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Suchasimulationmightnotbeabsolutelyexactbecauseof nonlineareffects.XANESfitsusinglinearcombinationsdonot accountsufficientlyfornonlinearabsorptioninmixtureswhen theend-memberscomponentshaveverydifferentmassabsorp- tions(differentdensitiesandabsorptioncharacteristics).O’Day etal. 46 havepreciselystudiedironspeciationinnaturalmixtures, andtheyhaveshownthattherelativeproportionsofeach componentofthetotalspectrumdependonthelocalbonding environment(s)aroundtheelementofinterest,onthetypeand numberofcomponentsinthemixture,andonthecomposition ofthematrix.Theynotedthattheobserveddifferencesbetweentrueandapparentfractionsdependonthetotalfluorescenceyield oftheironinthecompound,whichisproportionaltothemass fractionofironinthecompounds,andonmatrixabsorption andfluorescence.Sofluorescenceintensitiesofcomplex mixturescanbeinfluencedsignificantlybysampleandmatrix absorptionthatdoesnotvaryinasimplelinearfashionasa functionofconcentration.Thisstudysuggeststhatcalibration curvesshouldbeconstructedonacase-by-casebasisif componentfractionsofcomplexmixtureshavetobedetermined quantitativelyfromXANESfitsanditpointsouttheimportance ofahigh-quality,experimentallyconsistentwithstandard compoundslibrary.Inourstudy,thegoodagreementbetween trueandapparentfractionsmaybeduetoalowmatrixeffects. Indeed,thelinearcombinationswerecalculatedwithspectra ofstandardcompoundsdilutedinBNfortransmissionmode. TheBNdilutioncertainlymitigatesthematrixeffects. ACaseStudy:VanadiumOxidationStateandSymmetry withinAlteredSteelSlag.Analysisofthepre-edgefeatures ofstandardcompoundsshowedthatitispossibletoderive informationonthevanadiumspeciation(oxidationstateand symmetry)fromthecentroidpositionandthetotalareaofthe pre-edgepeak.Whenstudyingvanadiumspeciationinunknown sampleassteelslag,variationsinthesymmetryandinthe oxidationstatecanbeexpectedduetothedifferentalteredareas. Thismakestheanalysisofthepre-edgemuchmorechallenging. Averyhighsignal-to-noiseratio,thebestexperimentalresolu- tionpossible,andacarefulpre-edgepeakexaminationwiththe appropriateinterpretativemethodsarerequired. Theoxidationstateandsymmetryofvanadiuminleached steelslagweredeterminedusingthemethodpresentedprevi-ouslybasedonpre-edgeanalysis(Figure4)andcorrelations betweencentroidpositionandnormalizedarea.Thealteredsteel slagisaheterogeneousmatrixatthemicrometerscale,interm ofvanadiumvalenceandsymmetrydistribution.Bulk-XANES measurementsaretheninadequatetoassessaccuratelythe variationoftheoxidationstateandsymmetry.Wehaverecorded micro-XANESspectratoelucidatethemicrodistributionof vanadiuminsteelslagfromtheunalteredtothealteredlayer. SeveralROIwereselectedtorecordmicro-XANESspectrawith thefocusedbeam(Figures6and7).Becauseofahighvanadium releaseduringlaboratory-scaleleachingtests, 27 theoxidation ofvanadiumwassuspected.MoreoversinceV5 exhibitssome toxicity, 47 itiscrucialtoevaluatetheoxidationstateofvanadium withinalteredsteelslagintermsofenvironmentalimpacts especiallyifsteelslagisreusedasanaggregateinroad constructions.Theresults(Figure8)showthatvanadiumis mainlypresentinunalteredregionasoctahedralV3 (ROI4 6). Asignificantincreaseofthepre-edgepeakareaisobservedas wellasaslightpositiveshiftofitscentroidpositionforROI1, 2,and9.Itreflectsthelossofsymmetryofthevanadiumsite anditsoxidationduringleaching.Vanadiumispresentinalteredregioninthepentavalentformwithtetrahedralsymmetry.Then Figure8showsthatvanadiumispresentinlittlealteredregion (ROI3,7,and8)asamixtureofvariousoxidationstateand symmetrybutdoesnotallowarigorousdiscriminationofthe natureofthemixture.However,thecorrespondencewiththe mixingcurvecoulsonite/vanadinitesuggeststhatvanadiumis presentasamixtureofoctahedralV3 andtetrahedralV5 (around50:50mixture),showingthecomplexityofvanadium oxidationstatedistributioninleachedsteelslag.Butthe calibrationcurve(opentrianglesinFigure4)isthennotwell- adaptedbecauseofmatrixvariationbetweenXANESspectra ofstandardcompoundsandmicro-XANESspectracollectedin fluorescencemode. 46 Sothisobservationcannotbequantita- tivelyconfirmed.Micro-XANESresults(Figure8)showsthedistributionof vanadiumformsatthesub-micrometerlevelinalteredsteelslagandtheirevolutionduringleaching.Theseresultssuggestthat thevanadiumspeciationisdifferentinalteredorunaltered region.Theyprovideessentialinformationtoexplainvanadium releaseandtoidentifythemechanismsassociated.Wehave chosenvariousstandardcompoundsinthetypeofcoordinating atom(oxides,minerals).Thechoiceofstandardcompounds shouldbealwaysdonewithcare,andstandardcompoundswith similarchemicalbondingarepreferred.However,thenatureof standardcompoundsappearslesscrucialwiththisselected interpretationmethod.Thismethodiswelladaptedtothe assessmentofvanadiumspeciationinaheterogeneousunknown sample,suchassteelslag,evenwithstandardcompoundswith differentchemicalbonding. |
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4樓2010-06-02 08:14:01
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mengchuncai(金幣+200, 翻譯EPI+1): 2010-06-02 15:57:53
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mengchuncai(金幣+1): 2010-06-03 20:06:55
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由于非線性的原因這種模擬可能不完全準(zhǔn)確。當(dāng)各物質(zhì)有不同質(zhì)量的吸收（不同密度和吸收特征），適合使用線性組合的XANES不能充分的說明混合物中的非線性吸收。O’Day 等人[46]正研究在自然形態(tài)鐵的混合物，他們表明，每個(gè)總頻譜分量的相對(duì)比例取決于周圍元素的吸引力，類型和混合物的數(shù)量，以及基質(zhì)組成。他們指出可以觀察到的真實(shí)和明顯的化合物之間的差異，這種差異主要是鐵在混合物中的質(zhì)量分?jǐn)?shù)和基質(zhì)吸收和熒光。因此，復(fù)雜的混合物的熒光可被樣品顯著的影響，基體吸收對(duì)濃度的變化并不是一個(gè)簡(jiǎn)單的線性函數(shù)關(guān)系。這項(xiàng)研究表明校準(zhǔn)曲線應(yīng)該逐點(diǎn)畫出，如果復(fù)雜的混合物組成必須從定量的XANES確定，實(shí)驗(yàn)結(jié)果應(yīng)該是一致的。在我們的研究中，明顯的真實(shí)分?jǐn)?shù)之間的矩陣效果較好。事實(shí)上，線性組合可以通過標(biāo)準(zhǔn)化合物的光譜算的，BN稀釋同時(shí)也減輕了基體效應(yīng)。 案例分析：對(duì)稱性氧化釩與蝕變鋼渣。對(duì)標(biāo)準(zhǔn)化合物的邊緣特征的分析表明，得出了釩的形態(tài)（氧化態(tài)和對(duì)稱性）由質(zhì)心位置和前邊緣高峰相聯(lián)系。研究鋼鐵釩渣未知樣品的形態(tài)時(shí)可知，因區(qū)域的不同改變，而導(dǎo)致的對(duì)稱性和氧化狀態(tài)是可以預(yù)測(cè)的。這就使得前邊緣的分析更有挑戰(zhàn)性。最佳實(shí)驗(yàn)結(jié)果表明了埃弗里高信號(hào)與噪聲的比例。而且能夠解釋卡姆萊什沙方法和薩雷所需的峰值檢查的優(yōu)勢(shì)。通過采用前邊緣質(zhì)心位置分析的方法和相關(guān)性我們可以確定釩的氧化鈦和對(duì)稱性（圖4）。鋼渣的改變是因?yàn)橐粋�(gè)異構(gòu)微米尺度的釩的對(duì)稱分布的原因。大容量測(cè)量薩雷的XANES然后在適當(dāng)?shù)脑u(píng)估條件下得到他的氧化態(tài)和對(duì)稱性的變化情況。同時(shí)我們記錄闡明了XANES譜中從沒有改變過的渣鋼釩的微觀分布。幾個(gè)回收率和微XANES聚焦光束譜被記錄（圖6和7）。 有些亂先替你翻譯三分之二。 [ Last edited by liufengbao100 on 2010-6-2 at 14:38 ] |
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