ASTM E275-93
(Practice)Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers
Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers
SCOPE
1.1 This practice covers the description of requirements of spectrophotometric performance especially for ASTM methods, and the testing of the adequacy of available equipment for a specific method. The tests give a measurement of some of the important parameters controlling results obtained in spectrophotometric methods, but it is specifically not to be concluded that all the factors in instrument performance are measured.
1.1.1 This practice is not to be used (1) as a rigorous test of performance of instrumentation, or (2) to intercompare the quantitative performance of instruments of different design.
1.1.2 This practice is primarily directed to dispersive spectrophotometers used for transmittance measurements rather than instruments designed for diffuse transmission and diffuse reflection.
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Designation: E 275 – 93
Standard Practice for
Describing and Measuring Performance of Ultraviolet,
Visible, and Near-Infrared Spectrophotometers
This standard is issued under the fixed designation E 275; 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.
This standard has been approved for use by agencies of the Department of Defense.
INTRODUCTION
In developing a spectrophotometric method it is the responsibility of the originator to describe the
instrumentation and the performance required to duplicate the precision and accuracy of the method.
It is necessary to specify this performance in terms that may be used by others in applications of the
method.
The tests and measurements described in this practice are for the purpose of determining the
experimental conditions required for a particular analytical method. In using this practice an analyst
has either a particular analysis for which he describes requirements for instrument performance, or he
expects to test the capability of an instrument to perform a particular analysis. To accomplish either
of these objectives it is necessary that instrument performance be obtained in terms of the factors that
control the analysis. Unfortunately, it is true that not all the factors that can affect the results of an
analysis are readily measured and easily specified for the various types of spectrophotometric
equipment.
Of the many factors that control analytical results, this practice covers selection of the setting of
analytical wavelength, selection of slit width, photometric measurements, and characteristics of
absorption cells as the parameters of spectrophotometry that are likely to be affected by the analyst in
obtaining data. Other important factors, particularly those primarily dependent on instrument design,
are not covered in this practice.
1. Scope E 169 Practices for General Techniques of Ultraviolet-
Visible Quantitative Analysis
1.1 This practice covers the description of requirements of
E 387 Test Method for Estimating Stray Radiant Power
spectrophotometric performance especially for ASTM meth-
Ratio of Spectrophotometers by the Opaque Filter Method
ods, and the testing of the adequacy of available equipment for
a specific method. The tests give a measurement of some of the
3. Terminology
important parameters controlling results obtained in spectro-
3.1 Definitions:
photometric methods, but it is specifically not to be concluded
3.1.1 For definitions of terms used in this practice, refer to
that all the factors in instrument performance are measured.
Terminology E 131.
1.1.1 This practice is not to be used (1) as a rigorous test of
performance of instrumentation, or (2) to intercompare the
4. Significance and Use
quantitative performance of instruments of different design.
4.1 This practice permits an analyst to compare the general
2. Referenced Documents performance of his instrument, as he is using it in a specific
spectrophotometric method, with the performance of instru-
2.1 ASTM Standards:
ments used in developing the method.
E 131 Terminology Relating to Molecular Spectroscopy
E 168 Practices for General Techniques of Infrared Quanti-
5. Reference to This Practice in Standards
tative Analysis
5.1 Reference to this practice in any ASTM spectrophoto-
metric method (preferably in the section on apparatus where
This practice is under the jurisdiction of ASTM Committee E-13 on Molecular
the spectrophotometer is described) shall constitute due noti-
Spectroscopy and is the direct responsibility of Subcommittee E13.01 on Ultraviolet
fication that the adequacy of the spectrophotometer perfor-
and Visible Spectroscopy.
Current edition approved Dec. 15, 1993. Published February 1994. Originally
mance is to be evaluated by means of this practice. Perfor-
published as E 275 – 65 T. Last previous edition E 275 – 83 (1989).
mance is considered to be adequate when the instrument can be
Annual Book of ASTM Standards, Vol 03.06.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 275
operated in a manner to give test results equivalent to those recommendations. These tests should include a measurement
obtained on instruments used in establishing the method or in of the following factors in instrument operation, or their
cooperative testing of the method. equivalent:
5.2 It is recommended that the apparatus be described in 7.2.1 Ambient temperature,
terms of the results obtained on application of this practice to 7.2.2 Response time,
instruments used in establishing the method. This description 7.2.3 Signal-to-noise ratio,
should give a numerical value showing the wavelength accu- 7.2.4 Mechanical repeatability,
racy, wavelength repeatability, and photometric repeatability 7.2.5 Scanning speed for recording instruments, and
found to give acceptable results. A recommended spectral slit 7.2.6 Optical stability (for diode array).
width maximum should be given along with typical spectra of 7.3 Each of the factors in instrument operation is important
the components to be determined to indicate the resolution in the measurement of analytical wavelength and photometric
found to be adequate to perform the analysis. If it is considered data. For example, changes in wavelength precision and
necessary in a particular analysis, the use of only the linear accuracy can occur because of variation of ambient tempera-
portion of an analytical curve (absorbance per centimetre ture of various parts of a monochromator, particularly a crystal
versus concentration) may be specified, or if nonlinearity is prism. The correspondence of the absorbance to wavelength
encountered, the use of special calculation methods may be and any internal calculations (or corrections) can affect wave-
specified. However, it is not permissible to specify the amount length measurement for digital instruments. In recording spec-
of curvature if a nonlinear working curve is used. trophotometers there is always some lag between the recorded
reading and the correct reading. It is necessary to select the
6. Parameters in Spectrophotometry
conditions of operation to make this effect negligible or
6.1 Any spectrophotometer may be described as a source of
repeatable. Scanning speeds should be selected to make sure
radiant energy, a dispersing optical element, and a detector that the detecting system can follow the signal from narrow
together with a photometer for measuring relative radiant
emission lines or absorption bands. Too rapid scanning will
power. Accurate spectrophotometry involves a large number of displace the apparent wavelength toward the direction scanned
interrelated factors that determine the quality of the radiant
and peak absorbance readings will vary with speed of scan-
energy passing through a sample and the sensitivity and ning. A change in instrument response-time may produce
linearity with which this radiant energy may be measured.
apparent wavelength shifts. Mechanical repeatability of the
Assuming proper instrumentation and its use, the instrumental
various parts of the monochromator and recording system and
factors responsible for inaccuracies in spectrophotometry are
positioning of chart paper are important in wavelength mea-
resolution, linearity, stray radiant energy, and cell constants.
surement. Successive batches of chart paper should be checked
Rigorous measurement of these factors is beyond the scope of
for uniformity of the chart spectrum length, particularly if the
this practice. The measurement of stray radiant energy is
paper has been subjected to pronounced humidity changes.
described in Method E 387.
Instructions on obtaining proper mechanical repeatability are
6.2 Modern spectrophotometers are capable of more accu-
usually given in the manufacturer’s literature.
racy than most analysts obtain. The problem lies in the
7.4 Digital spectrophotometers and diode array spectropho-
selection and proper use of instrumentation. In order to ensure
tometers may require a calibration routine to be completed
proper instrumentation and its use in a specific spectrophoto-
prior to measurement of wavelength or absorbance accuracy.
metric method, it is necessary for an analyst to evaluate certain
Consult the manufacturer’s manual for any such procedures.
parameters that can control the results obtained. These param-
WAVELENGTH ACCURACY AND PRECISION
eters are wavelength accuracy and precision, spectral slit
width, photometry, and absorption-cell constants. Unsatisfac-
8. Nature of Test
tory measurement of any of these parameters may be due to
8.1 Most spectrophotometric methods employ pure com-
improper instrumentation or to improper use of available
pounds or known mixtures for the purpose of calibrating
instrumentation. It is therefore first necessary to determine that
instruments photometrically at specified analytical wave-
instrument operation is in accordance with the manufacturer’s
lengths. The wavelength at which an analysis is made is read
recommendations. Tests shall then be made to determine the
from the dial of the monochromator, from the digital readout,
performance of an instrument in terms of each of the param-
from an attached computer, or from a chart in recording
eters in 6.1 and 6.2.
instruments. To reproduce measurements properly, it is neces-
sary for the analyst to state the wavelength limits within which
7. Instrument Operation
the analytical wavelength is known.
7.1 In obtaining spectrophotometric data, the analyst must
8.2 The accompanying spectra are given to show the loca-
select the proper instrumental operating conditions in order to
tion of selected reference wavelengths which have been found
realize satisfactory instrument performance. Operating condi-
useful. Numerical values are given in wavelength units (na-
tions for individual instruments are best obtained from the
nometres or micrometres, measured in air). Reference (1)
manufacturer’s literature because of variations with instrument
tabulates additional reference wavelengths of interest.
design. A record should be kept to document the operating
conditions selected so that they may be duplicated.
7.2 Because tests for proper instrument operation vary with
The boldface numbers in parentheses refer to the list of references appended to
instrument design, it is necessary to rely on the manufacturer’s this practice.
E 275
9. Definitions ing element (polychomator) located after the sample compart-
ment.
9.1 wavelength accuracy—the deviation of the average
wavelength reading at an absorption band or emission band
NOTE 1—Several commercially available mercury arcs are satisfactory.
from the known wavelength of the band. They may differ, however, in the number of lines observed and in the
relative intensities of the lines because of differences in operating
9.2 wavelength precision—a measure of the ability of a
conditions. Low-pressure arcs such as the Beckman arc and the “Pen
spectrophotometer to return to the same spectral position as
Ray” quartz lamp or common germicidal lamps with transmitting enve-
measured by an absorption band or emission band of known
lopes have a high-intensity line at 253.65 nm, and other useful lines as
wavelength when the instrument is reset or read at a given
seen in Fig. 1 are satisfactory.
wavelength. The index of precision used in this practice is the
10.3 The absorption spectrum of holmium oxide glass (Fig.
standard deviation.
2) is obtained by measuring the transmittance or absorbance of
a piece of holmium oxide glass about 2 to 4 mm thick. The
10. Reference Wavelengths in the Ultraviolet Region
absorption spectrum of holmium oxide solution (Fig. 3) is
10.1 The most convenient spectra for wavelength calibra-
obtained similarly by measuring an approximately 4 % solu-
tion in the ultraviolet region are the emission spectrum of the
tion of holmium oxide in 1.4 M perchloric acid (40 g/L) with
low-pressure mercury arc (Fig. 1), the absorption spectra of
air as reference.
holmium oxide glass (Fig. 2), holmium oxide solution (Fig. 3),
10.4 The absorption spectrum of benzene is obtained by
and benzene vapor (Fig. 4).
measuring the absorbance of a 1-cm cell filled with vapor (Fig.
10.2 The mercury emission spectrum is obtained by illumi-
4). The sample is prepared by placing 1 or 2 drops of liquid
nating the entrance slit of the monochromator with a quartz
benzene in the cell, pouring out the excess liquid, and
mercury arc or by a mercury arc that has a transmitting
stoppering the cell. Some care must be exercised to ensure that
envelope (Note 1). It is not necessary, when using an arc
source, that the arc be in focus on the entrance slit of the
monochromator. However, it is advantageous to mount the
The Beckman mercury arc is manufactured by Beckman Instruments, Inc.,
lamp reasonably far from the entrance slit in order to minimize Fullerton, Calif., and is most useful on the Beckman DU Spectrophotometer. The
“Pen Ray” quartz lamp is manufactured by Ultraviolet Products, Inc., San Gabriel,
the scatter from the edges of the slit. Displacement of the
Calif. Its small size makes it convenient to use. Both are available from equipment
source will not shift the apparent wavelength as long as the slit
distributors.
widths used are small, that is, less than 0.1 mm. Reference
Holmium oxide glass is available as a polished filter, 2-mm thick, in 2-in.
(51-mm) squares designated as Corning Color Filter CS3-142, Glass No. 3131, from
wavelengths for diode array spectrophotometers can be ob-
F.J. Gray & Co., 139 Queens Blvd., Jamaica, NY 11435, and other industrial
tained by placing a low-pressure mercury discharge lamp in the
materials distributors.
sample compartment. It is not necessary to put the reference 6
Holmium oxide may be obtained from Lindsay Chemical Division, American
source in the lamp compartment for systems with the dispers- Potash and Chemical Corp., West Chicago, IL.
Line Number Wavelength, nm Line Number Wavelength, nm Line Number Wavelength, nm Line Number Wavelength, nm
1 253.65 4 313.16 7 404.66 10 546.07
2 296.73 5 334.15 8 407.78 11 576.96
3 302.15 6 365.01 9 435.84 12 579.07
Instrument: Cary Model 14 Slit Width: 0.03 mm
Scanning Speed: 2.5 A/s Spectral Slit Width: 0.10 to 0.15 nm
FIG. 1 Mercury Arc Emission Spectrum in the Ultraviolet and Visible Regions Showing Reference Wavelength
E 275
Band Number Wavelength, nm Band Number Wavelength, n
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