ASTM E1790-04(2010)
(Practice)Standard Practice for Near Infrared Qualitative Analysis
Standard Practice for Near Infrared Qualitative Analysis
SIGNIFICANCE AND USE
NIR spectroscopy is a widely used technique for quantitative analysis, and it is also becoming more widely used for the identification of organic materials, that is, qualitative analysis. In general, however, the concept of qualitative analysis as used in the NIR spectral region differs from that used in the mid-IR spectral region in that NIR qualitative analysis refers to the process of automated comparison of the spectra of unknown materials to the spectra of known materials in order to identify the unknown. This approach constitutes a library search method in which each user generates his own library.
Historically, NIR spectroscopy as practiced with classical UV-VIS-NIR instruments using methods similar to those described in Practice E1252 was not considered to be a strong technique for qualitative analysis. Although the positions and intensities of absorption bands in specific wavelength ranges were used to confirm the presence of certain functional groups, the spectra were not considered to be specific enough to allow unequivocal identification of unknown materials. A few important libraries of NIR spectra were developed for qualitative purposes, but the lack of suitable data handling facilities limited the scope of qualitative analysis severely. Furthermore, earlier work was limited almost entirely to liquid samples.
Currently, the mid-IR procedure of deducing the structure of an unknown material by method of analysis of the locations, strengths, and positional shifts of individual absorption bands is generally not used in the NIR.
With the development of specialized NIR instruments and mathematical algorithms for treating the data, it became possible to obtain a wealth of information from NIR spectra that had hitherto gone unused. While the mathematical algorithms described in this practice can be applied to spectral data in any region, this practice describes their application to the NIR.
The application of NIR spectroscopy to qualitative analysis ...
SCOPE
1.1 This practice covers the use of near-infrared (NIR) spectroscopy for the qualitative analysis of liquids and solids. The practice is written under the assumption that most NIR qualitative analyses will be performed with instruments designed specifically for this region and equipped with computerized data handling algorithms. In principle, however, the practice also applies to work with liquid samples using instruments designed for operation over the ultraviolet (UV), visible, and mid-infrared (IR) regions if suitable data handling capabilities are available. Many Fourier Transform Infrared (FTIR) (normally considered mid-IR instruments) have NIR capability, or at least extended-range beamsplitters that allow operation to 1.2 μm; this practice also applies to data from these instruments.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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.
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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
Designation: E1790 − 04 (Reapproved2010)
Standard Practice for
Near Infrared Qualitative Analysis
This standard is issued under the fixed designation E1790; 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.
1. Scope 3. Terminology
3.1 Definitions—For definitions of general terms and sym-
1.1 This practice covers the use of near-infrared (NIR)
bols pertaining to NIR spectroscopy and statistical
spectroscopy for the qualitative analysis of liquids and solids.
computations, refer to Terminology E131.
The practice is written under the assumption that most NIR
3.2 Definitions of Terms Specific to This Standard:
qualitative analyses will be performed with instruments de-
3.2.1 interactance, n—the phenomenon whereby radiant
signed specifically for this region and equipped with comput-
energy entering the surface of a material is scattered by the
erized data handling algorithms. In principle, however, the
material back to the surface, but at a different portion of the
practice also applies to work with liquid samples using
surface.
instruments designed for operation over the ultraviolet (UV),
3.2.1.1 Discussion—This differs from diffuse reflectance,
visible, and mid-infrared (IR) regions if suitable data handling
where the returning radiation exits the same portion of the
capabilities are available. Many Fourier Transform Infrared
surface of the material as the illuminating radiation entered.
(FTIR) (normally considered mid-IR instruments) have NIR
3.2.2 training sample (otherwise called a “reference
capability, or at least extended-range beamsplitters that allow
sample” or “standard”), n—a quantity of material of known
operation to 1.2 µm; this practice also applies to data from
composition or properties, or both, presented to an instrument
these instruments.
for measurement in order to find relationships between the
1.2 The values stated in SI units are to be regarded as
measurements and the composition or properties, or both, of
standard. No other units of measurement are included in this the sample.
standard.
3.2.2.1 Discussion—This term is typically used in conjunc-
tion with computerized methods for ascertaining the relation-
1.3 This standard does not purport to address all of the
ships.
safety concerns, if any, associated with its use. It is the
Training samples for quantitative analysis (also called
responsibility of the user of this standard to establish appro-
“calibration samples,” as in Practices E1655) have different
priate safety and health practices and determine the applica-
requirements than training samples used for qualitative
bility of regulatory limitations prior to use.
analysis.
2. Referenced Documents
4. Significance and Use
2.1 ASTM Standards:
4.1 NIR spectroscopy is a widely used technique for quan-
E131 Terminology Relating to Molecular Spectroscopy
titative analysis, and it is also becoming more widely used for
E1252 Practice for General Techniques for Obtaining Infra-
the identification of organic materials, that is, qualitative
red Spectra for Qualitative Analysis
analysis. In general, however, the concept of qualitative analy-
E1655 Practices for Infrared Multivariate Quantitative
sis as used in the NIR spectral region differs from that used in
Analysis
the mid-IR spectral region in that NIR qualitative analysis
refers to the process of automated comparison of the spectra of
unknown materials to the spectra of known materials in order
to identify the unknown. This approach constitutes a library
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
search method in which each user generates his own library.
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.11 on Multivariate Analysis.
4.2 Historically, NIR spectroscopy as practiced with classi-
Current edition approved March 1, 2010. Published April 2010. Originally
cal UV-VIS-NIR instruments using methods similar to those
approved in 1996. Last previous edition approved in 2004 as E1790 – 04. DOI:
10.1520/E1790-04R10.
described in Practice E1252 was not considered to be a strong
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
technique for qualitative analysis. Although the positions and
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
intensities of absorption bands in specific wavelength ranges
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. were used to confirm the presence of certain functional groups,
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1790 − 04 (2010)
the spectra were not considered to be specific enough to allow achievingupto100-mpathlengthsmayberequired.Spectraof
unequivocal identification of unknown materials.Afew impor- vapors and gases may be sensitive to the total sample pressure,
tant libraries of NIR spectra were developed for qualitative and this has to be determined for each type of sample.
purposes, but the lack of suitable data handling facilities
5.1.2 Unknownsamplestobeidentifiedmaybeprescreened
limited the scope of qualitative analysis severely. Furthermore,
based on criteria other than their NIR spectra (for example,
earlier work was limited almost entirely to liquid samples.
visual inspection). The training samples (that is, the “knowns”
used to teach the algorithm what different materials look like)
4.3 Currently, the mid-IR procedure of deducing the struc-
may also be similarly prescreened and grouped into libraries of
ture of an unknown material by method of analysis of the
similar materials (for example, liquids and solids). The un-
locations, strengths, and positional shifts of individual absorp-
known is then compared with only those materials in the
tion bands is generally not used in the NIR.
appropriate library. The prescreening will help reduce the
4.4 With the development of specialized NIR instruments
chance of false identification, although care must be taken that
and mathematical algorithms for treating the data, it became
an unknown material not in the library is not identified as a
possible to obtain a wealth of information from NIR spectra
similar material that is in the library.
that had hitherto gone unused. While the mathematical algo-
5.1.3 Measurements may be made by method of
rithms described in this practice can be applied to spectral data
transmission, reflection, or any other optical setup suitable for
in any region, this practice describes their application to the
collecting NIR spectra. In practice, only transmission and
NIR.
diffuse reflection have been in common use.
4.5 The application of NIR spectroscopy to qualitative
5.1.4 Determination of the relationships between absor-
analysis in the manner described is relatively new, and proce-
bances at different wavelengths for a set of materials and
dures for this application are still evolving. The application of
consolidation of these relationships into a set of criteria for
chemometric methods to spectroscopy has limitations, and the
identifying those materials requires the use of computerized
limitations are not all defined yet since the techniques are
learning algorithms. These algorithms can also take into
relatively new. One area of concern to some scientists is the
account extraneous variations such as are found, for example,
effect of low-level contaminants. Any analytical methodology
when measurements are made on powdered solids.
has its detection limits, and NIR is no different in this regard,
5.1.5 Instrumentation is commercially available for making
but neither would we expect it to be any worse. Since the
suitable measurements in the NIR spectral region. Manufac-
relatively broad character of NIR bands makes it unlikely that
turer’s instructions should be followed to ensure correct
a contaminant would not overlap any of the measured
operation,optimumaccuracy,andsafetybeforecollectingdata.
wavelengths, the question would only be one of degree:
whether a given amount of contaminant could be detected.The
5.1.6 NIR spectroscopy has, as one of its paradigms, that
user must be aware of the probable contaminants he is liable to
little or no sample preparation be required. In conformance
run into and account for the possibility of this occurring,
with that paradigm, sample preparation steps in other spectro-
perhaps by including deliberately contaminated samples in the
scopic technologies are replaced with sample presentation
training set.
methodologies in NIR analysis. The most common sample
presentation methods are the following:
5. General
5.1.6.1 Diffuse Reflectance—Solid materials are ground into
5.1 NIR qualitative analysis is conducted by comparison of
powder (or used as-is, if already in suitably fine powder form)
NIR absorption spectra of unknown materials with those of
and packed into a cup, which allows the surface of the sample
knownreferencematerials.Sincetheabsorptionbandsofmany
to be illuminated and the reflected radiant power measured.
substances of interest are less distinctive in the NIR than in the
5.1.6.2 “Transflectance”—Clear or scattering liquids are
mid-IR spectral region, the analytical capability of the tech-
placed in a cup containing a transparent window with a
nique relies heavily on the accuracy of the absorption mea-
diffusely reflecting material behind the sample. Any radiant
surements and the relationship of the relative absorbances at
energy passing through the sample is reflected diffusely by the
different wavelengths. Materials to be identified are measured
backingmaterial,sothenetmeasurementisjustlikethediffuse
by a NIR spectrometer, and the spectral data thus generated are
reflectance measurement of powdered solids.
saved in an auxiliary computer attached to the spectrometer
5.1.6.3 Transmission—Liquids or solids are placed in cells
proper. One of the several algorithms described in Section 6 is
with two transparent windows and measured by transmission.
then applied to the data in order to generate classification
criteria, which can then be applied to data from unknown 5.1.6.4 Fiber Probes—Illuminating and collecting fibers are
samples in order to classify (or identify) them as being the brought in parallel to the sample. A variety of optical configu-
same as one of the previously seen materials. Good chemical rations are used to couple the radiant energy from the fibers to
laboratory practice should be followed to help ensure repro- the sample and back again, in an optical “head” of some sort.
ducible results for each material. The preparation and presen- Transmittance, reflectance, and interactance have all been used
tation of samples to the instrument should be consistent within at the sample end of the fiber to couple the radiation to the
a library, and unknowns should be treated the same way that sample.Interactancemeasurementsaresometimesmadebythe
the training samples were. simple expedient of pressing the end of a fiber bundle
5.1.1 The technique is applicable to liquids, solids, and containing mixed illuminating and receiving fibers against the
gases. For analysis of gases, multipath vapor cells capable of sample surface.
E1790 − 04 (2010)
5.2 To connect the mathematics with the spectroscopy used, 5.3.3 The unknowns must also be treated in the same
the procedure can be generally described as follows: manner as the training samples. It is particularly important that
(1) The spectral measurements define some multidimen- if the samples must be ground, the unknown samples should be
sional space. The axes in that space are the absorbances at the groundtothesameparticlesizeastheknownsamplesincluded
in the library.
various wavelengths, or some mathematical transformation
thereof.
6. Algorithms Used
(2) Groups of spectra for the same material define some
region in the multidimensional space.
6.1 This section describes some of the computerized algo-
(3) The analysis involves determining which region the
rithms that have been found effective for qualitative analysis in
unknown falls in.
the NIR spectral region. This section is mainly for reference.
5.2.1 Problems with this type of analysis include the fol- Descriptionsofmultivariatemethodsofstatisticaldataanalysis
lowing:insufficientseparationofthegroupsinthemultidimen- tend to be inherently abstract mathematically and resistant to
sional space to allow for classification (indicating insufficient reduction to words. A number of books exist in both the
differences among the spectra of the materials involved), statistical and chemometric literature that describe methods of
inadequate representation of measurement variability within multivariate analysis at varying levels of mathematical abstrac-
groups during training (indicating an insufficient number or tion (see, for example, Refs (1-5), a useful starting point but
variety of training samples), or poor detection limits for minor far from exhaustive list); most of the algorithms used for NIR
contaminants. qualitative analysis are relatively straightforward applications
of these methods.
5.2.2 To optimize the methods against these potential prob-
6.1.1 Implementations of these algorithms are available in
lem areas, generation of a method occurs in three stages. In the
standard generic statistical software packages. Software pro-
first, or training stage, known samples are presented to the
grams designed for analysis of spectroscopic data may also
instrument. The data collected are then presented to one of the
contain implementations of these algorithms. In addition, the
various algorithms and are thus used to “train” the algorithm to
manufacturers of modern NIR spectrometers include imple-
recognize the various different materials.
mentations of these algorithms in their proprietary software
5.2.3 In the second, or validation stage, the ability of the
packages that run on the auxiliary computers supplied with the
algorithm to correctly recognize materials not in the training
spectrometers; this approach has the advantage that the soft-
set of samples is tested. Samples measured during the valida-
ware matches the format and nature of the data generated by
tion stage should preferably be in the same phase and physical
the spectrometer. In either case, the details of the algorithms
condition as the known samples were during the training stage.
and their implementations are usually transparent to the user. It
5.2.4 In the third, or use stage, unknown samples are
is th
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