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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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2楼2010-06-01 23:24:38
caoyongfan
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由于非线性的原因这种模拟可能不完全准确。当各物质有不同质量的吸收（不同密度和吸收特征），适合使用线性组合的XANES不能充分的说明混合物中的非线性吸收。O’Day 等人[46]正研究在自然形态铁的混合物，他们表明，每个总频谱分量的相对比例取决于周围元素的吸引力，类型和混合物的数量，以及基质组成。他们指出可以观察到的真实和明显的化合物之间的差异，这种差异主要是铁在混合物中的质量分数和基质吸收和荧光。因此，复杂的混合物的荧光可被样品显著的影响，基体吸收对浓度的变化并不是一个简单的线性函数关系。这项研究表明校准曲线应该逐点画出，如果复杂的混合物组成必须从定量的XANES确定，实验结果应该是一致的。在我们的研究中，明显的真实分数之间的矩阵效果较好。事实上，线性组合可以通过标准化合物的光谱算的，BN稀释同时也减轻了基体效应。 案例分析：对称性氧化钒与蚀变钢渣。对标准化合物的边缘特征的分析表明，得出了钒的形态（氧化态和对称性）由质心位置和前边缘高峰相联系。研究钢铁钒渣未知样品的形态时可知，因区域的不同改变，而导致的对称性和氧化状态是可以预测的。这就使得前边缘的分析更有挑战性。最佳实验结果表明了埃弗里高信号与噪声的比例。而且能够解释卡姆莱什沙方法和萨雷所需的峰值检查的优势。通过采用前边缘质心位置分析的方法和相关性我们可以确定钒的氧化钛和对称性（图4）。钢渣的改变是因为一个异构微米尺度的钒的对称分布的原因。大容量测量萨雷的XANES然后在适当的评估条件下得到他的氧化态和对称性的变化情况。同时我们记录阐明了XANES谱中从没有改变过的渣钢钒的微观分布。几个回收率和微XANES聚焦光束谱被记录（图6和7）。 有些乱先替你翻译三分之二。 [ Last edited by liufengbao100 on 2010-6-2 at 14:38 ] |
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