ASTM D6342-12(2017)e1

Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy

SIGNIFICANCE AND USE
5.1 General Utility:  
5.1.1 It is necessary to know the hydroxyl number of polyols in order to formulate polyurethane systems.  
5.1.2 This practice is suitable for research, quality control, specification testing, and process control.  
5.2 Limitations:  
5.2.1 Factors affecting the NIR spectra of the analyte polyols need to be determined before a calibration procedure is started. Chemical structure, interferences, any nonlinearities, the effect of temperature, and the interaction of the analyte with other sample components such as catalyst, water and other polyols needs to be understood in order to properly select samples that will model those effects which cannot be adequately controlled.  
5.2.2 Calibrations are generally considered valid only for the specific NIR instrument used to generate the calibration. Using different instruments (even when made by the same manufacturer) for calibration and analysis can seriously affect the accuracy and precision of the measured hydroxyl number. Procedures used for transferring calibrations between instruments are problematic and are to be utilized with caution following the guidelines in Section 16. These procedures generally require a completely new validation and statistical analysis of errors on the new instrument.  
5.2.3 The analytical results are statistically valid only for the range of hydroxyl numbers used in the calibration. Extrapolation to lower or higher hydroxyl values can increase the errors and degrade precision. Likewise, the analytical results are only valid for the same chemical composition as used for the calibration set. A significant change in composition or contaminants can also affect the results. Outlier detection, as discussed in Practices E1655, is a tool that can be used to detect the possibility of problems such as those mentioned above.
SCOPE
1.1 This standard covers a practice for the determination of hydroxyl numbers of polyols using NIR spectroscopy.  
1.2 Definitions, terms, and calibration techniques are described. Procedures for selecting samples, and collecting and treating data for developing NIR calibrations are outlined. Criteria for building, evaluating, and validating the NIR calibration model are also described. Finally, the procedure for sample handling, data gathering and evaluation are described.  
1.3 The implementation of this standard requires that the NIR spectrometer has been installed in compliance with the manufacturer's specifications.  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Note 1: This standard is equivalent ISO 15063.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Historical
• Publication Date: 31-Jul-2017
• ICS: 83.080.10 - Thermosetting materials
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.22 - Cellular Materials - Plastics and Elastomers

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
´1
Designation: D6342 − 12 (Reapproved 2017)
Standard Practice for
Polyurethane Raw Materials: Determining Hydroxyl Number
of Polyols by Near Infrared (NIR) Spectroscopy
This standard is issued under the fixed designation D6342; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Reapproved with editorial changes in August 2017.
1. Scope 2. Referenced Documents
1.1 This standard covers a practice for the determination of 2.1 ASTM Standards:
hydroxyl numbers of polyols using NIR spectroscopy. D883 Terminology Relating to Plastics
D4274 Test Methods for Testing Polyurethane Raw Materi-
1.2 Definitions, terms, and calibration techniques are de-
als: Determination of Hydroxyl Numbers of Polyols
scribed. Procedures for selecting samples, and collecting and
D4855 Practice for Comparing Test Methods (Withdrawn
treating data for developing NIR calibrations are outlined.
2008)
Criteria for building, evaluating, and validating the NIR
E131 Terminology Relating to Molecular Spectroscopy
calibration model are also described. Finally, the procedure for
E168 Practices for General Techniques of Infrared Quanti-
sample handling, data gathering and evaluation are described.
tative Analysis
1.3 The implementation of this standard requires that the
E222 Test Methods for Hydroxyl Groups Using Acetic
NIR spectrometer has been installed in compliance with the
Anhydride Acetylation
manufacturer’s specifications.
E275 Practice for Describing and Measuring Performance of
Ultraviolet and Visible Spectrophotometers
1.4 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this E456 Terminology Relating to Quality and Statistics
E1655 Practices for Infrared Multivariate Quantitative
standard.
Analysis
1.5 This standard does not purport to address all of the
E1899 Test Method for Hydroxyl Groups Using Reaction
safety concerns, if any, associated with its use. It is the
with p-ToluenesulfonylIsocyanate(TSI)andPotentiomet-
responsibility of the user of this standard to establish appro-
ric Titration with Tetrabutylammonium Hydroxide
priate safety and health practices and determine the applica-
2.2 ISO Standard:
bility of regulatory limitations prior to use.
ISO 15063 Plastics—Polyols for use in the production of
NOTE 1—This standard is equivalent ISO 15063.
polyurethanes—Determination of hydroxyl number by
1.6 This international standard was developed in accor-
NIR spectroscopy
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
3. Terminology
Development of International Standards, Guides and Recom-
3.1 Definitions—Terminology used in this practice follows
mendations issued by the World Trade Organization Technical
that defined in Terminology D883. For terminology related to
Barriers to Trade (TBT) Committee.
1 2
This practice is under the jurisdiction ofASTM Committee D20 on Plastics and For referenced ASTM standards, visit the ASTM website, www.astm.org, or
is the direct responsibility of Subcommittee D20.22 on Cellular Materials - Plastics contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
and Elastomers. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Aug. 1, 2017. Published August 2017. Originally the ASTM website.
approved in 1998. Last previous edition approved in 2012 as D6342 - 12. DOI: The last approved version of this historical standard is referenced on
10.1520/D6342-12R17E01. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D6342 − 12 (2017)
molecular spectroscopy methods, refer to Terminology E131. 5.2.3 Theanalyticalresultsarestatisticallyvalidonlyforthe
For terms relating to multivariate analysis, refer to Practice range of hydroxyl numbers used in the calibration. Extrapola-
E1655. tion to lower or higher hydroxyl values can increase the errors
and degrade precision. Likewise, the analytical results are only
3.2 Definitions of Terms Specific to This Standard:
valid for the same chemical composition as used for the
3.2.1 hydroxyl number—the milligrams of potassium hy-
calibrationset.Asignificantchangeincompositionorcontami-
droxide equivalent to the hydroxyl content of1gof sample.
nants can also affect the results. Outlier detection, as discussed
in Practices E1655, is a tool that can be used to detect the
4. Summary of Practice
possibility of problems such as those mentioned above.
4.1 Multivariate mathematics is applied to correlate the NIR
absorbance values for a set of calibration samples to the
6. Instrumentation
respective reference hydroxyl number for each sample. The
6.1 Introduction—A complete description of all applicable
resultant multivariate calibration model is then applied to the
types of NIR instrumentation is beyond the scope of this
analysis of unknown samples to provide an estimate of their
standard. Only a general outline is given here. A diagram of a
hydroxyl numbers.
typical NIR spectrometer is shown in Fig. 1.
4.2 Multilinear regression (MLR), principal components
6.2 Light Source and Detector—Tungsten-halogen lamps
regression (PCR), and partial least squares regression (PLS)
with quartz envelopes usually serve as the energy sources for
are the mathematical techniques used for the development of
NIR instruments. Most of the detectors used for NIR are
the calibration model.
solid-state semiconductors. PbS, PbSe, and InGaAs detectors
4.3 Statistical tests are used to detect outliers during the
are most commonly used.
development of the calibration model. Outliers can include
6.3 Light Dispersion—Spectrophotometers can be classified
high leverage samples and samples whose hydroxyl numbers
based on the procedure by which the instrument accomplishes
are inconsistent with the model.
wavelength selection.
4.4 Validation of the calibration model is performed by
6.3.1 Monochromator Instrument—Grating monochromator
using the model to analyze a set of validation samples. The
instruments, often called “dispersive” instruments, are com-
hydroxyl number estimates for the validation set are statisti-
monly used in the laboratory and for process applications. In a
cally compared to the reference hydroxyl number for this set to
holographic grating system, the grating is rotated so that only
test for agreement of the model with the reference method.
a narrow band of wavelengths is transmitted to a single
detector at a given time.
4.5 Statistical expressions are given for calculating the
6.3.2 Filter-Wheel Instrument—In this type of NIR
precision and bias of the NIR method relative to the reference
instrument, one or several narrow band filters are mounted on
method.
a turret wheel so that the individual wavelengths are presented
to a single detector sequentially.
5. Significance and Use
6.3.3 Acoustic Optic Tunable Filter (AOTF) Instrument—
5.1 General Utility:
TheAOTFisacontinuousvariantofthefixed-filterphotometer
5.1.1 It is necessary to know the hydroxyl number of
with no moving optical parts for wavelength selection. A
polyols in order to formulate polyurethane systems.
birefringent TeO crystal is used in a noncollinear configura-
5.1.2 This practice is suitable for research, quality control,
tion in which acoustic and optical waves move through the
specification testing, and process control.
crystal at different angles. Variations in the acoustic frequency
5.2 Limitations:
cause the crystal lattice spacing to change. That in turn causes
5.2.1 Factors affecting the NIR spectra of the analyte the crystal to act as a variable transmission diffraction grating
polyols need to be determined before a calibration procedure is
for one wavelength. The main advantage of using AOTF
started. Chemical structure, interferences, any nonlinearities, instruments is the speed. A wavelength or an assembly of
theeffectoftemperature,andtheinteractionoftheanalytewith
wavelengths can be changed hundreds of times per second
other sample components such as catalyst, water and other under computer control.
polyols needs to be understood in order to properly select
6.3.4 Light-Emitting Diode (LED) Instrument—Each wave-
samples that will model those effects which cannot be ad- length band is produced by a different diode. The major
equately controlled.
5.2.2 Calibrations are generally considered valid only for
the specific NIR instrument used to generate the calibration.
Using different instruments (even when made by the same
manufacturer) for calibration and analysis can seriously affect
the accuracy and precision of the measured hydroxyl number.
Procedures used for transferring calibrations between instru-
ments are problematic and are to be utilized with caution
following the guidelines in Section 16. These procedures
generally require a completely new validation and statistical
analysis of errors on the new instrument. FIG. 1 Schematic of a Near-IR System
´1
D6342 − 12 (2017)
advantages of the system are its small size and compactness, 6.5.5 The capability to perform transformations of log l/R
stability of construction with no moving parts, and low power optical data into derivatives, or other forms of mathematical
consumption. treatment, and to reverse the transformation,
6.5.6 The capability to compute multiple linear regression
6.3.5 Fourier Transfer (FT) Instrument—In FT-NIR
(MLR), principal component regression (PCR), and partial
instruments, the light is divided into two beams whose relative
least squares regression (PLS),
pathsarevariedbyuseofamovingopticalelement.Thebeams
6.5.7 The capability to store PCR or PLS loading, weights,
are recombined to produce an interference pattern that contains
scores or other desirable data, and to display these data for
all of the wavelengths of interest. The interference pattern is
subsequent examination and interpretation,
mathematically converted into spectral data using Fourier
6.5.8 The capability to enable the operator to evaluate the
transform. FT interferometer optics provide complete spectra
calibrationmodelbycomputingthestandarderrorofvalidation
with very high wavelength resolution. FTsignal averaging also
(SEV), coefficient of regression, and the root mean square
provides higher signal-to-noise ratios in general than can be
deviation (RMSD), and to display various plots,
achieved with other types of instruments.
6.5.9 The capability to perform cross-validation
6.4 Sampling System—Depending upon the applications,
automatically,
several different sampling systems can be used in the labora-
6.5.10 The capability to identify an outlier(s), and
tory or for on-line instruments, or both.
6.5.11 The capability to develop and save regression
6.4.1 Cuvette—Quartzorglasscuvetteswithfixedoradjust-
equations and analyze a sample to calculate a hydroxyl
able pathlengths can be used in the laboratory.
number.
6.4.2 Flow-Through Cell—This type of cell can be used for
6.6 Software Packages—Most NIR instruments provide
continuous or intermittent monitoring of liquid sample.
necessary software for collecting and modeling data. Several
6.4.3 Probes:
non-instrumentalcompaniesalsosupplychemometricsoftware
6.4.3.1 Transmission Probe—Transmission probes com- packages that can be used to analyze NIR data.
bined with optic fibers are ideal for analyzing clear liquids,
7. Near-IR Spectral Measurements
slurries, suspensions, and other high viscosity samples. Low
absorptivity in the NIR region permits sampling pathlengths of
7.1 NIR spectral measurements are based on Beer’s law,
up to 10 cm.
namely, the absorbance of a homogeneous sample containing
6.4.3.2 Immersion Probe—The immersion system uses a an absorbing substance is linearly proportional to the concen-
bi-directional optic fiber bundle and variable pathlength probe tration of the absorbing species. The absorbance of a sample is
for sample measurements. Radiation from the source is trans- defined as the logarithm to the base ten of the reciprocal of the
mitted to the sample by the inner ring of fibers, and diffuse Transmittance (T):
transmitted radiation is collected by the outer ring of fibers for
A 5 log 1/T (1)
~ !
detection.
where:
6.4.3.3 Attenuated Total Reflection (ATR) Probe—
T = the ratio of radiant power transmitted by the sample to
Attenuated total reflection occurs when an absorbing medium
the radiant power incident on the sample.
(the sample) is in close contact with the surface of a crystal
material of higher refractive index. At an optimized angle, the
7.1.1 For most types of instrumentation, the radiant power
NIR beam reflects internally along the crystal faces, penetrat-
incident on the sample cannot be measured directly. Instead, a
ing a few microns into the sample surface, where selective
reference (background) measurement of the radiant power is
absorption occurs. The resulting spectrum is very close to the
made without the sample being present in the light beam.
conventional transmission spectrum for the sample. There are
7.1.2 A measurement is then conducted with the sample
many designs ofATR plates and rods for specific applications.
present, and the ratio, T, is calculated. The background
Single or multiple reflection units are available.ATR sampling
measurement can be conducted in a variety of ways depending
accessories are available for the laboratory and, in the form of
on the application and instrumentation. The sample and its
fiber optic probes, can be used for on-line analysis. This is an
holder can be physically removed from the light beam and a
advantage when handling viscous liquids and highly absorbing
background measurement made on the “empty beam”. The
materials.
sample holder (cell) can be emptied, and a background
measurement taken for the empty cell. The cell can be filled
6.5 Software—The ideal software has the following capa-
with a material that has minimal absorption in the spectral
bilities:
range of interest, and the background measurement taken.
6.5.1 The capability to record all sample identification and
Alternatively, split the light beam or alternately pass the light
spectral data accurately and to access the reference data,
beam through the sample and through an empty beam, and
6.5.2 The capability to record the date and time of day that
empty cell, or a background material in the cell.
all spectra and files were recorded or created,
7.1.3 The particular background referencing scheme that is
6.5.3 The capability t
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: D6342 − 12 D6342 − 12 (Reapproved 2017)
Standard Practice for
Polyurethane Raw Materials: Determining Hydroxyl Number
of Polyols by Near Infrared (NIR) Spectroscopy
This standard is issued under the fixed designation D6342; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Reapproved with editorial changes in August 2017.
1. Scope*Scope
1.1 This standard covers a practice for the determination of hydroxyl numbers of polyols using NIR spectroscopy.
1.2 Definitions, terms, and calibration techniques are described. Procedures for selecting samples, and collecting and treating
data for developing NIR calibrations are outlined. Criteria for building, evaluating, and validating the NIR calibration model are
also described. Finally, the procedure for sample handling, data gathering and evaluation are described.
1.3 The implementation of this standard requires that the NIR spectrometer has been installed in compliance with the
manufacturer’s specifications.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
NOTE 1—This standard is equivalent ISO 15063.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D883 Terminology Relating to Plastics
D4274 Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols
D4855 Practice for Comparing Test Methods (Withdrawn 2008)
E131 Terminology Relating to Molecular Spectroscopy
E168 Practices for General Techniques of Infrared Quantitative Analysis
E222 Test Methods for Hydroxyl Groups Using Acetic Anhydride Acetylation
E275 Practice for Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
E456 Terminology Relating to Quality and Statistics
E1655 Practices for Infrared Multivariate Quantitative Analysis
E1899 Test Method for Hydroxyl Groups Using Reaction with p-Toluenesulfonyl Isocyanate (TSI) and Potentiometric Titration
with Tetrabutylammonium Hydroxide
2.2 ISO Standard:
ISO 15063 Plastics—Polyols for use in the production of polyurethanes—Determination of hydroxyl number by NIR
spectroscopy
This practice is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.22 on Cellular Materials - Plastics and
Elastomers.
Current edition approved Aug. 1, 2012Aug. 1, 2017. Published September 2012August 2017. Originally approved in 1998. Last previous edition approved in 20082012
as D6342 - 08.D6342 - 12. DOI: 10.1520/D6342-12.10.1520/D6342-12R17E01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D6342 − 12 (2017)
3. Terminology
3.1 Definitions—Terminology used in this practice follows that defined in Terminology D883. For terminology related to
molecular spectroscopy methods, refer to Terminology E131. For terms relating to multivariate analysis, refer to Practice E1655.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 hydroxyl number—the milligrams of potassium hydroxide equivalent to the hydroxyl content of 1 g of sample.
4. Summary of Practice
4.1 Multivariate mathematics is applied to correlate the NIR absorbance values for a set of calibration samples to the respective
reference hydroxyl number for each sample. The resultant multivariate calibration model is then applied to the analysis of unknown
samples to provide an estimate of their hydroxyl numbers.
4.2 Multilinear regression (MLR), principal components regression (PCR), and partial least squares regression (PLS) are the
mathematical techniques used for the development of the calibration model.
4.3 Statistical tests are used to detect outliers during the development of the calibration model. Outliers can include high
leverage samples and samples whose hydroxyl numbers are inconsistent with the model.
4.4 Validation of the calibration model is performed by using the model to analyze a set of validation samples. The hydroxyl
number estimates for the validation set are statistically compared to the reference hydroxyl number for this set to test for agreement
of the model with the reference method.
4.5 Statistical expressions are given for calculating the precision and bias of the NIR method relative to the reference method.
5. Significance and Use
5.1 General Utility:
5.1.1 It is necessary to know the hydroxyl number of polyols in order to formulate polyurethane systems.
5.1.2 This practice is suitable for research, quality control, specification testing, and process control.
5.2 Limitations:
5.2.1 Factors affecting the NIR spectra of the analyte polyols need to be determined before a calibration procedure is started.
Chemical structure, interferences, any nonlinearities, the effect of temperature, and the interaction of the analyte with other sample
components such as catalyst, water and other polyols needs to be understood in order to properly select samples that will model
those effects which cannot be adequately controlled.
5.2.2 Calibrations are generally considered valid only for the specific NIR instrument used to generate the calibration. Using
different instruments (even when made by the same manufacturer) for calibration and analysis can seriously affect the accuracy
and precision of the measured hydroxyl number. Procedures used for transferring calibrations between instruments are problematic
and are to be utilized with caution following the guidelines in Section 16. These procedures generally require a completely new
validation and statistical analysis of errors on the new instrument.
5.2.3 The analytical results are statistically valid only for the range of hydroxyl numbers used in the calibration. Extrapolation
to lower or higher hydroxyl values can increase the errors and degrade precision. Likewise, the analytical results are only valid
for the same chemical composition as used for the calibration set. A significant change in composition or contaminants can also
affect the results. Outlier detection, as discussed in Practices E1655, is a tool that can be used to detect the possibility of problems
such as those mentioned above.
6. Instrumentation
6.1 Introduction—A complete description of all applicable types of NIR instrumentation is beyond the scope of this standard.
Only a general outline is given here. A diagram of a typical NIR spectrometer is shown in Fig. 1.
6.2 Light Source and Detector—Tungsten-halogen lamps with quartz envelopes usually serve as the energy sources for NIR
instruments. Most of the detectors used for NIR are solid-state semiconductors. PbS, PbSe, and InGaAs detectors are most
commonly used.
FIG. 1 Schematic of a Near-IR System
´1
D6342 − 12 (2017)
6.3 Light Dispersion—Spectrophotometers can be classified based on the procedure by which the instrument accomplishes
wavelength selection.
6.3.1 Monochromator Instrument—Grating monochromator instruments, often called “dispersive” instruments, are commonly
used in the laboratory and for process applications. In a halographicholographic grating system, the grating is rotated so that only
a narrow band of wavelengths is transmitted to a single detector at a given time.
6.3.2 Filter-Wheel Instrument—In this type of NIR instrument, one or several narrow band filters are mounted on a turret wheel
so that the individual wavelengths are presented to a single detector sequentially.
6.3.3 Acoustic Optic Tunable Filter (AOTF) Instrument—The AOTF is a continuous variant of the fixed-filter photometer with
no moving optical parts for wavelength selection. A birefringent TeO crystal is used in a noncollinear configuration in which
acoustic and optical waves move through the crystal at different angles. Variations in the acoustic frequency cause the crystal lattice
spacing to change. That in turn causes the crystal to act as a variable transmission diffraction grating for one wavelength. The main
advantage of using AOTF instruments is the speed. A wavelength or an assembly of wavelengths can be changed hundreds of times
per second under computer control.
6.3.4 Light-Emitting Diode (LED) Instrument—Each wavelength band is produced by a different diode. The major advantages
of the system are its small size and compactness, stability of construction with no moving parts, and low power consumption.
6.3.5 Fourier Transfer (FT) Instrument—In FT-NIR instruments, the light is divided into two beams whose relative paths are
varied by use of a moving optical element. The beams are recombined to produce an interference pattern that contains all of the
wavelengths of interest. The interference pattern is mathematically converted into spectral data using Fourier transform. FT
interferometer optics provide complete spectra with very high wavelength resolution. FT signal averaging also provides higher
signal-to-noise ratios in general than can be achieved with other types of instruments.
6.4 Sampling System—Depending upon the applications, several different sampling systems can be used in the laboratory or for
on-line instruments, or both.
6.4.1 Cuvette—Quartz or glass cuvettes with fixed or adjustable pathlengths can be used in the laboratory.
6.4.2 Flow-Through Cell—This type of cell can be used for continuous or intermittent monitoring of liquid sample.
6.4.3 Probes:
6.4.3.1 Transmission Probe—Transmission probes combined with optic fibers are ideal for analyzing clear liquids, slurries,
suspensions, and other high viscosity samples. Low absorptivity in the NIR region permits sampling pathlengths of up to 10 cm.
6.4.3.2 Immersion Probe—The immersion system uses a bi-directional optic fiber bundle and variable pathlength probe for
sample measurements. Radiation from the source is transmitted to the sample by the inner ring of fibers, and diffuse transmitted
radiation is collected by the outer ring of fibers for detection.
6.4.3.3 Attenuated Total Reflection (ATR) Probe—Attenuated total reflection occurs when an absorbing medium (the sample) is
in close contact with the surface of a crystal material of higher refractive index. At an optimized angle, the NIR beam reflects
internally along the crystal faces, penetrating a few microns into the sample surface, where selective absorption occurs. The
resulting spectrum is very close to the conventional transmission spectrum for the sample. There are many designs of ATR plates
and rods for specific applications. Single or multiple reflection units are available. ATR sampling accessories are available for the
laboratory and, in the form of fiber optic probes, can be used for on-line analysis. This is an advantage when handling viscous
liquids and highly absorbing materials.
6.5 Software—The ideal software has the following capabilities:
6.5.1 The capability to record all sample identification and spectral data accurately and to access the reference data,
6.5.2 The capability to record the date and time of day that all spectra and files were recorded or created,
6.5.3 The capability to move or copy spectra, or both, from file to file,
6.5.4 The capability to add or subtract spectral data, and to average spectra,
6.5.5 The capability to perform transformations of log l/R optical data into derivatives, or other forms of mathematical
treatment, and to reverse the transformation,
6.5.6 The capability to compute multiple linear regression (MLR), principal component regression (PCR), and partial least
squares regression (PLS),
6.5.7 The capability to store PCR or PLS loading, weights, scores or other desirable data, and to display these data for
subsequent examination and interpretation,
6.5.8 The capability to enable the operator to evaluate the calibration model by computing the standard error of validation
(SEV), coefficient of regression, and the root mean square deviation (RMSD), and to display various plots,
6.5.9 The capability to perform cross-validation automatically,
6.5.10 The capability to identify an outlier(s), and
6.5.11 The capability to develop and save regression equations and analyze a sample to calculate a hydroxyl number.
6.6 Software Packages—Most NIR instruments provide necessary software for collecting and modeling data. Several
non-instrumental companies also supply chemometric software packages that can be used to analyze NIR data.
´1
D6342 − 12 (2017)
7. Near-IR Spectral Measurements
7.1 NIR spectral measurements are based on Beer’s law, namely, the absorbance of a homogeneous sample containing an
absorbing substance is linearly proportional to the concentration of the absorbing species. The absorbance of a sample is defined
as the logarithm to the base ten of the reciprocal of the Transmittance (T):
A 5 log 1/T (1)
~ !
where:
T = the ratio of radiant power transmitted by the sample to the radiant power incident on the sample.
7.1.1 For most types of instrumentation, the radiant power incident on the sample cannot be measured directly. Instead, a
reference (background) measurement of the radiant power is made without the sample being present in the light beam.
7.1.2 A measurement is then conducted with the sample present, and the ratio, T, is calculated. The background measurement
can be conduct
...