1、Designation: E 1184 02Standard Practice forElectrothermal (Graphite Furnace) Atomic AbsorptionAnalysis1This standard is issued under the fixed designation E 1184; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last rev
2、ision. 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 a procedure for the determinationof microgram per millilitre (g/mL) or lower concentrations ofelements i
3、n solution using an electrothermal atomization de-vice attached to an atomic absorption spectrophotometer. Ageneral description of the equipment is provided. Recommen-dations are made for preparing the instrument for measure-ments, establishing optimum temperature conditions and othercriteria which
4、should result in determining a useful calibrationconcentration range, and measuring and calculating the testsolution analyte concentration.1.2 This standard does not purport to address all of thesafety problems, if any, associated with its use. It is theresponsibility of the user of this standard to
5、 establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Specific safetyhazard statements are given in Section 9.NOTE 1This practice is a companion to Practice E 663.2. Referenced Documents2.1 ASTM Standards:E50 Practices for Appara
6、tus, Reagents, and Safety Precau-tions for Chemical Analysis of MetalsE 131 Terminology Relating to Molecular SpectroscopyE 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related Materials0E 406 Practice for Using Controlled Atmospheres in Spec-trochemical Analysis0E 416 Pract
7、ice for Planning and Safe Operation of a Spec-trochemical Laboratory0E 663 Practice for Flame Atomic Absorption Analysis0E 863 Practice for Describing Flame Atomic AbsorptionSpectroscopy Equipment03. Terminology3.1 Refer to Terminology E 131 and E 135 for the definitionof terms used in this practice
8、.3.2 Definitions of Terms Specific to This Standard:3.2.1 atomizationthe formation of ground state atoms thatabsorb radiation from a line emission source (see PracticeE 663). The atomization process in electrothermal atomicabsorption analysis is covered in greater detail in 6.2.3.2.2 pyrolysisthe pr
9、ocess of heating a specimen to atemperature high enough to remove or alter its original matrix,but not so high as to volatilize the element to be measured. Thepurpose of the pyrolysis step in electrothermal atomic absorp-tion analysis is to remove or alter the original specimen matrix,thereby reduci
10、ng or eliminating possible interferences to theformation of ground state atoms that are formed when thetemperature is increased during the atomization step. Manypublications and references will refer to pyrolysis as charringor ashing.3.2.3 pyrolytic graphite coatinga layer of pyrolytic graph-ite tha
11、t coats a graphite tube used in electrothermal atomicabsorption analysis. Pyrolytic graphite is formed by pyrolizinga hydrocarbon, for example, methane, at 2000C.3.2.4 rampinga slow, controlled increase of the tempera-ture in the graphite tube. Ramping will provide for an efficientbut not too rapid
12、removal or decomposition of the specimenmatrix. Most electrothermal atomizers allow for rampingduring the drying, pyrolysis, and atomization steps. It is usuallyemployed during the drying and pyrolysis steps. However,some instrument manufacturers may recommend rampingduring the atomization step depe
13、nding on the specimen matrixand the element being measured (for example, the analysis ofcadmium or lead in hair or blood). The power supplies for mostinstruments also allow the rate of the temperature increase tobe varied.1This practice is under the jurisidiction of ASTM Committee E01 on AnalyticalC
14、hemistry for Metals, Ores and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices and Measurement Traceabillity .Current edition approved Oct 10, 2002. Published June 2003. Originallypublished in 1987 as E 118487. Last previous edtion approved in 1998 as
15、 E1184-98.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Significance and Use4.1 This practice is intended for spectroscopists who areattempting to establish electrothermal atomic absorption pro-cedures. Used in conjunction with
16、Practice E 663, it should behelpful for establishing a complete atomic absorption analysisprogram.5. Theory of Atomic Absorption Spectroscopy5.1 Practice E 663 provides a brief discussion of the theoryof flame atomic absorption spectroscopy.Amore complete anddetailed discussion of atomic absorption
17、spectroscopy may befound in Dean and Rains (1).6. Theory of Electrothermal Atomic AbsorptionSpectroscopy6.1 Basic TechniqueA discrete amount of test solution isheated in an electrothermal device to produce a cloud of neutralatoms. Light, emitted by a specific element from a line sourceat a specific
18、wavelength, is passed through the cloud andneutral atoms of this same element in the cloud absorbs someof this light. Thus the intensity of the beam is decreased at thewavelengths characteristic of the element. This absorbance ofradiation from the external light source depends on thepopulation of th
19、e neutral atoms and is proportional to theconcentration of the element in the test solution.6.2 Electrothermal Atomization TheoryThermodynamicand kinetic theories must be considered to fully understand theatomization process that takes place in the electrothermalatomizer. Fuller (2) and also Campbel
20、l and Ottaway (3)provide a complete discussion of the thermodynamic theory.They also discuss thermal dissociation of metal oxides, reduc-tion of metal oxides, evaporation of metal oxides prior toatomization, and carbide formation. Several models have beenproposed to explain the theory of kinetic ato
21、mization. Twostudies provide a general summary of these models: oneprovides a discussion of atomization under increasing tempera-ture (4), the other discusses atomization under isothermalconditions (5). Additional discussion and clarification of thekinetic atomization theory is provided by Paveri-Fo
22、ntana et al(6) and Johnson et al (7).7. Apparatus7.1 Atomic Absorption SpectrophotometerMost flameatomic absorption spectrophotometers manufactured currentlycan be easily adapted for electrothermal analysis. Thesespectrophotometers are described in Practice E 863.7.1.1 Automatic background correctio
23、n is necessary for allspectrophotometers used with electrothermal devices. Whenelectrothermal atomizers, especially graphite furnaces, areheated to high temperatures, background from absorption isproduced within the graphite tube. Also, small amounts ofparticular matter in the furnace contribute to
24、the backgroundsignal. Therefore, it is essential to correct or compensate forthis background.7.2 Electrothermal AtomizersThe most commonly usedelectrothermal atomizer is the graphite tube furnace. Thisatomizer consists of a graphite tube positioned in a water-cooled unit designed to be placed in the
25、 optical path of thespectrophotometer so that the light from the hollow cathodelamp passes through the center of the tube. The tubes vary insize depending upon a particular instrument manufacturersfurnace design. These tubes are available with or withoutpyrolytic graphite coating. However, because o
26、f increased tubelife, tubes coated with pyrolytic graphite are commonly used.The water-cooled unit or atomizer head which holds thegraphite tube is constructed in such a way that an inert gas,usually argon or nitrogen, is passed over, around, or throughthe graphite tube to protect it from atmospheri
27、c oxidation. Theheating of all of these atomizers is controlled by powersupplies which make it possible to heat the graphite tube to3000C in less than 1 s. Temperatures and drying, pyrolysis,and atomization times are controlled by these power supplies(determination of these parameters are covered la
28、ter in Section10. The flow of the inert gas through the atomizer head also iscontrolled by the power supplies.7.2.1 Other types of atomizers and accessories such as thegraphite cup, graphite rod, Lvov platform, tantalum filament,and tantalum boat have been used and are covered in theliterature. With
29、 the exception of the Lvov platform, they havenot enjoyed the widespread and general use that the graphitetube atomizers have. Therefore, they will not be covered indetail within this practice. A good general description of theseother units is provided by Fuller (2).7.3 Signal Output SystemThe outpu
30、t signal resulting fromthe atomization of a specimen may be displayed by a strip chartrecorder, video display, digital computer, printer, or othersuitable device depending on the electronic capability of thespectrophotometer employed.7.3.1 If a strip chart recorder is used, it must have a fullscale
31、response of 0.5 s or less. Normally, when a strip chartrecorder is used, the absorption is determined by measuring thepeak height of the recorder tracing. This procedure is appro-priate because the absorption signal generated by an electro-thermal atomizer usually results in a very narrow peak (ab-s
32、orption versus time). However, some specimen matrices mayrequire instrumental parameters (for example, ramping), whichwill result in broad absorption versus time peaks. In such cases,peak area measurement may be more appropriate. The instru-ment manufacturers manual should be consulted to determinew
33、hich procedure is most suitable for the instrument being used.8. Reagents and Materials8.1 Picogram quantities of some elements can be deter-mined by means of electrothermal atomization. Therefore, allacids and water used to prepare calibration solutions and testsolutions shall be double distilled o
34、r ultra-pure in quality. Allreagents used in the preparation of calibration solutions andtest solutions shall be spectroscopically pure.9. Hazards9.1 Electrical HazardsThe power supplies for all electro-thermal atomizers require high-voltage (greater than 200 V)electrical service. Refer to Practice
35、E 416 when connecting andtesting this equipment.9.2 Compressed Gas HazardThe inert or nonoxidizingatmosphere required in the atomizer head during heating cyclesis usually maintained by using argon or nitrogen gas deliveredE1184022from portable gas cylinders. Practices E 406 and E 416 shouldbe follow
36、ed when handling the cylinders and connecting flowlines to the instrument.9.3 Chemical HazardPractice E50should be consultedfor recommendations and precautions concerning chemicalhazards.9.4 VentilationAsmall hood is required to carry away anytoxic fumes that may result from the atomization process.
37、 SeePractice E 416.9.5 LaboratoryThe laboratory in which the electrother-mal atomizer is operated shall be kept as clean as possible.Anyprocedures that may produce an atmosphere that is corrosive tothe instrumentation or detrimental to the analysis of thespecimen should be removed from the laborator
38、y.9.6 Laboratory ApparatusIt is imperative that all labora-tory apparatus and containers used in the preparation ofcalibration and test solutions be chemically clean. All labora-tory ware, including plastic tips used on micropipets for thetransfer of calibration solutions and test solutions to thegr
39、aphite tube, should be acid rinsed before being used. Oncelaboratory ware is acid rinsed, all of the items that come incontact with analytical solutions shall be isolated from subse-quent contact with fingers, clothing, bench tops, etc.9.7 Magnetic background correctionIf the Electothermal(Graphite
40、Furnace) Atomic Absorption unit is provided with abackground correction that does or can produce a magneticfield, the unit should not be operated by an individual whowears, internally or externally, a medical device such as apacemaker, that can be affected by the magnetic field, withoutthe approval
41、of the prescribing and/or installing physician. Inaddition an appropriate warning sign should warn visitors ofthe magnetic field.10. Preparation of Apparatus10.1 Atomizer ParametersAll electrothermal atomizersare resistance-heated by power supplies that provide individu-ally controlled heating stage
42、s for drying, pyrolysis, and atomi-zation. The means to control the times and temperatures ofthese stages will vary with instrumentation. Most manufactur-ers provide a listing of the atomizer parameters required for theelectrothermal analysis of numerous elements in the mostcommonly encountered matr
43、ices. The recommended atomizerparameters for a particular element should be verified for thespecific instrument being used with an appropriate solution.Also, for sample matrices that differ from those printed in themanufacturers list, the most appropriate time and temperaturesetting for each stage m
44、ust be calculated or determinedexperimentally (see 10.1.1, , and ).NOTE 2Ramping is normally used during the drying and pyrolysisstages. Some procedures may also recommend that ramping be usedduring the atomization stage, depending upon the specimen matrix and theelement being measured. Refer to the
45、 instrument manufacturers manualof the particular instrument for the recommended ramp rates, if any, forthe type of solution being analyzed.10.1.1 DryingThe drying stage is a low temperature stagein which the graphite tube is heated to a temperature highenough to evaporate, but not boil, any solvent
46、. The ideal dryingtemperature would be one just below the boiling point of thesolvent. Specimen spattering may occur if the temperature israised above the boiling point before evaporation is complete.The time, in seconds, required to completely dry a specimenmay be calculated by multiplying 1.5 to 2
47、 times the volume ofthe specimen, measured in microlitres (L) (2) . For example,a 10-L specimen would require a drying time of 15 to 20 s(see when the highest temperature during the atomization cyclehas been reached. If an auto-sampling device is to be used,adjust it to deposit the desired volume (i
48、n microlitres) in thegraphite tube (see Note 3). Deposit a measured amount of thereagent blank solution, prepared as directed in 11.1,inthegraphite tube. The volume should be the same as will be usedfor the test solution (see Note 3). Run through the heatingstages and adjust the readout system of th
49、e instrument to readzero absorbance during the atomization of the reagent blanksolution. If the spectrophotometer has an auto-zero capability,the auto-zero should be activated at this time. Atomize acalibration solution, prepared as directed in 11.3, containingthe analyte at a concentration that will yield an absorbance of0.1 to 0.3 and is anticipated to be within the linear absorbancerange of the procedure. Where applicable, refer to the instru-ment manufacturers instruction manual to determine an ap-proximation of the linear concentration range for the a
