ASTM E387-04(2022)

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) more than several spectral bandwidths from
4.2.1 Manufacturers typically specify stray light, meaning
the spectral position of the spectrophotometer (3).
SRPR, at one or more wavelengths. Where sharp cutoff SRP
3.2.19 specified wavelength (wavenumber)—thewavelength
filtersareused,thespecifiedwavelengthsshouldbenear,buta
(wavenumber) specified by the manufacturer of a spectropho-
little toward the lower transmittance side, of the cutoff wave-
tometer (or by the spectroscopist) as that at which the SRPR is
length of the chosen SRP filter. Other wavelengths can be
stated (or measured).
specified by the spectroscopist, according to the need of
3.2.20 SRP—stray radiant power.
particularanalyses,usingsharpcutofffilterslistedinTable1or
sharp cutoff filters that are now available from various manu-
3.2.21 SRPR—stray radiant power ratio.
facturers and distributors. Cutoff wavelengths of some solu-
3.2.22 stray light—the term used in much technical and
tion filters for the ultraviolet and cutoff wavenumbers of some
manufacturer’s literature to represent either SRP or SRPR.
solid filters for the mid-infrared are given in Table 1. Where
4. Summary of Test Method
4.1 The following test procedures are written for spectro-
Sources of solution filters in sealed cuvettes, interference filters, glass filters,
photometers that have provision for recording (that is, for
neutral density filters, and materials for mid-infrared filters can be found inAnnual
collecting and storing) spectral data digitally. Processing may
BuyersGuidesofseveralscientificorganizations,inadvertisementsintradejournals
bebybuilt-inprogramsorinaseparatecomputer.Datamaybe
that serve optical and spectroscopic disciplines, in catalogs of suppliers of optical
and spectroscopic materials, and by searching the Internet, using concatenations of
collected in either the transmittance or the absorbance mode.
selected terms: filter, optical, stray light, color, absorbing, solution (or liquid)
The data sets to be collected are: (1) open-beam spectrum:
cuvette, spectrophotometer cell, interference, cutoff, sharp cut, longpass, shortpass,
100% transmittance or zero absorbance; (2) blocked-beam
bandpass, neutral density; for mid infrared materials: infrared cells, infrared
spectrum: 0%T, transmittance mode only; and (3) SRP filter crystals, infrared accessories, fused silica.
E387 − 04 (2022)
narrow blocking-band filters are used, the filters themselves 5. Significance and Use
define the specified wavelength.
5.1 Stray radiant power can be a significant source of error
in spectrophotometric measurements. SRP usually increases
NOTE5—Insomecases,manufacturersstateSRPRatawavelengthwell
removedfromthecutoffwavelengthofthecitedSRPfilter.Thiscanresult
with the passage of time; therefore, testing should be per-
in an appreciable underestimate of the true SRPR at the specified
formed periodically. Moreover, the SRPR test is an excellent
wavelength.Usersarecautionedtonotecarefullythespecificinformation
indicator of the overall condition of a spectrophotometer. A
provided about the test used to determine the stated SRPR.
control-chartrecordoftheresultsofroutinelyperformedSRPR
4.2.2 The SRP filter materials are selected for sharp cutoff,
tests can be a useful indicator of need for corrective action or,
freedom from fluorescence, and sufficiently high absorption
at least, of the changing reliability of critical measurements.
that their transmittance in the stop band can be neglected.
5.2 This test method provides a means of determining the
Liquid (solution) filters should be visually clear and free of
stray radiant power ratio of a spectrophotometer at selected
bubbles; cuvette windows should be free of striae. SRP will
wavelengthsinaspectralrange,asdeterminedbytheSRPfilter
then set the limit to the minimum transmittance (maximum
used, thereby revealing those wavelength regions where sig-
absorbance) observed, unless an adverse signal-to-noise ratio
nificant photometric errors might occur. It does not provide a
orlimitingdynamicrangeofthespectrophotometerintervenes.
means of calculating corrections to indicated absorbance (or
4.2.3 Open-beam, blocked-beam (zero% transmittance),
transmittance) values. The test method must be used with care
and SRP filter spectra are recorded over the nominal wave-
and understanding, as erroneous results can occur, especially
length range of the spectrophotometer in which the specified
with respect to some modern grating instruments that incorpo-
wavelength lies, and the filter spectrum is corrected
rate moderately narrow bandpass SRP-blocking filters. This
(automatically, in the case of some instruments). The limiting
test method does not provide a basis for comparing the
transmittance (absorbance), indicated by the leveling off or
performance of different spectrophotometers.
increase (decrease) of the transmittance (absorbance)
NOTE 8—Kaye (3) discusses correction methods of measured transmit-
spectrum, is adjusted for the transmittance of the SRP filter in
tances (absorbances) that sometimes can be used if sufficient information
its high transmittance passband. This result is the estimated
on the properties and performance of the instrument can be acquired. See
SRPR. (If SRP is small enough that the limiting transmittance also A1.2.5.
(absorbance) is not observed, see 4.2.4.)
5.3 This test method describes the performance of a spec-
trophotometer in terms of the specific test parameters used.
NOTE 6—For a single monochromator instrument, inspection of the
When an analytical sample is measured, absorption by the
spectral curve may show, by where the transmittance (absorbance) levels
off or starts to rise (fall), the wavelength limit of reliable use of the
sample of radiation outside of the nominal bandpass at the
instrument. That limit might be set by SRP or by other instrumental
analytical wavelength can cause a photometric error, underes-
limitations (for example, dynamic range).
timating the transmittance or overestimating the absorbance,
4.2.4 SRP in double monochromator instruments is too
and correspondingly underestimating the SRPR.
small for the limiting transmittance to be observed without
5.4 The SRPR indicated by this test method using SRP
using increased reference attenuation.This is accomplished by
filters is almost always an underestimation of the true value
inserting a calibrated neutral filter into the reference beam of a
(see1.3).Avaluecitedinamanufacturer’sliteraturerepresents
double beam spectrophotometer and recording the spectrum of
the performance of a new instrument, tested exactly in accor-
the SRP filter. It might be necessary to increase slit width in
dancewiththemanufacturer’sspecification.Theimplicationis
order to obtain an acceptable signal-to-noise ratio (S/N). For a
that the manufacturer’s stated SRPR can serve as a benchmark
single beam spectrophotometer, the spectrum of the neutral
for future performance, provided that the user performs the
filterisrecordedandusedasadivisorofthecorrectedtestfilter
manufacturer’sspecifiedtest.Itisrecommendedthatuserstest
spectrum (this will succeed only if the dynamic range of
new instruments promptly, thereby establishing a comparative
instrument is adequate).
benchmarkintermsoftheirowntestingfacilities.Thesolution
NOTE7—Electronicscaleexpansionmaybeused,providedthattheS/N filter ratio method (4.3) is a convenient method for control-
is acceptable.
charting SRPR. Mielenz, et al., (4) show that its results tend to
correlate well with those of the specified wavelength method,
4.3 Solution Filter Ratio Method:
but for critical comparison with the manufacturer’s
4.3.1 This method (4) uses a solution filter from Table 1,
specification, the method used by the manufacturer must be
Part A, and so is intended for testing only in the ultraviolet
used. Because some instruments reduce SRP by incorporating
range of a spectrophotometer. The sample beam filter is a
moderately narrow bandpass SRP-blocking filters that are
10mm pathlength cuvette containing the solution, and the
changed as the wavelength range is scanned, it is possible for
reference beam filter is a 5mm pathlength cuvette containing
SRPR determinations to be highly inaccurate if the cutoff
the same solution. Alternatively, the reference beam filter can
wavelength of the SRP filter falls too close to the absorption
be a 10mm pathlength cuvette containing the solution diluted
edge of an instrument’s SRP-reducing filter (3).
to one-half concentration. This test can be performed with a
single beam instrument by recording the two solution filter
6. Apparatus and Materials
spectra sequentially and calculating their ratio. However, this
will not provide the benefit of reducing the needed dynamic 6.1 Liquidcellsfortheultravioletshouldhavelowfluoresc-
range of the instrument that is gained by the double beam ingfusedsilicawindows;thoseforthevisibleandnearinfrared
measurement. may be of less expensive glass. Neutral filters must be
E387 − 04 (2022)
approximately constant in transmittance over the full wave- 6.2.3 Plates of Alkali Halide, about 6mm thick for absorp-
length range of the photodetector’s sensitivity; that is, for tion cell windows are commonly on hand in analytical labora-
ultraviolet and visible testing, from the shortest usable wave- tories or can be obtained from dispensers of infrared cells, and
length in the ultraviolet to the long wavelength end of the the 80% transmittance points are specified for this thickness.
visible range. Recommended neutral (neutral density, ND) However, other thicknesses, over a range from about 4mm to
filters are the “metal-on-quartz” type, that is, evaporated metal 15mm, can be substituted without invalidating the test.
on fused silica substrate. Recommended optical densities are 6.2.4 Fused Silica, in the form of cell windows, is com-
1.0, 2.0, and 3.0. It should not be necessary to stack neutral monly available and is useful over a range of thickness of
filters to have optical densities greater than 3.0. If stacking 1mmto6mm.Crystalquartzshouldnotbeusedbecauseofits
must be done, separate these highly reflecting filters and tilt birefringence, which may cause apparent cyclical transmit-
them slightly to avoid multiple reflection into the beam path. tance variations with wavelength.
6.2 SRP Filter Materials,suchasshowninTable1,provide
7. Hazards
an array capable of covering nearly all normal ultraviolet and
7.1 Narrow blocking-band filters, referenced for use in
infrared spectral ranges. The first column shows the cutoff
A1.2.2, using benzene and, as described by Tunnicliff (5), hot
wavelength (wavenumber). The test wavelength to be used
mercury vapor, should be handled with proper precaution.
with any given SRP filter will depend on the design and
performance of the instrument under test, and so must be 8. Procedure
determined empirically (Note 3). The test wavelength shall be
8.1 Specified Wavelength Method:
that at which the true transmittance of the SRP filter becomes
8.1.1 Record an open-beam (100% transmittance or zero
a negligibly small fraction of the observed transmittance
absorbance) and a blocked-beam (0% transmittance spectrum
(Notes 5 and 6). The second column (Table 1) shows the
in the transmittance mode) over the nominal wavelength range
approximate 80% transmittance wavelength or wavenumber.
of the spectrophotometer that includes the specified wave-
Scanning for the following procedure should always begin at
length.
this point, or at one more remote from the test spectral range.
8.1.2 Insert the SRP filter into the sample beam (and,
optionally, a blank solution in the reference beam ).
NOTE9—OncethetestwavelengthhasbeenestablishedforaSRPfilter
8.1.3 For a single monochromator instrument, record the
and an instrument of any given design, the test is applicable to all
instruments of the same design.
SRPfilterspectrum.Correctitwithstoredopen-beamandzero
NOTE 10—The true transmittance of a SRPfilter can be determined by
transmittance spectra. Inspect the spectral curve for indication
measuringthespectrumofadilutesolutionorathinspecimenoftheSRP
of a limiting transmittance (absorbance). If such be present,
filter material and using Beer’s law to extrapolate the transmittance to the
calculate the SRPR. Otherwise, proceed in accordance with
concentration or thickness employed in the test for SRPR.
8.1.4.
NOTE 11—For testing grating spectrophotometers that use moderately
narrowbandpassSRP-blockingfilters,useaSRPfilterthatcutsoffsharply 8.1.4 For a double monochromator instrument (and a very
at a wavelength as near as possible to the edge of the bandpass of the
low SRP single monochromator instrument), insert into the
instrument’s SRP-blocking filter that is normally in the beam at the
reference beam a “neutral” beam attenuator, that is, a neutral
designated wavelength, if known. If necessary, consult the manufacturer,
filter (or a built-in optical attenuator, for example, a perforated
or test in accordance with the manufacturer’s stated method. In any case,
metal screen) of which the transmittance at or near the
it is strongly recommended that the test wavelength itself be as close as
possible to the transmission cutoff of the SRP filter in order to minimize specifiedwavelengthofthetestisknown.Recordthespectrum
absorption of SRP by the test filter.
of the SRP filter and correct it. If necessary to have adequate
S/N, increase the slit width and repeat the measurement.
6.2.1 SRP filters (and analytical samples) should be large
enough to cover the entire cross-sectional area of the optical
NOTE 12—As indicated in Annex A4, the change in slit width may
beam with a substantial safety margin. Radiation scattered in
change the value of SRP.
the sample compartment can sometimes bypass the SRP filter
8.2 Solution Filter Ratio Method:
(analytical sample), re-enter the optical beam, and reach the
8.2.1 Record the open-beam and blocked-beam (0% trans-
photodetector. If the determined SRPR appears to be large
mittance) spectra in accordance with 8.1.1.
enough to bias a measurement significantly, use an opaque
8.2.2 Select a solution from Table 1 that has a cutoff
mask in the sample compartment that intercepts any bypassing
wavelength at or near the desired wavelength for the test.
radiation, to test for this source of SRP.
8.2.3 Insert into the sample beam of the spectrophotometer
6.2.2 If there is any possibility that fluorescence of
a 10mm pathlength cuvette filled with the solution. Insert into
windows,cells,orsamplesolventsmaybecontributingtoSRP
th
...