1、Designation: E 1172 87 (Reapproved 2003)Standard Practice forDescribing and Specifying a Wavelength-Dispersive X-RaySpectrometer1This standard is issued under the fixed designation E 1172; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revi
2、sion, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the components of a wavelength-dispersive X-ray spectrometer that are basic to
3、 its operationand to the quality of its performance. It is not the intent of thispractice to specify component tolerances or performancecriteria, as these are unique for each instrument. The documentdoes, however, attempt to identify which of these are criticaland thus which should be specified.1.2
4、This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Specific safetyhaz
5、ard statements are given in 5.3.1.2 and 5.3.2.4, and inSection 7.1.3 There are several books and publications from theNational Institute of Standards and Technology2and the U.S.Government Printing Office3,4which deal with the subject ofX-ray safety. Refer also to Practice E 416.52. Referenced Docume
6、nts2.1 ASTM Standards:E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related Materials5E 416 Practice for Planning and Safe Operation of a Spec-trochemical Laboratory5E 876 Practice for Use of Statistics in the Evaluation ofSpectrometric Data53. Terminology3.1 For terminolog
7、y relating to X-ray spectrometry, refer toTerminology E 135.4. Significance and Use4.1 This practice describes the essential components of awavelength-dispersive X-ray spectrometer. This description ispresented so that the user or potential user may gain a cursoryunderstanding of the structure of an
8、 X-ray spectrometer sys-tem. It also provides a means for comparing and evaluatingdifferent systems as well as understanding the capabilities andlimitations of each instrument.5. Description of Equipment5.1 Types of SpectrometersX-ray spectrometers can beclassified as sequential, simultaneous, or a
9、combination ofthese two (hybrid).5.1.1 Sequential SpectrometersThe sequential spectrom-eter disperses and detects secondary X rays by means of anadjustable monochromator called a goniometer. In flat-crystalinstruments, secondary X rays are emitted from the specimenand nonparallel X rays are eliminat
10、ed by means of a Soller slit(collimator). The parallel beam of X rays strikes a flatanalyzing crystal which disperses the X rays according to theirwavelengths. The dispersed X rays are then measured bysuitable detectors. Adjusting the goniometer varies the anglebetween the specimen, crystal, and det
11、ector, permitting themeasurement of different wavelengths and therefore differentelements. Sequential instruments containing curved-crystaloptics are less common. This design substitutes curved for flatcrystals and entrance and exit slits for collimators.5.1.2 Simultaneous SpectrometersSimultaneous
12、spec-trometers use separate monochromators to measure each ele-ment. These instruments are for the most part of fixedconfiguration, although some simultaneous instruments have ascanning channel with limited function. A typical monochro-mator consists of an entrance slit, a curved (focusing) analyz-i
13、ng crystal, an exit slit, and a suitable detector. Secondary Xrays pass through the entrance slit and strike the analyzingcrystal, which diffracts the wavelength of interest and focuses1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores and Related
14、Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved June 10, 2003. Published July 2003. Originallyapproved in 1987. Last previous edition approved in 2001 as E 1172 87(2001).2NBS Handbook, X-Ray Protection, HB76, and NBS Handbook 111, AN
15、SIN43.2-1971, available from National Institute of Standards and Technology,Gaithersburg, MD 20899.3Radiation Safety Recommendations for X-Ray Diffraction and SpectrographicEquipment, No. MORP 68-14, 1968, available from U.S. Department of Health,Education, and Welfare, Rockville, MD 20850.4U.S. Gov
16、ernment Handbook 93, Safety Standards for Non-Medical X-Ray andSealed Gamma-Ray Sources, Part 1, General, Superintendent of Documents,available from U.S. Government Printing Office, Washington, DC 22025.5Annual Book of ASTM Standards, Vol 03.05.1Copyright ASTM International, 100 Barr Harbor Drive, P
17、O Box C700, West Conshohocken, PA 19428-2959, United States.it through the exit slit where it is measured by the detector.Some simultaneous instruments use flat crystals, but this is lesscommon.5.1.3 Hybrid SpectrometersHybrid spectrometers com-bine features found in sequential and simultaneous inst
18、ru-ments. They have both fixed channels and one or more fullyfunctional goniometers.5.2 Spectrometer Environment:5.2.1 Temperature StabilizationA means for stabilizingthe temperature of the spectrometer shall be provided. Thedegree of temperature control shall be specified by the manu-facturer. Temp
19、erature stability directly affects instrument sta-bility.5.2.2 Optical Path:5.2.2.1 A vacuum path is generally preferred, especially forthe analysis of light elements (long wavelengths). Instrumentscapable of vacuum operation shall have a vacuum gage toindicate vacuum level. An airlock mechanism sha
20、ll also beprovided to pump down the specimen chamber before openingit to the spectrometer. Pump down time shall be specified bythe manufacturer.5.2.2.2 A helium path is recommended when light elementanalysis is required and the specimen (such as a liquid) wouldbe disturbed by a vacuum. Instruments e
21、quipped for heliumoperation shall have an airlock for flushing the specimenchamber with helium before introducing the specimen into thespectrometer. Helium flushing time shall be specified by themanufacturer. The manufacturer shall also provide a means foraccurately controlling the pressure of the h
22、elium within thespectrometer.5.2.2.3 An air path is an option when the instrument is notequipped for vacuum or helium operation. Light elementanalysis and some lower detection limits are sacrificed whenoperating with an air optical path.5.3 ExcitationA specimen is excited by X rays generatedby an X-
23、ray tube which is powered by a high voltage generatorand is usually cooled by circulating water. The intensity of thevarious wavelengths of X rays striking the specimen is variedby changing the power settings to the tube or by inserting filtersinto the beam path.5.3.1 X-Ray TubeThe X-ray tube may be
24、 one of twotypes; end-window or side-window. Depending upon the in-strument, either the anode or the cathode is grounded. Cathodegrounding permits the window of the X-ray tube to be thinnerand thus affords more efficient transmittance of the longerexcitation wavelengths.5.3.1.1 X-ray tubes are produ
25、ced with a variety of targets.The choice of the target material depends upon the wave-lengths that require excitation. X rays from certain materialsexcite the longer wavelengths more efficiently. Other materialsare better for exciting the shorter wavelengths. Generally thechoice of target material i
26、s a compromise.5.3.1.2 X-ray tubes are rated according to maximum power,maximum current, and typical power settings. These should bespecified by the manufacturer. (WarningIt is important thatthe user be protected from exposure to harmful X rays.Standard warning labels shall warn the user of the poss
27、ibility ofexposure to X rays. Safety interlock circuits (7.3) shall shutdown power to the X-ray tube whenever protective shielding isremoved.)5.3.2 High Voltage GeneratorThe high voltage generatorsupplies power to the X-ray tube. Its stability is critical to theprecision of the instrument.5.3.2.1 Th
28、e dc voltage output of the high voltage generatoris typically adjustable within the range of 10 to 100 kV. Voltagestability, drift with temperature, and voltage ripple should bespecified. Voltage repeatability should be specified for aprogrammable generator, which is frequently used in sequen-tial s
29、ystems.5.3.2.2 The current to the X-ray tube is typically adjustablewithin the range of 5 to 100 mA. Current stability and thermaldrift should be specified. Current repeatability should bespecified for programmable generators.5.3.2.3 Voltage and current recovery times should be speci-fied for progra
30、mmable generators. The software routines whichcontrol the generator must delay measurement until the gen-erator recovers from voltage or current changes.5.3.2.4 Input power requirements should be specified by themanufacturer so the proper power can be supplied when theinstrument is installed. Maximu
31、m generator power outputshould be stated. (WarningSafety is a primary concernwhen dealing with high voltage. Safety interlock circuits (7.3)and warning labels shall protect the user from coming incontact with high voltage. The interlock system shall shutdown the generator when access to high voltage
32、 is attempted.Circuits shall be provided to protect the X-ray tube from powerand current overloads.)5.3.3 Water Cooling RequirementsThe X-ray tube andsome high voltage generators require cooling by either filteredtap water or a closed-loop heat exchanger system.5.3.3.1 The manufacturer shall specify
33、 water flow andquality requirements.5.3.3.2 To protect components from overheating, an inter-lock circuit that monitors either water coolant flow or tempera-ture or both shall shut down power to the X-ray tube wheneverthese requirements are not met.5.3.3.3 Water purity is especially critical in cath
34、ode-grounded systems since this requires the coolant to be noncon-ducting. A closed-loop heat exchanger is necessary to supplyhigh purity cooling water. A conductivity gage shall monitorwater coolant purity in these systems and shall shut downpower to the X-ray tube when coolant purity is below requ
35、ire-ments.5.3.4 Primary Beam FilterA primary beam filter is com-monly used in sequential spectrometers to filter out thecharacteristic emissions from the X-ray tubes target whenthese emissions might interfere with the measurement of ananalyte element. Primary beam filters are also useful forlowering
36、 the background in the longer wavelength portion ofthe spectrum. This serves to increase the peak to backgroundratio and offers greater detection of those longer wavelength Xrays.5.3.4.1 Primary beam filters are made of several differentmetals (depending upon the X-ray tubes target) and come inE 117
37、2 87 (2003)2various thicknesses. The manufacturer should specify the type,thickness, and location of the primary beam filter.5.4 Sample PositioningThe process of positioning aspecimen in a spectrometer for analysis involves severalcomponents; the specimen holder, the specimen changer, andthe specime
38、n rotation mechanism (spinner). These componentscontribute collectively to the reproducibility of positioning thespecimen in the optical path and thus, to instrument precision.The design of these components should therefore be regardedcritically.5.4.1 Reproducibility of the distance between the face
39、 of thespecimen and the window of the X-ray tube is especiallycritical and should be specified by the manufacturer.5.4.2 The spinner rotates the specimen while it is beingexposed to the X-ray beam and thus helps to minimize theinfluence of surface defects and specimen inhomogeneity onanalytical resu
40、lts.5.4.3 Imperfections in the surface of the specimen have thegreatest effect on analytical results in spectrometers with ashallow angle of irradiation or take-off angle. The manufac-turer shall specify these angles.5.4.4 Other important specifications include maximumspecimen size (thickness and di
41、ameter) and the specimenrotation speed (if the instrument is equipped with a spinner).5.5 DispersionThe analyzing crystal is the dispersivedevice in a wavelength-dispersive X-ray spectrometer. Variouscrystals having a variety of interplanar spacings are used todisperse the specimens characteristic w
42、avelengths.5.5.1 Sequential spectrometers may contain several differentcrystals mounted on a crystal changer mechanism. Thus, theanalyst is able to select a specific crystal for the wavelengthbeing measured.5.5.2 Each monochromator in a simultaneous instrumenthas a separate specified crystal. The se
43、lection is made inaccordance with the expected analytical requirements. Thecrystal is generally bent and ground to a curve or a logarithmicspiral in order to focus the analytical line through the mono-chromators exit slit.5.5.3 The manufacturer should specify each of the crystalsinstalled in a parti
44、cular spectrometer according to composition,location (which monochromator or crystal changer position),lattice orientation (when applicable), interplanar spacing, andshape (flat or curved).5.5.4 The manufacturer shall provide an adjustment forrotating the crystal to peak the spectrometer or monochro
45、ma-tor.5.6 Beam Moderating Devices:5.6.1 Soller SlitsSoller slits are provided for systemscontaining flat crystals. They are composed of a series ofclosely-spaced, thin, parallel plates or tubes. When a Soller slitis placed in the path of a beam of X rays, only those X rays thatare parallel to the p
46、lates or tubes will pass through the slit.Therefore, the X rays that exit the slit are parallel.5.6.1.1 Soller slits may be present in several locations. Aprimary Soller slit is always present in the optical path betweenthe specimen and the analyzing crystal. Auxiliary slits may beinstalled at the d
47、etector windows between the detector and theanalyzing crystal.5.6.1.2 It is common for a sequential spectrometer to haveboth coarse and fine primary Soller slits, installed and mountedin a changer mechanism. Better resolution is achieved with afine slit, but at the expense of a loss of intensity.5.6
48、.1.3 The manufacturer should specify the location andplate spacing of all Soller slits installed in a particularinstrument. A means shall be provided for adjusting (peaking)each slit.5.6.2 Entrance and Exit SlitsBoth entrance and exit slitsare required in a curved-crystal spectrometer. The curvedcry
49、stal establishes a focusing circle that is similar to theRowland circle defined by a grating in an optical emissionspectrograph. In an X-ray spectrometer, however, proper fo-cusing requires that both slits not only be on the focusing circlebut also have identical chordal distances from the slits to thecrystal. A detector is aimed at the crystal through the exit slit.5.6.2.1 The manufacturer should specify the size of theentrance and exit slit for each monochromator and shallprovide adjustments to peak each monochromator. Dependingupon the manufacturer, peaking may i
