ASTM E1655-00

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Designation: E 1655 – 00
Standard Practices for
Infrared Multivariate Quantitative Analysis
This standard is issued under the fixed designation E1655; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope analyzed. While these surrogate methods generally make use
of the multivariate mathematics described herein, they do not
1.1 These practices cover a guide for the multivariate
conform to procedures described herein, specifically with
calibration of infrared spectrometers used in determining the
respect to the handling of outliers. Surrogate methods may
physical or chemical characteristics of materials. These prac-
indicate that they make use of the mathematics described
tices are applicable to analyses conducted in the near infrared
herein, but they should not claim to follow the procedures
(NIR) spectral region (roughly 780 to 2500 nm) through the
described herein.
mid infrared (MIR) spectral region (roughly 4000 to 400
−1
1.7 This standard does not purport to address all of the
cm ).
safety concerns, if any, associated with its use. It is the
NOTE 1—While the practices described herein deal specifically with
responsibility of the user of this standard to establish appro-
mid-andnear-infraredanalysis,muchofthemathematicalandprocedural
priate safety and health practices and determine the applica-
detail contained herein is also applicable for multivariate quantitative
bility of regulatory limitations prior to use.
analysisdoneusingotherformsofspectroscopy.Theuseriscautionedthat
typicalandbestpracticesformultivariatequantitativeanalysisusingother
2. Referenced Documents
formsofspectroscopymaydifferfrompracticesdescribedhereinformid-
and near-infrared spectroscopies. 2.1 ASTM Standards:
D1265 Practice for Sampling Liquified Petroleum (LP)
1.2 Procedures for collecting and treating data for develop-
Gases (Manual Method)
ing IR calibrations are outlined. Definitions, terms, and cali-
D4057 Practice for Manual Sampling of Petroleum and
bration techniques are described. Criteria for validating the
Petroleum Products
performance of the calibration model are described.
D4177 Practice for Automatic Sampling of Petroleum and
1.3 The implementation of these practices require that the
Petroleum Products
IR spectrometer has been installed in compliance with the
D4855 Practices for Comparing Test Methods
manufacturer’s specifications. In addition, it assumes that, at
D6122 Practice for Validation of Multivariate Process In-
the times of calibration and of validation, the analyzer is
frared Spectrophotometers
operating at the conditions specified by the manufacturer.
D6299 Practice for Applying Statistical Quality Assurance
1.4 These practices cover techniques that are routinely
Techniques to Evaluate Analytical Measurement System
applied in the near and mid infrared spectral regions for
Performance
quantitative analysis. The practices outlined cover the general
D6300 Practice for Determination of Precision and Bias
cases for coarse solids, fine ground solids, and liquids. All
Data for Use in Test Methods for Petroleum Products and
techniques covered require the use of a computer for data
Lubricants
collection and analysis.
E131 Terminology Relating to Molecular Spectroscopy
1.5 These practices provide a questionnaire against which
E168 Practices for General Techniques of Infrared Quanti-
multivariate calibrations can be examined to determine if they
tative Analysis
conform to the requirements defined herein.
E275 Practice for Describing and Measuring Performance
1.6 For some multivariate spectroscopic analyses, interfer-
of Ultraviolet, Visible, and Near Infrared Spectrophotom-
encesandmatrixeffectsaresufficientlysmallthatitispossible
eters
to calibrate using mixtures that contain substantially fewer
chemical components than the samples that will ultimately be
Annual Book of ASTM Standards, Vol 05.01.
1 3
These practices are under the jurisdiction of ASTM Committee E13 on Annual Book of ASTM Standards, Vol 05.02.
Molecular Spectroscopy and are the direct responsibility of Subcommittee E13.11 Annual Book of ASTM Standards, Vol 07.02.
on Chemometrics.
Annual Book of ASTM Standards, Vol 05.04
Current edition approved Sept. 10, 2000. Published November 2000. Originally Annual Book of ASTM Standards, Vol 05.03.
published as E1655–97. Last previous edition E1655–99. Annual Book of ASTM Standards, Vol 03.06.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1655
E334 Practice for General Techniques of Infrared Mi- 3.2.9 reference values—the component concentrations or
croanalysis property values for the calibration or validation samples which
are measured by the reference analytical method.
E456 Terminology Relating to Quality and Statistics
3.2.10 spectrometer/spectrophotometer qualification,
E691 Practice for Conducting an Interlaboratory Study to
n—the procedures by which a user demonstrates that the
Determine the Precision of a Test Method
performance of a specific spectrometer/spectrophotometer is
E932 Practice for Describing and Measuring Performance
adequate to conduct a multivariate analysis so as to obtain
of Dispersive Infrared Spectrometers
precision consistent with that specified in the method.
E1421 PracticeforDescribingandMeasuringPerformance
3.2.11 surrogate calibration, n—a multivariate calibration
of Fourier Transform Infrared (FT-IR) Spectrometers:
that is developed using a calibration set which consists of
Level Zero and Level One Tests
mixtures which contain substantially fewer chemical compo-
E1866 Guide for Establishing Spectrophotometer Perfor-
nents than the samples which will ultimately be analyzed.
mance Tests
3.2.12 surrogate method, n—a standard test method that is
E1944 PracticeforDescribingandMeasuringPerformance
based on a surrogate calibration.
of Fourier Transform Near-Infrared (FT-NIR) Spectrom-
7 3.2.13 validation samples—a set of samples used in vali-
eters: Level Zero and Level One Tests
dating the model. Validation samples are not part of the set of
calibration samples. Reference component concentration or
3. Terminology
property values are known (measured by reference method),
3.1 Definitions—Forterminologyrelatedtomolecularspec-
and are compared to those estimated using the model.
troscopic methods, refer to Terminology E131. For terminol-
ogy relating to quality and statistics, refer to Terminology 4. Summary of Practices
E456.
4.1 Multivariate mathematics is applied to correlate the
3.2 Definitions of Terms Specific to This Standard:
absorbances measured for a set of calibration samples to
3.2.1 analysis—inthecontextofthispractice,theprocessof
reference component concentrations or property values for the
applying the calibration model to an absorption spectrum so as
set of samples. The resultant multivariate calibration model is
to estimate a component concentration value or property.
applied to the analysis of spectra of unknown samples to
3.2.2 calibration—a process used to create a model relating
provide an estimate of the component concentration or prop-
two types of measured data. In the context of this practice, a
erty values for the unknown sample.
process for creating a model that relates component concen-
4.2 Multilinear regression (MLR), principal components
trations or properties to absorbance spectra for a set of known
regression (PCR), and partial least squares (PLS) are examples
reference samples.
of multivariate mathematical techniques that are commonly
3.2.3 calibration model—the mathematical expression that
used for the development of the calibration model. Other
relates component concentrations or properties to absorbances
mathematical techniques are also used, but may not detect
for a set of reference samples.
outliers, and may not be validated by the procedure described
3.2.4 calibration samples—the set of reference samples
in these practices.
used for creating a calibration model. Reference component 4.3 Statistical tests are applied to detect outliers during the
concentration or property values are known (measured by
development of the calibration model. Outliers include high
referencemethod)forthecalibrationsamplesandcorrelatedto leverage samples (samples whose spectra contribute a statisti-
the absorbance spectra during the calibration.
cally significant fraction of one or more of the spectral
3.2.5 estimate—the value for a component concentration or variables used in the model), and samples whose reference
property obtained by applying the calibration model for the values are inconsistent with the model.
analysis of an absorption spectrum. 4.4 Validation of the calibration model is performed by
using the model to analyze a set of validation samples and
3.2.6 model validation—the process of testing a calibration
statistically comparing the estimates for the validation samples
model to determine bias between the estimates from the model
toreferencevaluesmeasuredforthesesamples,soastotestfor
and the reference method, and to test the expected agreement
bias in the model and for agreement of the model with the
between estimates made with the model and the reference
reference method.
method.
4.5 Statistical tests are applied to detect when values esti-
3.2.7 multivariate calibration—a process for creating a
mated using the model represent extrapolation of the calibra-
model that relates component concentrations or properties to
tion.
the absorbances of a set of known reference samples at more
4.6 Statistical expressions for calculating the repeatability
than one wavelength or frequency.
of the infrared analysis and the expected agreement between
3.2.8 reference method—the analytical method that is used
the infrared analysis and the reference method are given.
to estimate the reference component concentration or property
value which is used in the calibration and validation proce-
5. Significance and Use
dures.
5.1 These practices can be used to establish the validity of
theresultsobtainedbyaninfrared(IR)spectrometeratthetime
the calibration is developed. The ongoing validation of esti-
Annual Book of ASTM Standards, Vol 14.02. mates produced by analysis of unknown samples using the
E 1655
calibration model should be covered separately (see for ex- testsamplesbestatisticallycomparedtovaluesobtainedbythe
ample, Practice D 6122). reference method. The statistical tests to be applied for
5.2 These practices are intended for all users of infrared validation of the model are discussed in Section 18.
spectroscopy. Near-infrared spectroscopy is widely used for 6.1.6 Application of the Model for the Analysis of
quantitative analysis. Many of the general principles described Unknowns—The mathematical model is applied to the spectra
in these practices relate to the common modern practices of of unknown samples to estimate component concentrations or
near-infrared spectroscopic analysis. While sampling methods property values, or both, (see Section 13). Outlier statistics are
and instrumentation may differ, the general calibration meth- used to detect when the analysis involves extrapolation of the
odologies are equally applicable to mid-infrared spectroscopy. model (see Section 16).
New techniques are under study that may enhance those 6.1.7 Routine Analysis and Monitoring—Once the efficacy
discussedwithinthesepractices.Userswillfindthesepractices of calibration equations is established, the equations must be
to be applicable to basic aspects of the technique, to include monitored for continued accuracy and precision. Simulta-
sample selection and preparation, instrument operation, and neously, the instrument performance must be monitored so as
data interpretation. to trace any deterioration in performance to either the calibra-
5.3 The calibration procedures define the range over which tion model itself or to a failure in the instrumentation perfor-
measurements are valid and demonstrate whether or not the mance. Procedures for verifying the performance of the analy-
sensitivityandlinearityoftheanalysisoutputsareadequatefor sis are only outlined in Section 22 but are covered in detail in
providing meaningful estimates of the specific physical or Practice D 6122. The use of this method requires that a model
chemical characteristics of the types of materials for which the quality control material be established at the time the model is
calibration is developed. developed. The model QC material is discussed in Section 22.
For practices to compare reference methods and analyzer
6. Overview of Multivariate Calibration
methods, refer to Practices D4855.
6.1 The practice of infrared multivariate quantitative analy-
6.1.8 Transfer of Calibrations—Transferable calibrations
sis involves the following steps:
are equations that can be transferred from the original instru-
6.1.1 Selecting the Calibration Set—This set is also termed
ment, where calibration data were collected, to other instru-
thetrainingsetorspectrallibraryset.Thissetistorepresentall
ments where the calibrations are to be used to predict samples
of the chemical and physical variation normally encountered
forroutineanalysis.Inorderforacalibrationtobetransferable
forroutineanalysisforthedesiredapplication.Selectionofthe
it must perform prediction after transfer without a significant
calibration set is discussed in Section 17, after the statistical
decrease in performance, as indicated by established statistical
terms necessary to define the selection criteria have been
tests. In addition, statistical tests that are used to detect
defined.
extrapolation of the model must be preserved during the
6.1.2 Determination of Concentrations or Properties, or
transfer. Bias or slope adjustments, or both, are to be made
Both, for Calibration Samples—The chemical or physical
after transfer only when statistically warranted. Calibration
properties, or both, of samples in the calibration set must be
transfer, that is sometimes referred to as instrument standard-
accurately and precisely measured by the reference method in
ization, is discussed in Section 21.
order to accurately calibrate the infrared model for prediction
7. Infrared Instrumentation
of the unknown samples. Reference measurements are dis-
cussed in Section 9. 7.1 A complete description of all applicable types of infra-
6.1.3 The Collection of Infrared Spectra—The collection of red instrumentation is beyond the scope of these practices.
optical data must be per
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