ASTM E1172-87(1996)
(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.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 1172 – 87 (Reapproved 1996)
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 3. Terminology
1.1 This practice describes the components of a wavelength- 3.1 For terminology relating to X-ray spectrometry, refer to
dispersive X-ray spectrometer that are basic to its operation Terminology E 135.
and to the quality of its performance. It is not the intent of this
4. Significance and Use
practice to specify component tolerances or performance
4.1 This practice describes the essential components of a
criteria, as these are unique for each instrument. The document
wavelength-dispersive X-ray spectrometer. This description is
does, however, attempt to identify which of these are critical
and thus which should be specified. presented so that the user or potential user may gain a cursory
understanding of the structure of an X-ray spectrometer sys-
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the tem. It also provides a means for comparing and evaluating
different systems as well as understanding the capabilities and
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- limitations of each instrument.
bility of regulatory limitations prior to use. Specific safety
5. Description of Equipment
hazard statements are given in Note 1 and Note 2, and in
5.1 Types of Spectrometers—X-ray spectrometers can be
Section 7.
classified as sequential, simultaneous, or a combination of
1.3 There are several books and publications from the
these two (hybrid).
National Institute of Standards and Technology and the U.S.
3,4
5.1.1 Sequential Spectrometers—The sequential spectrom-
Government Printing Office which deal with the subject of
eter disperses and detects secondary X rays by means of an
X-ray safety. Refer also to Practice E 416 .
adjustable monochromator called a goniometer. In flat-crystal
2. Referenced Documents
instruments, secondary X rays are emitted from the specimen
and nonparallel X rays are eliminated by means of a Soller slit
2.1 ASTM Standards:
E 135 Terminology Relating to Analytical Chemistry for (collimator). The parallel beam of X rays strikes a flat
analyzing crystal which disperses the X rays according to their
Metals, Ores, and Related Materials
E 416 Practice for Planning and Safe Operation of a Spec- wavelengths. The dispersed X rays are then measured by
suitable detectors. Adjusting the goniometer varies the angle
trochemical Laboratory
between the specimen, crystal, and detector, permitting the
E 876 Practice for Use of Statistics in the Evaluation of
Spectrometric Data measurement of different wavelengths and therefore different
elements. Sequential instruments containing curved-crystal
optics are less common. This design substitutes curved for flat
1 crystals and entrance and exit slits for collimators.
This practice is under the jurisdiction of ASTM Committee E-1 on Analytical
Chemistry for Metals, Ores and Related Materials and is the direct responsibility of 5.1.2 Simultaneous Spectrometers—Simultaneous spec-
Subcommittee E01.20 on Fundamental Practices.
trometers use separate monochromators to measure each ele-
Current edition approved June 26, 1987. Published October 1987.
ment. These instruments are for the most part of fixed
NBS Handbook, X-Ray Protection, HB76, and NBS Handbook 111, ANSI
configuration, although some simultaneous instruments have a
N43.2-1971, available from National Institute of Standards and Technology,
Gaithersburg, MD 20899.
scanning channel with limited function. A typical monochro-
Radiation Safety Recommendations for X-Ray Diffraction and Spectrographic
mator consists of an entrance slit, a curved (focusing) analyz-
Equipment, No. MORP 68-14, 1968, available from U.S. Department of Health,
ing crystal, an exit slit, and a suitable detector. Secondary X
Education, and Welfare, Rockville, MD 20850.
rays pass through the entrance slit and strike the analyzing
U.S. Government Handbook 93, Safety Standards for Non-Medical X-Ray and
Sealed Gamma-Ray Sources, Part 1, General, Superintendent of Documents,
crystal, which diffracts the wavelength of interest and focuses
available from U.S. Government Printing Office, Washington, DC 22025.
it through the exit slit where it is measured by the detector.
Annual Book of ASTM Standards, Vol 03.05.
Annual Book of ASTM Standards, Vol 03.06.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1172
shut down power to the X-ray tube whenever protective shielding is
Some simultaneous instruments use flat crystals, but this is less
removed.
common.
5.3.2 High Voltage Generator—The high voltage generator
5.1.3 Hybrid Spectrometers—Hybrid spectrometers com-
supplies power to the X-ray tube. Its stability is critical to the
bine features found in sequential and simultaneous instru-
precision of the instrument.
ments. They have both fixed channels and one or more fully
5.3.2.1 The d-c voltage output of the high voltage generator
functional goniometers.
is typically adjustable within the range of 10 to 100 kV. Voltage
5.2 Spectrometer Environment:
stability, drift with temperature, and voltage ripple should be
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
5.2.2.1 A vacuum path is generally preferred, especially for
specified for programmable generators.
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.
be disturbed by a vacuum. Instruments equipped for helium
NOTE 2—Warning: Safety is a primary concern when dealing with
operation shall have an airlock for flushing the specimen
high voltage. Safety interlock circuits (7.3) and warning labels shall
chamber with helium before introducing the specimen into the
protect the user from coming in contact with high voltage. The interlock
spectrometer. Helium flushing time shall be specified by the
system shall shut down the generator when access to high voltage is
manufacturer. The manufacturer shall also provide a means for
attempted. Circuits shall be provided to protect the X-ray tube from power
and current overloads.
accurately controlling the pressure of the helium within the
spectrometer.
5.3.3 Water Cooling Requirements—The X-ray tube and
some high voltage generators require cooling by either filtered
5.2.2.3 An air path is an option when the instrument is not
tap water or a closed-loop heat exchanger system.
equipped for vacuum or helium operation. Light element
5.3.3.1 The manufacturer shall specify water flow and
analysis and some lower detection limits are sacrificed when
quality requirements.
operating with an air optical path.
5.3.3.2 To protect components from overheating, an inter-
5.3 Excitation—A specimen is excited by X rays generated
lock circuit that monitors either water coolant flow or tempera-
by an X-ray tube which is powered by a high voltage generator
ture or both shall shut down power to the X-ray tube whenever
and is usually cooled by circulating water. The intensity of the
these requirements are not met.
various wavelengths of X rays striking the specimen is varied
5.3.3.3 Water purity is especially critical in cathode-
by changing the power settings to the tube or by inserting filters
grounded systems since this requires the coolant to be noncon-
into the beam path.
ducting. A closed-loop heat exchanger is necessary to supply
5.3.1 X-Ray Tube—The X-ray tube may be one of two
high purity cooling water. A conductivity gage shall monitor
types; end-window or side-window. Depending upon the in-
water coolant purity in these systems and shall shut down
strument, either the anode or the cathode is grounded. Cathode
power to the X-ray tube when coolant purity is below require-
grounding permits the window of the X-ray tube to be thinner
ments.
and thus affords more efficient transmittance of the longer
5.3.4 Primary Beam Filter—A primary beam filter is com-
excitation wavelengths.
monly used in sequential spectrometers to filter out the
5.3.1.1 X-ray tubes are produced with a variety of targets.
characteristic emissions from the X-ray tube’s target when
The choice of the target material depends upon the wave-
these emissions might interfere with the measurement of an
lengths that require excitation. X rays from certain materials
analyte element. Primary beam filters are also useful for
excite the longer wavelengths more efficiently. Other materials
lowering the background in the longer wavelength portion of
are better for exciting the shorter wavelengths. Generally the
the spectrum. This serves to increase the peak to background
choice of target material is a compromise.
ratio and offers greater detection of those longer wavelength X
5.3.1.2 X-ray tubes are rated according to maximum power,
rays.
maximum current, and typical power settings. These should be
5.3.4.1 Primary beam filters are made of several different
specified by the manufacturer.
metals (depending upon the X-ray tube’s target) and come in
various thicknesses. The manufacturer should specify the type,
NOTE 1—Warning: It is important that the user be protected from
thickness, and location of the primary beam filter.
exposure to harmful X rays. Standard warning labels shall warn the user
of the possibility of exposure to X rays. Safety interlock circuits (7.3) shall 5.4 Sample Positioning—The process of positioning a
E 1172
specimen in a spectrometer for analysis involves several fine slit, but at the expense of a loss of intensity.
components; the specimen holder, the specimen changer, and
5.6.1.3 The manufacturer should specify the location and
the specimen rotation mechanism (spinner). These components
plate spacing of all Soller slits installed in a particular
contribute collectively to the reproducibility of positioning the
instrument. A means shall be provided for adjusting (peaking)
specimen in the optical path and thus, to instrument precision.
each slit.
The design of these components should therefore be regarded
5.6.2 Entrance and Exit Slits—Both entrance and exit slits
critically.
are required in a curved-crystal spectrometer. The curved
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-
5.4.2 The spinner rotates the specimen while it is being
cusing requires that both slits not only be on the focusing circle
exposed to the X-ray beam and thus helps to minimize the
but also have identical chordal distances from the slits to the
influence of surface defects and specimen inhomogeneity on
crystal. A detector is aimed at the crystal through the exit slit.
analytical results.
5.6.2.1 The manufacturer should specify the size of the
5.4.3 Imperfections in the surface of the specimen have the
entrance and exit slit for each monochromator and shall
greatest effect on analytical results in spectrometers with a
provide adjustments to peak each monochromator. Depending
shallow angle of irradiation or take-off angle. The manufac-
upon the manufacturer, peaking may involve movement of the
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
device in a wavelength-dispersive X-ray spectrometer. Various
this aperture is fixed, but in most it can be varied, either by
crystals having a variety of interplanar spacings are used to
manual replacement or a mechanical changer. The manufac-
disperse the specimen’s characteristic wavelengths.
turer should specify the sizes of any apertures installed in the
5.5.1 Sequential spectrometers may contain several different
spectrometer.
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
5.5.2 Each monochromator in a simultaneous instrument
counting range. An attenuator, positioned between the speci-
has a separate specified crystal. The selection is made in
men and the analyzing crystal, absorbs some of the secondary
accordance with the expected analytical requirements. The
X rays and thus lowers the intensity. The manufacturer should
crystal is generally bent and ground to a curve or
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