ASTM E958-93(2005)
(Practice)Standard Practice for Measuring Practical Spectral Bandwidth of Ultraviolet-Visible Spectrophotometers
Standard Practice for Measuring Practical Spectral Bandwidth of Ultraviolet-Visible Spectrophotometers
SIGNIFICANCE AND USE
This practice should be used by a person who develops an analytical method to ensure that the spectral bandwidths cited in the practice are actually the ones used.
Note 1—The method developer should establish the spectral bandwidths that can be used to obtain satisfactory results.
This practice should be used to determine whether a spectral bandwidth specified in a method can be realized with a given spectrophotometer and thus whether the instrument is suitable for use in this application.
This practice allows the user of a spectrophotometer to determine the actual spectral bandwidth of the instrument under a given set of conditions and to compare the result to the spectral bandwidth calculated from data given in the manufacturer’literature or indicated by the instrument.
Instrument manufacturers can use this practice to measure and describe the practical spectral bandwidth of an instrument over its entire wavelength operating range. This practice is highly prefered to the general practice of stating the limiting or the theoretical spectral bandwidth at a single wavelength.
SCOPE
1.1 This practice describes a procedure for measuring the practical spectral bandwidth of a spectrophotometer in the wavelength region of 185 to 820 nm. Practical spectral bandwidth is the spectral bandwidth of an instrument operated at a given integration period and a given signal-to-noise ratio.
1.2 This practice is applicable to instruments that utilize servo-operated slits and maintain a constant period and a constant signal-to-noise ratio as the wavelength is automatically scanned. It is also applicable to instruments that utilize fixed slits and maintain a constant servo loop gain by automatically varying gain or dynode voltage. In this latter case, the signal-to-noise ratio varies with wavelength. It can also be used on instruments that utilize some combination of the two designs, as well as on those that vary the period during the scan. For digitized instruments, refer to the manufacturer's manual.
1.3 This practice does not cover the measurement of limiting spectral bandwidth, defined as the minimum spectral bandwidth achievable under optimum experimental conditions.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E958 − 93 (Reapproved2005)
Standard Practice for
Measuring Practical Spectral Bandwidth of Ultraviolet-
Visible Spectrophotometers
This standard is issued under the fixed designation E958; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This practice describes a procedure for measuring the
3.1 Definitions:
practical spectral bandwidth of a spectrophotometer in the
3.1.1 integration period, n—the time, in seconds, required
wavelength region of 185 to 820 nm. Practical spectral
for the pen or other indicator to move 98.6 % of its maximum
bandwidth is the spectral bandwidth of an instrument operated
travel in response to a step function.
at a given integration period and a given signal-to-noise ratio.
3.1.2 practical spectral bandwidth, designated by the sym-
1.2 This practice is applicable to instruments that utilize
bol:
servo-operated slits and maintain a constant period and a
π
constant signal-to-noise ratio as the wavelength is automati- ∆λ (1)
~ !
S/N
cally scanned. It is also applicable to instruments that utilize
where:
fixed slits and maintain a constant servo loop gain by auto-
∆λ = spectral bandwidth,
maticallyvaryinggainordynodevoltage.Inthislattercase,the
π = integration period, and
signal-to-noiseratiovarieswithwavelength.Itcanalsobeused
S/N = signal-to-noise ratio measured at or near 100 % T.
on instruments that utilize some combination of the two
designs, as well as on those that vary the period during the
3.1.3 signal-to-noise ratio, n—the ratio of the signal, S,to
scan. For digitized instruments, refer to the manufacturer’s
the noise, N, as indicated by the readout indicator. The
manual.
recommended measure of noise is the maximum peak-to-peak
excursion of the indicator averaged over a series of five
1.3 This practice does not cover the measurement of limit-
successive intervals, each of duration ten times the integration
ing spectral bandwidth, defined as the minimum spectral
bandwidthachievableunderoptimumexperimentalconditions. period. (This measure of noise is about five times the root-
mean-square noise.)
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.1.4 spectral bandwidth, n—the wavelength interval of
responsibility of the user of this standard to establish appro-
radiation leaving the exit slit of a monochromator measured at
priate safety and health practices and determine the applica-
halfthepeakdetectedradiantpower.Itisnotsynonymouswith
bility of regulatory limitations prior to use.
spectral slit width, which is the product of the mechanical slit
width and the reciprocal linear dispersion of the spectropho-
2. Referenced Documents
tometer.
2.1 ASTM Standards:
E131 Terminology Relating to Molecular Spectroscopy
4. Summary of Practice
E275 Practice for Describing and Measuring Performance of
4.1 The pen period and signal-to-noise ratio are set at the
Ultraviolet and Visible Spectrophotometers
desired values when the instrument is operated with its normal
light source and adjusted to read close to 100 % T. The
mechanical slit width, or the indicated spectral bandwidth,
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
Spectroscopy and Separation Science and is the direct responsibility of Subcom- required to give the desired signal-to-noise ratio is recorded.
mittee E13.01 on Ultra-Violet, Visible, and Luminescence Spectroscopy.
The continuum source is replaced with a line emission source,
Current edition approved April 1, 2005. Published April 2005. Originally
such as a mercury lamp, and the apparent half-intensity
approved in 1983. Last previous edition approved in 1999 as E958 – 93 (1999).
DOI: 10.1520/E0958-93R05. bandwidth of an emission line occurring in the wavelength
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
region of interest is measured using the same slit width, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
indicated spectral bandwidth, as was used to establish the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. signal-to-noise ratio with the continuum source.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E958 − 93 (2005)
TABLE 1 Emission Lines Useful for Measuring Spectral Bandwidth
Reference Line,
Emitter Intensity Nearest Neighbor, nm Separation, nm I /I Weak Neighbor, nm
Neighbor Reference
nm
184.91 Hg 8 194.17 9.26 0.13
194.17 Hg 8 184.91 9.26 0.13 197.33
205.29 Hg 4 202.70 2.59 0.08
226.22 Hg 5 237.83 11.61 0.06 226.03
253.65 Hg 10 . . . 253.48
275.28 Hg 5 280.35 5.07 0.08
289.36 Hg 6 296.73 7.37 0.42
296.73 Hg 8 302.15 5.42 0.04
318.77 He 5 294.51 24.26 0.06
334.15 Hg 7 313.18 20.97 0.70
341.79 Ne 5 344.77 2.98 0.20
359.35 Ne 5 352.05 7.30 0.14 360.02
388.87 He 7 447.15 58.28 0.04
404.66 Hg 8 407.78 3.12 0.04
427.40 Kr 5 431.96 4.56 0.28 428.30
435.95 Hg 9 407.78 28.17 0.02 435.75
447.15 He 5 471.31 24.16 0.04
471.31 He 4 492.19 20.88 0.25
486.0 D . . . .
486.13 H 6 492.87 6.74 0.03 485.66
501.57 He 5 492.19 9.38 0.06
546.07 Hg 8 577.12 31.05 0.04
557.03 Kr 3 587.09 30.06 0.30 556.22
587.56 He 7 706.52 118.96 0.03 667.82
603.00 Ne 5 607.43 4.43 0.54
614.31 Ne 7 616.36 2.05 0.04
626.65 Ne 6 630.48 3.83 0.07
640.23 Ne 7 638.30 1.93 0.11
656.1 D . . . .
656.28 H 7 . . . 656.99
667.82 He 5 706.52 38.70 0.50
692.95 Ne 6 703.24 10.29 0.45
703.24 Ne 7 692.95 10.29 0.06 702.41
724.52 Ne 5 703.24 21.28 0.02 717.39
743.89 Ne 4 724.52 19.37 1.4 748.89
785.48 Kr 3 769.45 16.03 0.7
819.01 Kr 2 811.29 7.72 3.1
5. Significance and Use instruments at the levels of resolution encountered in most
commercial instruments.All of the lines listed have widths less
5.1 This practice should be used by a person who develops
than 0.02 nm, suitable for measuring spectral bandwidths of
an analytical method to ensure that the spectral bandwidths
greater than 0.2 nm. The wavelengths of these lines in
cited in the practice are actually the ones used.
nanometres are listed in the first column. Values refer to
NOTE 1—The method developer should establish the spectral band-
measurements in standard air (760 nm, 15°C) except for the
widths that can be used to obtain satisfactory results.
two lines below 200 nm. The wavelength for these lines refer
5.2 This practice should be used to determine whether a
to a nitrogen atmosphere at 760 nm and 15°C.
spectral bandwidth specified in a method can be realized with
6.1.1 The second column in Table 1 lists the emitter gas of
a given spectrophotometer and thus whether the instrument is
six sources. Only sources operating at low pressure should be
suitable for use in this application.
used, as line broadening can introduce errors. The hydrogen,
deuterium, and mercury lamps used to obtain these data were
5.3 This practice allows the user of a spectrophotometer to
determine the actual spectral bandwidth of the instrument
under a given set of conditions and to compare the result to the
spectral bandwidth calculated from data given in the manufac-
turer’s literature or indicated by the instrument.
5.4 Instrument manufacturers can use this practice to mea-
sure and describe the practical spectral bandwidth of an
instrument over its entire wavelength operating range. This
practice is highly prefered to the general practice of stating the
limiting or the theoretical spectral bandwidth at a single
wavelength.
6. Test Materials and Apparatus
6.1 Table 1 lists reference emission lines that may be used
FIG. 1 Comparison of Measured and Calculated Spectral Band-
for measuring the spectral bandwidth of ultraviolet/visible widths
E958 − 93 (2005)
Beckman lamps operated on Beckman spectrophotometer determine the slit widths required to yield a given signal-to-
power supplies. The other lamps are all of the “pencil-lamp” noise ratio throughout the region of interest using the standard
3 4
type. Amercury vapor Pen-Ray lamp was used to obtain the continuum source of the instrument. Then use appropriate line
data shown in Fig. 1. In many applications the mercury and sources to illuminate the monochromator, and record the
hydrogen (or deuterium) lines suffice. spectral bandwidths obtained at the appropriate mechanical slit
6.1.2 Relativeintensitydataforthereferencelinesaregiven widths for the wavelengths in question.
in the third column of Table 1. The data refer to measurements
7.1.1 Although the integration period may be indicated on
made with a double prism-grating spectrophotometer equipped
the instrument or in the manufacturer’s literature, check the
with a silica window S-20 photomultiplier (RCA-C70109E).
value as follows:
These intensities will be different when using detectors of
7.1.1.1 For recording instruments, set the wavelength at any
different spectral sensitivity. They may also vary somewhat
convenient position and adjust the 0 and 100 % T controls for
amongsources.Allofthelinesareintenseones,butallmaynot
normal recorder presentation. Using 100 % T as the base line,
alwaysbesufficientlyintensetoallowthespectrophotometerto
block the sample beam and measure the time required for the
be operated with very narrow slit widths.
pen to reach the 2 % T level (Note 2).
6.1.3 Informationonnearestneighborsofappreciableinten-
NOTE 2—The time may be measured with a stopwatch or from the
sity is needed in order to set an upper limit on the measurable
distance the chart moves, if a fast chart speed recorder is being used.
spectral bandwidth. If the resolution of the instrument in
Integrationperiodsof1sorlesscanonlybeestimatedbyeithertechnique,
question is so poor that two lines or bands of the test source or
but generally this estimate is adequate to determine if the indicated period
sample overlap, the measured half bandwidth will not indicate is approximately correct.
the spectral bandwidth of the instrument. Very few of the lines
7.1.1.2 For instruments that can be operated only in the
listed in Table 1 are so well isolated from other lines of
absorbance mode, follow the same procedure, with the excep-
appreciable intensity that they could always be used without
tion that 0 A replaces 100 % T and 1.7 A replaces 2 % T.
interference or overlap. The atomic hydrogen (deuterium) line
7.1.2 The signal-to-noise ratio is measured as follows:
at 656 nm and the very intense mercury resonance line at 253
7.1.2.1 Set the instrument at a convenient wavelength and
nm fall in a category of “isolation,” but in all other cases
adjust the pen to read either 100 % T or 0 A. For low-noise
interfering lines are nearby. The nearest neighboring lines
levels use an expanded scale, if available.
having an intensity more than 15 % of the reference lines are
7.1.2.2 Adjust the slit width either to its normal value or to
given in the fourth column of Table 1. The separation in
a value that gives the desired signal-to-noise ratio.
nanometers between the reference and nearest neighbor lines is
7.1.2.3 Disengage the wavelength drive, start the chart
listed in the fifth column. In general, lines cannot be used for
drive, and allow the pen to record for at least 2 min or 50
a spectral bandwidth test when the spectral bandwidth exceeds
integration periods, whichever is longer.
one half the separation between reference and nearest neighbor
7.1.2.4 Divide the recording into five approximately equal
lines.
segments and determine the maximum peak-to-peak excursion
6.1.4 To some extent this rule can be modified by the
in each segment (Note 3).
relative intensities of neighbor to reference lines. This ratio,
I /I , is listed in column 6. Neighboring lines having
reference
NOTE 3—Care should be taken that the noise level is not partially
neighbor
an intensity less than 15 % of the reference lines will not
obscured by a detectable recorder dead zone.
seriously distort bandwidth measurements. However, to ac-
7.1.2.5 Average the five readings to obtain the noise, N.
commodate the possible situation of sources with intensity
7.1.2.6 If a % T recording is being used, divide 100 by N to
relationships different from that encountered in this study,
obtain the signal-to-noise ratio, S/N. If an absorbance record-
neighboring lines weaker than 15 % are tabulated in the
ing is being used, divide 0.43 by N to determine S/N.
seventh column under the heading “weak neighbor.”
7.1.2.7 The signal-to-noise ratio should be independent of
wavelength for a given source and detector combination, but it
7. Procedure
is advisable to check this point experimentally. For example,
7.1 Instruments with Servo-Operated Slits—These instru-
many instruments are operated with different slit programs in
ments maintain a constant period and signal-to-noise ratio as
the ultraviolet and visible regions and thus exhibit different
wavelength is automatically scanned. The determination of
signal-to-noise ratios in the two regions.
practical spectral bandwidth requires a preliminary determina-
7.1.3 Set the period and signal-to-noise ratio to the values
tion of the mechanical slit width necessary to yield a given
used in 7.1.1 and 7.1.2, scan to the wavelength of interest (see
signal-to-noise at a given integration period. This is best
Table 1), and record the resulting mechanical slit widths or
accomplished by first establishing the desired period. Next
spectral bandwidth (Note 4).
NOTE 4—It may be desirable to scan the entire wavelength range of the
instrument and record the slit width at suitable intervals so that a curve of
Suitable lamps are available from laboratory supply houses as well as
slit width versus wavelength may be constructed (usually 25- and 50-nm
manufacturers, which include UVP, Inc., 5100 Walnut Grove Ave., P.O. Box 1501,
San Gabriel, CA 91778-1501; Spectronics Corp., 956 Brush Hollow Rd., P.O. Box intervals are satisfactory for the ultraviolet and visible regions, respec-
483, Westbury, NY 11590-0483; Jelight Co., Inc., 23052Alcalde, Unit E, P.O. Box tively).
2632, Laguna Hills, CA 92653-2632; and BHK, Inc., 2885 Metropolitan Place,
7.1.4 Measure the spectral bandwidth of the instrument as
Pomona, CA 91767.
Available from UVP, Inc. follows:
E958 − 93 (2005)
The neighboring 302-nm line is clearly evident. It introduces a small
7.1.4.1 Position the appropriate line source so that it illumi-
error into the measure
...
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