ASTM E1944-98(2007)
(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
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.
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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Designation: E1944 − 98 (Reapproved2007)
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 3. Terminology
1.1 This practice covers two levels of tests to measure the 3.1 For definitions of terms used in this practice, refer to
performance of laboratory Fourier transform near infrared Terminology E131. All identifications of spectral regions and
(FT-NIR) spectrometers. This practice applies to the short- absorbance band positions are given in nanometers (nm), and
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wave near infrared region, approximately 800 nm (12 500 cm wavenumbers (cm ); and spectral energy, transmittance,
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) to 1100 nm (9090.91 cm ); and the long-wavelength near reflectance, and absorbance are signified by the letters E, T, R
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infrared region, approximately 1100 nm (9090.91 cm )to and A respectively. A subscripted number signifies a spectral
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2500 nm (4000 cm ). This practice is intended mainly for position in nanometers, with wavenumbers in parenthesis (for
1940(5154.64)
transmittance measurements of gases and liquids, although it is example, A , denotes the absorbance at 1940 nm or
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broadly applicable for reflectance measurements. 5154.64 cm ).
1.2 The values stated in SI units are to be regarded as
4. Significance and Use
standard.
4.1 This practice permits an analyst to compare the general
1.3 This standard does not purport to address all of the
performance of a laboratory instrument on any given day with
safety concerns, if any, associated with its use. It is the
the prior performance of that instrument. This practice is not
responsibility of the user of this standard to establish appro-
intended for comparison of different instruments with each
priate safety and health practices and determine the applica-
other, nor is it directly applicable to dedicated process FT-NIR
bility of regulatory limitations prior to use.
analyzers. This practice requires the use of a check sample
compatible with the instrument under test as described in 5.3.
2. Referenced Documents
5. Test Conditions
2.1 ASTM Standards:
5.1 Operating Conditions—In obtaining spectrophotometric
E131 Terminology Relating to Molecular Spectroscopy
data for the check sample, the analyst must select the proper
E168 Practices for General Techniques of Infrared Quanti-
instrumental operating conditions in order to realize satisfac-
tative Analysis
tory instrument performance. Operating conditions for indi-
E932 Practice for Describing and Measuring Performance of
vidual instruments are best obtained from the manufacturer’s
Dispersive Infrared Spectrometers
instructional literature due to the variations with instrument
E1252 Practice for General Techniques for Obtaining Infra-
design. It should be noted that many FT-NIR instruments are
red Spectra for Qualitative Analysis
designedtoworkbestifleftinstandbymodewhentheyarenot
E1421 Practice for Describing and Measuring Performance
in use. A record should be kept to document the operating
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
eters: Level Zero and Level One Tests conditions selected during a test so that they can be duplicated
for future tests. Note that spectrometers are to be tested only
withintheirrespectiverecommendedmeasurementwavelength
(wavenumber) ranges.
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
5.2 Instrumental characteristics can influence these mea-
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
surements in several ways. Vignetting of the beam (that is, the
Current edition approved Dec. 1, 2007. Published December 2007. Originally
aperture of the sample cell is smaller than the diameter of the
approved in 1998. Last previous edition approved in 2002 as E1944 - 98 (2002).
DOI: 10.1520/E1944-98R07.
near infrared beam at the focus) reduces the transmittance
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
value measured in nonabsorbing regions, and on most instru-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ments can change the apparent wavelength (or wavenumber)
Standards volume information, refer to the standard’s Document Summary page on
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the ASTM website. scale by a small amount, usually less than 0.01 nm (0.1 cm ).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1944 − 98 (2007)
Focus changes can also change transmittance values, so the sample temperature, purge time, warm-up time, beam splitter
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 18
approximating 100 % R.
psig pressure in a 10 cm gas cell; for liquids, pure spectro-
6.5.2 Reference 2 is a transmittance spectrum of the check
scopic grade hydrocarbon compounds (for example, toluene,
sample. For reflectance measurements this spectrum is a
decane, isooctane, etc.), or precise mixtures of these pure
reflectance spectrum of the check sample.
compounds; for reflectance measurements of solids, rare earth
oxides mixed with white halon powder, or Spectralon -based
6.6 Repeatability of Procedures—Care should be taken that
rare earth oxide reflectance standards. Reference reflectance
each of the spectral measurements is made in a consistent and
standards yielding a featureless, near 100 % reflectance spec-
repeatable manner, including sample orientation (although,
trum are pure powdered sulfur, halon, or Spectralon.
different spectral measurements do not necessarily use the
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
designed to uncover malfunctions or other changes in instru-
6.7 Measurements—Three test spectra will be acquired and
ment operation but not to specifically diagnose or quantita-
stored.The test spectra will be identified hereafter as Spectrum
tively assess any malfunction. For Level Zero tests, a resolu-
1, Spectrum 2, and Spectrum 3.
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tion of 4 cm and a nominal measurement time of 30 s is
6.7.1 Spectrum 1—An empty-beam spectrum stored as a
recommended. Resolution and measurement times can be
Fourier-transformed single beam energy spectrum (or as an
specified to match conditions used for routine measurement
interferogram). If stored as an interferogram, it must be
applications.The exact measurement time, along with the date,
transformed before use in the ensuing tests.
time, sample identification, number of scans, and operator’s
6.7.2 Spectrum 2—An empty-beam spectrum taken imme-
name, should always be recorded.
diately after Spectrum 1. This spectrum should be stored as
either a Fourier-transformed single-beam energy spectrum or
6.2 Philosophy—The philosophy of the tests is to use
as a transmittance spectrum ratioed against Spectrum 1.
previously stored test results as bases for comparison and the
6.7.3 Spectrum3—Aspectrumofthechecksampleobtained
visual display screen or plotter to overlay the current test
reasonably soon after Spectrum 2. This spectrum should be
results with the reference results (known to be good). If the old
stored as a transmittance spectrum (or reflectance spectrum,
and new results agree, they are simply reported as no change.
when applicable) ratioed against either Spectrum 1 or Spec-
Level Zero consists of three tests. Run the tests under the same
trum 2, or as a single-beam energy spectrum. To reproducibly
conditionsthatyouwouldnormallyusetorunasample(thatis,
insert the sample, the serial number (or other identifying
information) should be right side up facing the instrument
detector (or aligned in a manner that allows repeatable mea-
Spectralon, available from Labsphere, Inc., P.O. Box 70, Shaker St., North
Sutton, NH 03260-0070, has been found satisfactory for this purpose. surements each time the check sample is measured).
E1944 − 98 (2007)
7. Level Zero Test Procedures One tests. Report the RMS (preferred) or peak-to-peak noise
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levelsatovera ;8-18nm(100cm )rangecenteredat800nm
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Refer-
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(12 500 cm ), 1000 nm (10 000 cm ), 1500 nm (6666.67
ence 1. Note any changes in energy level across the spectrum.
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cm ), 2000 nm (5000 cm ), 2500 nm (4000 cm ). If the
Ratio Spectrum 1 to Reference 1.Video display resolution may
instrument wavelength (wavenumber) range does not include
limit the accuracy to which this test can be interpreted if the
some of these, substitute the nearest measurable wavelength
comparisonismadeon-screen.Inaddition,iftheinterferogram
(frequency).
was saved, it may be displayed or plotted and the center burst
7.2.2 Interpretation—Excessive noise may result from mis-
height recorded. Changes in the interferogram height are
alignment or source malfunction (refer to the energy spectrum
difficult to interpret since minor decreases in source tempera-
test) or from a malfunction in the detector or the electronics.
ture that only affect high frequencies can result in changes in
Repetitive noise patterns (for example, spikes or sinusoids)
interferogram height. These changes do not affect photometric
sometimes indicate digital problems. Isolated noise spikes may
accuracy.
be digital malfunctions or they can indicate electromagnetic
7.1.1 Reportage—Report by making an overlay plot of
interference. Positive or negative bands often indicate a rapid
Spectrum 1 energy ratioed against Reference 1 energy over the
change in purge quality. Simultaneously positive and negative
range of 95 to 105 % T, and by reporting the following energy
sharp bands in the water region may indicate instrumental
ratios:
problems or excessive water vapor within the spectrometer.
For short 2 wave near infrared: (1)
Deviations from the 100 % level (usually at lower wavelengths
(higher wavenumbers) indicate interferometer, detector, or
800/1000~12 500/10 000! 800/1000~12 500/10 000!
RATIO 5 E
source instability (see Practice E1421).
For long 2 wave near infrared:
7.3 CheckSampleTest—RatioSpectrum3toSpectrum2(or
1) to produce a check sample transmittance spectrum (or
1500/2000 6666.67/5000 1500/2000 6666.67/5000
~ ! ~ !
RATIO 5 E
reflectance spectrum, when applicable). Convert all spectra to
2000/2500 5000/4000 2000/2500 5000/4000 absorbance spectra. Subtract the stored absorbance check
~ ! ~ !
RATIO 5 E
sample spectrum from this new absorbance check sample
Report the date and time of both spectra used, and the actual
spectrum. Note any changes.
numbers of scans and measurement times, as well as details of
7.3.1 Reportage—Plot the check sample absorbance spec-
the instrument set up conditions.
trum over the reported dynamic range of the instrument. Plot
7.1.2 Interpretation—An overall drop in the energy level in
the subtraction result as a full scale spectrum.
which the largest percentage of change occurs at higher
7.3.2 Interpretation—Additional sharp features in the water
wavenumbers usually indicates interferometer misalignment or
vapor absorption regions indicate excessive water vapor in the
a reduction in source temperature. An overall drop in the
sample compartment. Instrumental problems may include Jac-
energy level without wavelength (wavenumber) dependence
quinot vignetting, source optics or laser misalignment, or
suggests beam obstruction (vignetting) or misalignment of
interferometer scan problems. In the subtraction spectrum,
...
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