1、Designation: E168 16Standard Practices forGeneral Techniques of Infrared Quantitative Analysis1This standard is issued under the fixed designation E168; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A n
2、umber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 These practices cover the techniques most often us
3、ed ininfrared quantitative analysis. Practices associated with thecollection and analysis of data on a computer are included aswell as practices that do not use a computer.1.2 This practice does not purport to address all of theconcerns associated with developing a new quantitativemethod. It is the
4、responsibility of the developer to ensure thatthe results of the method fall in the desired range of precisionand bias.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This standard does not purport to address all of thes
5、afety 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 regulatory limitations prior to use. Specific hazardstatements appear in Section 6, Note A4.7, Note A4.11, a
6、ndNote A5.6.2. Referenced Documents2.1 ASTM Standards:2E131 Terminology Relating to Molecular SpectroscopyE334 Practice for General Techniques of Infrared Micro-analysisE932 Practice for Describing and Measuring Performance ofDispersive Infrared SpectrometersE1252 Practice for General Techniques for
7、 Obtaining Infra-red Spectra for Qualitative AnalysisE1421 Practice for Describing and Measuring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters: Level Zero and Level One TestsE1655 Practices for Infrared Multivariate QuantitativeAnalysis3. Terminology3.1 For definitions of term
8、s and symbols, refer to Terminol-ogy E131.4. Significance and Use4.1 These practices are intended for all infrared spectrosco-pists. For novices, these practices will serve as an overview ofpreparation, operation, and calculation techniques. For experi-enced persons, these practices will serve as a
9、review whenseldom-used techniques are needed.5. Apparatus5.1 The infrared techniques described here assume that theequipment is of at least the usual commercial quality and meetsthe standard specifications of the manufacturer. For dispersiveinstruments, also refer to Practice E932. For Fourier Trans
10、formand dispersive instruments, also refer to Practices E1421 andE932 respectively, and for microanalysis with these instru-ments see Practice E334.5.2 In developing a spectroscopic method, it is the respon-sibility of the originator to describe the instrumentation and theperformance required to dup
11、licate the precision and bias of amethod. It is necessary to specify this performance in termsthat can be used by others in applications of the method.6. Hazards6.1 Users of these practices must be aware that there areinherent dangers associated with the use of electricalinstrumentation, infrared ce
12、lls, solvents, and other chemicals,and that these practices cannot and will not substitute for apractical knowledge of the instrument, cells, and chemicalsused in a particular analysis.7. Considerations for Quantitative InfraredMeasurements7.1 Quantitative infrared analysis is commonly done withgrat
13、ing, filter, prism, or interferometer instruments. The fol-lowing guidelines for setting up an analytical procedure areappropriate:1These practices are under the jurisdiction of ASTM Committee E13 onMolecular Spectroscopy and Separation Science and are the direct responsibility ofSubcommittee E13.03
14、 on Infrared and Near Infrared Spectroscopy.Current edition approved April 1, 2016. Published June 2016. Originallyapproved in 1964. Last previous edition approved in 2006 as E168 06 which waswithdrawn January 2015 and reinstated in April 2016. DOI: 10.1520/E0168-16.2For referenced ASTM standards, v
15、isit 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 International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-
16、2959. United States17.1.1 Always operate the instrument in the most stable andreproducible conditions attainable. This includes instrumentwarm-up time, sample temperature equilibration, and exactreproduction of instrument performance tests for both stan-dards and samples.After calibration, use equiv
17、alent settings foranalyses. For all infrared instruments, refer to the manufactur-ers recommendations for the instrument settings. Aftercalibration, use these same settings for analysis.7.1.2 The absorbance values at analytical wavenumbersshould fall within the acceptably accurate range of the parti
18、cu-lar spectrometer used. In general, a single absorbance measure-ment will have the best signal-to-noise ratio when it is in therange from 0.3 to 0.8 absorbance units (AU) (1).3Thesensitivity of Fourier transform (FT-IR) spectrometers is suchthat lower absorbance values can be used quite effectivel
19、y,provided that the baseline can be estimated accurately (seeSection 12). Absorbances greater than 0.8 AU should beavoided wherever possible because of the possibility ofinstrumentally-caused non-linearity, both for dispersive (2) andFT-IR (3,4) spectrometers. Variation of the concentration andsampl
20、e path length can be used to adjust absorbance values intothe optimum range. When multiple components are determinedin a particular sample, it is acceptable to use absorbance valuesoutside the optimum range, (5) however, absorbances greaterthan 1.5AU should be avoided (2-4). Weaker absorption bandso
21、f high concentration components may be selected to provideabsorbance values within the optimal range.7.1.3 The most accurate analytical methods are imple-mented with samples in solution. With liquid samples that arenot exceptionally viscous, best results are obtained if the cell isnot moved after th
22、e first sample is introduced into the instru-ment (the fixed-cell method). The reason is that sample cellposition is difficult to reproduce accurately by insertion intotypical cell holders. Suitable fittings and tubes can be attachedto the cell to allow sample changing in a flow-through manner.When
23、it is not practical to use a flow-through cell, the cellshould fit tightly in the holder so that lateral and tilting motionsare restricted.7.1.4 Unless there is reason to suspect deposition on orcontamination of the cell from the samples, it is generallypreferable to wash out the current sample with
24、 the next sample,if sufficient sample is available. The volume of sample used toflush the cell should be at least five times (and preferably more,for example, 20 times) the volume between the sample inletand cell exit points.7.1.5 For some bands, the wavenumber of the maximumabsorbance changes as a
25、function of concentration. Similarly,the position of the baseline points may change with concen-tration. Selection of baseline points must be done carefully toaccount for the shift of the absorbance maximum. The questionarises whether it is preferable to measure absorbances at fixedwavenumber locati
26、ons or at the observed maximum of theanalytical band. The best approach is empirical testing of boththe fixed point and the tracking methods of evaluation.7.1.6 Whenever possible, working directly in absorbance ispreferable. That is, either the instrument or associated dataprocessor makes the necess
27、ary conversion from transmittanceto absorbance. If spectra cannot be obtained in absorbance,then EqA12.1 and A12.2 in AnnexA12 can be used to convertthe data.7.1.7 Use spectral regions offering the most information onthe analyte. Select analytical wavenumbers where the compo-nent has a relatively la
28、rge absorptivity. In addition, otheranalytes should have minimal effect on the measured absor-bance.7.1.8 The performance of the spectrometer should be suffi-ciently good to give adequate linearity of response for thedesired range of concentrations. The signal-to-noise ratio, S/N,should be acceptabl
29、e for the desired precision.7.1.9 Select analytical wavenumbers such that the linearityof the absorbance-concentration relationship is least affectedby molecular interaction, dispersion in refractive index, andspectrometer nonlinearity.8. Theory for a Single-Compound Analysis8.1 Quantitative spectro
30、metry is based on the Beer-Bouguer-Lambert (henceforth referred to as Beers) law, whichis expressed for the one component case as:A 5 abc (1)where:A = absorbance of the sample at a specified wavenumber,a = absorptivity of the component at this wavenumber,b = sample path length, andc = concentration
31、of the component.Since spectrometers measure transmittance, T, of the radia-tion through a sample, it is necessary to convert T to A asfollows:A 52logT 52logPP0(2)where:P0= input radiant power at the sample, andP = radiant power transmitted through the sample.9. Calibration for a Single-Component De
32、termination9.1 Proper sample preparation is essential to quantitativeanalysis. See Annex A4.9.1.1 Quantitative analysis has two distinct parts: calibra-tion and analysis. For a simple one-component analysis, selectan appropriate solvent that is essentially free from interferingabsorptions at the ana
33、lytical wavenumber.9.1.2 For calibration, measure the absorbances, A,oftheanalyte solutions at several known concentrations, c.Absorptivities, a, are then calculated, using Eq 1 with thebaseline corrections as described in Sections 1214.Alternatively, the absorbances, A, of a single solution in seve
34、ralcells of different, but accurately known, path lengths may bemeasured; however, interaction effects will not be elucidated inthis fashion.9.1.3 Calculate the average of the several a values for futureuse, or draw an analytical working curve by graphing absor-bance versus concentration for a const
35、ant path length asdemonstrated in Fig. 1. Use the linear part of the curve to3The boldface numbers in parentheses refer to the list of references at the end ofthese practices.E168 162calculate a. The calculation of a where curvature is present willbe discussed in 18.1 and 18.2.NOTE 1In practice, the
36、 calibration curve may not have a y intercept ofzero. This could be due to a variety of factors including, but not limited to,incompletely resolved analyte bands, reflection losses, and solvent inter-ferences. It is important that the method used to calculate the calibrationcurve not force the y int
37、ercept to be zero.9.1.4 For analysis, dissolve the unknown in the solvent,measure the absorbance, A, and determine the concentration, c,of the analyte graphically or by calculation. Convert thisconcentration in solution to the concentration in the unknownsample.9.1.5 Both analysis time and chance of
38、 error are less if theconcentrations of the unknowns and the cell path length arekept the same over a series of analyses, and the concentrationsof the calibration solutions have bracketed the expected highand low values of the unknown solutions (6, 7).10. Theory for Multicomponent Analysis10.1 Beers
39、 law is expressed for a mixture of n indepen-dently absorbing components at a single path length and singlewavenumber as:A 5 a1bc11a2bc211anbcn(3)Eq 3 defines an absorbance at a wavenumber as being due tothe sum of the independent contributions of each component.In order to solve for the n component
40、 concentrations, nindependent equations containing n absorbance measurementsat n wavenumbers are necessary. This is expressed for constantpath length as follows:A15 a11bc11a12bc211a1nbcn(4)A25 a21bc21a22bc211a2nbcn Ai5 ai1bc11ai2bc211ainbcnwhere:Ai= total absorbance at wavenumber i,ain= absorptivity
41、 at the wavenumber i of component n,b = path length of the cell in which the mixture is sampled,andcn= concentration of component n in the mixture.10.2 During calibration, concentrations cnare known, andbaseline corrected absorbances A are measured. The experi-mental absorptivity-path length product
42、s ainb are then calcu-lated (see Note 2). During analysis, the absorptivity-path lengthproducts ainb are known, and the absorbances A are measured.The unknown concentrations are then calculated (see Section17). Therefore, accurate calibration generally requires thatexperimental absorptivity values b
43、e 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 independentcomponents, n.NOTE 2All ab
44、sorbance conversions use transmittance (that is, thedecimal value), not percent transmittance. Regardless of form (that is,decimal or percent), the term transmittance refers to the term P/P0of Eq2, and should not be called transmission. (See Terminology E131).10.3 The first requirement allows the an
45、alyst to use morethan the minimum number of standards. Over-determination ofstandards permits error estimation in the analytical result. Thesecond requirement allows the use of more than the minimumnumber of peaks for specifying a chemical system, where atleast one distinctive band is selected for e
46、ach component(7-10).10.4 The procedures used in multicomponent analysis willbe discussed further in the following section which is also anintroduction to general solution phase analyses.11. Multicomponent Solution Analysis11.1 For the quantitative analysis of mixtures, Eq 4 isapplicable. The absorpt
47、ivities ainof the n components of themixture at the ith analytical wavenumber are determined fromabsorbance measurements made on each component takenindividually. These absorbances must be measured underconditions (sample path length, temperature, pressure, andsolvent) identical to those used for th
48、e unknowns, and theyshould be corrected for baselines as discussed in Sections 12 14.Absorbance measurements are made with concentrations ofthe analyte bracketing the amounts expected in the unknownsamples.11.2 Where possible, prepare samples as dilute solutionsand place in cells of appropriate path
49、 lengths (typically 0.2 to1.0 mm). Use lower concentrations in longer path length cellsrather than higher concentrations in shorter path length cells toobtain absorbance values in the 0.3 to 0.8 range. LowerFIG. 1 An Analytical Working CurveE168 163concentrations 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 mu
