ASTM E1421-99

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 1421 – 99
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 E 1421; 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. Scope E 1944 Practice for Describing and Measuring Performance
of Fourier Transform Near-Infrared (FT-NIR) Spectrom-
1.1 This practice describes two levels of tests to measure the
eters: Level Zero and Level One Tests
performance of laboratory Fourier transform mid-infrared
(FT-MIR) spectrometers equipped with a standard sample
3. Terminology
holder used for transmission measurements.
3.1 Definitions—For definitions of terms used in this prac-
1.2 This practice is not directly applicable to FT-IR spec-
tice, refer to Terminology E 131. All identifications of spectral
trometers equipped with various specialized sampling acces-
regions and absorption band positions are given in wavenum-
sories such as flow cells or reflectance optics, nor to Fourier
−1
bers (cm ), and spectral energy, transmittance, and absorbance
Transform Near-Infrared (FT-NIR) spectrometers, nor to FT-IR
are signified in equations by the letters E, T and A respectively.
spectrometers run in step scan mode.
The ratio of two transmittance or absorbance values, and the
1.2.1 If the specialized sampling accessory can be removed
ratio of energy levels at two different wavenumbers are
and replaced with a standard transmission sample holder, then
signified by the letter R. A subscripted number signifies a
this practice can be used. However, the user should recognize
spectral position in wavenumbers (for example, A , the
that the performance measured may not reflect that which is
−1
absorbance at 3082 cm ).
achieved when the specialized accessory is in use.
3.1.1 level one (1) test, n—a simple series of measurements
1.2.2 If the specialized sampling accessory cannot be re-
designed to provide quantitative data on various aspects of
moved, then it may be possible to employ a modified version
instrument performance and information on which to base the
of this practice to measure spectrometer performance. The user
diagnosis of problems.
is referred to Guide E 1866 for a discussion of how these tests
3.1.2 level zero (0) test, n—a routine check of instrument
may be modified.
performance, that can be done in a few minutes, designed to
1.2.3 Spectrometer performance tests for FT-NIR spectrom-
visually detect significant changes in instrument performance
eters are described in Practice E 1944.
and provide a database to determine instrument function over
1.2.4 Performance tests for dispersive MIR instruments are
time.
described in Practice E 932.
1.2.5 For FT-IR spectrometers run in a step scan mode,
4. Significance and Use
variations on this practice and information provided by the
4.1 This practice permits an analyst to compare the general
instrument vendor should be used.
performance of an instrument on any given day with the prior
2. Referenced Documents performance of an instrument. This practice is not necessarily
meant for comparison of different instruments with each other
2.1 ASTM Standards:
2 even if the instruments are of the same type and model. This
E 131 Terminology Relating to Molecular Spectroscopy
practice is not meant for comparison of the performance of one
E 932 Practice for Describing and Measuring Performance
2 instrument operated under differing conditions.
of Dispersive Infrared Spectrophotometers
E 1866 Guide for Establishing Spectrophotometer Perfor-
5. Test Conditions
mance Tests
5.1 Operating Conditions—A record should be kept to
document the operating conditions selected so that they can be
This practice is under the jurisdiction of ASTM Committee E-13 on Molecular duplicated. In obtaining spectrophotometric data, the analyst
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
must select proper instrumental operating conditions such as
Spectroscopy.
warm-up time, purge rate, and beam splitter alignment in order
Current edition approved Oct. 10, 1999. Published December 1999. Originally
published as E 1421 - 91. Last previous edition E 1421 - 94.
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.
E 1421
to realize satisfactory instrument performance. Operating con- designed to uncover malfunctions or other changes in instru-
ditions for individual instruments are best obtained from the ment operation but not to specifically diagnose or quantita-
manufacturer’s literature because of variations with instrument
tively assess any malfunction. It is recommended that the level
design. It should be noted that many FT-IR instruments are
zero tests be conducted at the highest (smallest numerical
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. A nominal measurement time of 30 s should
respective wavenumber ranges.
be used. The exact measurement time, along with the date,
time, sample identification, number of scans, exact data col-
NOTE 1—This practice is designed to be used in situations where the
lection and computation parameters, and operator’s name,
detector is not saturated. In some instruments, with some combinations of
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-
6.2 Philosphy—The philosophy of the tests is to use previ-
ated in the spectrometer or sample compartment to eliminate detector
ously stored test results as bases for comparison and the visual
saturation. Consult your instrument manual or discuss appropriate attenu-
display screen or plotter to overlay the current test results with
ation techniques with the instrument vendor.
the known, good results. If the old and new results agree, they
5.2 The environment in which a spectrometer is operated
are simply reported as no change. Level zero consists of three
can affects its performance. Spectrometers should only be
tests. The tests are run under the same conditions that are
operated in environments consistent with manufacturer’s rec-
normally used to run a sample (that is, purge time, warm-up
ommendations. Changes in the instrument environment includ-
time, detector, etc.).
ing variations in temperature, vibration or sound levels, elec-
6.3 Variations in Operating Procedure for Different
trical power or magnetic fields should be recorded.
Instruments—Most of the existing FT-IR instruments should
5.3 Instrumental characteristics can influence these mea-
be able to use the tests in this practice without modification.
surements in several ways.
However, a few instruments may not be able to perform the
5.3.1 Vignetting of the beam reduces the transmittance
tests exactly as they are written. In these cases, it should be
value measured in nonabsorbing regions, and on most instru-
possible to obtain the same final data using a slightly different
ments can change the apparent wavenumber scale by a small
−1
procedure. Practice E 1866 and the FT-IR manufacturer should
amount, usually less than 0.1 cm . Make sure that the film
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
6.5 Reference Spectra—Two spectra acquired and stored
affects apparent transmittance. Changes to the optical path
following the last major instrument maintenance are used as
including the introduction of samples can alter the acceptance
references. Major maintenance could include changes in
angle.
source, laser, detector, or optical alignment. These spectra will
5.3.4 Heating of the sample by the beam or by the higher
be identified as Reference 1 and Reference 2.
temperatures which exist inside most spectrometers changes
6.5.1 Reference Spectrum 1 is a single-beam energy spec-
absorbances somewhat, and even changes band ratios and
trum of an empty beam. (In this and all later usage, empty
locations slightly. Allow the sample to come to thermal
beam means that nothing is in the sample path except air or
equilibrium before measurement.
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
used for these tests is approximately 38-μm (1.5-mil) thick film
Reference Spectrum 1 should also be saved.
mounted on a card. The sample is mounted in a 2.5-cm (1-in.)
6.5.2 Reference Spectrum 2 is a transmittance spectrum of
circular aperture centered within the 5-cm (2.5-in.) width of the
the polystyrene sample. Optionally, an absorbance spectrum
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 may also be stored.
individually and unambiguously identified. A polystyrene 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 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
6. Level Zero Tests
and reproducible manner, including sample orientation (al-
though different spectral measurements do not necessarily use
6.1 Nature of Tests—Routine checks of instrument perfor-
the identical procedure). In particular, for those instruments
mance, these tests can be performed in a few minutes. They are
having more than one sample beam or path in the main sample
compartment, all of the test spectra always should be measured
SRM 1921 is available from the Standard Reference Materials Program,
using the same path. It may be desirable to repeat the tests on
Building 202, Room 204, National Institute of Standards and Technology, Gaith-
ersburg, MD 20899-0001. each path.
E 1421
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-
nonphysical energy will be observed. If the centerburst is small relative to
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
R 5 E /E (1)
4000/2000 4000 2000
polystyrene sample reasonably soon after Spectrum 2. This
R 5 E /E
spectrum should be stored as a transmittance spectrum calcu-
2000/1000 2000 1000
lated using either Spectrum 1 or Spectrum 2 as a background.
If possible, from Spectrum 1, report the ratio between the
Optionally, Spectrum 3 may also be stored as an absorbance
apparent energy in the wavenumber region below the instru-
spectrum. To reproducibly insert the sample, the serial number
ment cutoff and the energy in the maximum-energy region of
(or other identifying information) should be right side up
the spectrum, for example:
facing the instrument detector.
R 5 E /E (2)
nonphysical 150 max
NOTE 4—If the instrument software will not allow for subtraction of
Report the date and time of both spectra used, and the actual
transmittance spectra, Spectrum 2 should be saved as an absorbance
numbers of scans and measurement times.
spectrum.
7.1.2 Interpretation—An overall drop in the energy level in
7. Level Zero Test Procedures
which the largest percentage of change occurs at higher
wavenumbers usually indicates interferometer misalignment or
7.1 Energy Spectrum Test—Overlay Spectrum 1 and Refer-
a reduction in source temperature. An example of the affect of
ence 1. Note any change in energy level across the spectrum.
misalignment is shown in Fig. 1.
Ratio Spectrum 1 to Reference Spectrum 1 to produce a
7.1.2.1 If the instrument has been exposed to high humidity,
transmittance spectrum, and look for significant changes from
this drop in energy level may reflect beamsplitter or window
100 %, especially at high wavenumber. Video display resolu-
fogging.
tion may limit the accuracy to which this test can be interpreted
7.1.2.2 An overall drop in the energy level without wave-
if the comparison is made on-screen. In addition, if the
number dependence suggests beam obstruction or misalign-
interferogram for Spectrum 1 was saved, it may be displayed or
ment of noninterferometer optical components.
plotted and the center burst height recorded and compared to
the allowable range for the instrument. Use caution in inter- 7.1.2.3 The appearance of bands or other features indicates
preting this because minor changes in interferogram height purge gas contributions, beam obstruction by a partially
FIG. 1 Effect of Misalignment on Single-Beam Energy Spectra
...