ASTM E1944-98(2021)
(Practice)Standard Practice for Describing and Measuring Performance of Laboratory Fourier Transform Near-Infrared (FT-NIR) Spectrometers: Level Zero and Level One Tests
Standard Practice for Describing and Measuring Performance of Laboratory Fourier Transform Near-Infrared (FT-NIR) Spectrometers: Level Zero and Level One Tests
SIGNIFICANCE AND USE
4.1 This practice permits an analyst to compare the general performance of a laboratory instrument on any given day with the prior performance of that instrument. This practice is not intended for comparison of different instruments with each other, nor is it directly applicable to dedicated process FT-NIR analyzers. This practice requires the use of a check sample compatible with the instrument under test as described in 5.3.
SCOPE
1.1 This practice covers two levels of tests to measure the performance of laboratory Fourier transform near infrared (FT-NIR) spectrometers. This practice applies to the short-wave near infrared region, approximately 800 nm (12 500 cm–1) to 1100 nm (9090.91 cm–1); and the long-wavelength near infrared region, approximately 1100 nm (9090.91 cm–1) to 2500 nm (4000 cm–1). This practice is intended mainly for transmittance measurements of gases and liquids, although it is broadly applicable for reflectance measurements.
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 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
Relations
Standards Content (Sample)
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.
Designation: E1944 − 98 (Reapproved 2021)
Standard Practice for
Describing and Measuring Performance of Laboratory
Fourier Transform Near-Infrared (FT-NIR) Spectrometers:
Level Zero and Level One Tests
This standard is issued under the fixed designation E1944; 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 E932 Practice for Describing and Measuring Performance of
Dispersive Infrared Spectrometers
1.1 This practice covers two levels of tests to measure the
E1252 Practice for General Techniques for Obtaining Infra-
performance of laboratory Fourier transform near infrared
red Spectra for Qualitative Analysis
(FT-NIR) spectrometers. This practice applies to the short-
E1421 Practice for Describing and Measuring Performance
wave near infrared region, approximately 800 nm
–1 –1 of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
(12 500 cm ) to 1100 nm (9090.91 cm ); and the long-
eters: Level Zero and Level One Tests
wavelength near infrared region, approximately 1100 nm
–1 –1
(9090.91 cm ) to 2500 nm (4000 cm ). This practice is
3. Terminology
intended mainly for transmittance measurements of gases and
liquids, although it is broadly applicable for reflectance mea-
3.1 For definitions of terms used in this practice, refer to
surements.
Terminology E131. All identifications of spectral regions and
absorbance band positions are given in nanometers (nm), and
1.2 The values stated in SI units are to be regarded as
–1
wavenumbers (cm ); and spectral energy, transmittance,
standard. No other units of measurement are included in this
reflectance, and absorbance are signified by the letters E, T, R
standard.
and A respectively. A subscripted number signifies a spectral
1.3 This standard does not purport to address all of the
position in nanometers, with wavenumbers in parenthesis (for
safety concerns, if any, associated with its use. It is the
1940(5154.64)
example, A , denotes the absorbance at 1940 nm or
responsibility of the user of this standard to establish appro-
-1
5154.64 cm ).
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
4. Significance and Use
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
4.1 This practice permits an analyst to compare the general
ization established in the Decision on Principles for the
performance of a laboratory instrument on any given day with
Development of International Standards, Guides and Recom-
the prior performance of that instrument. This practice is not
mendations issued by the World Trade Organization Technical
intended for comparison of different instruments with each
Barriers to Trade (TBT) Committee.
other, nor is it directly applicable to dedicated process FT-NIR
analyzers. This practice requires the use of a check sample
2. Referenced Documents
compatible with the instrument under test as described in 5.3.
2.1 ASTM Standards:
E131 Terminology Relating to Molecular Spectroscopy 5. Test Conditions
E168 Practices for General Techniques of Infrared Quanti-
5.1 Operating Conditions—In obtaining spectrophotometric
tative Analysis
data for the check sample, the analyst must select the proper
instrumental operating conditions in order to realize satisfac-
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
tory instrument performance. Operating conditions for indi-
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
vidual instruments are best obtained from the manufacturer’s
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
instructional literature due to the variations with instrument
Current edition approved April 1, 2021. Published April 2021. Originally
design. It should be noted that many FT-NIR instruments are
approved in 1998. Last previous edition approved in 2013 as E1944 – 98 (2013).
DOI: 10.1520/E1944-98R21.
designedtoworkbestifleftinstandbymodewhentheyarenot
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
in use. A record should be kept to document the operating
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
conditions selected during a test so that they can be duplicated
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. for future tests. Note that spectrometers are to be tested only
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1944 − 98 (2021)
withintheirrespectiverecommendedmeasurementwavelength time, sample identification, number of scans, and operator’s
(wavenumber) ranges. name, should always be recorded.
5.2 Instrumental characteristics can influence these mea- 6.2 Philosophy—The philosophy of the tests is to use
previously stored test results as bases for comparison and the
surements in several ways. Vignetting of the beam (that is, the
aperture of the sample cell is smaller than the diameter of the visual display screen or plotter to overlay the current test
results with the reference results (known to be good). If the old
near infrared beam at the focus) reduces the transmittance
value measured in nonabsorbing regions, and on most instru- and new results agree, they are simply reported as no change.
Level Zero consists of three tests. Run the tests under the same
ments can change the apparent wavelength (or wavenumber)
–1
scale by a small amount, usually less than 0.01 nm (0.1 cm ). conditionsthatyouwouldnormallyusetorunasample(thatis,
sample temperature, purge time, warm-up time, beam splitter
Focus changes can also change transmittance values, so the
sample should be positioned in the same location in the sample type, detector configuration, etc.)
compartment for each measurement. The angle of acceptance
6.3 Variations in Operating Procedure for Different
(established by the f number) of the optics between the sample
Instruments—Most of the existing FT-NIR instruments should
and detector significantly affects apparent transmittance. Heat-
be able to use the tests in this procedure without modification.
ing of the sample by the beam or by the higher temperatures
However, a few instruments may not be able to perform the
which exist inside most spectrometers changes absorbances
tests exactly as they were written. In these cases, it should be
somewhat, and even changes band ratios and locations slightly.
possible to obtain the same final data using a slightly different
Allow the sample to come to thermal equilibrium prior to
procedure. The FT-NIR manufacturer should be consulted for
measurement.
appropriate alternative procedures.
5.3 The recommended check sample should meet the fol-
6.4 Sample—The check sample used for performance tests
lowing requirements: the check sample should be fully com-
is described in 5.3.The same sample should be used for all test
patible with the requirements for repeatable sample presenta-
comparisons (note serial number, or other identifying
tion to the measuring spectrophotometer. The check sample
information, of sample) as well as orientation of the sample
should consist of a single pure compound or precisely known
within the sample compartment during test measurements.
mixture of compounds which is spectroscopically stable over
6.5 Reference Spectra—Two spectra acquired and stored
months or years. The spectra obtained from such a check
during the last major instrument calibration are used as
sample should be known to indicate changes in the
references. These spectra will be identified as Reference 1 and
spectrophotometer, not the check sample itself. It is recom-
Reference 2.
mended that independent verification of the integrity of the
6.5.1 Reference 1 is a Fourier-transformed single-beam
check sample be used prior to test measurement. The check
energy spectrum of an empty beam (in this and all later usage,
sample should be measured under precisely the sample mea-
empty beam means that nothing is in the sample path except
surement conditions of temperature, humidity, and instrument
dry air or the purge gas normally present within the spectrom-
set up configuration. Suggested check samples may include,
eter sample compartment). For reflectance measurements this
but are not limited to the following: for gases, water vapor at
spectrum is a spectrum of a flat, pure reflectance standard
5.89 Torr and 1 atmosphere ina2mgas cell, or methane at
approximating 100 % R.
18 psig pressure in a 10 cm gas cell; for liquids, pure
6.5.2 Reference 2 is a transmittance spectrum of the check
spectroscopic grade hydrocarbon compounds (for example,
sample. For reflectance measurements this spectrum is a
toluene, decane, isooctane, etc.), or precise mixtures of these
reflectance spectrum of the check sample.
pure compounds; for reflectance measurements of solids, rare
6.6 Repeatability of Procedures—Care should be taken that
earth oxides mixed with white halon powder, or Spectralon -
each of the spectral measurements is made in a consistent and
based rare earth oxide reflectance standards. Reference reflec-
repeatable manner, including sample orientation (although,
tance standards yielding a featureless, near 100 % reflectance
different spectral measurements do not necessarily use the
spectrum are pure powdered sulfur, halon, or Spectralon.
identicalprocedure).Inparticular,forthoseinstrumentshaving
6. Level Zero Tests more than one sample beam or path in the main sample
compartment, all of the test spectra always should be measured
6.1 Nature of Tests—Routine checks of instrument perfor-
using the same optical path.
mance can be performed within a few minutes. They are
6.7 Measurements—Three test spectra will be acquired and
designed to uncover malfunctions or other changes in instru-
stored.The test spectra will be identified hereafter as Spectrum
ment operation but not to specifically diagnose or quantita-
1, Spectrum 2, and Spectrum 3.
tively assess any malfunction. For Level Zero tests, a resolu-
–1
6.7.1 Spectrum 1—An empty-beam spectrum stored as a
tion of 4 cm and a nominal measurement time of 30 s is
Fourier-transformed single beam energy spectrum (or as an
recommended. Resolution and measurement times can be
interferogram). If stored as an interferogram, it must be
specified to match conditions used for routine measurement
transformed before use in the ensuing tests.
applications.The exact measurement time, along with the date,
6.7.2 Spectrum 2—An empty-beam spectrum taken imme-
diately after Spectrum 1. This spectrum should be stored as
either a Fourier-transformed single-beam energy spectrum or
Spectralon, available from Labsphere, Inc., P.O. Box 70, Shaker St., North
Sutton, NH 03260-0070, has been found satisfactory for this purpose. as a transmittance spectrum ratioed against Spectrum 1.
E1944 − 98 (2021)
6.7.3 Spectrum3—Aspectrumofthechecksampleobtained 7.2 One Hundred Percent Line Test—Ratio Spectrum 2 to
reasonably soon after Spectrum 2. This spectrum should be Spectrum 1. Note the noise level and any variations from
stored as a transmittance spectrum (or reflectance spectrum, 100 % transmittance (or reflectance) across the spectrum.
when applicable) ratioed against either Spectrum 1 or Spec-
7.2.1 Reportage—Make an overlay plot of Spectra 1 and 2.
trum 2, or as a single-beam energy spectrum. To reproducibly
Then ratio the two and plot the 100 % transmittance (or
insert the sample, the serial number (or other identifying
reflectance) line. The ordinate range should be 99 to 101 %
information) should be right side up facing the instrument
T/R. If the noise or baseline drift exceeds these bounds, make
detector (or aligned in a manner that allows repeatable mea-
a plot from 90 to 110 % T/R and consider performing Level
surements each time the check sample is measured).
One tests. Report the RMS (preferred) or peak-to-peak noise
–1
levelsatovera ;8-18nm(100cm )rangecenteredat800nm
–1 –1
7. Level Zero Test Procedures
(12 500 cm ), 1000 nm (10 000 cm ), 1500 nm (6666.67
–1 –1 –1
cm ), 2000 nm (5000 cm ), 2500 nm (4000 cm ). If the
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Refer-
instrument wavelength (wavenumber) range does not include
ence 1. Note any changes in energy level across the spectrum.
some of these, substitute the nearest measurable wavelength
Ratio Spectrum 1 to Reference 1.Video display resolution may
(frequency).
limit the accuracy to which this test can be interpreted if the
7.2.2 Interpretation—Excessive noise may result from mis-
comparisonismadeon-screen.Inaddition,iftheinterferogram
alignment or source malfunction (refer to the energy spectrum
was saved, it may be displayed or plotted and the center burst
test) or from a malfunction in the detector or the electronics.
height recorded. Changes in the interferogram height are
Repetitive noise patterns (for example, spikes or sinusoids)
difficult to interpret since minor decreases in source tempera-
sometimes indicate digital problems. Isolated noise spikes may
ture that only affect high frequencies can result in changes in
be digital malfunctions or they can indicate electromagnetic
interferogram height. These changes do not affect photometric
interference. Positive or negative bands often indicate a rapid
accuracy.
change in purge quality. Simultaneously positive and negative
7.1.1 Reportage—Report by making an overlay plot of
sharp bands in the water region may indicate instrumental
Spectrum 1 energy ratioed against Reference 1 energy over the
problems or excessive water vapor within the spectrometer.
range of 95 to 105 % T, and by reporting the following energy
Deviations from the 100 % level (usually at lower wavelengths
ratios:
(higher wavenumbers) indicate interferometer, detector, or
For short 2 wave near infrared: (1)
source instability (see Practice E1421).
800/1000 12 500/10 000 800/1000 12 500/10 000
~ ! ~ !
RATIO 5 E
7.3 CheckSampleTest—RatioSpectrum3toSpectrum2(or
1) to produce a check sample transmittance spectrum (or
For long 2 wave near infrared:
reflectance spectrum, when applicable). Convert all spectra to
absorbance spectra. Subtract the stored absorbance check
1500/2000~6666.67/5000! 1500/2000~6666.67/5000!
RATIO 5 E
sample spectrum from this new absorbance check sample
2000/2500~5000/4000! 2000/2500~50
...
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