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UW-Madison G 777 - Trace Element Analysis

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Electron probe microanalysis EPMAWhat’s the point?DefinitionsTrace elements .... and trace elementsA little backgroundHow low can we go?MDL Equations - 1MDL Equations - 2Deriving the MDL equation-1Deriving the MDL equation-2Deriving the MDL equation-3Deriving the MDL equation-4MDL in olivine - single lineMDL in olivine - average“Figure of Merit”Keys to low detection levelsBackgrounds: traces can overlap tracesBackgrounds: Pb Ma in MonaziteBackgrounds ... holesPowerPoint PresentationWhere is the ...Arsenic?Pb in Cast IronX-ray mapping of irregularly positioned/shaped zircon grainsHuckleberry Ridge Tuff Zircon grain A (2 Ma, 2500 km3, normal d18O)Standards: validating trace element procedureComparison: Trace elements by WDS vs EDSElectron probe microanalysisEPMA Trace Element AnalysisMod 11/15/07What’s the point?What’s the minimum detection limit for a particular element – or said otherwise, at what point can be be sure that a small inflection above the surrounding background really is a peak? What kind of confidence level should be place on such a number?Definitions“Generally, WDS can achieve limits of detection of 100 ppm in favorable cases, with 10 ppm in ideal situations where there are no peak interferences and negligible matrix absorption.” (Goldstein et al., p. 341)Major >10 wt%Minor 1-10 wt%Trace <1 wt%wt% ppm100 100000010 1000001 100000.1 10000.01 1000.001 100.0001 1“No” Zn ... but at what level of confidence?Trace elements .... and trace elements In the real world, the definition of “trace element analysis” is sometimes broader than the strict quantitative analysis of ppm level elements in one microvolume (~micron interaction volume). Many individuals desire to use EPMA to tell them about the “distribution of trace elements” in their materials, e.g. where the 30 ppm of Pb is in a cast iron. There are two possibilities: the 30 ppm is spread uniformly throughout the material, or in fact most of the material has probably <1 ppm of Pb, but a small fraction of the volume has phases that have Pb at major element levels.The question is then, are they at least the size of the interaction volume and if so, where are they. Our discussion here will deal with all these aspects.A little backgroundInterest in trace elements dove tails with the develop of techniques that could achieve better/quicker/cheaper/more precise/small volumes of said elements. • From the 1960s on, geochemists and petrologists developed increasing interest in trace element partitioning between fluids/melts and minerals. The electron microprobe became the instrument of choice for characterizing the trace levels in doped experiments .• There has been an interest in “trace elements” in certain minerals to assist in the search for ore bodies that contain said elements. A related research field is locating the naturally-occurring minerals that are responsible for certain levels of groundwater contamination (e.g. As).How low can we go?The USNM olivine standard above (San Carlos, Mg.9 Fe.1SiO4) has a published Ca content of <0.04 wt% (= 400 ppm). This scan was acquired at 20 keV, 30 nA, with 10 seconds per channel. Clearly there is a peak at the Ca Ka position (24 cps), somewhat above the background (~10 cps). At what point can we say with 99% confidence that there is a statistically significant peak?MDL Equations - 1The key concept here is minimum detection limit (MDL), i.e., what is the lowest concentration of the element present that is statistically above the background continuum level by 3 sigma (commonly accepted level).There are (at least) two equations used to define the MDL:• the first* uses the Student t test values:where the detection limit CMDL is in wt%, CS=std wt %,bar NS=ave. peak cts on std,bar NSB=bkg cts on std, SC= std dev of measured values and n=number of data points• the second†, which is probably more wider used, was developed by Ziebold (1967):where n=number of measurements, T=seconds per measurement, P=pure element count rate, P/B= for pure element, and =matrix correction ( -factor or ZAF).CMDL=CSN S−N SB21/ 2(tn−11−α)SCn1/ 2CMDL=(3.29)⋅α(nTP⋅PB)1/ 2* Goldstein et al., p. 500, equation 9.84 † Goldstein et al., p. 500, equation 9.85MDL Equations - 2There are several points to be made about these two equations:• the first (Student t test) equation only works for the average of several measurements, since it uses SC, the average of several measurement. This calculation is useful in that special case.• however, as many times as not, a specific area or region is only measured once (e.g., a linear traverse across a zoned crystal), and the second equation is the appropriate one to use.• note in the second equation, the term P times P/B appears in the denominator. As P2/B increases, the MDL decreases (the lower, the better!). This P2/B term is called the ‘figure of merit” for trace element work. • following some discussions with John Valley about the traditional (second) equation and how the peak and background used in it were from the pure element standard—not the unknown, I went back to first principles and derived the equation.Deriving the MDL equation-11. We need to determine the precision of the background value for the unknown, i.e. for a given background value, how big is the statistical error bar (counting statistic, 3 sigmas) above it. So here, is it large as on the right below, or is it small as on the LEFT?Deriving the MDL equation-22. Let us consider Ca Ka peak on our olivine. We measure the background and get 9 cts/sec. The 1 sigma value however is calculated from TOTAL counts, NOT count rate. So we must multiply 9 cts/sec by the time, 10 seconds, and we get 90 counts. 1=Sq.Rt. of 90 =9.5 counts, so 3 =28.5. We’ll use 3  for now, the 99.7% confidence level. Ergo, our MDL for Ca in the olivine is 29 counts above background, over 10 seconds (or if plotted on the wavescan where data are in cts/sec, it would be a value of 3 cps (the left purple marker).Note: we haven’t said one word about count rate on a standard, and we have figured out the minimum detection limit for Ca in our unknown -- though we don’t know what that mdl of 3 cts/sec translates to in ppm or wt%).Deriving the MDL equation-33. However, we usually want to “translate” those raw counts into a more usable number, i.e. so many ppm. For that, we need some reference intensity counts for Ca Ka. We then count Ca ka peak


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UW-Madison G 777 - Trace Element Analysis

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