ASTM E1421-99(2009)
(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
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 E 1866 for a discussion of how these tests may be modified.
1.2.3 Spectrometer performance tests for FT-NIR spectrometers are described in Practice E 1944.
1.2.4 Performance tests for dispersive MIR instruments are described in Practice E 932.
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.
General Information
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Designation: E1421 − 99 (Reapproved2009)
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.
1. Scope E932PracticeforDescribingandMeasuringPerformanceof
Dispersive Infrared Spectrometers
1.1 Thispracticedescribestwolevelsofteststomeasurethe
E1866Guide for Establishing Spectrophotometer Perfor-
performance of laboratory Fourier transform mid-infrared
mance Tests
(FT-MIR) spectrometers equipped with a standard sample
E1944Practice for Describing and Measuring Performance
holder used for transmission measurements.
of Laboratory Fourier Transform Near-Infrared (FT-NIR)
1.2 This practice is not directly applicable to Fourier trans-
Spectrometers: Level Zero and Level One Tests
form infrared (FT-IR) spectrometers equipped with various
specialized sampling accessories such as flow cells or reflec-
3. Terminology
tance optics, nor to Fourier transform near-infrared (FT-NIR)
3.1 Definitions—For definitions of terms used in this
spectrometers, nor to FT-IR spectrometers run in step scan
practice, refer to Terminology E131. All identifications of
mode.
spectral regions and absorption band positions are given in
1.2.1 If the specialized sampling accessory can be removed −1
wavenumbers (cm ), and spectral energy, transmittance, and
and replaced with a standard transmission sample holder, then
absorbance are signified in equations by the letters E, T, and A
this practice can be used. However, the user should recognize
respectively. The ratio of two transmittance or absorbance
that the performance measured may not reflect that which is
values, and the ratio of energy levels at two different wave-
achieved when the specialized accessory is in use.
numbers are signified by the letter R. A subscripted number
1.2.2 If the specialized sampling accessory cannot be
signifies a spectral position in wavenumbers (for example,
removed,thenitmaybepossibletoemployamodifiedversion −1
A , the absorbance at 3082 cm ).
ofthispracticetomeasurespectrometerperformance.Theuser
3.1.1 level one (1) test, n—a simple series of measurements
is referred to Guide E1866 for a discussion of how these tests
designed to provide quantitative data on various aspects of
may be modified.
instrument performance and information on which to base the
1.2.3 SpectrometerperformancetestsforFT-NIRspectrom-
diagnosis of problems.
eters are described in Practice E1944.
3.1.2 level zero (0) test, n—a routine check of instrument
1.2.4 Performance tests for dispersive MIR instruments are
performance, that can be done in a few minutes, designed to
described in Practice E932.
visually detect significant changes in instrument performance
1.2.5 For FT-IR spectrometers run in a step scan mode,
and provide a database to determine instrument function over
variations on this practice and information provided by the
time.
instrument vendor should be used.
4. Significance and Use
2. Referenced Documents
4.1 This practice permits an analyst to compare the general
2.1 ASTM Standards:
E131Terminology Relating to Molecular Spectroscopy performance of an instrument on any given day with the prior
performance of an instrument. This practice is not necessarily
This practice is under the jurisdiction ofASTM Committee E13 on Molecular meant for comparison of different instruments with each other
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
even if the instruments are of the same type and model. This
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
practiceisnotmeantforcomparisonoftheperformanceofone
Current edition approved March 1, 2009. Published March 2009. Originally
instrument operated under differing conditions.
approved in 1991. Last previous edition approved in 2004 as E1421–99(2004).
DOI: 10.1520/E1421-99R09.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 5. Test Conditions
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
5.1 Operating Conditions—A record should be kept to
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. document the operating conditions selected so that they can be
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1421 − 99 (2009)
duplicated. In obtaining spectrophotometric data, the analyst 6. Level Zero Tests
must select proper instrumental operating conditions such as
6.1 Nature of Tests—Routine checks of instrument
warm-uptime,purgerate,andbeamsplitteralignmentinorder
performance, these tests can be performed in a few minutes.
to realize satisfactory instrument performance. Operating con-
Theyaredesignedtouncovermalfunctionsorotherchangesin
ditions for individual instruments are best obtained from the
instrument operation but not to specifically diagnose or quan-
manufacturer’sliteraturebecauseofvariationswithinstrument
titatively assess any malfunction. It is recommended that the
design. It should be noted that many FT-IR instruments are
levelzerotestsbeconductedatthehighest(smallestnumerical
designed to work best when left on or in the standby mode.
value) resolution at which the instrument is typically used in
Also note that spectrometers are to be tested only within their
normal operation.Anominal measurement time of 30 s should
respective wavenumber ranges.
be used. The exact measurement time, along with the date,
NOTE 1—This practice is designed to be used in situations where the time, sample identification, number of scans, exact data col-
detector is not saturated. In some instruments, with some combinations of
lection and computation parameters, and operator’s name,
optics and detectors, the detector electronics are saturated with an empty
should always be recorded.
beam. These instruments are designed to have the infrared beam attenu-
ated in the spectrometer or sample compartment to eliminate detector
6.2 Philosphy—The philosophy of the tests is to use previ-
saturation. Consult your instrument manual or discuss appropriate attenu-
ously stored test results as bases for comparison and the visual
ation techniques with the instrument vendor.
display screen or plotter to overlay the current test results with
5.2 The environment in which a spectrometer is operated
the known, good results. If the old and new results agree, they
can affects its performance. Spectrometers should only be
are simply reported as no change. Level zero consists of three
operated in environments consistent with manufacturer’s rec-
tests. The tests are run under the same conditions that are
ommendations.Changesintheinstrumentenvironmentinclud-
normally used to run a sample (that is, purge time, warm-up
ing variations in temperature, vibration or sound levels, elec-
time, detector, etc.).
trical power or magnetic fields should be recorded.
6.3 Variations in Operating Procedure for Different
5.3 Instrumental characteristics can influence these mea-
Instruments—MostoftheexistingFT-IRinstrumentsshouldbe
surements in several ways.
able to use the tests in this practice without modification.
5.3.1 Vignetting of the beam reduces the transmittance
However, a few instruments may not be able to perform the
value measured in nonabsorbing regions, and on most instru-
tests exactly as they are written. In these cases, it should be
ments can change the apparent wavenumber scale by a small
possible to obtain the same final data using a slightly different
−1
amount, usually less than 0.1 cm . Make sure that the film
procedure. Practice E1866 and the FT-IR manufacturer should
holder does not vignet the beam.
be consulted for appropriate alternative procedures.
5.3.2 Focus changes can also change transmittance values,
6.4 Sample—The recommended sample is described in 5.3.
so the sample should be positioned in approximately the same
It is a matte-finish polystyrene film (approximately 38-µm
location in the sample compartment each time.
thick, in a 2.5-cm aperture). The same sample should be used
5.3.3 The angle of acceptance (established by the f number)
for all comparisons (note serial number).
of the optics between the sample and detector significantly
affects apparent transmittance. Changes to the optical path
6.5 Reference Spectra—Two spectra acquired and stored
including the introduction of samples can alter the acceptance
following the last major instrument maintenance are used as
angle.
references. Major maintenance could include changes in
5.3.4 Heating of the sample by the beam or by the higher
source, laser, detector, or optical alignment. These spectra will
temperatures which exist inside most spectrometers changes
be identified as Reference 1 and Reference 2.
absorbances somewhat, and even changes band ratios and
6.5.1 Reference Spectrum 1 is a single-beam energy spec-
locations slightly. Allow the sample to come to thermal
trum of an empty beam. (In this and all later usage, empty
equilibrium before measurement.
beam means that nothing is in the sample path except air or
the purge gas normally present within the spectrometer sample
5.4 The recommended sample of matte-finish polystyrene
compartment). If possible, the interferogram corresponding to
usedforthesetestsisapproximately38-µm(1.5-mil)thickfilm
Reference Spectrum 1 should also be saved.
mounted on a card. The sample is mounted in a 2.5-cm (1-in.)
circularaperturecenteredwithinthe5-cm(2.5-in.)widthofthe 6.5.2 Reference Spectrum 2 is a transmittance spectrum of
the polystyrene sample. Optionally, an absorbance spectrum
card,andcentered3.8cm(1.5in.)fromthebottomofthecard.
The card should be approximately 0.25-cm (0.1-in.) thick and may also be stored.
individually and unambiguously identified.Apolystyrene film
NOTE 3—If the instrument software will not allow for subtraction of
meeting these requirements is available from the National
transmittance spectra, Reference Spectrum 2 should be saved as an
Institute of Standards and Technology (NIST) as SRM 1921.
absorbance spectrum.
NOTE 2—Very small beam diameters can defeat the interference fringe
6.6 Reproducibility of Procedures—Care should be taken
suppression provided by the matte finish on the sample.
that each of the spectral measurements is made in a consistent
and reproducible manner, including sample orientation (al-
though different spectral measurements do not necessarily use
SRM 1921 is available from the Standard Reference Materials Program,
the identical procedure). In particular, for those instruments
NationalInstituteofStandardsandTechnology(NIST),100BureauDr.,Stop1070,
Gaithersburg, MD 20899-1070, http://www.nist.gov. having more than one sample beam or path in the main sample
E1421 − 99 (2009)
FIG. 1 Effect of Misalignment on Single-Beam Energy Spectra
compartment,allofthetestspectraalwaysshouldbemeasured interferogramforSpectrum1wassaved,itmaybedisplayedor
using the same path. It may be desirable to repeat the tests on plotted and the center burst height recorded and compared to
each path. the allowable range for the instrument. Use caution in inter-
preting this because minor changes in interferogram height
6.7 Measurements—Acquire and store three test spectra.
only affect performance at high wavenumbers, and do not
The test spectra will be identified hereafter as Spectrum 1,
necessarily affect photometric performance.
Spectrum 2, and Spectrum 3.
6.7.1 Spectrum 1—Acquire and store a single-beam energy
NOTE 5—If the centerburst height exceeds the dynamic range of the
spectrum of any empty beam. When possible, the interfero- analog-to-digital converter, the energy profile is distorted and significant
nonphysicalenergywillbeobserved.Ifthecenterburstissmallrelativeto
gram of Spectrum 1 should also be stored. If Spectrum 1 is
the dynamic range, then the signal-to-noise of the measurement may be
stored only as an interferogram, it must be transformed before
less than optimal.
use in the ensuing tests.
7.1.1 Reportage—Report by (1) making an overlay plot of
6.7.2 Spectrum 2—Acquire and store an empty-beam spec-
Spectrum 1 and Reference 1, (2) plotting the transmittance
trum taken immediately after Spectrum 1. This spectrum
spectrum of Spectrum 1 ratioed against Reference 1 over the
should be stored as a transmittance spectrum ratioed against
range of 95 to 105% T, and by reporting the following energy
Spectrum 1.
ratios:
6.7.3 Spectrum 3—Acquire and store a spectrum of the
polystyrene sample reasonably soon after Spectrum 2. This R 5 E /E (1)
4000/2000 4000 2000
spectrum should be stored as a transmittance spectrum calcu-
R 5 E /E
2000/1000 2000 1000
lated using either Spectrum 1 or Spectrum 2 as a background.
Optionally, Spectrum 3 may also be stored as an absorbance
If possible, from Spectrum 1, report the ratio between the
spectrum.To reproducibly insert the sample, the serial number
apparent energy in the wavenumber region below the instru-
(or other identifying information) should be right side up
ment cutoff and the energy in the maximum-energy region of
facing the instrument detector.
the spectrum, for example:
NOTE 4—If the instrument software will not allow for subtraction of
R 5 E /E (2)
nonphysical 150 max
transmittance spectra, Spectrum 2 should be saved as an absorbance
spectrum. Reportthedateandtimeofbothspectraused,andtheactual
numbers of scans and measurement times.
7. Level Zero Test Procedures
7.1.2 Interpretation—An overall drop in the energy level in
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Refer- which the largest percentage of change occurs at higher
wavenumbersusuallyindicatesinterferometermisalignmentor
ence 1. Note any change in energy level across the spectrum.
Ratio Spectrum 1 to Reference Spectrum 1 to produce a a reduction in source temperature.An example of the affect of
misalignment is shown in Fig. 1.
transmittance spectrum, and look for significant changes from
100%, especially at high wavenumber. Video display resolu- 7.1.2.1 Iftheinstrumenthasbeenexposedtohighhumidity,
tionmaylimittheaccuracytowhichthistestcanbeinterpreted this drop in energy level may reflect beamsplitter or window
if the comparison is made on-screen. In addition, if the fogging.
E1421 − 99 (2009)
FIG. 2 Example of Nonphysical Energy
7.1.2.2 An overall drop in the energy level without wave- somet
...
This document is not anASTM standard and is intended only to provide the user of anASTM 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.
Designation:E1421–99(Reapproved2004) Designation:E1421–99(Reapproved2009)
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.
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-Infraredtransform
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.
2. Referenced Documents
2.1 ASTM Standards:
E131 Terminology Relating to Molecular Spectroscopy
E932 Practice for Describing and Measuring Performance of Dispersive Infrared SpectrophotometersSpectrometers
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—Fordefinitionsoftermsusedinthispractice,refertoTerminologyE131.Allidentificationsofspectralregions
−1
and absorption band positions are given in wavenumbers (cm ), and spectral energy, transmittance, and absorbance are signified
inequationsbythelettersE,Tand,andArespectively.Theratiooftwotransmittanceorabsorbancevalues,andtheratioofenergy
levelsattwodifferentwavenumbersaresignifiedbytheletterR.Asubscriptednumbersignifiesaspectralpositioninwavenumbers
−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.
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 Feb.March 1, 2004.2009. Published March 2004.2009. Originally approved in 1991. Last previous edition approved in 19992004 as
E1421-–99(2004).
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.ForAnnualBookofASTMStandards
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.
E1421–99 (2009)
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
iftheinstrumentsareofthesametypeandmodel.Thispracticeisnotmeantforcomparisonoftheperformanceofoneinstrument
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.Inobtainingspectrophotometricdata,theanalystmustselectproperinstrumentaloperatingconditionssuchaswarm-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
benotedthatmanyFT-IRinstrumentsaredesignedtoworkbestwhenleftonorinthestandbymode.Alsonotethatspectrometers
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
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
mountedonacard.Thesampleismountedina2.5-cm(1-in.)circularaperturecenteredwithinthe5-cm(2.5-in.)widthofthecard,
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.Apolystyrene 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
designedtouncovermalfunctionsorotherchangesininstrumentoperationbutnottospecificallydiagnoseorquantitativelyassess
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.
6.2 Philosphy—Thephilosophyofthetestsistousepreviouslystoredtestresultsasbasesforcomparisonandthevisualdisplay
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. Practice E1866
and the FT-IR manufacturer should be consulted for appropriate alternative procedures.
SRM 1921 is available from the Standard Reference Materials Program, Building 202, Room 204, National Institute of Standards and Technology, Gaithersburg, MD
20899-0001.
SRM1921isavailablefromtheStandardReferenceMaterialsProgram,NationalInstituteofStandardsandTechnology(NIST),100BureauDr.,Stop1070,Gaithersburg,
MD 20899-1070, http://www.nist.gov.
E1421–99 (2009)
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 ReferenceSpectrum1isasingle-beamenergyspectrumofanemptybeam.(Inthisandalllaterusage,emptybeammeans
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
Spectrum1shouldalsobestored.IfSpectrum1isstoredonlyasaninterferogram,itmustbetransformedbeforeuseintheensuing
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
Spectrum1toReferenceSpectrum1toproduceatransmittancespectrum,andlookforsignificantchangesfrom100%,especially
athighwavenumber.Videodisplayresolutionmaylimittheaccuracytowhichthistestcanbeinterpretedifthecomparisonismade
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—Reportby(1)makinganoverlayplotofSpectrum1andReference1,(2)plottingthetransmittancespectrum
of Spectrum 1 ratioed against Reference 1 over the range of 95 to 105% T, and by reporting the following energy ratios:
R 5 E /E (1)
4000/2000 4000 2000
R 5 E /E
2000/1000 2000 1000
Ifpossible,fromSpectrum1,reporttheratiobetweentheapparentenergyinthewavenumberregionbelowtheinstrumentcutoff
and the energy in the maximum-energy region of the spectrum, for example:
R 5 E /E (2)
nonphysical 150 max
Report the date and time of both spectra used, and the actual numbers of scans and measurement times.
7.1.2 Interpretation— An overall drop in the energy level in which the largest percentage of change occurs at higher
wavenumbers usually indicates interferometer misalignment or a reduction in source temperature. An example of the affect of
misalignment is shown in Fig. 1.
7.1.2.1 If the instrument has
...
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