ASTM E1507-98
(Guide)Standard Guide for Describing and Specifying the Spectrometer of an Optical Emission Direct-Reading Instrument
Standard Guide for Describing and Specifying the Spectrometer of an Optical Emission Direct-Reading Instrument
SCOPE
1.1 This guide lists and discusses features of a spectrometer or polychromator used for optical emission, direct-reading, spectrochemical analysis. A polychromator in this sense consists of a spectrometer with an extended and fixed wavelength range and an array of fixed exit slits to isolate the spectral lines of the elements to be measured.
1.1.1 This guide does not apply to direct-reading systems that employ echelle spectrometers and vidicon or other detectors, where the design parameters are quite different.
1.2 This guide covers only the optical portion of the instrument, from excitation stand to photomultipliers.
1.2.1 Only general statements are made about source units.
1.2.2 Photomultipliers are included to the extent that they are mounted within the spectrometer to convert optical intensities to electrical signals, and establish the instrumental precision of each channel as a light measuring device. Readout systems are not included.
1.3 It is not the purpose of this guide to establish binding specifications or tolerances, but rather, to call attention to important parameters that manufacturers should include in their literature, to provide methods for measuring those parameters, and to assign values that are indicative of acceptably good performance. Because of the great variety of demands imposed by spectrochemical techniques, rigid performance criteria are not feasible.
1.4 The values stated in SI units are to be regarded as the standard.
1.5 This standard does not purport to address all of the safety problems, 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.
1.6 A partial listing of the information in this guide includes the following:
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 1507 – 98
Standard Guide for
Describing and Specifying the Spectrometer of an Optical
Emission Direct-Reading Instrument
This standard is issued under the fixed designation E 1507; 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.
1. Scope
Background Equivalent Concentration (BEC) 3.2.1
Channel 3.2.2
1.1 This guide lists and discusses features of a spectrometer
Readability 3.2.3
or polychromator used for optical emission, direct-reading,
Scattered Radiation 3.2.4
Shot Noise 3.2.5
spectrochemical analysis. A polychromator in this sense con-
White Light Precision 3.2.6
sists of a spectrometer with an extended and fixed wavelength
Working Resolution 3.2.7
range and an array of fixed exit slits to isolate the spectral lines Fundamental Spectrochemical Objectives 5
Accuracy 5.2
of the elements to be measured.
Precise Photometry 5.2.1
1.1.1 This guide does not apply to direct-reading systems
Bias in Photometry 5.2.2
that employ echelle spectrometers and vidicon or other detec- Detection and Determination Limits 5.3
Minimization of Interference 5.4
tors, where the design parameters are quite different.
Parameters of Spectrometer and Associated Components 6
1.2 This guide covers only the optical portion of the
Dispersion, Reciprocal Linear 6.2
instrument, from excitation stand to photomultipliers.
Working Resolution 6.3
Speed or Radiation Throughput 6.4
1.2.1 Only general statements are made about source units.
White Light Precision 6.5
1.2.2 Photomultipliers are included to the extent that they
Scattered Light 6.6
are mounted within the spectrometer to convert optical inten- Slits 6.7
Accuracy of Positioning Exit Slits 6.7.3
sities to electrical signals, and establish the instrumental
Wavelength Coverage and Focal Curve Length 6.8
precision of each channel as a light measuring device. Readout
Secondary Optics 6.9
systems are not included. Optical Stability 6.10
Spectrometer Illumination 6.11
1.3 It is not the purpose of this guide to establish binding
Astigmatic Image 6.12
specifications or tolerances, but rather, to call attention to
Excitation Stands 6.13
Vacuum Systems 6.14
important parameters that manufacturers should include in
Flushing With Transparent Gas (Nitrogen) 6.15
their literature, to provide methods for measuring those param-
Other, Including Maintenance Features 6.16-6.18
eters, and to assign values that are indicative of acceptably
Measuring Specified Polychromator Parameters 7
good performance. Because of the great variety of demands Working Resolution 7.1-7.4
Line Interference 7.5
imposed by spectrochemical techniques, rigid performance
White Light Precision 7.6
criteria are not feasible.
Scattered Light 7.7
1.4 The values stated in SI units are to be regarded as the Optical Alignment and Focus 7.8
Optical Stability 7.9
standard.
Precision and Accuracy 7.10
1.5 This standard does not purport to address all of the
Describing the Spectrometer in Analytical Methods 8
Wavelength Coverage and Reciprocal Dispersion 8.2.1
safety problems, if any, associated with its use. It is the
Working Resolution 8.2.2
responsibility of the user of this standard to establish appro-
Entrance and Exit Slit Widths 8.2.3
priate safety and health practices and determine the applica-
Low Concentrations 8.2.4
High Concentrations 8.2.5
bility of regulatory limitations prior to use.
Varying Parameters on Working Resolution and Throughput Appendix X1
1.6 A partial listing of the information in this guide includes
Dispersion X1.2
the following:
Size of Spectrometer X1.3
Effects of Slit Widths X1.4
Section
Recommended Spectral Lines for Measuring Working Reso- Appendix X2
ution
Terminology 3
Photographic Speed Versus Photoelectric Throughput Appendix X3
Photographic Speed X3.1
Polychromator Radiation Throughput X3.2
Calculation of Radiation Throughput X3.3
This guide is under the jurisdiction of ASTM Committee E-1 on Analytical Typical Radiation Throughputs X3.4
Chemistry for Metals, Ores, and Related Materials and is the direct responsibility of
Subcommittee E01.20 on Fundamental Practices.
Current edition approved May. 10, 1998. Published July 1998.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1507
2. Referenced Documents 4. Significance and Use
2.1 ASTM Standards: 4.1 Direct-reading polychromators are instruments com-
E 135 Terminology Relating to Analytical Chemistry for
monly used for multi-element spectrochemical analysis. This
Metals, Ores, and Related Materials guide seeks to describe those aspects of such instruments that
E 172 Practice for Describing and Specifying the Excitation
are of significance in achieving useful spectrochemical perfor-
Source in Emission Spectrochemical Analysis mance. Awareness of parameters described in this practice will
E 356 Practices for Describing and Specifying the Spec-
make manufacturers cognizant of factors they should consider
trograph in designing instruments, assist purchasers of instruments in
E 380 Practice for Use of the International System of Units
making intelligent comparisons of competing designs, and
(SI) (The Modernized Metric System) make users aware of the compromises they must make in
E 520 Practice for Describing Detectors in Emission and
performing particular determinations.
Absorption Spectroscopy 4.2 Adequate description of spectrometers permits forming
E 876 Practice for Use of Statistics in the Evaluation of
qualified appraisals on three important performance character-
Spectrometric Data istics: accuracy of analysis, detection limits, and freedom from
line interferences.
3. Terminology
3.1 Definitions—For definitions of terms used in this guide, 5. Fundamental Spectrochemical Objectives
refer to Terminology E 135.
5.1 The analyst is interested in three important performance
3.2 Definitions of Terms Specific to This Standard:
characteristics of an overall direct-reading system: precision,
3.2.1 background equivalent concentration (BEC)—the
particularly in how it affects the accuracy of a signal; detection;
concentration of an element at which the signal due to the
and freedom from or minimization of interferences.
analytical line is equal to the signal from a specimen with zero
5.2 Accuracy depends on precision and the absence of bias.
concentration of that element.
In a spectrometer, accuracy depends on the precision of
3.2.2 channel—the combination of exit slit and photomul-
photometric measurement with minimal bias introduced by
tiplier positioned to receive the radiation of a specific spectral
scattered radiation or line interference.
line. It includes any mirror, refractor plate, filter, or other items
5.2.1 Precise photometry is the ability to closely repeat
in the exit optical path.
readings from the excitation of a homogeneous specimen on
3.2.3 readability—the minimum difference between signals
both short term and long term applications.
that can be perceived or distinguished.
5.2.1.1 Short term, and particularly long term, precision are
improved by the use of wide slits and with a marked difference
NOTE 1—Readability is of concern for earlier direct-reading spectrom-
eters that use analog or digital voltmeter displays. For direct-reading
between the widths of the entrance and exit slits. These
spectrometers that are interfaced with a computer, readability is replaced
conditions conflict with the requirements for sensitivity and
by standard deviation.
freedom from interference.
3.2.4 scattered radiation—that portion of the reading result-
5.2.1.2 Precision is improved by a compact, rugged spec-
ing from radiation at wavelengths different from the wave-
trometer construction that remains stable despite severe me-
length being measured, as a result of scatter by the dispersing
chanical vibration. Long term precision may require either a
medium or by surfaces within the spectrometer.
temperature-controlled spectrometer, a closely regulated tem-
3.2.5 shot noise—the minimum deviation in the measure-
perature in the laboratory, or a spectrometer construction that
ment of a signal due to the discreteness of the events being
adjusts or compensates for shifts in the spectrum that occur as
observed. In an optical emission spectrometer, the 88events” are
a result of changes in ambient temperature. In an air spectrom-
photons hitting a photomultiplier. Since the minimum standard
eter, atmospheric pressure can have a small effect on stability
deviation of the detection of photons is the square root of the
and precision.
total available photons, relative standard deviation is reduced
5.2.1.3 An important factor in short term stability is the
as the signal intensity increases.
“white light precision” for each of the channels. See descrip-
3.2.6 white light precision—the relative standard deviation
tion of white light in 6.5 and its precision measurement in 7.6.
of at least ten readings from a channel, either as an absolute
This precision is limited by the stability of the photomultipli-
intensity or as a ratio to an internal standard channel, when
ers. Frequently the most stable tubes are the least sensitive.
exposed to a stable radiation source.
5.2.1.4 Analytical precision depends on excitation condi-
3.2.7 working resolution—the ability of an exit slit to isolate
tions and specimen homogeneity, which are outside the scope
the spectral line being measured from possible nearby inter-
of this practice. Practice E 172 describes applicable excitation
fering lines of other elements. Working resolution can only be
source units. See also 5.4.1 on spectral interference.
measured with sharp spectral lines and may be finer than the
5.2.2 Bias in photometry signals can be minimized by
practical resolution imposed by the source conditions required
spectrometer design and selection of spectral lines.
for actual determinations.
5.2.2.1 Scattered radiation can be reduced by having fewer
components in the optical paths of radiation passing through
exit slits, by dulling neutral surfaces, or by masking.
Annual Book of ASTM Standards, Vol 03.05.
5.2.2.2 Bias is minimized when a selected analytical line
Annual Book of ASTM Standards, Vol 03.06.
Annual Book of ASTM Standards, Vol 14.02. shows an optimum response to changes in a concentration of
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1507
E 380, but is used throughout this guide because the most common
the element of interest, with little effect from other elements.
˚
wavelength tables employ angstroms. One A 5 0.1 nanometre.
5.3 Detection and determination limits, which are discussed
in Practice E 876, are favored by:
6.3 Working Resolution—The resolving power of the spec-
5.3.1 Improvement in the precision with which the overall
trometer has little bearing on the ability of the instrument to
signal plus background can be measured and distinguished
separate an analytical line from a nearby interfering line.
from the background alone.
Frequently it will seem to indicate a performance as much as
5.3.2 A higher signal-to-background ratio. Although im-
ten times better than can be achieved in practice. The more
provement in the ratio may be realized by the use of narrow
appropriate term is working resolution, expressed as the
slits, this may adversely affect precision and stability.
half-width of a line, as described and illustrated in 7.1. If there
5.3.3 Use of the most sensitive analytical line for each
is a significant difference, the half-width should be specified at
element.
the center and ends of the focal curve. Parameters that affect
5.3.3.1 In some cases, the most sensitive line of an element
working resolution are discussed in X1.2.2 and X1.3, and X1.4.
may have an interference from a strong line of another element.
Recommended spectral lines for measuring working resolution
In these cases, the best practical detection may be obtained by
are given in Appendix X2.
using weaker emission lines that do not suffer from interfer-
6.3.1 A specification for working resolution might be: the
ence.
half-widths listed below were obtained with a flat specimen of
5.3.3.2 For some sets of elements, access to all the desirable
low alloy steel, excited by a 5 A, d-c arc in air, with entrance
emission lines may require an extended wavelength range,
and exit slits of 25 and 50 μm, respectively:
which involves either reduced dispersion, a broader focal
˚ ˚ ˚
Wavelength: 1930 A 3110 A 4358 A
˚ ˚ ˚
plane, or auxiliary monochromators, or spectrometers. Re-
Half-Width: 0.20 A 0.35 A 0.45 A
duced dispersion may impair detection.
6.4 Speed or Radiation Throughput—It is a common mis-
5.4 Minimization of Interference—Interference in the signal
conception that a spectrometer with a high aperture ratio or
of a particular element may occur due to neighboring lines of
radiation throughput will yield improved detection. Measure-
other elements or molecular species. Interference may be
ment precision, however, which does affect detection, requires
reduced by the use of narrow slits or higher dispersion. Narrow
an adequate radiation throughput. The various factors that
slits, however, may affect photometric precision and detection.
determine the radiation throughput of a polychromator are
5.4.1 Different types of excitation sources can enhance or
described in Appendix X3. An interrelation of working reso-
suppress certain spectral interferences on identical spectrom-
lution and throughput is discussed in Appendix X1.
eters. A practical evaluation of a direct-reading instrument
6.4.1 Radiation throughput varies from channel to channel,
must consider excitation conditions. See also 5.2.1.4 on ana-
due primarily to the combination of entrance and exit slit
lytical precision.
widths and the blaze of the grating, but the signal that is
5.5 Analytical Compromises—The individual parameters of
obtained from a channel depends more on the characteristics of
a spectrometer are closely interrelated, and there is no one best
an individual photomultiplier than on the optical system. For
general solution. The characteristics may be modified to suite
optimum detection, the selection of a photomultiplier and the
the intended application.
voltage used to drive it must produce a signal of sufficient
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.