ASTM E1832-2008 Standard Practice for Describing and Specifying a Direct Current Plasma Atomic Emission Spectrometer《描述直流原子光谱发射计特性的标准操作》.pdf

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1、Designation: E 1832 08Standard Practice forDescribing and Specifying a Direct Current Plasma AtomicEmission Spectrometer1This standard is issued under the fixed designation E 1832; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, th

2、e year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes the components of a directcurrent plasma (DCP) atomic emission spectrometer. Thispra

3、ctice does not attempt to specify component tolerances orperformance criteria. This practice does, however, attempt toidentify critical factors affecting bias, precision, and sensitivity.A prospective user should consult with the vendor beforeplacing an order to design a testing protocol for demonst

4、ratingthat the instrument meets all anticipated needs.1.2 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

5、of regulatory limitations prior to use. Specific hazardsstatements are give in Section 9.2. Referenced Documents2.1 ASTM Standards:2E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related MaterialsE 158 Practice for Fundamental Calculations to ConvertIntensities into Concentr

6、ations in Optical Emission Spec-trochemical Analysis3E 172 Practice for Describing and Specifying the ExcitationSource in Emission Spectrochemical Analysis3E 406 Practice for Using Controlled Atmospheres in Spec-trochemical AnalysisE 416 Practice for Planning and Safe Operation of a Spec-trochemical

7、 Laboratory3E 520 Practice for Describing Photomultiplier Detectors inEmission and Absorption SpectrometryE 528 Practices for Grounding Basic Optional EmissionSpectrochemical Equipment3E 1097 Guide for Direct Current Plasma-Atomic EmissionSpectrometry Analysis3. Terminology3.1 For terminology relati

8、ng to emission spectrometry, referto Terminology E 135.4. Significance and Use4.1 This practice describes the essential components of theDCP spectrometer. This description allows the user or potentialuser to gain a basic understanding of this system. It alsoprovides a means of comparing and evaluati

9、ng this system withsimilar systems, as well as understanding the capabilities andlimitations of each instrument.5. Overview5.1 A DCP spectrometer is an instrument for determiningconcentration of elements in solution. It typically is comprisedof several assemblies including a direct current (dc) elec

10、tricalsource, a sample introduction system, components to form andcontain the plasma, an entrance slit, elements to disperseradiation emitted from the plasma, one or more exit slits, oneor more photomultipliers for converting the emitted radiationinto electrical current, one or more electrical capac

11、itors forstoring this current as electrical charge, electrical circuitry formeasuring the voltage on each storage device, and a dedicatedcomputer with printer. The liquid sample is introduced into aspray chamber at a right angle to a stream of argon gas. Thesample is broken up into a fine aerosol by

12、 this argon stream andcarried into the plasma produced by a dc-arc discharge betweena tungsten electrode and two or more graphite electrodes.When the sample passes through the plasma, it is vaporizedand atomized, and many elements are ionized. Free atoms andions are excited from their ground states.

13、 When electrons ofexcited atoms and ions fall to a lower-energy state, photons ofspecific wavelengths unique to each emitting species areemitted. This radiation, focussed by a lens onto the entrance slitof the spectrometer and directed to an echelle grating andquartz prism, is dispersed into higher

14、orders of diffraction.Control on the diffraction order is accomplished by the1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition ap

15、proved May 1, 2008. Published June 2008. Originallyapproved in 1996. Last previous edition approved in 2003 as E 1832 03, whichwas withdrawn October 2004 and reinstated in May 2008.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.or

16、g. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.low-dispersion echelle grating. Radiation of specific

17、wave-length or wavelengths passes through exit slits and impinges ona photomultiplier or photomultipliers. The current outputscharge high-quality capacitors, and the voltages thus generatedare measured and directed to the computer. Using calibrationsolutions, a calibration curve is generated for eac

18、h element ofinterest. The computer compares the signals arising from themany elements in the sample to the appropriate calibrationcurve and then calculates the concentration of each element.Over seventy elements may be determined. Detection limits ina simple aqueous solution are less than 1 mg/L for

19、 most ofthese elements. Mineral acids or organic liquids also may beused as solvents, and detection limits are usually within anorder of magnitude of those obtained with water. Detectionlimits may be improved by using preconcentration procedures.Solid samples are dissolved before analysis.6. Descrip

20、tion of Equipment6.1 Echelle SpectrometerComponents of the equipmentshown in Fig. 1 and described in this section are typical of acommercially available spectrometer.Although a specific spec-trometer is described herein, other spectrometers having equalor better performance may be satisfactory. The

21、spectrometer isa Czerny-Turner mount and consists of a condensing lens infront of an entrance slit, a collimating mirror, combineddispersing elements (grating and prism), focus mirror, exit slits,photomultipliers, control panel, and wavelength selectormechanism.6.1.1 Condensing Lens, placed between

22、the DCP source andthe entrance slit. It should have a focal length capable offocusing an image of the source on the entrance slit and withsufficient diameter to fill the aperture of the spectrometer withradiant energy.6.1.2 Entrance Slit, although available with fixed width andheight, a slit variabl

23、e in both width and height provides greaterflexibility. Typical values are 0.025 mm to 0.500 mm in widthand 0.100 mm to 0.500 mm in height. Adjustable slit widthsand heights are useful in obtaining optimal spectral band widthand radiant energy entering the spectrometer for the require-ments of the a

24、nalytical method.6.1.3 Collimating Mirror, renders all rays parallel afterentering the spectrometer. These parallel rays illuminate thecombined dispersing elements. The focal length and f numbershould be specified. Typical focal length and f number are 750mm and f/13.6.1.4 Combined Dispersing Compon

25、ents, positioned so thatthe radiant energy from the collimating mirror passes throughthe prism, is refracted and reflected by a plane grating and backthrough the prism. Specify the ruling on the grating (forexample, 79 grooves/mm).6.1.5 Focus Mirror, placed to focus the radiant energy fromthe combin

26、ed dispersing elements on a flat two-dimensionalfocal plane where the exit slits are located.6.1.6 Fixed Exit Slits, mounted in a removable fixture calledan optical cassette for multielement capability. A two-mirrorperiscope behind each exit slit directs the radiant energy to acorresponding photomul

27、tiplier. For single element capability,energy for one wavelength usually passes through its exit slitdirectly to the photomultiplier without the need for a periscope.Select the specific exit slit width before installation. Provide asingle channel cassette with one exit slit variable from 0.025mm to

28、0.200 mm in width and from 0.100 mm to 0.500 mm inlength.6.1.7 Photomultipliers, up to twenty end-on tubes, aremounted behind the focal plane in a fixed pattern. Considersensitivity at specific wavelength and dark current in theselection of appropriate photomultipliers. Provide variablevoltage to ea

29、ch photomultiplier to change its response asrequired by the specific application. A typical range is from550 V to 1000 V in 50-V steps. A survey of the properties ofphotomultipliers is given in Practice E 520.6.1.8 Control Panels, are provided to perform several func-tions and serve as input to micr

30、oprocessors to control theoperation of the spectrometer. Provide a numeric keyboard toenter high and low concentrations of reference materials forcalibration and standardization of each channel and to displayentered values for verification. Provide a switch on this panelto set the mode either to int

31、egrate during analysis or to measureinstantaneous intensity. The latter mode is required to obtainthe peak position for a specific channel by seeking maximumintensity by wavelength adjustment and verifying by wave-length scanning. Conduct interference and background inves-tigations with this mode. S

32、canning is required if automaticbackground correction is to be performed. Provide othernecessary switches for the following purposes: to calibrate orstandardize the spectrometer, start analysis, interrupt the func-tion being performed, set integration time and the number ofreplicate analyses, and di

33、rect the output to a printer, display, orstorage medium. Impose a fixed time delay of 10 s beforeintegration can begin to ensure that the solution being analyzedFIG. 1 Echelle Grating SpectrometerE1832082is aspirated into the DCP discharge. Provide digital and analogvoltmeters for displaying the ins

34、tantaneous or integrated inten-sities during peaking, scanning, or analysis. If a computer is anintegral part of the spectrometer, most of the control functionsare accomplished with software.6.1.9 Wavelength Adjustment, provided to adjust the wave-length range and diffraction order for peaking the s

35、pectrometerbecause a two-dimensional spectrum is produced. Both coarseand final control of these adjustments are required. To maintainoptical alignment, the spectrometer should be thermally iso-lated from the DCP source or heated. A heated base on whichthe spectrometer rests has been satisfactory fo

36、r this purpose.6.1.10 Dispersion and Spectral Band PassTypical disper-sion and spectral band pass with a 0.025-mm slit width varyfrom 0.061 nm/mm and 0.0015 nm at 200 nm to 0.244 nm/mmand 0.0060 nm at 800 nm, respectively.6.2 DCP Source, composed of several distinct parts, namelythe electrode, direc

37、t current power supply, gas flow, sampleintroduction, exhaust, water cooling, and safety systems. Referto Practice E 172 for a list of the electrical source parametersthat should be specified in a DCP method.6.2.1 Electrode System, Fig. 2, consists of two graphiteanodes fixed in a vertical plane and

38、 at a typical angle of 60 toone another, and a tungsten cathode fixed in a horizontal planeat an angle of 45 to the optic axis. In their operating position,the tips of the two anodes are separated by a distance of 1316in., (3.0 cm), and the tungsten cathode is 158 in., (4.1 cm),above the anode tips.

39、 Each electrode is recessed in a ceramicsleeve fitted into water-cooled anode and cathode blocks.Because the electrodes are of special design to fit into and beheld by these blocks, the user must follow the manufacturersrecommendations for these electrodes. The electrode systemshall provide mechanis

40、m to adjust the electrodes vertically andhorizontally across the optic axis to properly project the imageof the excitation region onto the entrance slit and obtain amaximum signal-to-noise ratio. Sometimes a visible excitationregion is not produced when some specimens are aspirated intothis source.

41、Iron solutions, as well as solutions of several otherelements, however, are satisfactory for this purpose.6.2.2 Direct Current Power Supply, capable of maintaininga constant current of 7 A dc in the discharge with a voltage of40 V to 50 V dc between the anodes and cathodes. Theresulting discharge ha

42、s the shape of an inverted letter Y with aluminous zone in the crotch of the Y.6.2.3 Gas Flow System, (Refer to Practice E 406) shall becapable of the following:6.2.3.1 Providing argon gas delivered at a pressure of 80 psi(5.62 kg/cm2) to the discharge sustaining gas and samplenebulization.6.2.3.2 P

43、roviding a pneumatic system to extend the anodeand cathode out of their sleeves and move the cathode blockdownwards so that the cathode electrode makes contact withone of the anodes and initiates the plasma.6.2.3.3 Providing gas pressures of 15 psi to 30 psi (1.05kg/cm2to 2.01 kg/cm2) for nebulizati

44、on and 50 psi (3.52kg/cm2) for other functions. Needle valves are used to adjustthese pressures, as well as provide for division of gas flowsamong three electrode blocks. A balance among the gas flowsthrough these blocks and past the electrodes is necessary toproduce and maintain a symmetrical disch

45、arge and atriangular- or arrowhead-shaped excitation region where thespecimens spectrum is generated.6.2.3.4 Providing isolation of the gas flow system from theambient atmosphere. For good analytical performance, ensurethat all tubing connections are tight and O-rings are in goodcondition.6.2.4 Samp

46、le Introduction System is required to control theflow of sample solution. This typically involves placing aflexible tube in the sample container, which aspirates thesample solution into a nebulizer, usually a cross-flow design.Aperistaltic pump is used to pump the sample solution to thenebulizer.As

47、a specimen drop is formed at the nebulizer orifice(0.02 in. or 0.05 cm), it is removed by the argon stream andbroken into several smaller drops. Most of these impinge onthe walls of the spray chamber running down to collect in awaste reservoir. Typically, about 20 % of the nebulized speci-men is car

48、ried by the argon stream as an aerosol into theplasma. The liquid in the waste reservoir is removed continu-ously by the same peristaltic pump used to feed the nebulizer,and passes the waste through a second tube to be safelydisposed. If this is not done, the volume of liquid waste in thereservoir a

49、nd the spray chamber is increased, increasing the gaspressure and volume of the specimen injected into the plasma,thus extinguishing the plasma. Because this pump crushesthese tubes with use, daily damage inspection is required foroptimum performance.6.2.5 Exhaust SystemProvide a small hood connected toan exhaust fan above the plasma cabinet to remove the wastegases. The fan should have a capacity to move 100 ft3/min(47.2 L/s). The flow rate should be adjustable to efficientlyremove these gases, and the hood, duct, and fan should becompatible for use with chemicals contained