ASTM E1944-98(2002)
(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 the 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)
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Designation:E1944–98 (Reapproved2002)
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 E 1944; 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 (e) indicates an editorial change since the last revision or reapproval.
-1
1. Scope wavenumbers (cm ); and spectral energy, transmittance, re-
flectance, and absorbance are signified by the letters E, T, R
1.1 This practice covers two levels of tests to measure the
and A respectively. A subscripted number signifies a spectral
performance of laboratory Fourier transform near infrared
position in nanometers, with wavenumbers in parenthesis (for
(FT-NIR) spectrometers. This practice applies to the short-
example,
wave near infrared region, approximately 800 nm (12 500
1940(5154.64)
-1 -1
A , denotes the absorbance at 1940 nm or 5154.64
cm ) to 1100 nm (9090.91 cm ); and the long-wavelength
-1
-1
cm ).
near infrared region, approximately 1100 nm (9090.91 cm )to
-1
2500 nm (4000 cm ). This practice is intended mainly for
4. Significance and Use
transmittance measurements of gases and liquids, although it is
4.1 This practice permits an analyst to compare the general
broadly applicable for reflectance measurements.
performance of a laboratory instrument on any given day with
1.2 The values stated in SI units are to be regarded as the
the prior performance of that instrument. This practice is not
standard.
intended for comparison of different instruments with each
1.3 This standard does not purport to address all of the
other, nor is it directly applicable to dedicated process FT-NIR
safety concerns, if any, associated with its use. It is the
analyzers. This practice requires the use of a check sample
responsibility of the user of this standard to establish appro-
compatible with the instrument under test as described in 5.3.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
5. Test Conditions
5.1 OperatingConditions—Inobtainingspectrophotometric
2. Referenced Documents
data for the check sample, the analyst must select the proper
2.1 ASTM Standards:
2 instrumental operating conditions in order to realize satisfac-
E 131 Terminology Relating to Molecular Spectroscopy
tory instrument performance. Operating conditions for indi-
E 168 Practices for General Techniques of Infrared Quanti-
2 vidual instruments are best obtained from the manufacturer’s
tative Analysis
instructional literature due to the variations with instrument
E 932 Practices for Describing and Measuring Performance
2 design. It should be noted that many FT-NIR instruments are
of Dispersive Infrared Spectrometers
designedtoworkbestifleftinstandbymodewhentheyarenot
E 1252 Practice for General Techniques for Qualitative
2 in use. A record should be kept to document the operating
Analysis
conditions selected during a test so that they can be duplicated
E 1421 Practice for Describing and Measuring Performance
for future tests. Note that spectrometers are to be tested only
of Fourier Transform Infrared (FT-IR) Spectrometers:
withintheirrespectiverecommendedmeasurementwavelength
Level Zero and Level One Tests
(wavenumber) ranges.
3. Terminology 5.2 Instrumental characteristics can influence these mea-
surements in several ways. Vignetting of the beam (that is, the
3.1 For definitions of terms used in this practice, refer to
aperture of the sample cell is smaller than the diameter of the
Terminology E 131. All identifications of spectral regions and
near infrared beam at the focus) reduces the transmittance
absorbance band positions are given in nanometers (nm), and
value measured in nonabsorbing regions, and on most instru-
ments can change the apparent wavelength (or wavenumber)
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This practice is under the jurisdiction ofASTM Committee E-13 on Molecular
scale by a small amount, usually less than 0.01 nm (0.1 cm ).
Spectroscopy and is the direct responsibility of Subcommittee E 13.03 on Infrared
Focus changes can also change transmittance values, so the
Spectroscopy.
sample should be positioned in the same location in the sample
Current edition approved March 10, 1998. Published August 1998.
compartment for each measurement. The angle of acceptance
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.
E1944–98 (2002)
(established by the f number) of the optics between the sample be able to use the tests in this procedure without modification.
and detector significantly affects apparent transmittance. Heat- However, a few instruments may not be able to perform the
ing of the sample by the beam or by the higher temperatures tests exactly as they were written. In these cases, it should be
which exist inside most spectrometers changes absorbances possible to obtain the same final data using a slightly different
somewhat, and even changes band ratios and locations slightly. procedure. The FT-NIR manufacturer should be consulted for
Allow the sample to come to thermal equilibrium prior to appropriate alternative procedures.
measurement. 6.4 Sample—The check sample used for performance tests
5.3 The recommended check sample should meet the fol- is described in 5.3.The same sample should be used for all test
lowing requirements: the check sample should be fully com- comparisons (note serial number, or other identifying informa-
patible with the requirements for repeatable sample presenta- tion, of sample) as well as orientation of the sample within the
tion to the measuring spectrophotometer. The check sample sample compartment during test measurements.
should consist of a single pure compound or precisely known 6.5 Reference Spectra—Two spectra acquired and stored
mixture of compounds which is spectroscopically stable over during the last major instrument calibration are used as
months or years. The spectra obtained from such a check references. These spectra will be identified as Reference 1 and
sample should be known to indicate changes in the spectro- Reference 2.
photometer,notthechecksampleitself.Itisrecommendedthat 6.5.1 Reference 1 is a Fourier-transformed single-beam
independent verification of the integrity of the check sample be energy spectrum of an empty beam. (in this and all later usage,
used prior to test measurement. The check sample should be empty beam means that nothing is in the sample path except
measured under precisely the sample measurement conditions dry air or the purge gas normally present within the spectrom-
of temperature, humidity, and instrument set up configuration. eter sample compartment). For reflectance measurements this
Suggested check samples may include, but are not limited to spectrum is a spectrum of a flat, pure reflectance standard
the following: for gases, water vapor at 5.89 Torr and 1 approximating 100 % R.
atmosphere ina2mgas cell, or methane at 18 psig pressure in 6.5.2 Reference 2 is a transmittance spectrum of the check
a 10 cm gas cell; for liquids, pure spectroscopic grade sample. For reflectance measurements this spectrum is a
hydrocarbon compounds (for example, toluene, decane, isooc- reflectance spectrum of the check sample.
tane, etc.), or precise mixtures of these pure compounds; for 6.6 Repeatability of Procedures—Care should be taken that
reflectance measurements of solids, rare earth oxides mixed each of the spectral measurements is made in a consistent and
withwhitehalonpowder,orSpectralon -basedrareearthoxide repeatable manner, including sample orientation (although,
reflectance standards. Reference reflectance standards yielding different spectral measurements do not necessarily use the
a featureless, near 100 % reflectance spectrum are pure identicalprocedure).Inparticular,forthoseinstrumentshaving
powdered sulfur, halon, or Spectralon. more than one sample beam or path in the main sample
compartment, all of the test spectra always should be measured
6. Level Zero Tests
using the same optical path.
6.7 Measurements—Three test spectra will be acquired and
6.1 Nature of Tests—Routine checks of instrument perfor-
stored.The test spectra will be identified hereafter as Spectrum
mance can be performed within a few minutes. They are
1, Spectrum 2, and Spectrum 3.
designed to uncover malfunctions or other changes in instru-
6.7.1 Spectrum 1—An empty-beam spectrum stored as a
ment operation but not to specifically diagnose or quantita-
tively assess any malfunction. For Level Zero tests, a resolu- Fourier-transformed single beam energy spectrum (or as an
-1
interferogram). If stored as an interferogram, it must be
tion of 4 cm and a nominal measurement time of 30 s is
recommended. Resolution and measurement times can be transformed before use in the ensuing tests.
6.7.2 Spectrum 2—An empty-beam spectrum taken imme-
specified to match conditions used for routine measurement
applications.The exact measurement time, along with the date, diately after Spectrum 1. This spectrum should be stored as
either a Fourier-transformed single-beam energy spectrum or
time, sample identification, number of scans, and operator’s
name, should always be recorded. as a transmittance spectrum ratioed against Spectrum 1.
6.7.3 Spectrum 3—A spectrum of the check sample ob-
6.2 Philosophy—The philosophy of the tests is to use
previously stored test results as bases for comparison and the tained reasonably soon after Spectrum 2.This spectrum should
be stored as a transmittance spectrum (or reflectance spectrum,
visual display screen or plotter to overlay the current test
results with the reference results (known to be good). If the old when applicable) ratioed against either Spectrum 1 or Spec-
trum 2, or as a single-beam energy spectrum. To reproducibly
and new results agree, they are simply reported as no change.
Level Zero consists of three tests. Run the tests under the same insert the sample, the serial number (or other identifying
information) should be right side up facing the instrument
conditionsthatyouwouldnormallyusetorunasample(thatis,
sample temperature, purge time, warm-up time, beam splitter detector (or aligned in a manner that allows repeatable mea-
surements each time the check sample is measured).
type, detector configuration, etc.)
6.3 Variations in Operating Procedure for Different
7. Level Zero Test Procedures
Instruments—Most of the existing FT-NIR instruments should
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Refer-
ence 1. Note any changes in energy level across the spectrum.
Ratio Spectrum 1 to Reference 1.Video display resolution may
Spectralon, available from Labsphere, Inc., P.O. Box 70, Shaker St., North
Sutton, NH 03260-0070, has been found satisfactory for this purpose. limit the accuracy to which this test can be interpreted if the
E1944–98 (2002)
comparisonismadeon-screen.Inaddition,iftheinterferogram Repetitive noise patterns (for example, spikes or sinusoids)
was saved, it may be displayed or plotted and the center burst sometimes indicate digital problems. Isolated noise spikes may
height recorded. Changes in the interferogram height are be digital malfunctions or they can indicate electromagnetic
difficult to interpret since minor decreases in source tempera- interference. Positive or negative bands often indicate a rapid
ture that only affect high frequencies can result in changes in change in purge quality. Simultaneously positive and negative
interferogram height. These changes do not affect photometric sharp bands in the water region may indicate instrumental
accuracy. problems or excessive water vapor within the spectrometer.
Deviations from the 100 % level (usually at lower wavelengths
7.1.1 Reportage—Report by making an overlay plot of
(higher wavenumbers) indicate interferometer, detector, or
Spectrum 1 energy ratioed against Reference 1 energy over the
source instability (see Practice E 1421).
range of 95 to 105 % T, and by reporting the following energy
ratios: 7.3 Check Sample Test—Ratio Spectrum 3 to Spectrum 2
(or 1) to produce a check sample transmittance spectrum (or
For short2wave near infrared:
800/1000~12 500/10 000! 800/1000~12 500/10 000!
reflectance spectrum, when applicable). Convert all spectra to
RATIO 5 E (1)
absorbance spectra. Subtract the stored absorbance check
For long2wave near infrared:
sample spectrum from this new absorbance check sample
1500/2000~6666.67/5000! 1500/2000~6666.67/5000!
RATIO 5 E
spectrum. Note any changes.
2000/2500~5000/4000! 2000/2500~5000/4000!
RATIO 5 E
7.3.1 Reportage—Plot the check sample absorbance spec-
Report the date and time of both spectra used, and the actual
trum over the reported dynamic range of the instrument. Plot
numbers of scans and measurement times, as well as details of
the subtraction result as a full scale spectrum.
the instrument set up conditions.
7.3.2 Interpretation—Additional sharp features in the water
7.1.2 Interpretation—An overall drop in the energy level in
vapor absorption regions indicate excessive water vapor in the
which the largest percentage of change occurs at higher
sample compartment. Instrumental problems may include Jac-
wavenumbers usually indicates interferometer misalignment or
quinot vignetting, source optics or laser misalignment, or
a reduction in source temperature. An overall drop in the
interferometer scan problems. In the subtraction spectrum,
energy level without wavelength (wavenumber) dependence
first-derivative-like bandshapes that correspond to absorption
suggests beam obstruction (vignetting) or misalignment of
band positions indicate these instrumental problems. Artifacts
non-interferometer optical components. The appearance of
appearing only at the positions of the strongest (completely
bands or other features indicates purge gas contributions, beam
absorbing) bands may indicate phasing or other problems
obstruction by a partially transmitting object, oil or smoke
associated with detector non-linearity. Artifacts at both me-
depositiononmirrorsorwindows,oraforgottensamplewithin
dium and strong band positions indicate analog electronic,
the beam. With cooled detectors (for example InSb), the
ADC, or computer problems, or sampling jitter, (Zachor-
-1
appearance of a broad band around 1940 nm (5154.64 cm )
Aaronsen distortion).
indicates ice deposition on the detector surface. Non-zero
energy levels below the detector cut-off (more than 0.2 % of
8. Level One Tests
the maximum energy-leve
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