ASTM E1770-1995(2006) Standard Practice for Optimization of Electrothermal Atomic Absorption Spectrometric Equipment《电热原子吸收光谱仪器优化的标准实用规程》.pdf

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1、Designation: E 1770 95 (Reapproved 2006)Standard Practice forOptimization of Electrothermal Atomic AbsorptionSpectrometric Equipment1This standard is issued under the fixed designation E 1770; the number immediately following the designation indicates the year oforiginal adoption or, in the case of

2、revision, 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 optimization of electrothermalatomic absorption spectrometers and the

3、checking of spectrom-eter performance criteria.1.2 The values stated in SI units are to be regarded as thestandard.1.3 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

4、safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E50 Practices for Apparatus, Reagents, and Safety Consid-erations for Chemical Analysis of Metals, Ores, andRelated MaterialsE 876 Practice for Use of Statis

5、tics in the Evaluation ofSpectrometric Data3E 1184 Practice for Electrothermal (Graphite Furnace)Atomic Absorption AnalysisE 1452 Practice for Preparation of Calibration Solutions forSpectrophotometric and for Spectroscopic Atomic Analy-sis33. Significance and Use3.1 This practice is for optimizing

6、the parameters used inthe determination of trace elements in metals and alloys by theelectrothermal atomic absorption spectrometric method. It alsodescribes the practice for checking the spectrometer perfor-mance. The work is expected to be performed in a properlyequipped laboratory by trained opera

7、tors and appropriatedisposal procedures are to be followed.4. Apparatus4.1 Atomic Absorption Spectrometer with ElectrothermalAtomizer, equipped with an appropriate background corrector,a signal output device such as a video display screen (VDS), adigital computer, a printer or strip chart recorder,

8、and anautosampler.4.2 Grooved Pyrolytic Graphite-Coated Graphite Tubes,conforming to the instrument manufacturers specifications.4.3 Pyrolytic Graphite Platforms, Lvov design, fitted to thetubes specified in 4.2.4.4 Pyrolytic Graphite-Coated Graphite Tubes, platform-less, conforming to the instrumen

9、t manufacturers specifica-tions.4.5 Radiation Source for the AnalyteA hollow cathodelamp or electrodeless discharge lamp is suitable.NOTE 1The use of multi-element lamps is not generally recom-mended, since they may be subject to spectral line overlaps.4.6 For general discussion of the theory and in

10、strumentalrequirements of electrothermal atomic absorption spectromet-ric analysis, see Practice E 1184.5. Reagents5.1 Purity and Concentration of ReagentsThe purity andconcentration of common chemical reagents shall conform toPractices E50. The reagents should be free of or containminimal amounts (

11、0.01 g/g) of the analyte of interest.5.2 Magnesium Nitrate Solution 2 g/L Mg(NO3)2Dissolve 0.36 6 0.01 g high-purity Mg(NO3)26H2O in about50 mL of water, in a 100-mL beaker, and transfer the solutioninto a 100-mL volumetric flask. Dilute to mark with water andmix. Store in polypropylene or high-dens

12、ity polyethylenebottle.5.3 Calibration SolutionsRefer to the preparation of cali-bration solutions in the relevant analytical method for thedetermination of trace elements in the specific matrix. Calibra-tion solution S0represents the calibration solution containingno analyte; S1the least concentrat

13、ed calibration solution; S2thecalibration solution with the next highest concentration;1This 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 Fundamental Practices.Current ed

14、ition approved June 1, 2006. Published June 2006. Originallyapproved in 1995. Last previous edition approved in 2001 as E 1770 95 (2001).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume i

15、nformation, 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.through Sk, the most concentrated calibration solution. Alsorefer to Practice E 1452.5.4 Matrix Mo

16、difiersRefer to the relevant analyticalmethod for the determination of trace elements in the specificmatrix.6. Initial Checks and Adjustments6.1 Turn on power, cooling water, gas supplies, and fumeexhaust system.6.2 Open the furnace to inspect the tube and contacts.Replace graphite components, if we

17、ar or contamination isevident. Inspect windows and clean or replace as required.6.2.1 New graphite contacts or new tubes should be condi-tioned prior to use, in accordance with the heating programrecommended by the manufacturer.6.2.1.1 In the absence of manufacturers recommendations,a conditioning p

18、rogram for a graphite furnace is shown inTable 1.7. Radiation Source7.1 Install and operate hollow cathode lamps or electrode-less discharge lamps in accordance with the manufacturersinstructions.7.2 After the manufacturers prescribed warm-up time, thesignal from the radiation source should not devi

19、ate by morethan 0.5 % from the maximum value (that is, by not more than0.002 absorbance units) over a period of 15 min. Significantlygreater fluctuations are usually indicative of a faulty lamp orpower supply.8. Spectrometer Parameters8.1 Wavelength, as specified by the appropriate procedure.8.2 Sli

20、t Width, as recommended by the manufacturer. Wheretwo slit height settings are available, select the shorter height.8.3 Background Correction:8.3.1 Zeeman Background Correction System:8.3.1.1 Ensure that the poles of the magnet are clean andsecurely tightened.8.3.1.2 If necessary, set the optical te

21、mperature sensor inaccordance with the instrument manufacturers instructions.8.3.2 Continuum Background System:8.3.2.1 Select the background correction option and allowlamps to stabilize for 30 min. Verify that the energies of theanalyte lamp and the deuterium lamp are balanced withintolerances reco

22、mmended by the manufacturer.8.3.2.2 If necessary, set the optical temperature sensor inaccordance with the instrument manufacturers recommenda-tion.8.3.3 To check the performance of the background correc-tion system, measure the atomic background absorbance of 20L of 2 g/L magnesium nitrate solution

23、 at a wavelength in the200 to 250 nm region (for example, Bi 223.1 nm) using a drytemperature of 120C, a pyrolysis temperature of 950C, andan atomization temperature of 1800C. A large backgroundsignal should be observed with no over-or under-correction ofthe atomic signal.NOTE 2In general, Zeeman sy

24、stems should compensate for back-ground levels as high as 1.0 to 1.5 absorbance units. A continuumcorrection system should be able to correct for the broad-band backgroundabsorbance up to 0.5 to 0.6 absorbance units.8.4 AutosamplerCheck operation of the autosampler. Payparticular attention to the co

25、ndition of the pipette tip andposition of the tip during sample deposition. Clean the pipettetip with methanol. Adjust in accordance with the manufactur-ers instructions.NOTE 3Use of an appropriate surfactant in the rinse water mayenhance operation. If a surfactant is used, it should be checked for

26、thepresence of all the analytes to be determined.9. Optimization of the Furnace Heating Program9.1 Optimization of the furnace heating program is essen-tial. Furnace programs recommended by the manufacturers areoften designed for samples of a completely unrelated matrix.The analyst shall optimize th

27、e furnace program for a particularsample matrix (for example, steel, nickel alloys, etc.) andmodifier system in accordance with the following procedure:Furnace Step SectionDrying 9.2Pyrolysis 9.3Atomization 9.4Clean-out 9.59.2 Drying Step:9.2.1 Select the graphite tube type (Lvov or platformless)and

28、 measurement mode (peak height or integrated peak area).Then select the same heating parameters used in 8.3.3. Opti-mize the drying parameters using any of the calibrationsolutions (see 5.3) and the procedure given in either 9.2.2 or9.2.3.9.2.2 Samples Deposited on the Tube WallFor wall-deposited sa

29、mples, a drying temperature of 120C is satisfac-tory. To avoid spattering, a 20 s ramping time should be usedto reach the 120C temperature and then held at that tempera-ture. The holding time will depend on the volume of the sampleintroduced. Typical holding times are as follows:Injected Volume, L H

30、olding Time, s10 1540 309.2.3 Samples Deposited on the Lvov Platform:9.2.3.1 When using a Lvov platform, a two-stage dryingprocess is beneficial to prevent spattering.9.2.3.2 In the first stage, heat the sample rapidly to 80C,usinga1sramp and then hold the temperature at 80C for ashort time. The hol

31、ding time depends upon the volume of thesolution injected. Typical holding times are shown in 9.2.2.TABLE 1 Program for Graphite Furnace ConditioningStep Temperature, C Ramp, s Hold, sGas flow, mL/min1 1500 60 20 3002 20 1 10 3003 2000 60 20 3004 20 1 10 3005 2600 60 10 3006 20 1 10 3007 2650 2 5 0E

32、 1770 95 (2006)29.2.3.3 For the second stage, the temperature is ramped overa period of 20 to 30 s, to a value 20 to 40C above the boilingpoint of the solvent. The holding times should be the same asgiven in 9.2.3.2.9.2.4 In both cases, select a preliminary set of dryingconditions and monitor the dr

33、ying process visually with the aidof a dental mirror, to ensure that it proceeds without spattering.Hold the mirror directly above the sample introduction port(avoid touching the magnet), or near the end windows of thegraphite tube. Observe vapor formation on the mirror as dryingproceeds. Vapor evol

34、ution should cease at approximately 10 sbefore the end of the drying step. Adjust the hold timesaccordingly to accomplish this.9.2.4.1 Warning:To prevent serious eye injury, do not viewthe tube directly during the atomization or clean-out steps.9.3 Pyrolysis Step:9.3.1 During this step, volatile com

35、ponents of the matrix aredriven off and precursory reactions occur (for example, reduc-tion of the analyte oxide to the elemental state and theformation of matrix refractory oxides and carbides).NOTE 4Because of the low volatility of most metal alloy matrices,most of the matrix will remain in the fu

36、rnace after the pyrolysis.9.3.2 Use the optimum drying conditions as determined in9.2.NOTE 5At this stage, both the optimum pyrolysis and atomizationtemperatures are unknown. An estimate of the suitable atomizationtemperature shall be made and entered into the instrument prior tooptimization of the

37、pyrolysis temperature.9.3.3 Set atomizing temperature according to the manufac-turers instructions. Select the “Gas Interrupt” (or “Gas Stop”)option. Select an atomization integration time of 10 s.9.3.4 Select a pyrolysis time of 30 s ramp and 30 s hold.9.3.5 Select a calibration solution (see 5.3)

38、which will givean absorbance reading of 0.2 to 0.4 absorbance units.9.3.6 Vary the pyrolysis temperature in increments of100C, throughout the range from 500 to 1400C, taking threeabsorbance measurements at each step for the calibrationsolution selected in accordance with 9.3.5.9.3.7 Calculate the me

39、an of the three absorbance measure-ments obtained for each temperature step. Plot a graph relatingthe pyrolysis temperature to the mean absorbance. Note thetemperature at which the absorbance starts to decline. Subtract50C from this value to obtain the optimum pyrolysis tempera-ture.NOTE 6The 50C al

40、lowance accommodates the day-to-day variationsin the working temperature of the system.9.3.8 Use the optimum pyrolysis temperature found, andvary the hold time over the range of 15 to 60 s (15 s intervals).Take three absorbance measurements at each step for thecalibration solution selected in accord

41、ance with 9.3.5. Monitorthe background signal during the process, and note the time atwhich the background signal returns to the baseline.9.3.9 Calculate the mean of the three absorbance measure-ments. There should be no evidence of analyte loss (indicatedby lower absorbances for the longer hold tim

42、es).9.3.10 Provided the pyrolysis condition in 9.3.7 is satisfied,select the shortest time in which the background signal returnsto the baseline and add 10 s to this value to obtain the optimumhold time.NOTE 7A slow ramp time of 30 s and a hold time of 30 s is usuallysufficient for all pretreatment

43、reactions to occur. Short ramp times mayprovoke explosive loss of sample in the tube.9.4 Atomization Step:9.4.1 This step involves the production of gaseous analyteatoms inside the graphite tube.9.4.2 The analyst should determine the optimum atomiza-tion temperature and integration time experimental

44、ly, using thesame “Gas Interrupt option,” graphite tube type, and measuringmode combination selected before the optimization of thedrying step.NOTE 8Although it is possible to optimize the Lvov platform usingthe peak height measurement mode, the atomization step shall beoptimized in such a manner th

45、at the conditions required for stabilizedtemperature platform furnace (STPF) operation are satisfied. In addition tothe use of “Gas Interrupt” and matrix modification (inherent in certainprocedures), the following additional conditions are to be satisfied: (1) Thetemperature difference between the p

46、yrolysis step and the atomization stepshould be as small as possible (less than 1000C). This allows the furnaceto approach the near isothermal conditions quickly and reduces theamount of matrix volatilized; (2) Peak area integration measurement modeshall be used; and (3) There shall be zero ramping

47、time between thepyrolysis and the atomization steps. The “Gas Interrupt” will interrupt theflow of inert gas through the graphite tube from a few seconds prior to thestart of the atomization cycle to the end of the atomization cycle.9.4.3 Use the optimum drying and pyrolysis conditions.9.4.4 Select

48、an atomization temperature of 1200C and anintegration time of 20 s.9.4.5 Obtain three absorbance measurements with the cali-bration solution used in 9.3.5.9.4.6 Repeat varying the atomization temperature in 100Cincrements up to 2500C. It is not necessary to continue raisingthe atomization temperatur

49、e once the absorbance reaches aplateau.9.4.7 Plot the mean of the three absorbance measurementsobtained for each step against the atomization temperature.9.4.8 Examine the graph and determine the lowest atomiza-tion temperature where maximum absorbance was obtained.Add 200C to this value to obtain the optimum atomizationtemperature.NOTE 9At the lowest atomization temperature that gives maximumabsorbance when using the peak area integration mode, the peaks may bebroad with considerable trailing. The extra 200C will overcome thispr