalyses.For all infrared instruments,refer to the manufactur-the data.ers recommendations for the instrument settings.After cali-7.1.7 Use spectral regions offering the most information onbration,use these same settings for analysis.the analyte.Select analytical wavenumbers where the compo-7.1.2 The absorbance values at analytical wavenumbersnent has a relatively large absorptivity.In addition,othershould fall within the acceptably accurate range of the particu-analytes should have minimal effect on the measured absor-lar spectrometer used.In general,a single absorbance measure-bance.ment will have the best signal-to-noise ratio when it is in the7.1.8 The performance of the spectrometer should be suffi-range from 0.3 to 0.8 absorbance units(AU)(1).3 Theciently good to give adequate linearity of response for thesensitivity of Fourier transform(FT-IR)spectrometers is suchdesired range of concentrations.The signal-to-noise ratio,S/N,that lower absorbance values can be used quite effectively.should be acceptable for the desired precision.provided that the baseline can be estimated accurately(see7.1.9 Select analytical wavenumbers such that the linearitySection 12).Absorbances greater than 0.8 AU should beof the absorbance-concentration relationship is least affectedavoided wherever possible because of the possibility ofby molecular interaction,dispersion in refractive index,andinstrumentally-caused non-linearity,both for dispersive(2)andspectrometer nonlinearity.FT-IR(3,4)spectrometers.Variation of the concentration andsample path length can be used to adjust absorbance values into8.Theory for a Single-Compound Analysisthe optimum range.When multiple components are determined8.1 Quantitative spectrometry is based on the Beer-in a particular sample,it is acceptable to use absorbance valuesBouguer-Lambert(henceforth referred to as Beers)law,whichoutside the optimum range,(5)however,absorbances greateris expressed for the one component case as:than 1.5 AU should be avoided(2-4).Weaker absorption bandsA=abc(1)of high concentration components may be selected to provideabsorbance values within the optimal range.where:7.1.3 The most accurate analytical methods are imple-A=absorbance of the sample at a specified wavenumber,mented with samples in solution.With liquid samples that area=absorptivity of the component at this wavenumber,not exceptionally viscous,best results are obtained if the cell isb=sample path length,andnot moved after the first sample is introduced into the instru-c=concentration of the component.ment(the fixed-cell method).The reason is that sample cellSince spectrometers measure transmittance,T,of the radia-position is difficult to reproduce accurately by insertion intotion through a sample,it is necessary to convert T to A astypical cell holders.Suitable fittings and tubes can be attachedfollows:to the cell to allow sample changing in a flow-through manner.PA=-log T=-log Po(2)When it is not practical to use a flow-through cell,the cellshould fit tightly in the holder so that lateral and tilting motionswhere:are restricted.Po=input radiant power at the sample,and7.1.4 Unless there is reason to suspect deposition on orP=radiant power transmitted through the sample.contamination of the cell from the samples,it is generallypreferable to wash out the current sample with the next sample.9.Calibration for a Single-Component Determinationif sufficient sample is available.The volume of sample used to9.1 Proper sample preparation is essential to quantitativeflush the cell should be at least five times(and preferably more.analysis.See Annex A4.for example,20 times)the volume between the sample inlet9.1.1 Quantitative analysis has two distinct parts:calibra-and cell exit points.tion and analysis.For a simple one-component analysis,select7.1.5 For some bands,the wavenumber of the maximuman appropriate solvent that is essentially free from interferingabsorbance changes as a function of concentration.Similarly,absorptions at the analytical wavenumber.the position of the baseline points may change with concen-9.1.2 For calibration,measure the absorbances,A,of thetration.Selection of baseline points must be done carefully toanalyte solutions at several known concentrations,c.Absorp-account for the shift of the absorbance maximum.The questiontivities,a,are then calculated,using Eq 1 with the baselinearises whether it is preferable to measure absorbances at fixedcorrections as described in Sections 12-14.Alternatively,thewavenumber locations or at the observed maximum of theabsorbances,A,of a single solution in several cells of different,analytical band.The best approach is empirical testing of bothbut accurately known,path lengths may be measured;however,the fixed point and the tracking methods of evaluation.interaction effects will not be elucidated in this fashion.7.1.6 Whenever possible,working directly in absorbance is9.1.3 Calculate the average of the several a values for futurepreferable.That is,either the instrument or associated datause,or draw an analytical working curve by graphing absor-processor makes the necessary conversion from transmittancebance versus concentration for a constant path length asdemonstrated in Fig.1.Use the linear part of the curve tocalculate a.The calculation of a where curvature is present willbe discussed in 18.1 and 18.2.3 The boldface numbers in parentheses refer to the list of references at the end ofthese practices.NoTE 1-In practice,the calibration curve may not have a y intercept ofE168-06where:A:=total absorbance at wavenumber i,ain=absorptivity at the wavenumber i of component n,b=path length of the cell in which the mixture issampled,andA2c,=concentration of component n in the mixture.10.2 During calibration,concentrations c,are known,andbaseline corrected absorbances A are measured.The experi-mental absorptivity-path length products ab are then calcu-8500080A1lated(see Note 2).During analysis,the absorptivity-path lengthproducts ab are known,and the absorbances A are measured.The unknown concentrations are then calculated(see Section17).Therefore,accurate calibration generally requires thatexperimental absorptivity values be obtained from at least nstandards.The following requirements must be met:10.2.1 The number of standards must be equal to or greaterthan the number of analytes,n,and10.2.2 The number of analytical wavenumbers,i,must beequal to or greater than the number of independent compo-nents,n.C1c2NoTE 2-All absorbance conversions use transmittance(that is,theConcentrationdecimal value),not percent transmittance.Regardless of form(that is,FIG.1 An Analytical Working Curvedecimal or percent),the term transmittance refers to the term P/P of Eq2,and should not be called transmission.(See Terminology E 131).zero.This could be due to a variety of factors including,but not limited to,10.3 The first requirement allows the analyst to use moreincompletely resolved analyte bands,reflection losses,and solvent inter-than the minimum number of standards.Over-determination offerences.It is important that the method used to calculate the calibrationstandards permits error estimation in the analytical result.Thecurve not force the y intercept to be zero.second requirement allows the use of more than the minimum9.1.4 For analysis,dissolve the unknown in the solvent,number of peaks for specifying a chemical system,where atmeasure the absorbance,A,and determine the concentration,c,least one distinctive band is selected for each componentof the analyte graphically or by calculation.Convert this(7-10).concentration in solution to the concentration in the unknown10.4 The procedures used in multicomponent analysis willsample.be discussed further in the following section which is also an9.1.5 Both analysis time and chance of error are less if theintroduction to general solution phase analyses.concentrations of the unknowns and the cell path length arekept the same over a series of analyses,and the concentrations11.Multicomponent Solution Analysisof the calibration solutions have bracketed the expected high11.1 For the quantitative analysis of mixtures,Eq 4 isand low values of the unknown solutions(6,7).applicable.The absorptivities a of the n components of themixture at the ith analytical wavenumber are determined from10.Theory for Multicomponent Analysisabsorbance measurements made on each component taken10.1 Beers law is expressed for a mixture of n indepen-individually.These absorbances must be measured underdently absorbing components at a single path length and singleconditions(sample path length,temperature,pressure,andwavenumber as:solvent)identical to those used for the unknowns,and theyshould be corrected for baselines as discussed in SectionsA=aibc1+a2bc2+anbcn(3)12-14.Absorbance measurements are made with concentra-Eq 3 defines an absorbance at a wavenumber as being due totions of the analyte bracketing the amounts expected in thethe sum of the independent contributions of each component.unknown samples.In order to solve for the n component concentrations,n11.2 Where possible,prepare samples as dilute solutionsindependent equations containing n absorbance measurementsand place in cells of appropriate path lengths(typically 0.2 toat n wavenumbers are necessary.This is expressed for constant1.0 mm).Use lower concentrations in longer path length cellspath length as follows:rather than higher concentrations in shorter path length cells toA1=a1ubc1+a12bc2+-+anbcn(4)obtain absorbance values in the 0.3 to 0.8 range.LowerA2=a2bc2+a22bc2+a2mbcnconcentrations will minimize nonlinear effects due to disper-sion(that is,change of refractive index with wavenumber).Where freedom from intermolecular effects is uncertain orwhere intermolecular effects are known to be present,calibra-tion must be based on measurements taken from syntheticAi=anbe+anbe2+aimbenmixtures of all components as described in 15.1.2.E168-06thickness of each film and apply a proportional correction fordeviations from standard thickness.NoTE 4-The spectra of films and pellets can be complicated by thepresence of a fringe pattern.For pellets and films,follow the suggestionsAin A4.5.1.2 and Note A5.1,respectively.A fringe pattern is undesirablebecause analyte absorbance values can be altered by its presence.15.2.2 In cases where all components of a mixture aredetermined to a total of 100%,it is usually sufficient to800804determine only the ratios of absorbances.In such cases,it is notnecessary to know the thickness of the sample layer;it is onlynecessary to know the ratio of the components.However,aknowledge of the thickness is needed to determine the presenceof impurities because the total then will be less than 100%.15.2.3 The above procedure for films is also used withpowders prepared as mulls.Measurement of thickness can beaccomplished by an internal standard technique as described inA3A4.4.2.This involves the addition to the sample of a knownA2.3A2weight ratio of a compound having an absorption band ofknown absorptivity that does not overlap the bands of theW3W1W2sample.15.2.4 When powders are measured as pressed plates orWavenumber,cm-1pellets,analytical curves are prepared in the same manner asFIG.4 A Two-Point Baselinesolutions,see Sections 9 and 11.15.3 Gases:15.Nonsolution Analyses15.3.1 All calibration measurements for a given analysis15.1 Liquids:must be made at a fixed total pressure.This pressure must be15.1.1 Analyzing a liquid mixture without the use of aequal to the total pressure employed in the analysis.Andiluting solvent is sometimes complicated by intermolecularanalysis may be set up in either of two ways:forces.An absorption band may undergo intensity changes or15.3.1.1 Method 1-A fixed sample pressure is establishedfrequency shifts,or both,relative to the same absorption bandthat is a fraction of the total pressure obtained by addition of aof the component in solution.The absorbance contribution of anonabsorbing diluent ponent in a mixture can seldom be calculated from its15.3.1.2 Method 2-A fixed sample pressure is used as theabsorbance measured in the pure state.It is desirable tototal pressure.Analytical curves are prepared by introducing adetermine the absorptivities from known mixtures havingpure component at various measured pressures which bracketproportions near those of the samples.the expected component pressures in the sample.A diluent gas15.1.2 Prepare mixtures having known concentrations of theis then added to bring the total pressure up to the establishedvarious components covering the expected ranges.Measurevalue.baseline corrected absorbances at each of the wavelengths15.3.2 In Method 2,the analytical curve preparation doeschosen for the analysis and substitute them(along with thenot allow for the possibility of band broadening for differentknown concentrations)in Eq 4.Solve for the absorptivity-pathcomponents.This factor is more properly addressed by follow-length products,ab directly from the set of n simultaneousing Method 1 where the same diluent gas is employed forequations,or use a multivariant method(see Annex A8)ifsample preparation and calibration.Low molecular weightsufficient data are available.gases frequently produce very strong,sharp absorption fea-15.1.3 If the concentrations in the unknowns vary widely.tures.Addition of a diluent gas and use of pressure less thancalculation of a second set of the ab products is recom-atmospheric may be necessary.Absorbances are measured formended.A second set may be necessary due to the presence ofeach standard at the wavenumbers selected for analysis.Whereintermolecular influences,and the differences in the values ofpossible,integrated absorbances(see Annex A3)are preferredthe absorptivities thus determined will indicate the extent ofto offset the effect of small pressure variations.The absor-these influences.bances are plotted against the partial pressures(or mole15.1.4 A single set of absorptivities may not suffice tofractions)to produce analytical curves.analyze mixtures throughout all possible concentration rangesof the components,in which case,narrowing the range of16.Difference Methodconcentrations is recommended.16.1 Spectral subtraction using a computer is a common15.1.5 Since the ab products are calculated directly in thispractice in qualitative infrared analysis.This technique is alsoprocedure,it is not necessary to plot analytical curves.used to perform quantitative infrared analyses.The advantage15.2 Solids:of spectral subtraction(the difference method)is that small15.2.1 For cast films,pressed films,or pellets,follow theconcentration differences can be measured with greater accu-same general procedure as for liquids(see 15.1).Measure theracy than is possible on superimposed bands.5