1、Designation: D 3919 08Standard Practice forMeasuring Trace Elements in Water by Graphite FurnaceAtomic Absorption Spectrophotometry1This standard is issued under the fixed designation D 3919; the number immediately following the designation indicates the year oforiginal adoption or, in the case of r
2、evision, the 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 covers the general considerations for thequantitative determination of trace elemen
3、ts in water andwastewater by graphite furnace atomic absorption spectropho-tometry. Furnace atomizers are a most useful means of extend-ing detection limits; however, the practice should only be usedat concentration levels below the optimum range of directflame aspiration atomic absorption spectroph
4、otometry. Be-cause of differences between various makes and models ofsatisfactory instruments, no detailed operating instructions canbe provided for each instrument. Instead, the analyst shouldfollow the instructions provided by the manufacturer of aparticular instrument.1.2 Wavelengths, estimated d
5、etection limits, and optimumconcentration ranges are given in the individual methods.Ranges may be increased or decreased by varying the volumeof sample injected or the instrumental settings or by the use ofa secondary wavelength. Samples containing concentrationshigher than those given in the optim
6、um range may be dilutedor analyzed by other techniques.1.3 This technique is generally not applicable to brines andseawater. Special techniques such as separation of the traceelements from the salt, careful temperature control throughramping techniques, or matrix modification may be useful forthese
7、samples.1.4 The analyst is encouraged to consult the literature asprovided by the instrument manufacturer as well as varioustrade journals and scientific publications.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This
8、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.2. Referenced Documents2
9、.1 ASTM Standards:2D 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed ConduitsD 4841 Practice for Estimation of Holding Ti
10、me for WaterSamples Containing Organic and Inorganic ConstituentsD 5810 Guide for Spiking into Aqueous SamplesD 5847 Practice for Writing Quality Control Specificationsfor Standard Test Methods for Water Analysis3. Terminology3.1 DefinitionsFor definitions of terms used in this prac-tice, refer to T
11、erminology D 1129.3.2 Definitions of Terms Specific to This Standard:3.2.1 graphite furnacean electrothermal graphite devicecapable of reaching the specified temperatures required by theelement being determined.3.2.2 platform or similar device a flat, grooved or un-grooved piece of pyrolytic graphit
12、e inserted in the graphitetube on which the sample is placed (1).34. Summary of Practice4.1 The element is determined by an atomic absorptionspectrophotometer used in conjunction with a graphite furnace.The principle is essentially the same as with direct flameaspiration atomic absorption except a f
13、urnace, rather than aflame, is used to atomize the sample. The elemental atoms to bemeasured are placed in the beam of radiation by increasing thetemperature of the furnace, thereby causing the injected speci-men to be volatilized. Radiation from a given excited elementis passed through the vapor co
14、ntaining ground-state atoms ofthat element. The decrease in intensity of the transmitted1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.05 on Inorganic Constituents inWater.Current edition approved Nov. 15, 2008. Published N
15、ovember 2008. Originallyapproved in 1980. Last previous edition approved in 2004 as D 3919 04.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document
16、 Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.radiation is a measure of the amount of the ground-state
17、element in the vapor. A monochromator isolates the character-istic radiation from the hollow-cathode lamp and a photosen-sitive device measures the attenuated transmitted radiation.4.2 Dissolved elements are determined on a filtered samplewith no pretreatment. See 9.5.4.3 Total recoverable elements
18、are determined followingacid digestion and filtration. If suspended material is notpresent, this digestion and filtration may be omitted.5. Significance and Use5.1 Elemental constituents in potable water, receiving water,and wastewater need to be identified for support of effectivepollution control
19、programs. Currently, one of the most sensitiveand practical means for measuring low concentrations of traceelements is by graphite furnace atomic absorption spectropho-tometry.6. Interferences6.1 Background absorption is caused by the formation ofmolecular species from the sample matrix that absorb
20、or scatterthe light emitted by the hollow cathode or electrodelessdischarge line source. Without correction, this will cause theanalytical results to be erroneously high. Three approachesexist for simultaneous background correction: continuumsource, Zeeman, and Smith-Hieftje.6.1.1 Continuum SourceTh
21、e continuum source procedureinvolves the use of a deuterium arc source for the ultraviolet ora tungsten halide lamp for the visible region of the spectrum.Light from the primary spectral source and the appropriatecontinuum source are alternately passed through the graphitefurnace. Narrow-band emissi
22、on of the primary source isaffected by the scatter and background absorption from thematrix as well as the absorption of light by analyte atoms. Thebroadband emission of the continuum source is affected onlyby the background absorption. The effect of the background isremoved by taking a ratio of the
23、 energy of the two sources.6.1.2 Zeeman CorrectionThe Zeeman correction systeminvolves the use of an external magnetic field to split theatomic spectral line. When the magnetic field is off, bothsample and background are measured. When the magnetic fieldis applied, the absorption line is shifted and
24、 only the back-ground absorption is measured. Background correction isperformed by electronically comparing the field-off and field-on measurements, yielding an analyte-only absorption re-sponse.6.1.3 Smith-Hieftje SystemThis system involves cyclingthe atomic line source at high currents for brief i
25、ntervals. Theseintervals cause nonexcited atoms of the source element toundergo the process of self-reversal by emitting light atwavelengths other than those of the analyte. This light isabsorbed only by the background, so that interspersing periodsof high and low source current permit correction of
26、 thebackground.6.2 Some types of interference problems encountered indirect aspiration atomic absorption spectrophotometry can beobserved with the furnace technique. Although quite rare,spectral interference may be encountered. When this occurs,the use of another wavelength is suggested. Additionall
27、y, thefurnace technique is subject to chemical and matrix interfer-ence and the composition of the sample matrix can have amajor effect on the analysis. Therefore, for each differentmatrix encountered, the possibility of these interferencesshould be considered. The tests as outlined in 6.2.1-6.2.5 a
28、rerecommended prior to reporting analytical data. These testswill provide indication whether positive or negative interfer-ence effects are operative in any way on the analyte elementsthereby distorting the accuracy of the reported values.6.2.1 Spiking VerificationWhen the sample absorbance is40 % o
29、r less of the absorbance of the highest standard on thestandard curve, the amount of spike added to the sample shouldresult in a net increase equal to 50 % of the highest standardconcentration. The purpose of adding a large spike is todifferentiate between matrix interferences and random errors.The
30、recovery of the spike must be between 90 and 110 % forverification of the original determination. If the result of theoriginal determination is above 40 % on the curve, two aliquotsshould be withdrawn and diluted at least 1 + 1. One of thealiquots should be spiked before dilution with an amountresul
31、ting in a net increase over the unspiked aliquot equivalentto 50 % of the highest standard concentration. The reportedresult should be based on the analysis of the diluted aliquot.For verification of this result, the spike recovery must bebetween 90 and 110 %. For spiking verification to be valid in
32、either situation in the presence of nonspecific absorbance,simultaneous background correction must be used duringanalysis. If the result of the determination cannot be verified,the sample should be treated in one or more of the followingways:6.2.2 Serial DilutionSuccessively dilute and reanalyzethe
33、sample using spiking verifications to determine if theinterference can be eliminated. This assumes that the analyteoccurs at a sufficiently high concentration.6.2.3 Matrix ModificationMatrix modifiers are frequentlyused to stabilize volatile or moderately volatile analyte metalssuch as lead, cadmium
34、, chromium, and nickel. Metals such asthese begin to volatilize at very low temperatures and requirethat the charring/ashing temperature be lowered. Lowercharring/ashing temperatures reduce the chance of removingpotential interferents from the matrix during the charring/ashing step. Adding certain c
35、hemical compounds or combina-tions of chemical compounds will reduce the volatility ofselected metals by the formation of less volatile compoundsduring the charring/ashing process. The use of ammoniumdihydrogen phosphate or phosphoric acid results in highervolatilization temperatures for many elemen
36、ts, thus permittingthe use of higher charring/ashing temperatures to remove orreduce matrix interferences. Nickel nitrate has been shown toperform the same role for arsenic and selenium by forminghigh temperature arsenides and selenides. An alternate ap-proach to the same problem is to reduce the te
37、mperature atwhich the matrix volatilizes, permitting it to be removed at alower charring/ashing temperature. Sodium chloride in seawa-ter can be volatilized by adding ammonium nitrate as a matrixmodifier. The sodium nitrate and ammonium chloride formedare more volatile than the sodium chloride and c
38、an bevolatilized at much lower charring/ashing temperatures. OtherD3919082matrix modifiers include various organic acids such as citricand ascorbic acid. These acids are believed to reduce matrixinterferences by preventing the formation of large salt crystalsthat can occlude the analyte. A table of
39、additional matrixmodifiers is given in Appendix X1. See also the literature(218).6.2.4 Platform FurnacesThe pseudo-constant tempera-ture furnace design suggested by LVov (1) has minimizedmatrix and gas phase interference problems. LVov placed agraphite platform inside the graphite tube furnace to ap
40、proxi-mate a constant temperature design. Since the platform isheated by radiation, it lags behind the tube walls in tempera-ture, and delays the atomization of the analyte until the tubeatmosphere is at a higher, more constant temperature. Thisresults in reduced vapor-phase condensation and reduces
41、 theeffect of the sample matrix on the analyte signal. The integratedabsorbance signal is proportional to the number of atoms in thesample, independent of the rate at which atomization occurs.This type of furnace is commercially available or the modifi-cation can be made by the user (19).6.2.5 Stand
42、ard AdditionsAnalyze the sample by methodof standard additions while noting the precautions and limita-tions of its use. See 12.4.6.3 Gases generated in the furnace during the atomizationmay have molecular absorption bands encompassing the ana-lytical wavelength. When this occurs, either using backg
43、roundcorrection or choosing an alternative wavelength outside theabsorption band should eliminate this interference. Nonspecificbroadband absorption interference can also be compensated bybackground correction.6.4 Memory effects occur if, during atomization, all theanalyte is not volatilized and rem
44、oved from the furnace. Thiscondition is dependent on several factors, such as the volatilityof the element and its chemical form, whether pyrolyticgraphite is used, the rate of atomization, and furnace design. Ifthis situation is detected through blank burns, the tube must becleaned by operating the
45、 furnace at full power for the requiredtime period at regular intervals in the analytical scheme.6.5 Interference from a smoke-producing sample matrix cansometimes be reduced by extending the charring time at ahigher temperature. Also, some instruments utilize an ashingcycle in the presence of air.
46、Take care, however, to prevent lossof analyte.6.6 Samples containing large amounts of organic materialshould be oxidized by conventional acid digestion prior tobeing placed in the furnace. In this way, broadband absorptionwill be minimized. The use of expendable-type laboratory wareshould be conside
47、red to limit contamination.6.7 Carbide formation, resulting from the chemical environ-ment of the furnace, has been observed with certain elementsthat form carbides at high temperatures. Barium, molybdenum,nickel, titanium, and vanadium may be cited as examples.When this takes place, the element wil
48、l be released very slowlyfrom the carbide and longer atomization times may be requiredbefore the signal returns to baseline levels. This problem isgreatly reduced and sensitivity increases with the use ofpyrolytically coated graphite.6.8 Ionization interferences have to date not been reportedwith fu
49、rnace techniques.6.9 Contamination of the sample can be a major source oferror because of the extreme sensitivities achieved with thefurnace. Keep the sample preparation work area scrupulouslyclean (see 9.1). Clean all glassware with dilute HNO3(1 + 1).Pipette tips have been known to be a source of contamination.If suspected, acid soak them with HNO3(1 + 1) and rinsethoroughly with water. The use of only high-quality pipette tipsgreatly reduces this problem. It is very important that specialattention be given to reagent blanks in both the analysis and thecorrection of a
