ASTM E1421-99(2015)e1
(Practice)Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
SIGNIFICANCE AND USE
4.1 This practice permits an analyst to compare the general performance of an instrument on any given day with the prior performance of an instrument. This practice is not necessarily meant for comparison of different instruments with each other even if the instruments are of the same type and model. This practice is not meant for comparison of the performance of one instrument operated under differing conditions.
SCOPE
1.1 This practice describes two levels of tests to measure the performance of laboratory Fourier transform mid-infrared (FT-MIR) spectrometers equipped with a standard sample holder used for transmission measurements.
1.2 This practice is not directly applicable to Fourier transform infrared (FT-IR) spectrometers equipped with various specialized sampling accessories such as flow cells or reflectance optics, nor to Fourier transform near-infrared (FT-NIR) spectrometers, nor to FT-IR spectrometers run in step scan mode.
1.2.1 If the specialized sampling accessory can be removed and replaced with a standard transmission sample holder, then this practice can be used. However, the user should recognize that the performance measured may not reflect that which is achieved when the specialized accessory is in use.
1.2.2 If the specialized sampling accessory cannot be removed, then it may be possible to employ a modified version of this practice to measure spectrometer performance. The user is referred to Guide E1866 for a discussion of how these tests may be modified.
1.2.3 Spectrometer performance tests for FT-NIR spectrometers are described in Practice E1944.
1.2.4 Performance tests for dispersive MIR instruments are described in Practice E932.
1.2.5 For FT-IR spectrometers run in a step scan mode, variations on this practice and information provided by the instrument vendor should be used.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3.1 Exception—Informational inch-pound units are provided in 5.4.
1.4 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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´1
Designation: E1421 − 99 (Reapproved 2015)
Standard Practice for
Describing and Measuring Performance of Fourier
Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero
and Level One Tests
This standard is issued under the fixed designation E1421; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Units statement was inserted in 1.3 editorially in July 2015.
1. Scope 1.3.1 Exception—Informational inch-pound units are pro-
vided in 5.4.
1.1 Thispracticedescribestwolevelsofteststomeasurethe
1.4 This standard does not purport to address all of the
performance of laboratory Fourier transform mid-infrared
safety concerns, if any, associated with its use. It is the
(FT-MIR) spectrometers equipped with a standard sample
responsibility of the user of this standard to establish appro-
holder used for transmission measurements.
priate safety and health practices and determine the applica-
1.2 This practice is not directly applicable to Fourier trans-
bility of regulatory limitations prior to use.
form infrared (FT-IR) spectrometers equipped with various
specialized sampling accessories such as flow cells or reflec-
2. Referenced Documents
tance optics, nor to Fourier transform near-infrared (FT-NIR)
2.1 ASTM Standards:
spectrometers, nor to FT-IR spectrometers run in step scan
E131Terminology Relating to Molecular Spectroscopy
mode.
E932PracticeforDescribingandMeasuringPerformanceof
1.2.1 If the specialized sampling accessory can be removed
Dispersive Infrared Spectrometers
and replaced with a standard transmission sample holder, then
E1866Guide for Establishing Spectrophotometer Perfor-
this practice can be used. However, the user should recognize
mance Tests
that the performance measured may not reflect that which is
E1944Practice for Describing and Measuring Performance
achieved when the specialized accessory is in use.
of Laboratory Fourier Transform Near-Infrared (FT-NIR)
1.2.2 If the specialized sampling accessory cannot be
Spectrometers: Level Zero and Level One Tests
removed,thenitmaybepossibletoemployamodifiedversion
ofthispracticetomeasurespectrometerperformance.Theuser
3. Terminology
is referred to Guide E1866 for a discussion of how these tests
3.1 Definitions—For definitions of terms used in this
may be modified.
practice, refer to Terminology E131. All identifications of
1.2.3 SpectrometerperformancetestsforFT-NIRspectrom-
spectral regions and absorption band positions are given in
eters are described in Practice E1944.
−1
wavenumbers (cm ), and spectral energy, transmittance, and
1.2.4 Performance tests for dispersive MIR instruments are
absorbance are signified in equations by the letters E, T, and A
described in Practice E932.
respectively. The ratio of two transmittance or absorbance
1.2.5 For FT-IR spectrometers run in a step scan mode,
values, and the ratio of energy levels at two different wave-
variations on this practice and information provided by the
numbers are signified by the letter R. A subscripted number
instrument vendor should be used.
signifies a spectral position in wavenumbers (for example,
−1
1.3 The values stated in SI units are to be regarded as
A , the absorbance at 3082 cm ).
standard. No other units of measurement are included in this
3.1.1 level one (1) test, n—a simple series of measurements
standard.
designed to provide quantitative data on various aspects of
instrument performance and information on which to base the
diagnosis of problems.
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.03 on Infrared and Near Infrared Spectroscopy. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedMay1,2015.PublishedJuly2015.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1991. Last previous edition approved in 2009 as E1421–99(2009). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E1421-99R15E01. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
E1421 − 99 (2015)
3.1.2 level zero (0) test, n—a routine check of instrument locations slightly. Allow the sample to come to thermal
performance, that can be done in a few minutes, designed to equilibrium before measurement.
visually detect significant changes in instrument performance
5.4 The recommended sample of matte-finish polystyrene
and provide a database to determine instrument function over
usedforthesetestsisapproximately38-µm(1.5-mil)thickfilm
time.
mounted on a card. The sample is mounted in a 2.5-cm (1-in.)
circularaperturecenteredwithinthe5-cm(2.5-in.)widthofthe
4. Significance and Use
card,andcentered3.8cm(1.5in.)fromthebottomofthecard.
4.1 This practice permits an analyst to compare the general
The card should be approximately 0.25-cm (0.1-in.) thick and
performance of an instrument on any given day with the prior
individually and unambiguously identified.Apolystyrene film
performance of an instrument. This practice is not necessarily
meeting these requirements is available from the National
meant for comparison of different instruments with each other
Institute of Standards and Technology (NIST) as SRM 1921.
even if the instruments are of the same type and model. This
NOTE 2—Very small beam diameters can defeat the interference fringe
practiceisnotmeantforcomparisonoftheperformanceofone
suppression provided by the matte finish on the sample.
instrument operated under differing conditions.
6. Level Zero Tests
5. Test Conditions
5.1 Operating Conditions—A record should be kept to 6.1 Nature of Tests—Routine checks of instrument
performance, these tests can be performed in a few minutes.
document the operating conditions selected so that they can be
duplicated. In obtaining spectrophotometric data, the analyst Theyaredesignedtouncovermalfunctionsorotherchangesin
instrument operation but not to specifically diagnose or quan-
must select proper instrumental operating conditions such as
warm-uptime,purgerate,andbeamsplitteralignmentinorder titatively assess any malfunction. It is recommended that the
levelzerotestsbeconductedatthehighest(smallestnumerical
to realize satisfactory instrument performance. Operating con-
ditions for individual instruments are best obtained from the value) resolution at which the instrument is typically used in
normal operation.Anominal measurement time of 30 s should
manufacturer’sliteraturebecauseofvariationswithinstrument
design. It should be noted that many FT-IR instruments are be used. The exact measurement time, along with the date,
time, sample identification, number of scans, exact data col-
designed to work best when left on or in the standby mode.
lection and computation parameters, and operator’s name,
Also note that spectrometers are to be tested only within their
respective wavenumber ranges. should always be recorded.
6.2 Philosphy—The philosophy of the tests is to use previ-
NOTE 1—This practice is designed to be used in situations where the
detector is not saturated. In some instruments, with some combinations of
ously stored test results as bases for comparison and the visual
optics and detectors, the detector electronics are saturated with an empty
display screen or plotter to overlay the current test results with
beam. These instruments are designed to have the infrared beam attenu-
the known, good results. If the old and new results agree, they
ated in the spectrometer or sample compartment to eliminate detector
are simply reported as no change. Level zero consists of three
saturation. Consult your instrument manual or discuss appropriate attenu-
tests. The tests are run under the same conditions that are
ation techniques with the instrument vendor.
normally used to run a sample (that is, purge time, warm-up
5.2 The environment in which a spectrometer is operated
time, detector, etc.).
can affects its performance. Spectrometers should only be
operated in environments consistent with manufacturer’s rec- 6.3 Variations in Operating Procedure for Different
ommendations.Changesintheinstrumentenvironmentinclud- Instruments—MostoftheexistingFT-IRinstrumentsshouldbe
ing variations in temperature, vibration or sound levels, elec- able to use the tests in this practice without modification.
trical power or magnetic fields should be recorded. However, a few instruments may not be able to perform the
tests exactly as they are written. In these cases, it should be
5.3 Instrumental characteristics can influence these mea-
possible to obtain the same final data using a slightly different
surements in several ways.
procedure.GuideE1866andtheFT-IRmanufacturershouldbe
5.3.1 Vignetting of the beam reduces the transmittance
consulted for appropriate alternative procedures.
value measured in nonabsorbing regions, and on most instru-
ments can change the apparent wavenumber scale by a small 6.4 Sample—The recommended sample is described in 5.3.
−1
amount, usually less than 0.1 cm . Make sure that the film It is a matte-finish polystyrene film (approximately 38-µm
holder does not vignet the beam. thick, in a 2.5-cm aperture). The same sample should be used
5.3.2 Focus changes can also change transmittance values, for all comparisons (note serial number).
so the sample should be positioned in approximately the same
6.5 Reference Spectra—Two spectra acquired and stored
location in the sample compartment each time.
following the last major instrument maintenance are used as
5.3.3 The angle of acceptance (established by the f number)
references. Major maintenance could include changes in
of the optics between the sample and detector significantly
source, laser, detector, or optical alignment. These spectra will
affects apparent transmittance. Changes to the optical path
be identified as Reference 1 and Reference 2.
including the introduction of samples can alter the acceptance
angle.
5.3.4 Heating of the sample by the beam or by the higher
SRM 1921 is available from the Standard Reference Materials Program,
temperatures which exist inside most spectrometers changes
NationalInstituteofStandardsandTechnology(NIST),100BureauDr.,Stop1070,
absorbances somewhat, and even changes band ratios and Gaithersburg, MD 20899-1070, http://www.nist.gov.
´1
E1421 − 99 (2015)
6.5.1 Reference Spectrum 1 is a single-beam energy spec- only affect performance at high wavenumbers, and do not
trum of an empty beam. (In this and all later usage, empty necessarily affect photometric performance.
beam means that nothing is in the sample path except air or
NOTE 5—If the centerburst height exceeds the dynamic range of the
the purge gas normally present within the spectrometer sample
analog-to-digital converter, the energy profile is distorted and significant
compartment). If possible, the interferogram corresponding to
nonphysicalenergywillbeobserved.Ifthecenterburstissmallrelativeto
the dynamic range, then the signal-to-noise of the measurement may be
Reference Spectrum 1 should also be saved.
less than optimal.
6.5.2 Reference Spectrum 2 is a transmittance spectrum of
7.1.1 Reportage—Report by (1) making an overlay plot of
the polystyrene sample. Optionally, an absorbance spectrum
Spectrum 1 and Reference 1, (2) plotting the transmittance
may also be stored.
spectrum of Spectrum 1 ratioed against Reference 1 over the
NOTE 3—If the instrument software will not allow for subtraction of
range of 95 to 105% T, and by reporting the following energy
transmittance spectra, Reference Spectrum 2 should be saved as an
ratios:
absorbance spectrum.
R 5 E /E (1)
4000/2000 4000 2000
6.6 Reproducibility of Procedures—Care should be taken
that each of the spectral measurements is made in a consistent
R 5 E /E
2000/1000 2000 1000
and reproducible manner, including sample orientation (al-
though different spectral measurements do not necessarily use If possible, from Spectrum 1, report the ratio between the
apparent energy in the wavenumber region below the instru-
the identical procedure). In particular, for those instruments
having more than one sample beam or path in the main sample ment cutoff and the energy in the maximum-energy region of
the spectrum, for example:
compartment,allofthetestspectraalwaysshouldbemeasured
using the same path. It may be desirable to repeat the tests on
R 5 E /E (2)
nonphysical 150 max
each path.
Reportthedateandtimeofbothspectraused,andtheactual
6.7 Measurements—Acquire and store three test spectra.
numbers of scans and measurement times.
The test spectra will be identified hereafter as Spectrum 1,
7.1.2 Interpretation—An overall drop in the energy level in
Spectrum 2, and Spectrum 3.
which the largest percentage of change occurs at higher
6.7.1 Spectrum 1—Acquire and store a single-beam energy
wavenumbersusuallyindicatesinterferometermisalignmentor
spectrum of any empty beam. When possible, the interfero-
a reduction in source temperature.An example of the affect of
gram of Spectrum 1 should also be stored. If Spectrum 1 is
misalignment is shown in Fig. 1.
stored only as an interferogram, it must be transformed before
7.1.2.1 Iftheinstrumenthasbeenexposedtohighhumidity,
use in the ensuing tests.
this drop in energy level may reflect beamsplitter or window
6.7.2 Spectrum 2—Acquire and store an empty-beam spec-
fogging.
trum taken immediately after Spectrum 1. This spectrum
7.1.2.2 An overall drop in the energy level without wave-
should be stored as a transmittance spectrum ratioed against
number dependence suggests beam obstruction or misalign-
Spectrum 1.
ment of noninterferometer optical components.
6.7.3 Spectrum 3—Acquire and store a spectrum of the
7.1.2.3 The appearance of bands or other features indicates
polystyrene sample reasonably soon after Spectrum 2. This
purge gas contributions, beam obstruction by a partially
spectrum should be stored as a transmittance spectrum calcu-
transmitting object, oil, or smoke deposition on mirrors or
lated using either Spectrum 1 or Spectrum 2 as a background.
windows, or a forgotten sample in the beam.
Optionally, Spectrum 3 may also be stored as an absorbance
7.1.2.4 With cooled detectors, the appearance of a band
−1
spectrum.To reproducibly insert the sample, the serial number
around 3440 cm indicates ice deposition on the detector
(or other identifying information) should be right side up
surface.
facing the instrument detecto
...
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: E1421 − 99 (Reapproved 2009) E1421 − 99 (Reapproved 2015)
Standard Practice for
Describing and Measuring Performance of Fourier
Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero
and Level One Tests
This standard is issued under the fixed designation E1421; 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—Units statement was inserted in 1.3 editorially in July 2015.
1. Scope
1.1 This practice describes two levels of tests to measure the performance of laboratory Fourier transform mid-infrared
(FT-MIR) spectrometers equipped with a standard sample holder used for transmission measurements.
1.2 This practice is not directly applicable to Fourier transform infrared (FT-IR) spectrometers equipped with various
specialized sampling accessories such as flow cells or reflectance optics, nor to Fourier transform near-infrared (FT-NIR)
spectrometers, nor to FT-IR spectrometers run in step scan mode.
1.2.1 If the specialized sampling accessory can be removed and replaced with a standard transmission sample holder, then this
practice can be used. However, the user should recognize that the performance measured may not reflect that which is achieved
when the specialized accessory is in use.
1.2.2 If the specialized sampling accessory cannot be removed, then it may be possible to employ a modified version of this
practice to measure spectrometer performance. The user is referred to Guide E1866 for a discussion of how these tests may be
modified.
1.2.3 Spectrometer performance tests for FT-NIR spectrometers are described in Practice E1944.
1.2.4 Performance tests for dispersive MIR instruments are described in Practice E932.
1.2.5 For FT-IR spectrometers run in a step scan mode, variations on this practice and information provided by the instrument
vendor should be used.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3.1 Exception—Informational inch-pound units are provided in 5.4.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:
E131 Terminology Relating to Molecular Spectroscopy
E932 Practice for Describing and Measuring Performance of Dispersive Infrared Spectrometers
E1866 Guide for Establishing Spectrophotometer Performance Tests
E1944 Practice for Describing and Measuring Performance of Laboratory Fourier Transform Near-Infrared (FT-NIR)
Spectrometers: Level Zero and Level One Tests
3. Terminology
3.1 Definitions—For definitions of terms used in this practice, refer to Terminology E131. All identifications of spectral regions
−1
and absorption band positions are given in wavenumbers (cm ), and spectral energy, transmittance, and absorbance are signified
This practice is under the jurisdiction of ASTM Committee E13 on Molecular Spectroscopy and Separation Science and is the direct responsibility of Subcommittee
E13.03 on Infrared and Near Infrared Spectroscopy.
Current edition approved March 1, 2009May 1, 2015. Published March 2009July 2015. Originally approved in 1991. Last previous edition approved in 20042009 as
E1421 – 99 (2004).(2009). DOI: 10.1520/E1421-99R09.10.1520/E1421-99R15E01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
E1421 − 99 (2015)
in equations by the letters E,T, and A respectively. The ratio of two transmittance or absorbance values, and the ratio of energy
levels at two different wavenumbers are signified by the letter R. A subscripted number signifies a spectral position in wavenumbers
−1
(for example, A , the absorbance at 3082 cm ).
3.1.1 level one (1) test, n—a simple series of measurements designed to provide quantitative data on various aspects of
instrument performance and information on which to base the diagnosis of problems.
3.1.2 level zero (0) test, n—a routine check of instrument performance, that can be done in a few minutes, designed to visually
detect significant changes in instrument performance and provide a database to determine instrument function over time.
4. Significance and Use
4.1 This practice permits an analyst to compare the general performance of an instrument on any given day with the prior
performance of an instrument. This practice is not necessarily meant for comparison of different instruments with each other even
if the instruments are of the same type and model. This practice is not meant for comparison of the performance of one instrument
operated under differing conditions.
5. Test Conditions
5.1 Operating Conditions—A record should be kept to document the operating conditions selected so that they can be
duplicated. In obtaining spectrophotometric data, the analyst must select proper instrumental operating conditions such as warm-up
time, purge rate, and beam splitter alignment in order to realize satisfactory instrument performance. Operating conditions for
individual instruments are best obtained from the manufacturer’s literature because of variations with instrument design. It should
be noted that many FT-IR instruments are designed to work best when left on or in the standby mode. Also note that spectrometers
are to be tested only within their respective wavenumber ranges.
NOTE 1—This practice is designed to be used in situations where the detector is not saturated. In some instruments, with some combinations of optics
and detectors, the detector electronics are saturated with an empty beam. These instruments are designed to have the infrared beam attenuated in the
spectrometer or sample compartment to eliminate detector saturation. Consult your instrument manual or discuss appropriate attenuation techniques with
the instrument vendor.
5.2 The environment in which a spectrometer is operated can affects its performance. Spectrometers should only be operated
in environments consistent with manufacturer’s recommendations. Changes in the instrument environment including variations in
temperature, vibration or sound levels, electrical power or magnetic fields should be recorded.
5.3 Instrumental characteristics can influence these measurements in several ways.
5.3.1 Vignetting of the beam reduces the transmittance value measured in nonabsorbing regions, and on most instruments can
− 1−1
change the apparent wavenumber scale by a small amount, usually less than 0.1 cm . Make sure that the film holder does not
vignet the beam.
5.3.2 Focus changes can also change transmittance values, so the sample should be positioned in approximately the same
location in the sample compartment each time.
5.3.3 The angle of acceptance (established by the f number) of the optics between the sample and detector significantly affects
apparent transmittance. Changes to the optical path including the introduction of samples can alter the acceptance angle.
5.3.4 Heating of the sample by the beam or by the higher temperatures which exist inside most spectrometers changes
absorbances somewhat, and even changes band ratios and locations slightly. Allow the sample to come to thermal equilibrium
before measurement.
5.4 The recommended sample of matte-finish polystyrene used for these tests is approximately 38-μm (1.5-mil) thick film
mounted on a card. The sample is mounted in a 2.5-cm (1-in.) circular aperture centered within the 5-cm (2.5-in.) width of the card,
and centered 3.8 cm (1.5 in.) from the bottom of the card. The card should be approximately 0.25-cm (0.1-in.) thick and
individually and unambiguously identified. A polystyrene film meeting these requirements is available from the National Institute
of Standards and Technology (NIST) as SRM 1921.
NOTE 2—Very small beam diameters can defeat the interference fringe suppression provided by the matte finish on the sample.
6. Level Zero Tests
6.1 Nature of Tests—Routine checks of instrument performance, these tests can be performed in a few minutes. They are
designed to uncover malfunctions or other changes in instrument operation but not to specifically diagnose or quantitatively assess
any malfunction. It is recommended that the level zero tests be conducted at the highest (smallest numerical value) resolution at
which the instrument is typically used in normal operation. A nominal measurement time of 30 s should be used. The exact
measurement time, along with the date, time, sample identification, number of scans, exact data collection and computation
parameters, and operator’s name, should always be recorded.
SRM 1921 is available from the Standard Reference Materials Program, National Institute of Standards and Technology (NIST), 100 Bureau Dr., Stop 1070, Gaithersburg,
MD 20899-1070, http://www.nist.gov.
´1
E1421 − 99 (2015)
6.2 Philosphy—The philosophy of the tests is to use previously stored test results as bases for comparison and the visual display
screen or plotter to overlay the current test results with the known, good results. If the old and new results agree, they are simply
reported as no change. Level zero consists of three tests. The tests are run under the same conditions that are normally used to run
a sample (that is, purge time, warm-up time, detector, etc.).
6.3 Variations in Operating Procedure for Different Instruments—Most of the existing FT-IR instruments should be able to use
the tests in this practice without modification. However, a few instruments may not be able to perform the tests exactly as they
are written. In these cases, it should be possible to obtain the same final data using a slightly different procedure. PracticeGuide
E1866 and the FT-IR manufacturer should be consulted for appropriate alternative procedures.
6.4 Sample—The recommended sample is described in 5.3. It is a matte-finish polystyrene film (approximately 38-μm thick, in
a 2.5-cm aperture). The same sample should be used for all comparisons (note serial number).
6.5 Reference Spectra—Two spectra acquired and stored following the last major instrument maintenance are used as
references. Major maintenance could include changes in source, laser, detector, or optical alignment. These spectra will be
identified as Reference 1 and Reference 2.
6.5.1 Reference Spectrum 1 is a single-beam energy spectrum of an empty beam. (In this and all later usage, empty beam means
that nothing is in the sample path except air or the purge gas normally present within the spectrometer sample compartment). If
possible, the interferogram corresponding to Reference Spectrum 1 should also be saved.
6.5.2 Reference Spectrum 2 is a transmittance spectrum of the polystyrene sample. Optionally, an absorbance spectrum may
also be stored.
NOTE 3—If the instrument software will not allow for subtraction of transmittance spectra, Reference Spectrum 2 should be saved as an absorbance
spectrum.
6.6 Reproducibility of Procedures—Care should be taken that each of the spectral measurements is made in a consistent and
reproducible manner, including sample orientation (although different spectral measurements do not necessarily use the identical
procedure). In particular, for those instruments having more than one sample beam or path in the main sample compartment, all
of the test spectra always should be measured using the same path. It may be desirable to repeat the tests on each path.
6.7 Measurements—Acquire and store three test spectra. The test spectra will be identified hereafter as Spectrum 1, Spectrum
2, and Spectrum 3.
6.7.1 Spectrum 1—Acquire and store a single-beam energy spectrum of any empty beam. When possible, the interferogram of
Spectrum 1 should also be stored. If Spectrum 1 is stored only as an interferogram, it must be transformed before use in the ensuing
tests.
6.7.2 Spectrum 2—Acquire and store an empty-beam spectrum taken immediately after Spectrum 1. This spectrum should be
stored as a transmittance spectrum ratioed against Spectrum 1.
6.7.3 Spectrum 3—Acquire and store a spectrum of the polystyrene sample reasonably soon after Spectrum 2. This spectrum
should be stored as a transmittance spectrum calculated using either Spectrum 1 or Spectrum 2 as a background. Optionally,
Spectrum 3 may also be stored as an absorbance spectrum. To reproducibly insert the sample, the serial number (or other
identifying information) should be right side up facing the instrument detector.
NOTE 4—If the instrument software will not allow for subtraction of transmittance spectra, Spectrum 2 should be saved as an absorbance spectrum.
7. Level Zero Test Procedures
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Reference 1. Note any change in energy level across the spectrum. Ratio
Spectrum 1 to Reference Spectrum 1 to produce a transmittance spectrum, and look for significant changes from 100 %, especially
at high wavenumber. Video display resolution may limit the accuracy to which this test can be interpreted if the comparison is made
on-screen. In addition, if the interferogram for Spectrum 1 was saved, it may be displayed or plotted and the center burst height
recorded and compared to the allowable range for the instrument. Use caution in interpreting this because minor changes in
interferogram height only affect performance at high wavenumbers, and do not necessarily affect photometric performance.
NOTE 5—If the centerburst height exceeds the dynamic range of the analog-to-digital converter, the energy profile is distorted and significant
nonphysical energy will be observed. If the centerburst is small relative to the dynamic range, then the signal-to-noise of the measurement may be less
than optimal.
7.1.1 Reportage—Report by (1) making an overlay plot of Spectrum 1 and Reference 1, (2) plotting the transmittance spectrum
of Spectrum 1 ratioed against Reference 1 over the range of 95
...
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