ASTM E387-04(2022)
(Test Method)Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method
Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method
SIGNIFICANCE AND USE
5.1 Stray radiant power can be a significant source of error in spectrophotometric measurements. SRP usually increases with the passage of time; therefore, testing should be performed periodically. Moreover, the SRPR test is an excellent indicator of the overall condition of a spectrophotometer. A control-chart record of the results of routinely performed SRPR tests can be a useful indicator of need for corrective action or, at least, of the changing reliability of critical measurements.
5.2 This test method provides a means of determining the stray radiant power ratio of a spectrophotometer at selected wavelengths in a spectral range, as determined by the SRP filter used, thereby revealing those wavelength regions where significant photometric errors might occur. It does not provide a means of calculating corrections to indicated absorbance (or transmittance) values. The test method must be used with care and understanding, as erroneous results can occur, especially with respect to some modern grating instruments that incorporate moderately narrow bandpass SRP-blocking filters. This test method does not provide a basis for comparing the performance of different spectrophotometers.
Note 8: Kaye (3) discusses correction methods of measured transmittances (absorbances) that sometimes can be used if sufficient information on the properties and performance of the instrument can be acquired. See also A1.2.5.
5.3 This test method describes the performance of a spectrophotometer in terms of the specific test parameters used. When an analytical sample is measured, absorption by the sample of radiation outside of the nominal bandpass at the analytical wavelength can cause a photometric error, underestimating the transmittance or overestimating the absorbance, and correspondingly underestimating the SRPR.
5.4 The SRPR indicated by this test method using SRP filters is almost always an underestimation of the true value (see 1.3). A value cited in a manufacturer’s li...
SCOPE
1.1 Stray radiant power (SRP) can be a significant source of error in spectrophotometric measurements, and the danger that such error exists is enhanced because its presence often is not suspected (1-4).2 This test 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 bandpass transmitted through the monochromator of an absorption spectrophotometer. Test-filter materials are described that discriminate between the desired wavelengths and those that contribute most to SRP for conventional commercial spectrophotometers used in the ultraviolet, the visible, the near infrared, and the mid-infrared ranges. These procedures apply to instruments of conventional design, with usual sources, detectors, including array detectors, and optical arrangements. The vacuum ultraviolet and the far infrared present special problems that are not discussed herein.
Note 1: Research (3) has shown that particular care must be exercised in testing grating spectrophotometers that use moderately narrow bandpass SRP-blocking filters. Accurate calibration of the wavelength scale is critical when testing such instruments. Refer to Practice E275.
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 very short wavelengths in the ultraviolet, or of lower frequencies in the infrared. Sharp cutoff longpass filters are available for testing for shorter wavelength SRP in the visible and the near infrared, and sharp cutoff shortpass filters are available for testing at longer visible wavelengths. The procedures 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...
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This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E387 − 04 (Reapproved 2022)
Standard Test Method for
Estimating Stray Radiant Power Ratio of Dispersive
Spectrophotometers by the Opaque Filter Method
This standard is issued under the fixed designation E387; 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.
NOTE 2—Instruments with array detectors are inherently prone to
1. Scope
having higher levels of SRP. SeeAnnexes for the use of filters to reduce
1.1 Strayradiantpower(SRP)canbeasignificantsourceof
SRP.
error in spectrophotometric measurements, and the danger that
1.3 TheproportionofSRP(thatis,SRPR)encounteredwith
such error exists is enhanced because its presence often is not
a well-designed monochromator, used in a favorable spectral
suspected (1-4). This test method affords an estimate of the
region, typically is 0.1% transmittance or better, and with a
relative radiant power, that is, the Stray Radiant Power Ratio
-6
double monochromator it can be less than 1×10 , even with a
(SRPR), at wavelengths remote from those of the nominal
broadband continuum source. Under these conditions, it may
bandpasstransmittedthroughthemonochromatorofanabsorp-
be difficult to do more than determine that it falls below a
tion spectrophotometer. Test-filter materials are described that
certain level. Because SRP test filters always absorb some of
discriminate between the desired wavelengths and those that
theSRP,andmayabsorbanappreciableamountifthespecified
contribute most to SRP for conventional commercial spectro-
measurement wavelength is not very close to the cutoff
photometers used in the ultraviolet, the visible, the near
wavelength of the SRP filter, this test method underestimates
infrared, and the mid-infrared ranges. These procedures apply
the true SRPR. However, actual measurement sometimes
to instruments of conventional design, with usual sources,
requires special techniques and instrument operating condi-
detectors, including array detectors, and optical arrangements.
tions that are not typical of those occurring during use. When
The vacuum ultraviolet and the far infrared present special
absorption measurements with continuum sources are being
problems that are not discussed herein.
made, it can be that, owing to the effect of slit width on SRP
NOTE1—Research (3)hasshownthatparticularcaremustbeexercised
in testing grating spectrophotometers that use moderately narrow band-
inadoublemonochromator,thesetestproceduresmayoffsetin
pass SRP-blocking filters.Accurate calibration of the wavelength scale is
some degree the effect of absorption by the SRP filter; that is,
critical when testing such instruments. Refer to Practice E275.
because larger slit widths than normal might be used to admit
1.2 These procedures are neither all-inclusive nor infallible.
enough energy to the monochromator to permit evaluation of
Because of the nature of readily available filter materials, with
the SRP, the stray proportion indicated could be greater than
afewexceptions,theproceduresareinsensitivetoSRPofvery
wouldnormallybeencounteredinuse(buttheneteffectisstill
short wavelengths in the ultraviolet, or of lower frequencies in
more likely to be an underestimation of the true SRPR).
the infrared. Sharp cutoff longpass filters are available for
Whether the indicated SRPR equals or differs from the
testing for shorter wavelength SRP in the visible and the near
normal-use value depends on how much the SRP is increased
infrared, and sharp cutoff shortpass filters are available for
with the wider slits and on how much of the SRP is absorbed
testing at longer visible wavelengths. The procedures are not
by the SRPfilter. What must be accepted is that the numerical
necessarily valid for “spike” SRP nor for “nearby SRP.” (See
valueobtainedfortheSRPRisacharacteristicoftheparticular
Annexesforgeneraldiscussionanddefinitionsoftheseterms.)
test conditions as well as of the performance of the instrument
However, they are adequate in most cases and for typical
in normal use. It is an indication of whether high absorbance
applications. They do cover instruments using prisms or
measurements of a sample are more or less likely to be biased
gratings in either single or double monochromators, and with
by SRP in the neighborhood of the analytical wavelength
single and double beam instruments.
where the sample test determination is made.
1.4 The principal reason for a test procedure that is not
This test method is under the jurisdiction of ASTM Committee E13 on
exactly representative of normal operation is that the effects of
Molecular Spectroscopy and Separation Science and is the direct responsibility of
SRP are “magnified” in sample measurements at high absor-
Subcommittee E13.01 on Ultra-Violet, Visible, and Luminescence Spectroscopy.
Current edition approved Nov. 1, 2022. Published November 2022. Originally
bance. It might be necessary to increase sensitivity in some
approvedin1969.Lastpreviouseditionapprovedin2014asE387–04(2014).DOI:
way during the test in order to evaluate the SRP adequately.
10.1520/E0387-04R22.
This can be accomplished by increasing slit width and so
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. obtaining sufficient energy to allow meaningful measurement
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E387 − 04 (2022)
of the SRPafter the monochromatic energy has been removed wavelength absorption edge. The rate of change of transmit-
by the SRP filter. However, some instruments automatically tance in the absorption edge may not be as fast as for sharp
increase sensitivity by increasing dynode voltages of the cutoff filters.
photomultiplier detector. This is particularly true of high-end
3.2.2 blocked-beam spectrum—aspectrumrecordedwithan
double monochromator instruments in their ultraviolet and
opaque (that is, transmittance less than 0.001%) object in the
visible ranges. A further reason for increasing energy or
sample beam; the level of opacity must exist over the range of
sensitivity can be that many instruments have only absorbance
wavelengths where the photodetector is sensitive.
scales, which obviously do not extend to zero transmittance.
3.2.3 corrected spectrum—the transmittance (absorbance)
Even a SRP-proportion as large as 1% may fall outside the
spectrum of a SRPfilter after the measured spectrum has been
measurement range.
adjusted for the offset of the open-beam spectrum and (trans-
NOTE 3—Instruments that have built-in optical attenuators to balance
sample absorption may make relatively inaccurate measurements below mittance mode) of the blocked-beam spectrum.
10% transmittance, because of poor attenuator linearity. The spectropho-
3.2.4 cutoff wavelength (wavenumber)—the wavelength
tometer manufacturer should be consulted on how to calibrate transmit-
(wavenumber) at which the transmittance of a sharp cutoff
tance of the attenuator at such lower level of transmittance.
filter is 0.01%.
1.5 High accuracy in SRP measurement is not always
required; a measurement reliable within 10 or 20% may be
3.2.5 filter, longpass—an optical filter having high transmit-
sufficient. However, regulatory requirements, or the needs of a
tance at wavelengths longer than its absorption edge.
particular analysis, may require much higher accuracy. Pains-
3.2.6 filter, moderately narrow bandpass SRP-blocking—a
taking measurements are always desirable.
filter used to reduce remote SRP by transmitting efficiently
1.6 The values stated in SI units are to be regarded as
over a limited band of wavelengths within a nominal wave-
standard. No other units of measurement are included in this
length range of a spectrophotometer.
standard.
3.2.7 filter, narrow blocking-band—an optical filter having
1.7 This standard does not purport to address all of the
high transmittance at shorter and at longer wavelengths than a
safety concerns, if any, associated with its use. It is the
narrow band within which the transmittance is very low (that
responsibility of the user of this standard to establish appro-
is, less than 0.001%).
priate safety, health, and environmental practices and deter-
3.2.8 filter, narrow transmission band—an optical filter
mine the applicability of regulatory limitations prior to use.
having very low transmittance at shorter and longer wave-
1.8 This international standard was developed in accor-
lengths than those of a narrow band within which some
dance with internationally recognized principles on standard-
transmittances exceed 10%.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- 3.2.9 filter, neutral (also, neutral density: ND)—a filter that
mendations issued by the World Trade Organization Technical attenuates the radiant power reaching the detector by the same
Barriers to Trade (TBT) Committee. factor at all wavelengths within a prescribed wavelength
region.
2. Referenced Documents
3.2.10 filter, opaque—an optical filter that has transmit-
2.1 ASTM Standards: tances less than 0.01% over a specified band of wavelengths.
E131Terminology Relating to Molecular Spectroscopy
3.2.11 filter, sharp cutoff—an optical filter that has a very
E275PracticeforDescribingandMeasuringPerformanceof
rapid transition in wavelengths (wavenumbers) from a state of
Ultraviolet and Visible Spectrophotometers
high transmittance to a state of very low transmittance (that is,
less than 0.001%) and that continues in that low transmittance
3. Terminology
state to at least the end of the spectral region that is being
3.1 Definitions: tested.
3.1.1 For definitions of terms used in this test method, refer
3.2.12 filter, shortpass—a sharp cutoff filter having a high
to Terminology E131.
transmittance at wavelengths shorter than its absorption edge.
3.2 Definitions of Terms Specific to This Standard:
3.2.13 filter, SRP—a test filter for determining SRPR.
3.2.1 absorption edge—of a sharp cutoff filter: the wave-
3.2.14 limiting transmittance (absorbance)—the minimum
length interval over which the transmittance changes rapidly
transmittance (maximum absorbance) of the SRP filter that is
from high to very low (that is, less than 0.01%).
observed in the SRPR test; the transmittance (absorbance)
3.2.1.1 Discussion—Thebandpasstransmittancefiltersused
indicatedwhenthespectralcurvelevelsofforstartstoincrease
in some spectrophotometers to reduce SRP within their band-
(decrease).
passareconsideredtohavebothashortwavelengthandalong
3.2.15 near SRP—stray radiant power of wavelengths
(wavenumbers) within several spectral bandwidths from the
For referenced ASTM standards, visit the ASTM website, www.astm.org, or spectral position of the spectrophotometer (3).
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.2.16 open-beam spectrum—the spectrum recorded with
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. no attenuating medium in the sample beam.
E387 − 04 (2022)
TABLE 1 Filters for Tests for Stray Radiant Power Ratio
B
Cutoff Wavelength,
Transmittance, 80 % C D E
Filter Source Detector
A
Wavenumber Wavelength or Wavenumber
A. Sharp Cutoff Types
F
173.5 nm 183 nm 0.01 cm H O UV UV
F
183.5 nm 195 nm 1.00 cm H O UV UV
F
200 nm 214 nm 1.00 cm 12 g/L KCl aqueous UV UV
F
223 nm 232 nm 1.00 cm 10 g/L NaBr aqueous UV UV
259 nm 271 nm 1.00 cm 10 g/L NaI aqueous UV UV
259 nm 271 nm 1.00 cm 10 g/L KI aqueous UV UV
325 nm 339 nm 1.00 cm acetone UV UV
385 nm 420 nm 1.00 cm 50 g/L NaNO aqueous VIS UV
-1 -1 G
1200 cm 2800 cm 2.0-mm fused silica (2) IR IR
-1 -1
800 cm 1760 cm 6 mm LiF IR IR
-1 -1
600 cm 1240 cm 6mmCaF IR IR
-1 -1
400 cm 1030 cm 6 mm NaF IR IR
-1 -1
250 cm 650 cm 6 mm NaCl IR IR
-1 -1
200 cm 420 cm 6 mm KBr IR IR
B. Passband Filters
Approximate Stop Band . 1.00 cm 0.005 % (mass fraction) VIS VIS or NIR
H
600 nm to 660 nm methylene blue aqueous
I
1.66 mm to 1.75 mm . 5.0 cm CH Br NIR NIR
2 2
A -4
The wavelength (or wavenumber, for infrared range) gives 10 transmittance point.
B
Transmittance value not corrected for reflection loss.
C
Solution filters should be placed in sample cuvettes appropriate to the range covered. Solid filters are best-retained in metal holders.
D
Under “source” is tabulated the usual and appropriate source for each spectral range.
E
Considerable flexibility in detectors selected is common.
F
Apparentabsorbanceisstronglyaffectedbydissolvedoxygen.Bubblepurenitrogenthroughliquidforseveralminutesimmediatelybeforeuse.Useonlyrecentlydistilled
(not demineralized) water. Alternatively, use commercially available solution-in-sealed-cuvette filters.
G
Filterssuchasthese,whichabsorboverawiderangeintheinfrared,maybewarmedsufficientlybythesourcebeamtoreradiate,andsoproducesignificantzeroshifts
whichvarywithwavelengthandwithtimeofexposuretothebeam.Thiseffectisgreatlyreducedbyusingtwofilters,separatedbyatleast1cmalongthebeamaxis.The
re-radiation from the first is then mostly absorbed by the second.Aslightly less effective alternative is to use a LiF disc for the first filter. If zero shift is troublesome with
the LiF filter, a CaF disk can be used ahead of the LiF filter.
H
Passes blue to yellow light efficiently. The 0.005 % (mass fraction) methylene blue solution must be made up freshly from a 0.5 % (mass fraction) stock solutionin2%
(mass fraction) KH PO , preserved with 0.002 % (mass fraction) phenylmercuric acetate solution. User should test performance, which may vary with source of the
2 4
chemicals.
I
Passes from ultraviolet to 1.5 µm radiant power efficiently, except for a narrow, intense band at 1.4 µm, which is suitable for “nearby stray” evaluation in NIR grating
monochromators. Users should test performance, which may vary with source of the chemicals.
3.2.17 passband—of a monochromator, the band of wave- spectra. Filter spectra are assumed to have been corrected in
lengthsaroundthespectralpositionofthemonochromatorthat the following discussion.
NOTE 4—For instruments that lack digital recording capability, tradi-
are preferentially transmitted; of a sharp cutoff filter: the
tional methods of correcting open-beam and blocked-beam spectra must
wavelength region of high transmittance of the filter.
be applied.
3.2.18 remote SRP—stray radiant power of wavelengths
4.2 Specified Wavelength Method:
(wavenumbers) mor
...
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