ASTM E1507-98(2003)
(Guide)Standard Guide for Describing and Specifying the Spectrometer of an Optical Emission Direct-Reading Instrument (Withdrawn 2003)
Standard Guide for Describing and Specifying the Spectrometer of an Optical Emission Direct-Reading Instrument (Withdrawn 2003)
SIGNIFICANCE AND USE
Direct-reading polychromators are instruments commonly used for multi-element spectrochemical analysis. This guide seeks to describe those aspects of such instruments that are of significance in achieving useful spectrochemical performance. Awareness of parameters described in this practice will make manufacturers cognizant of factors they should consider in designing instruments, assist purchasers of instruments in making intelligent comparisons of competing designs, and make users aware of the compromises they must make in performing particular determinations.
Adequate description of spectrometers permits forming qualified appraisals on three important performance characteristics: accuracy of analysis, detection limits, and freedom from line interferences.
SCOPE
1.1 This guide covers 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.
WITHDRAWN RATIONALE
This guide covered 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.
Formerly under the jurisdiction of Committee E01 on Analytical Chemistry for Metals, Ores, and Related Materials, this guide was withdrawn in May 2012 because the information contained therein is obsolete and is not useful.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E1507 – 98 (Reapproved 2003)
Standard Guide for
Describing and Specifying the Spectrometer of an Optical
Emission Direct-Reading Instrument
This standard is issued under the fixed designation E1507; 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.
1. Scope 1.6 Apartiallistingoftheinformationinthisguideincludes
the following:
1.1 This guide covers features of a spectrometer or poly-
Section
chromator used for optical emission, direct-reading, spectro-
chemical analysis.Apolychromator in this sense consists of a
Terminology 3
spectrometerwithanextendedandfixedwavelengthrangeand
Background Equivalent Concentration (BEC) 3.2.1
Channel 3.2.2
an array of fixed exit slits to isolate the spectral lines of the
Readability 3.2.3
elements to be measured.
Scattered Radiation 3.2.4
1.1.1 This guide does not apply to direct-reading systems
Shot Noise 3.2.5
White Light Precision 3.2.6
that employ echelle spectrometers and vidicon or other detec-
Working Resolution 3.2.7
tors where the design parameters are quite different.
Fundamental Spectrochemical Objectives 5
1.2 This guide covers only the optical portion of the Accuracy 5.2
Precise Photometry 5.2.1
instrument, from excitation stand to photomultipliers.
Bias in Photometry 5.2.2
1.2.1 Only general statements are made about source units.
Detection and Determination Limits 5.3
1.2.2 Photomultipliers are included to the extent that they Minimization of Interference 5.4
Parameters of Spectrometer and Associated Components 6
are mounted within the spectrometer to convert optical inten-
Dispersion, Reciprocal Linear 6.2
sities to electrical signals, and establish the instrumental
Working Resolution 6.3
precisionofeachchannelasalightmeasuringdevice.Readout Speed or Radiation Throughput 6.4
White Light Precision 6.5
systems are not included.
Scattered Light 6.6
1.3 It is not the purpose of this guide to establish binding
Slits 6.7
specifications or tolerances, but rather, to call attention to Accuracy of Positioning Exit Slits 6.7.3
Wavelength Coverage and Focal Curve Length 6.8
important parameters that manufacturers should include in
Secondary Optics 6.9
theirliterature,toprovidemethodsformeasuringthoseparam-
Optical Stability 6.10
eters, and to assign values that are indicative of acceptably Spectrometer Illumination 6.11
Astigmatic Image 6.12
good performance. Because of the great variety of demands
Excitation Stands 6.13
imposed by spectrochemical techniques, rigid performance
Vacuum Systems 6.14
Flushing With Transparent Gas (Nitrogen) 6.15
criteria are not feasible.
Other, Including Maintenance Features 6.16-6.18
1.4 The values stated in SI units are to be regarded as the
Measuring Specified Polychromator Parameters 7
standard.
Working Resolution 7.1-7.4
1.5 This standard does not purport to address all of the Line Interference 7.5
White Light Precision 7.6
safety problems, if any, associated with its use. It is the
Scattered Light 7.7
responsibility of the user of this standard to establish appro-
Optical Alignment and Focus 7.8
Optical Stability 7.9
priate safety and health practices and determine the applica-
Precision and Accuracy 7.10
bility of regulatory limitations prior to use.
Describing the Spectrometer in Analytical Methods 8
Wavelength Coverage and Reciprocal Dispersion 8.2.1
Working Resolution 8.2.2
Entrance and Exit Slit Widths 8.2.3
Low Concentrations 8.2.4
This guide is under the jurisdiction of ASTM Committee E01 on Analytical
High Concentrations 8.2.5
ChemistryforMetals,Ores,andRelatedMaterialsandisthedirectresponsibilityof
Varying Parameters on Working Resolution and Throughput Appendix X1
Subcommittee E01.20 on Fundamental Practices.
Dispersion X1.2
Current edition approved June 10, 2003. Published September 2003. Originally
Size of Spectrometer X1.3
approved in 1993. Last previous edition approved in 1998 as E1507-98. DOI:
Effects of Slit Widths X1.4
10.1520/E1507-98R03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1507 – 98 (2003)
intensity or as a ratio to an internal standard channel, when
Recommended Spectral Lines for Measuring Working Reso- Appendix X2
ution
exposed to a stable radiation source.
Photographic Speed Versus Photoelectric Throughput Appendix X3
3.2.7 working resolution—theabilityofanexitslittoisolate
Photographic Speed X3.1
the spectral line being measured from possible nearby inter-
Polychromator Radiation Throughput X3.2
Calculation of Radiation Throughput X3.3
fering lines of other elements. Working resolution can only be
Typical Radiation Throughputs X3.4
measured with sharp spectral lines and may be finer than the
practical resolution imposed by the source conditions required
2. Referenced Documents
for actual determinations.
2.1 ASTM Standards:
E135 Terminology Relating to Analytical Chemistry for
4. Significance and Use
Metals, Ores, and Related Materials
E172 Practice for Describing and Specifying the Excitation 4.1 Direct-reading polychromators are instruments com-
Source in Emission Spectrochemical Analysis monly used for multi-element spectrochemical analysis. This
E356 Practices for Describing and Specifying the Spec-
guide seeks to describe those aspects of such instruments that
trograph are of significance in achieving useful spectrochemical perfor-
E380 Practice for Use of the International System of Units
mance.Awareness of parameters described in this practice will
(SI) (The Modernized Metric System) make manufacturers cognizant of factors they should consider
E520 Practice for Describing Photomultiplier Detectors in
in designing instruments, assist purchasers of instruments in
Emission and Absorption Spectrometry making intelligent comparisons of competing designs, and
E876 Practice for Use of Statistics in the Evaluation of
make users aware of the compromises they must make in
Spectrometric Data performing particular determinations.
4.2 Adequate description of spectrometers permits forming
3. Terminology
qualified appraisals on three important performance character-
3.1 Definitions—For definitions of terms used in this guide,
istics:accuracyofanalysis,detectionlimits,andfreedomfrom
refer to Terminology E135.
line interferences.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 background equivalent concentration (BEC)—the
5. Fundamental Spectrochemical Objectives
concentration of an element at which the signal due to the
5.1 The analyst is interested in three important performance
analytical line is equal to the signal from a specimen with zero
characteristics of an overall direct-reading system: precision,
concentration of that element.
particularlyinhowitaffectstheaccuracyofasignal;detection;
3.2.2 channel—the combination of exit slit and photomul-
and freedom from or minimization of interferences.
tiplier positioned to receive the radiation of a specific spectral
5.2 Accuracy depends on precision and the absence of bias.
line.Itincludesanymirror,refractorplate,filter,orotheritems
In a spectrometer, accuracy depends on the precision of
in the exit optical path.
photometric measurement with minimal bias introduced by
3.2.3 readability—the minimum difference between signals
scattered radiation or line interference.
that can be perceived or distinguished.
5.2.1 Precise photometry is the ability to closely repeat
NOTE 1—Readability is of concern for earlier direct-reading spectrom-
readings from the excitation of a homogeneous specimen on
eters that use analog or digital voltmeter displays. For direct-reading
both short term and long term applications.
spectrometers that are interfaced with a computer, readability is replaced
5.2.1.1 Short term, and particularly long term, precision are
by standard deviation.
improvedbytheuseofwideslitsandwithamarkeddifference
3.2.4 scattered radiation—thatportionofthereadingresult-
between the widths of the entrance and exit slits. These
ing from radiation at wavelengths different from the wave-
conditions conflict with the requirements for sensitivity and
length being measured, as a result of scatter by the dispersing
freedom from interference.
medium or by surfaces within the spectrometer.
5.2.1.2 Precision is improved by a compact, rugged spec-
3.2.5 shot noise—the minimum deviation in the measure-
trometer construction that remains stable despite severe me-
ment of a signal due to the discreteness of the events being
chanical vibration. Long term precision may require either a
observed.Inanopticalemissionspectrometer,the“events”are
temperature-controlled spectrometer, a closely regulated tem-
photons hitting a photomultiplier. Since the minimum standard
perature in the laboratory, or a spectrometer construction that
deviation of the detection of photons is the square root of the
adjusts or compensates for shifts in the spectrum that occur as
total available photons, relative standard deviation is reduced
aresultofchangesinambienttemperature.Inanairspectrom-
as the signal intensity increases.
eter, atmospheric pressure can have a small effect on stability
3.2.6 white light precision—the relative standard deviation
and precision.
of at least ten readings from a channel, either as an absolute
5.2.1.3 An important factor in short term stability is the
“white light precision” for each of the channels. See descrip-
tion of white light in 6.5 and its precision measurement in 7.6.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This precision is limited by the stability of the photomultipli-
Standards volume information, refer to the standard’s Document Summary page on
ers. Frequently the most stable tubes are the least sensitive.
the ASTM website.
5.2.1.4 Analytical precision depends on excitation condi-
Withdrawn. The last approved version of this historical standard is referenced
on www.astm.org. tions and specimen homogeneity, which are outside the scope
E1507 – 98 (2003)
of this practice. Practice E172 describes applicable excitation mator, and should be specified in all analytical methods. All
source units. See also 5.4.1 on spectral interference. gratings display small changes of dispersion for different
5.2.2 Bias in photometry signals can be minimized by wavelengths. The calculation of dispersion is described in
spectrometer design and selection of spectral lines. X1.2. Specify diffraction order as well as the reciprocal linear
5.2.2.1 Scattered radiation can be reduced by having fewer dispersion at a wavelength critical to an application. For
˚
components in the optical paths of radiation passing through example: Reciprocal linear dispersion at 3000 A: in the first
˚
exit slits by dulling neutral surfaces or by masking. order, 4.1 A/mm; in the second order, 2.0 A/mm.
5.2.2.2 Bias is minimized when a selected analytical line
NOTE 2—The angstrom is a depreciated unit according to Practice
shows an optimum response to changes in a concentration of
E380, 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=0.1 nanometre.
5.3 Detection and determination limits, which are discussed
6.3 Working Resolution—The resolving power of the spec-
in Practice E876, are favored by:
trometer has little bearing on the ability of the instrument to
5.3.1 Improvement in the precision with which the overall
separate an analytical line from a nearby interfering line.
signal plus background can be measured and distinguished
Frequently it will seem to indicate a performance as much as
from the background alone.
ten times better than can be achieved in practice. The more
5.3.2 A higher signal-to-background ratio. Although im-
appropriate term is working resolution, expressed as the
provement in the ratio may be realized by the use of narrow
half-width of a line, as described and illustrated in 7.1. If there
slits, this may adversely affect precision and stability.
is a significant difference, the half-width should be specified at
5.3.3 Use of the most sensitive analytical line for each
the center and ends of the focal curve. Parameters that affect
element.
workingresolutionarediscussedinX1.2.2andX1.3,andX1.4.
5.3.3.1 In some cases, the most sensitive line of an element
Recommendedspectrallinesformeasuringworkingresolution
mayhaveaninterferencefromastronglineofanotherelement.
are given in Appendix X2.
In these cases, the best practical detection may be obtained by
6.3.1 A specification for working resolution might be: the
using weaker emission lines that do not suffer from interfer-
half-widths listed below were obtained with a flat specimen of
ence.
low alloy steel, excited by a 5 A, dc arc in air, with entrance
5.3.3.2 Forsomesetsofelements,accesstoallthedesirable
and exit slits of 25 and 50 µm, respectively:
emission lines may require an extended wavelength range,
˚ ˚ ˚
Wavelength: 1930 A 3110 A 4358 A
which involves either reduced dispersion, a broader focal
˚ ˚ ˚
Half-Width: 0.20 A 0.35 A 0.45 A
plane, or auxiliary monochromators, or spectrometers. Re-
6.4 Speed or Radiation Throughput—It is a common mis-
duced dispersion may impair detection.
conception that a spectrometer with a high aperture ratio or
5.4 Minimization of Interference—Interference in the signal
radiation throughput will yield improved detection. Measure-
of a particular element may occur due to neighboring lines of
ment precision, however, which does affect detection, requires
other elements or molecular species. Interference may be
an adequate radiation throughput. The various factors that
reducedbytheuseofnarrowslitsorhigherdispersion.Narrow
determine the radiation throughput of a polychromator are
slits, however, may affect photometric precision and detection.
described in Appendix X3. An interrelation of working reso-
5.4.1 Different types of excitation sources can enhance or
lution and throughput is discussed in Appendix X1.
suppress certain spectral interferences on identical spectrom-
6.4.1 Radiation throughput varies from channel to channel,
eters. A practical evaluation of a direct-reading instrument
due primarily to the combination of entrance and exit slit
must consider excitation conditions. See also 5.2.1.4 on ana-
widths and the blaze of the grating, but the signal that is
lytical precision.
obtainedfromachanneldependsmoreonthecharacteristicsof
5.5 Analytical Compromises—The individual parameters of
a
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