ASTM E387-84(1995)e1
(Test Method)Standard Test Method for Estimating Stray Radiant Power Ratio of Spectrophotometers by the Opaque Filter Method
Standard Test Method for Estimating Stray Radiant Power Ratio of Spectrophotometers by the Opaque Filter Method
SCOPE
1.1 This method affords an estimate of the relative radiant power (that is, the Stray Radiant Power Ratio (SRPR) at wavelengths remote from those of the nominal pass band transmitted through the monochromator of an absorption spectrophotometer. Test-filter materials are described that discriminate between the desired wavelengths and those which contribute most to SRP for conventional commercial spectrophotometers used in the ultraviolet, the visible, the near infrared, and the alkali halide infrared ranges. These procedures apply to instruments of conventional design, with usual sources, detectors, and optical designs. The vacuum ultraviolet and the far infrared present special problems that are not discussed herein. Note 1-An equivalent test method (1,2) for use in the ultraviolet that is easier and faster to perform might be preferred, especially when frequent testing is indicated. Note 2-Recent research (3) has shown that particular care must be exercised in testing grating spectrophotometers that use moderately narrow bandpass blocking filters. See 4.2.
1.2 These procedures are neither all-inclusive nor infallible. Because of the nature of readily available filter materials, with a few exceptions the procedures are insensitive to SRP of shorter wavelengths in the ultraviolet or visible, or of lower frequencies in the infrared, and they are not necessarily valid for "spike" SRP nor for "nearby SRP." (See Annexes for general discussion and definitions of these terms.) However, they are adequate in most cases and for typical applications. They do cover instruments using prisms or gratings, in either single or double monochromators.
1.3 The proportion of SRP (that is, SRPR) encountered with a well-designed monochromator, used in a favorable spectral region, is a small fraction of 1%. With a double monochromator it may easily be less than 1 ppm even with a broad-band continuum source. Under these conditions, it may be difficult to do more than determine that it falls below a certain level. Actual measurement often requires special techniques and instrument operating conditions that are not typical of those occurring during use. When absorption measurements with continuum sources are being made, it is frequently true that, owing to the effect of slit width on SRP in a double monochromator, these test procedures tend to give "conservative" results; that is, because larger slit widths than normal must be used to admit enough energy to the monochromator to permit evaluation of the SRP, the stray proportion indicated is greater than would normally be encountered in use.
1.4 The principal reason for a test procedure that is not exactly representative of normal operation is that the effects of SRP are "magnified" in sample measurements at high absorbance. It is usually necessary to increase sensitivity in some way during the test in order to evaluate the SRP adequately. This is usually accomplished by increasing slit width and so obtaining sufficient energy to allow meaningful measurement of the SRP after the monochromatic energy has been removed by the SRP (test) filter. A further reason for increasing energy or sensitivity can be that many instruments have only absorbance scales, which obviously do not extend to zero transmittance. Even a SRP proportion as large as 1% may fall outside the measurement range.
1.5 Many instruments that use optical attenuators to balance sample absorption make relatively inaccurate measurements below 10% transmittance, because of poor attenuator linearity. All instruments should be carefully and frequently checked if used below about 1% transmittance because of the possibility of zero shift due to electrical pickup or other causes. For these reasons, the test procedure specifies that the measurements be made within 100% and 10% on the scale. Obviously this is not required if reliable photometric linearity and zero checks are made under the conditions prevailing during the ...
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e1
Designation: E 387 – 84 (Reapproved 1995)
Standard Test Method for
Estimating Stray Radiant Power Ratio of
Spectrophotometers by the Opaque Filter Method
This standard is issued under the fixed designation E 387; 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.
e NOTE—Section 9 was added editorially in November 1995.
1. Scope 1.3 The proportion of SRP (that is, SRPR) encountered with
a well-designed monochromator, used in a favorable spectral
1.1 This test method affords an estimate of the relative
region, is a small fraction of 1 %. With a double monochro-
radiant power (that is, the Stray Radiant Power Ratio (SRPR)
mator it may easily be less than 1 ppm even with a broad-band
at wavelengths remote from those of the nominal pass band
continuum source. Under these conditions, it may be difficult to
transmitted through the monochromator of an absorption
do more than determine that it falls below a certain level.
spectrophotometer. Test-filter materials are described that dis-
Actual measurement often requires special techniques and
criminate between the desired wavelengths and those which
instrument operating conditions that are not typical of those
contribute most to SRP for conventional commercial spectro-
occurring during use. When absorption measurements with
photometers used in the ultraviolet, the visible, the near
continuum sources are being made, it is frequently true that,
infrared, and the alkali halide infrared ranges. These proce-
owing to the effect of slit width on SRP in a double mono-
dures apply to instruments of conventional design, with usual
chromator, these test procedures tend to give “conservative”
sources, detectors, and optical designs. The vacuum ultraviolet
results; that is, because larger slit widths than normal must be
and the far infrared present special problems that are not
used to admit enough energy to the monochromator to permit
discussed herein.
evaluation of the SRP, the stray proportion indicated is greater
NOTE 1—An equivalent test method (1,2) for use in the ultraviolet that
than would normally be encountered in use.
is easier and faster to perform might be preferred, especially when
1.4 The principal reason for a test procedure that is not
frequent testing is indicated.
exactly representative of normal operation is that the effects of
NOTE 2—Recent research (3) has shown that particular care must be
SRP are “magnified” in sample measurements at high absor-
exercised in testing grating spectrophotometers that use moderately
narrow bandpass blocking filters. See 4.2. bance. It is usually necessary to increase sensitivity in some
way during the test in order to evaluate the SRP adequately.
1.2 These procedures are neither all-inclusive nor infal-
3 This is usually accomplished by increasing slit width and so
lible. Because of the nature of readily available filter materi-
obtaining sufficient energy to allow meaningful measurement
als, with a few exceptions the procedures are insensitive to
of the SRP after the monochromatic energy has been removed
SRP of shorter wavelengths in the ultraviolet or visible, or of
by the SRP (test) filter. A further reason for increasing energy
lower frequencies in the infrared, and they are not necessarily
or sensitivity can be that many instruments have only absor-
valid for “spike” SRP nor for “nearby SRP.” (See Annexes for
bance scales, which obviously do not extend to zero transmit-
general discussion and definitions of these terms.) However,
tance. Even a SRP proportion as large as 1 % may fall outside
they are adequate in most cases and for typical applications.
the measurement range.
They do cover instruments using prisms or gratings, in either
1.5 Many instruments that use optical attenuators to balance
single or double monochromators.
sample absorption make relatively inaccurate measurements
below 10 % transmittance, because of poor attenuator linearity.
All instruments should be carefully and frequently checked if
This method is under the jurisdiction of ASTM Committee E-13 on Molecular
used below about 1 % transmittance because of the possibility
Spectroscopy and is the direct responsibility of Subcommittee E13.01 on Ultraviolet
of zero shift due to electrical pickup or other causes. For these
and Visible Spectroscopy.
reasons, the test procedure specifies that the measurements be
Current edition approved March 30, 1984. Published May 1984. Originally
published as E 387 – 69 T. Last previous edition E 387 – 72(1977).
made within 100 % and 10 % on the scale. Obviously this is
The boldface numbers in parentheses refer to the list of references at the end of
not required if reliable photometric linearity and zero checks
this method.
are made under the conditions prevailing during the test, and
Currently in preparation by E13.01 are test methods that reduce the potential for
error that occurs when the test-filter absorbs an appreciable portion of the SRP. use of the 10 and 1 % range may give lower and more
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 387
accurately representative SRP values. In marginal cases this value of slit width should be used at the last step of the
more elaborate test procedure, incorporating accepted ASTM reference beam attenuator, as this corresponds most nearly with
tests for photometric linearity, may be justified. the normal operating conditions of the spectrophotometer.
1.6 High accuracy in SRP measurement is not required. A 2.7 Some instruments scan automatically only in one direc-
measurement reliable within 10 or 20 % is always sufficient, tion, which may not be the one specified in the test procedure.
but often many readings must be cascaded to obtain the final Manual control or scanning by hand may be necessary.
result, so that painstaking measurements are desirable. Particularly note any precautions called for by the manufac-
1.7 This standard does not purport to address the safety turer’s instructions in this case. If, for mechanical reasons,
problems associated with its use. It is the responsibility of the manual scanning is not feasible, the test procedure can be
user of this standard to establish appropriate safety and health reversed, after making the checks of 100 % line and zero, by
practices and determine the applicability of regulatory limita- going to the desired wavelength within the filter stop band,
tions prior to use. inserting neutral attenuators in the reference beam until the pen
lies between 10 and 100 % transmittance (preferably near
2. Summary of Test Method
10 %, widening the slits while doing so to restore normal pen
2.1 The following test procedure is written for
response, scanning in reverse until 100 % transmittance is
transmittance-recording double-beam instruments. Modifica-
reached, and removing the reference beam attenuators one at a
tion for absorbance recording will be obvious. It is readily
time while measuring their attenuation, until all are out and a
adapted for point-by-point operation by measuring at enough
region where the filter is transparent is reached.
points to allow reasonably accurate interpolation between the
2.7.1 The only difficulty with this procedure lies in finding
points so as to provide essentially continuous information over
the narrower slit width that will give normal pen response after
the wavelength region covered, or by direct measurement at the
each reference beam attenuator is removed, and finding the
desired specific wavelength.
transmittance change at that slit width. Several trials may be
2.2 After establishing that the instrument is adequately free
required.
of zero transmittance error, and measuring the baseline
2.7.2 When done carefully, this procedure gives the same
(100 %) over the appropriate spectral range, insert a filter into
result as the normal one, and in fact affords a way of checking
the sample beam and record its transmittance curve, starting in
the standard test results whenever both scan directions are
a region where it is nearly transparent and scanning slowly
permitted.
toward increasing absorption. The filter materials are selected
3. Significance and Use
for sharp cutoff, freedom from fluorescence, and sufficiently
high absorption that their transmittance in the stop band can be
3.1 Stray radiant power can be a significant source of error
neglected. They should be visually clear and free of bubbles in spectrophotometric measurements, and the danger that such
and striae. SRP will then set the limit to the minimum
error exists is enhanced because its presence is often unsus-
transmittance observed, unless an adverse signal-to-noise ratio, pected. SRP usually increases with the passage of time;
a scale limitation of the photometer, or a false electrical zero
therefore testing should be performed periodically. This test
intervenes. method provides a means of determining the stray radiant
2.3 These limitations may usually be overcome by increas- power ratio of a spectrophotometer, and so revealing those
ing the slit width to admit more incident radiant power on the wavelength regions where significant photometric errors might
filter. In a double-beam instrument, this may be accomplished occur. It does not provide a means of calculating corrections to
by attenuating the reference beam with a screen, mask, r indicated absorbance (or transmittance) values. The test
neutral filter. The following procedure assumes the use of method must be used with care and understanding, as errone-
etched screens, and measures their transmittance by their effect ous results can occur, especially with respect to some modern
on the instrument reading. grating instruments that incorporate blocking filters.
2.4 The apparent transmittance of the filter is sensitive to
4. Apparatus and Material
spectral bandwidth. Since bandwidth increases with slit width,
4.1 Screens of Etched Nickel, useful for reference beam
it is necessary that the transmittance of the screen be measured
attenuators are listed in Table 1 and footnote 4. They may also
at a fixed slit width each time. The slits may then be allowed
to open for the following steps of the test. The effect of such
changes in slit width on the apparent transmittance of the filter
Available in sheet form from Perforated Products, Inc., 68 Harvard St.,
becomes insignificant at the end of the test when the SRP
Brookline, Mass. 02146. Also as cut disks in holders from Varian Instrument Service
becomes the dominant source of signal. Center, 670 East Arques Ave., Sunnyvale, CA 94086. These holders fit the Cary
spectrophotometer cell blocks for cylindrical cells. They do not fit holders for
2.5 The scanning is continued, and the reference beam is
rectangular cells. The holders are also useful for glass filters for Bouguer’s law
attenuated further in as many steps as required, until the steeply
absorbance linearity tests.
falling transmittance curve levels off because of SRP. The
Screen and Holder Approximate Identification Nominal
product of all the reference attenuation fractions times the
Part Number Mesh Number Absorbance
value of the transmittance reading at the plateau, corrected by
1404111 30 25 K 0.5
the baseline value and the filter reflection loss, is taken as the
1404112 60 30 R 1.1
SRPR.
1404113 120 40 T 1.5
1404114 270 50 W 2.0
2.6 The SRPR can in some cases increase with increasing
slit width (see Annex A4). Therefore, the smallest practicable
E 387
TABLE 1 Perforated Screens
exact transmittance varies with location in the beam, hence
Nominal Nominal they should be mounted so they can be positioned reproduc-
A
Identification Number
Transmittance Absorbance
ibly, and mechanical stability of all parts of the spectrometer
20G 0.50 0.30
should be established. The transmittance of two or more
15H 0.41 0.39
screens used in cascade or tandem is rarely equal to the product
15L 0.29 0.54
of their separate transmittances, and may be quite sensitive to
10N 0.20 0.70
12 ⁄2 P 0.14 0.84
lateral displacements with respect to each other, due to the
10S 0.07 1.20
moiré effect.
30T 0.03 1.50
4.2 Test Filter Materials, shown in Table 2, provide an array
20W 0.01 2.00
A
capable of covering nearly all normal ultraviolet and infrared
The identification number is that of Perforated Products, Inc., 68 Harvard St.,
Brookline, Mass. 02146. Screens from this manufacturer are the only ones so far
spectral ranges. The first column shows the approximate
tested in committee work. Screens may be available from other manufacturers.
spectral range over which remote SRPR determinations can be
made. The test wavelength to be used with any given test filter
be convenient for “quick check” transmittance “standards.”
will depend on the design and performance of the instrument
The screens should be placed as far as possible from each under test, and so must be determined empirically (Note 3).
other, and preferably at a point along the beam where it is as The test wavelength shall be that at which the true transmit-
wide as possible. Usually a 1-cm spacing between screens tance of the test filter becomes a negligibly small fraction of the
suffices to allow acceptable reproducibility of results. Their observed transmittance (Note 4 and Note 5). The second
TABLE 2 Filters for Tests for Stray Radiant Power Ratio
B
Transmittance, 80 % Wavelength or
A C D E
Spectral Range Filter Source Detector
Wavenumber
A. Sharp Cutoff Types
F
165 to 173.5 nm 183 nm 0.01 cm H O UV UV
F
170 to 183.5 nm 195 nm 1.00 cm H O UV UV
F
175 to 200 nm 214 nm 1.00 cm 12 g/L KCl aqueous UV UV
F
195 to 223 nm 232 nm 100 cm 10 g/L NaBr aqueous UV UV
210 to 259 nm 271 nm 1.00 cm 10 g/L NaI aqueous UV UV
250 to 320 nm 339 nm 1.00 cm acetone UV UV
300 to 385 nm 420 nm 1.00 cm 50 g/L NaNO aqueous VIS UV
−1 −1 G
2050 to 1200 cm 2800 cm 2.0-mm fused silica (2) IR IR
−1 −1
1140 to 800 cm 1760 cm 6 mm LiF IR IR
−1 −1
760 to 600 cm 1240 cm 6mmCaF IR IR
−1 −1
630 to 400 cm 1030 cm 6 mm NaF IR IR
−1 −1
410 to 250 cm 650 cm 6 mm NaCl IR IR
−1 −1
240 to 200 cm 420 cm 6 mm KBr IR IR
B. Pass-Band Filters
600 to 660 nm . 1.00 cm 0.005 % methylene blue VIS VIS or NIR
H
aqueous
I
1.66 to 1.75 μm . 5.0 cm CH Br NIR NIR
2 2
A
The shorter wavelength (or lower wavenumber) limits given are nominal. The selection of a test filter should be made in accordance with 4.2. Longer wavelength (or
−4
higher wavenumber, for infrared range) gives 10 transmittance point.
B
Transmittance value not corrected for refl
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