ASTM E1172-87(2001)
(Practice)Standard Practice for Describing and Specifying a Wavelength-Dispersive X-Ray Spectrometer
Standard Practice for Describing and Specifying a Wavelength-Dispersive X-Ray Spectrometer
SCOPE
1.1 This practice describes the components of a wavelength-dispersive X-ray spectrometer that are basic to its operation and to the quality of its performance. It is not the intent of this practice to specify component tolerances or performance criteria, as these are unique for each instrument. The document does, however, attempt to identify which of these are critical and thus which should be specified.
1.2 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. Specific safety hazard statements are given in 5.3.1.2 and 5.3.1.2, and in Section 7.
1.2 There are several books and publications from the National Institute of Standards and Technology and the U.S. Government Printing Office, which deal with the subject of X-ray safety. Refer also to Practice E416.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: E 1172 – 87 (Reapproved 2001)
Standard Practice for
Describing and Specifying a Wavelength-Dispersive X-Ray
Spectrometer
This standard is issued under the fixed designation E 1172; 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 4. Significance and Use
1.1 This practice describes the components of a wavelength- 4.1 This practice describes the essential components of a
dispersive X-ray spectrometer that are basic to its operation wavelength-dispersive X-ray spectrometer. This description is
and to the quality of its performance. It is not the intent of this presented so that the user or potential user may gain a cursory
practice to specify component tolerances or performance understanding of the structure of an X-ray spectrometer sys-
criteria, as these are unique for each instrument. The document tem. It also provides a means for comparing and evaluating
does, however, attempt to identify which of these are critical different systems as well as understanding the capabilities and
and thus which should be specified. limitations of each instrument.
1.2 This standard does not purport to address all of the
5. Description of Equipment
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 5.1 Types of Spectrometers—X-ray spectrometers can be
classified as sequential, simultaneous, or a combination of
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. Specific safety these two (hybrid).
5.1.1 Sequential Spectrometers—The sequential spectrom-
hazard statements are given in 5.3.1.2 and 5.3.2.4, and in
Section 7. eter disperses and detects secondary X rays by means of an
adjustable monochromator called a goniometer. In flat-crystal
1.3 There are several books and publications from the
National Institute of Standards and Technology and the U.S. instruments, secondary X rays are emitted from the specimen
3,4
and nonparallel X rays are eliminated by means of a Soller slit
Government Printing Office which deal with the subject of
X-ray safety. Refer also to Practice E 416 . (collimator). The parallel beam of X rays strikes a flat
analyzing crystal which disperses the X rays according to their
2. Referenced Documents
wavelengths. The dispersed X rays are then measured by
2.1 ASTM Standards: suitable detectors. Adjusting the goniometer varies the angle
E 135 Terminology Relating to Analytical Chemistry for between the specimen, crystal, and detector, permitting the
Metals, Ores, and Related Materials measurement of different wavelengths and therefore different
E 416 Practice for Planning and Safe Operation of a Spec- elements. Sequential instruments containing curved-crystal
trochemical Laboratory optics are less common. This design substitutes curved for flat
E 876 Practice for Use of Statistics in the Evaluation of crystals and entrance and exit slits for collimators.
Spectrometric Data 5.1.2 Simultaneous Spectrometers—Simultaneous spec-
trometers use separate monochromators to measure each ele-
3. Terminology
ment. These instruments are for the most part of fixed
3.1 For terminology relating to X-ray spectrometry, refer to configuration, although some simultaneous instruments have a
Terminology E 135.
scanning channel with limited function. A typical monochro-
mator consists of an entrance slit, a curved (focusing) analyz-
ing crystal, an exit slit, and a suitable detector. Secondary X
This practice is under the jurisdiction of ASTM Committee E01 on Analytical
Chemistry for Metals, Ores and Related Materials and is the direct responsibility of rays pass through the entrance slit and strike the analyzing
Subcommittee E01.20 on Fundamental Practices.
crystal, which diffracts the wavelength of interest and focuses
Current edition approved June 26, 1987. Published October 1987.
it through the exit slit where it is measured by the detector.
NBS Handbook, X-Ray Protection, HB76, and NBS Handbook 111, ANSI
Some simultaneous instruments use flat crystals, but this is less
N43.2-1971, available from National Institute of Standards and Technology,
Gaithersburg, MD 20899.
common.
Radiation Safety Recommendations for X-Ray Diffraction and Spectrographic
5.1.3 Hybrid Spectrometers—Hybrid spectrometers com-
Equipment, No. MORP 68-14, 1968, available from U.S. Department of Health,
bine features found in sequential and simultaneous instru-
Education, and Welfare, Rockville, MD 20850.
U.S. Government Handbook 93, Safety Standards for Non-Medical X-Ray and ments. They have both fixed channels and one or more fully
Sealed Gamma-Ray Sources, Part 1, General, Superintendent of Documents,
functional goniometers.
available from U.S. Government Printing Office, Washington, DC 22025.
5.2 Spectrometer Environment:
Annual Book of ASTM Standards, Vol 03.05.
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 1172
5.2.1 Temperature Stabilization—A means for stabilizing specified. Voltage repeatability should be specified for a
the temperature of the spectrometer shall be provided. The programmable generator, which is frequently used in sequen-
degree of temperature control shall be specified by the manu- tial systems.
facturer. Temperature stability directly affects instrument sta-
5.3.2.2 The current to the X-ray tube is typically adjustable
bility.
within the range of 5 to 100 mA. Current stability and thermal
5.2.2 Optical Path: drift should be specified. Current repeatability should be
specified for programmable generators.
5.2.2.1 A vacuum path is generally preferred, especially for
the analysis of light elements (long wavelengths). Instruments 5.3.2.3 Voltage and current recovery times should be speci-
capable of vacuum operation shall have a vacuum gage to fied for programmable generators. The software routines which
indicate vacuum level. An airlock mechanism shall also be control the generator must delay measurement until the gen-
provided to pump down the specimen chamber before opening erator recovers from voltage or current changes.
it to the spectrometer. Pump down time shall be specified by
5.3.2.4 Input power requirements should be specified by the
the manufacturer.
manufacturer so the proper power can be supplied when the
5.2.2.2 A helium path is recommended when light element instrument is installed. Maximum generator power output
analysis is required and the specimen (such as a liquid) would should be stated. Warning: Safety is a primary concern when
be disturbed by a vacuum. Instruments equipped for helium dealing with high voltage. Safety interlock circuits (7.3) and
operation shall have an airlock for flushing the specimen warning labels shall protect the user from coming in contact
chamber with helium before introducing the specimen into the with high voltage. The interlock system shall shut down the
spectrometer. Helium flushing time shall be specified by the generator when access to high voltage is attempted. Circuits
manufacturer. The manufacturer shall also provide a means for shall be provided to protect the X-ray tube from power and
accurately controlling the pressure of the helium within the current overloads.
spectrometer.
5.3.3 Water Cooling Requirements—The X-ray tube and
5.2.2.3 An air path is an option when the instrument is not some high voltage generators require cooling by either filtered
equipped for vacuum or helium operation. Light element tap water or a closed-loop heat exchanger system.
analysis and some lower detection limits are sacrificed when
5.3.3.1 The manufacturer shall specify water flow and
operating with an air optical path.
quality requirements.
5.3 Excitation—A specimen is excited by X rays generated
5.3.3.2 To protect components from overheating, an inter-
by an X-ray tube which is powered by a high voltage generator
lock circuit that monitors either water coolant flow or tempera-
and is usually cooled by circulating water. The intensity of the
ture or both shall shut down power to the X-ray tube whenever
various wavelengths of X rays striking the specimen is varied
these requirements are not met.
by changing the power settings to the tube or by inserting filters
5.3.3.3 Water purity is especially critical in cathode-
into the beam path.
grounded systems since this requires the coolant to be noncon-
5.3.1 X-Ray Tube—The X-ray tube may be one of two
ducting. A closed-loop heat exchanger is necessary to supply
types; end-window or side-window. Depending upon the in-
high purity cooling water. A conductivity gage shall monitor
strument, either the anode or the cathode is grounded. Cathode
water coolant purity in these systems and shall shut down
grounding permits the window of the X-ray tube to be thinner
power to the X-ray tube when coolant purity is below require-
and thus affords more efficient transmittance of the longer
ments.
excitation wavelengths.
5.3.4 Primary Beam Filter—A primary beam filter is com-
5.3.1.1 X-ray tubes are produced with a variety of targets.
monly used in sequential spectrometers to filter out the
The choice of the target material depends upon the wave-
characteristic emissions from the X-ray tube’s target when
lengths that require excitation. X rays from certain materials
these emissions might interfere with the measurement of an
excite the longer wavelengths more efficiently. Other materials
analyte element. Primary beam filters are also useful for
are better for exciting the shorter wavelengths. Generally the
lowering the background in the longer wavelength portion of
choice of target material is a compromise.
the spectrum. This serves to increase the peak to background
5.3.1.2 X-ray tubes are rated according to maximum power, ratio and offers greater detection of those longer wavelength X
maximum current, and typical power settings. These should be rays.
specified by the manufacturer. Warning: It is important that
5.3.4.1 Primary beam filters are made of several different
the user be protected from exposure to harmful X rays.
metals (depending upon the X-ray tube’s target) and come in
Standard warning labels shall warn the user of the possibility of
various thicknesses. The manufacturer should specify the type,
exposure to X rays. Safety interlock circuits (7.3) shall shut
thickness, and location of the primary beam filter.
down power to the X-ray tube whenever protective shielding is
5.4 Sample Positioning—The process of positioning a
removed.
specimen in a spectrometer for analysis involves several
5.3.2 High Voltage Generator—The high voltage generator
components; the specimen holder, the specimen changer, and
supplies power to the X-ray tube. Its stability is critical to the
the specimen rotation mechanism (spinner). These components
precision of the instrument.
contribute collectively to the reproducibility of positioning the
5.3.2.1 The d-c voltage output of the high voltage generator specimen in the optical path and thus, to instrument precision.
is typically adjustable within the range of 10 to 100 kV. Voltage The design of these components should therefore be regarded
stability, drift with temperature, and voltage ripple should be critically.
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 1172
5.4.1 Reproducibility of the distance between the face of the crystal establishes a focusing circle that is similar to the
specimen and the window of the X-ray tube is especially Rowland circle defined by a grating in an optical emission
critical and should be specified by the manufacturer. spectrograph. In an X-ray spectrometer, however, proper fo-
cusing requires that both slits not only be on the focusing circle
5.4.2 The spinner rotates the specimen while it is being
but also have identical chordal distances from the slits to the
exposed to the X-ray beam and thus helps to minimize the
crystal. A detector is aimed at the crystal through the exit slit.
influence of surface defects and specimen inhomogeneity on
analytical results. 5.6.2.1 The manufacturer should specify the size of the
entrance and exit slit for each monochromator and shall
5.4.3 Imperfections in the surface of the specimen have the
greatest effect on analytical results in spectrometers with a provide adjustments to peak each monochromator. Depending
upon the manufacturer, peaking may involve movement of the
shallow angle of irradiation or take-off angle. The manufac-
turer shall specify these angles. crystal, the exit slit, or both.
5.4.4 Other important specifications include maximum 5.6.3 Apertures—In flat-crystal spectrometers, an aperture
specimen size (thickness and diameter) and the specimen is placed at the point in the optical path where the secondary X
rotation speed (if the instrument is equipped with a spinner). rays exit the specimen. The aperture limits the area of the
5.5 Dispersion—The analyzing crystal is the dispersive specimen seen by the detector. In some instruments, the size of
this aperture is fixed, but in most it can be varied, either by
device in a wavelength-dispersive X-ray spectrometer. Various
crystals having a variety of interplanar spacings are used to manual replacement or a mechanical changer. The manufac-
turer should specify the sizes of any apertures installed in the
disperse the specimen’s characteristic wavelengths.
spectrometer.
5.5.1 Sequential spectrometers may contain several different
crystals mounted on a crystal changer mechanism. Thus, the 5.6.4 Attenuators—When an element is present at a high
analyst is able to select a specific crystal for the wavelength concentration, it is sometimes desirable to decrease the inten-
being measured. sity of its emissions to avoid exceeding the detector’s linear
counting range. An attenuator, positioned between the speci-
5.5.2 Each monochromator in a simultaneous instrument
men and the analyzing crystal, absorbs some of the secondary
has a separate specified crystal. The selection is made in
X rays and thus lowers the intensity. The manufacturer should
accordance with the expected analytical requirements. The
specify the location (which monochromator) and the attenua-
crystal is generally bent and ground to a curve or a logarithmic
tion
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