ASTM E1421-99(2004)
(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 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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E1421–99(Reapproved2004)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E1866 Guide for Establishing Spectrophotometer Perfor-
mance Tests
1.1 Thispracticedescribestwolevelsofteststomeasurethe
E1944 PracticeforDescribingandMeasuringPerformance
performance of laboratory Fourier transform mid-infrared
of Fourier Transform Near-Infrared (FT-NIR) Spectrom-
(FT-MIR) spectrometers equipped with a standard sample
eters: Level Zero and Level One Tests
holder used for transmission measurements.
1.2 This practice is not directly applicable to FT-IR spec-
3. Terminology
trometers equipped with various specialized sampling acces-
3.1 Definitions—For definitions of terms used in this prac-
sories such as flow cells or reflectance optics, nor to Fourier
tice, refer to Terminology E131.All identifications of spectral
TransformNear-Infrared(FT-NIR)spectrometers,nortoFT-IR
regions and absorption band positions are given in wavenum-
spectrometers run in step scan mode.
−1
bers(cm ),andspectralenergy,transmittance,andabsorbance
1.2.1 If the specialized sampling accessory can be removed
aresignifiedinequationsbytheletters E, Tand Arespectively.
and replaced with a standard transmission sample holder, then
The ratio of two transmittance or absorbance values, and the
this practice can be used. However, the user should recognize
ratio of energy levels at two different wavenumbers are
that the performance measured may not reflect that which is
signified by the letter R. A subscripted number signifies a
achieved when the specialized accessory is in use.
spectral position in wavenumbers (for example, A , the
1.2.2 If the specialized sampling accessory cannot be re-
−1
absorbance at 3082 cm ).
moved, then it may be possible to employ a modified version
3.1.1 level one (1) test, n—a simple series of measurements
ofthispracticetomeasurespectrometerperformance.Theuser
designed to provide quantitative data on various aspects of
is referred to Guide E1866 for a discussion of how these tests
instrument performance and information on which to base the
may be modified.
diagnosis of problems.
1.2.3 SpectrometerperformancetestsforFT-NIRspectrom-
3.1.2 level zero (0) test, n—a routine check of instrument
eters are described in Practice E1944.
performance, that can be done in a few minutes, designed to
1.2.4 Performance tests for dispersive MIR instruments are
visually detect significant changes in instrument performance
described in Practice E932.
and provide a database to determine instrument function over
1.2.5 For FT-IR spectrometers run in a step scan mode,
time.
variations on this practice and information provided by the
instrument vendor should be used.
4. Significance and Use
2. Referenced Documents 4.1 This practice permits an analyst to compare the general
2 performance of an instrument on any given day with the prior
2.1 ASTM Standards:
performance of an instrument. This practice is not necessarily
E131 Terminology Relating to Molecular Spectroscopy
meant for comparison of different instruments with each other
E932 Practice for Describing and Measuring Performance
even if the instruments are of the same type and model. This
of Dispersive Infrared Spectrophotometers
practiceisnotmeantforcomparisonoftheperformanceofone
instrument operated under differing conditions.
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
5. Test Conditions
Spectroscopy.
5.1 Operating Conditions—A record should be kept to
Current edition approved Feb. 1, 2004. Published March 2004. Originally
approved in 1991. Last previous edition approved in 1999 as E1421-99. document the operating conditions selected so that they can be
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
duplicated. In obtaining spectrophotometric data, the analyst
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
must select proper instrumental operating conditions such as
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.
E1421–99 (2004)
warm-uptime,purgerate,andbeamsplitteralignmentinorder 6. Level Zero Tests
to realize satisfactory instrument performance. Operating con-
6.1 Nature of Tests—Routine checks of instrument perfor-
ditions for individual instruments are best obtained from the
mance,thesetestscanbeperformedinafewminutes.Theyare
manufacturer’sliteraturebecauseofvariationswithinstrument
designed to uncover malfunctions or other changes in instru-
design. It should be noted that many FT-IR instruments are
ment operation but not to specifically diagnose or quantita-
designed to work best when left on or in the standby mode.
tively assess any malfunction. It is recommended that the level
Also note that spectrometers are to be tested only within their
zero tests be conducted at the highest (smallest numerical
respective wavenumber ranges.
value) resolution at which the instrument is typically used in
NOTE 1—This practice is designed to be used in situations where the normal operation.Anominal measurement time of 30 s should
detector is not saturated. In some instruments, with some combinations of
be used. The exact measurement time, along with the date,
optics and detectors, the detector electronics are saturated with an empty
time, sample identification, number of scans, exact data col-
beam. These instruments are designed to have the infrared beam attenu-
lection and computation parameters, and operator’s name,
ated in the spectrometer or sample compartment to eliminate detector
should always be recorded.
saturation. Consult your instrument manual or discuss appropriate attenu-
ation techniques with the instrument vendor. 6.2 Philosphy—The philosophy of the tests is to use previ-
ously stored test results as bases for comparison and the visual
5.2 The environment in which a spectrometer is operated
display screen or plotter to overlay the current test results with
can affects its performance. Spectrometers should only be
the known, good results. If the old and new results agree, they
operated in environments consistent with manufacturer’s rec-
are simply reported as no change. Level zero consists of three
ommendations.Changesintheinstrumentenvironmentinclud-
tests. The tests are run under the same conditions that are
ing variations in temperature, vibration or sound levels, elec-
normally used to run a sample (that is, purge time, warm-up
trical power or magnetic fields should be recorded.
time, detector, etc.).
5.3 Instrumental characteristics can influence these mea-
6.3 Variations in Operating Procedure for Different
surements in several ways.
Instruments—Most of the existing FT-IR instruments should
5.3.1 Vignetting of the beam reduces the transmittance
be able to use the tests in this practice without modification.
value measured in nonabsorbing regions, and on most instru-
However, a few instruments may not be able to perform the
ments can change the apparent wavenumber scale by a small
tests exactly as they are written. In these cases, it should be
−1
amount, usually less than 0.1 cm . Make sure that the film
possible to obtain the same final data using a slightly different
holder does not vignet the beam.
procedure.PracticeE1866andtheFT-IRmanufacturershould
5.3.2 Focus changes can also change transmittance values,
be consulted for appropriate alternative procedures.
so the sample should be positioned in approximately the same
6.4 Sample—The recommended sample is described in 5.3.
location in the sample compartment each time.
It is a matte-finish polystyrene film (approximately 38-µm
5.3.3 The angle of acceptance (established by the f number)
thick, in a 2.5-cm aperture). The same sample should be used
of the optics between the sample and detector significantly
for all comparisons (note serial number).
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
5.4 The recommended sample of matte-finish polystyrene
the purge gas normally present within the spectrometer sample
usedforthesetestsisapproximately38-µm(1.5-mil)thickfilm
compartment). If possible, the interferogram corresponding to
mounted on a card. The sample is mounted in a 2.5-cm (1-in.)
Reference Spectrum 1 should also be saved.
circularaperturecenteredwithinthe5-cm(2.5-in.)widthofthe
6.5.2 Reference Spectrum 2 is a transmittance spectrum of
card,andcentered3.8cm(1.5in.)fromthebottomofthecard.
the polystyrene sample. Optionally, an absorbance spectrum
The card should be approximately 0.25-cm (0.1-in.) thick and
may also be stored.
individually and unambiguously identified.Apolystyrene film
meeting these requirements is available from the National
NOTE 3—If the instrument software will not allow for subtraction of
Institute of Standards and Technology as SRM 1921.
transmittance spectra, Reference Spectrum 2 should be saved as an
absorbance spectrum.
NOTE 2—Very small beam diameters can defeat the interference fringe
suppression provided by the matte finish on the sample.
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 (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
Building 202, Room 204, National Institute of Standards and Technology, Gaith-
ersburg, MD 20899-0001. having more than one sample beam or path in the main sample
E1421–99 (2004)
FIG. 1 Effect of Misalignment on Single-Beam Energy Spectra
compartment,allofthetestspectraalwaysshouldbemeasured plotted and the center burst height recorded and compared to
using the same path. It may be desirable to repeat the tests on
the allowable range for the instrument. Use caution in inter-
each path.
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.
NOTE 5—If the centerburst height exceeds the dynamic range of the
6.7.1 Spectrum 1—Acquire and store a single-beam energy
analog-to-digital converter, the energy profile is distorted and significant
spectrum of any empty beam. When possible, the interfero-
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.
6.7.2 Spectrum 2—Acquire and store an empty-beam spec-
7.1.1 Reportage—Report by (1) making an overlay plot of
trum taken immediately after Spectrum 1. This spectrum
Spectrum 1 and Reference 1, (2) plotting the transmittance
should be stored as a transmittance spectrum ratioed against
spectrum of Spectrum 1 ratioed against Reference 1 over the
Spectrum 1.
range of 95 to 105% T, and by reporting the following energy
6.7.3 Spectrum 3—Acquire and store a spectrum of the
ratios:
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
lated using either Spectrum 1 or Spectrum 2 as a background. 2000/1000 2000 1000
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
ence 1. Note any change in energy level across the spectrum.
wavenumbersusuallyindicatesinterferometermisalignmentor
Ratio Spectrum 1 to Reference Spectrum 1 to produce a
a reduction in source temperature.An example of the affect of
transmittance spectrum, and look for significant changes from
misalignment is shown in Fig. 1.
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
interferogramforSpectrum1wassaved,itmaybedisplayedor fogging.
E1421–99 (2004)
FIG. 2 Example of Nonphysical Energy
7.1.2.2 An overall drop in the energy level without wave- be digital malfunctions or they can indicate electromagnetic
number dependence suggests beam obstruction or misalign- interference. Positive
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