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What’s the point

发布时间:2013-12-07 14:02:02  

Electron probe microanalysisEPMA

Trace Element Analysis

Mod 11/8/10

What‘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)

“No” Zn ... but at what

level of confidence?Major>10 wt%Minor 1-10 wt%Trace <1 wt%

Trace elements .... and trace elementsIn 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 background-1

Interest 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).

A little background-2

Also…

?In both material science and geology, diffusion processes are studied, and EPMA is a prime technique. As you go further and further from the boundary between the two

phases, the elements drop to trace element levels. But where do they drop below detection? Or how do you set up the

EPMA conditions to reach down to a desired very low level?

How low can we go?

The USNM olivine standard above (San Carlos, Mg.9Fe.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 -1

The 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:?where the detection limit CMDLSScts on std, bar NSB=bkg cts on std, S

C= 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 elementcount rate, P/B= for pure element, and a=matrix correction (a-factor or ZAF).

* Goldstein et al., p. 500, equation 9.84 ?Goldstein et al., p. 500, equation 9.85

MDL Equations -2

There are several points to be made about these two equations:

?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 (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 ?‖ 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-1

Deriving the MDL equation-2

2. 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. 1s=Sq.Rt. of 90 =9.5 counts, so 3s=28.5. We‘ll use 3 sfor 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-3

3. 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 and backgrounds for the same time (10 sec) on CaSiO3(38.6 wt% Ca) and find a count rate of 6415 cts/sec on the peak and 16 cts/sec on the background.

4. So what is 2.9 cts/sec equal to in elemental wt%? We create a pseudo k-ratio where we take the statistical uncertainty of the background counts (square root, i.e. 1 sigma) divided by the Peak-Bkg of the standard counts on the element peak of interestthen by the composition C of the standard.The mdl will be in whatever units C is in.

Deriving the MDL equation-4

This is ??virtually the same result as the ―single line‖ detection limit provided by Probe for Windows (0.015 wt%, shown on next slide), derived from the Ziebold equation.

It would appear that the Ziebold equation is not exactly correct, for we must really be concerned with the background precision of the unknown, and the background level of the standard could be several times higher or lower. Going back and re-reading Ziebold, we find two interesting statements: that the

equation ―gives a measure of the detectability limit‖ and ―there is more than one way to define a detectability limit‖. Both are correct, and yes, the equation gives an approximation of the detection limit --but not the limit per se.

MDL in olivine -single line

MDL in olivine -average

―Figure of Merit‖Variation of figure-of-merit (P2/F) with accelerating voltage for various elements in different matrices. Scaled so 15 kV=1, no ZAF corrections.

Probers in Australia have much interest

in pushing the lower limits of EPMA They utilize a ―figure of merit‖ of detection, for mineral exploration P

2/B as a measure of how to achieve research.Utilizing extreme operating lower mdl (the higher the P2/B ).conditions (50 kV, 475 nA, 10 minute From our first principles derivation, counts) they have achieved mdl‘s below we can see that the P comes from the 5 ppm for some elements.standard, the B from the unknown. Variation of figure-of-merit with accelerating voltage, each element relative to its level at 15 kV. Detection limits (3 sigma) calculated for 100 sec counts, 50 kV, 450 nA. Data are all k-ratios, not ZAF corrected.From Advances in Electron Microprobe Trace-Element Analysis by B.W. Robinson and J. Graham, 1992, ACEM-12

Keys to low detection levels

?Maximize counts by utilizing

?Highest currents feasible (concern: beam damage)?Highest E0as feasible (concern: increased

penetration/range)

?Longer count times

?Correctly determine background locations

?Correct for unavoidable on-peak interferences the matrix correction*

*Donovan, Snyder and Rivers, 1993, An improved interference correction for trace element analysis, Microbeam Analysis, 2, 23-28.

Backgrounds: traces can overlap traces

Correct locating of

background

positions is

particularly

important in trace

element work, as

both first order and

higher order peaks

can cause incorrect

assessment of

background level.

Here, scans of the 3

Caltech/MAS trace

element glass

standards are

overlain. (Xe L

edge present as a

Xe gas sealed counter used.)From Carpenter, Counce, Kluk, and Nabelek, Characterization of Corning Standard

Glasses 95IRV, 95IRW and 95IRX: NIST/MAS Workshop, April 2002.

Backgrounds: Pb Ma in Monazite

Here the Th

Mz1 and 2nd

order La La

peaks fall close 1

to potential

backgrounds for

Pb Ma.

Monazite (Ce,La,REE,Th)PO4has been used for age dating, using U, Th and Pb concentrations.

Backgrounds ... holes

Probers in Australia,

interested in

detecting trace levels

of gold in certain

minerals, discovered

a ―hole in the

background‖ about

200 sin theta units

below the Au La

position.

(This scan was on

SrTiO3, on the LIF

crystal).Au La

Trace elements as fingerprints:

apatite in bentonites

Crystals were separated

from clay; mixed

population (zircons,

white; apatites, yellow

in false color mosaic

BSE image) mounts in

4 mm plug (above).A range of trace elements were analyzed in bentonites (very old volcanic ash), in order to verify common stratigraphic horizons in Ordovician sediments. 40-60 ppm

mdls were achieved with 20 second counts and 60 nA currents.

Where is the ...Arsenic?

Some groundwaters in

northeast Wisconsin have

elevated Arsenic (8 mg/L),

and EPMA is being used to

help understand the source.

Aquifer strata contain

mineralized zones (500-80

ppm whole rock), mainly

marcasite (FeS2) and quartz.

X-ray maps (PfW-MAN)

were acquired overnight for

Fe, S, Si, O and As. They

showed that As is located on

the edge of some quartz

grains.

Here, a rectangular area was

mapped at 10 mm intervals.

(Research of Toni Simo,Katie Thornberg, Selena Mederos)

Pb in Cast IronC KaPb MaFe Ka

(Research of Jun Park, Carl Loper and John Fournelle.)This cast iron has 100 ppm of Pb in the bulk analysis, and the question was which phase did it reside in. The working hypothesis was that it was associated with graphite dendrites. A full quantitative X-ray map (backgrounds acquired) was acquired overnight (conditions 15 keV, 300 nA, 150 seconds each on Pb peak and bkg). The mdl for Pb is 200 ppm (.02 wt%).

X-ray mapping of irregularly positioned/shaped zircon grains

?Mounted in epoxy: need to

avoid melting epoxy with

high currents!

?Define polygon boundary

?Select point spacing

interval

?Fully automated

quantitative EPMA

?Software contouring or 3D

surface mapping (“Surfer”)

CL

Huckleberry Ridge Tuff BSEBSE

Zircon grain A

(2 Ma, 2500 km3, normal d18O)

U wt%Th wt%(Research of Ilya Bindeman, John Valley and John Fournelle)

Standards: validating trace

element procedure

?There is an issue of trace element accuracy on unknowns, where the standard for the element of interest was at a high level. Such a standard should be used for peaking the spectrometer and acquiring standard counts, but it is recommended that a be also analyzed to validate the procedure.

?Such secondary standards could be

?Synthetic glasses such as the Caltech/MAS 95IRV,W and X glasses; NIST glasses and metals; Ni-Cr diopside glass, etc.

?

Minerals and glasses analyzed by ion probe

Comparison:

Trace elements by WDS vs EDS

WDS is clearly the better method for acquiring trace element data, by

an order of magnitude or so compared to EDS.

Goldstein et al, 1992, p. 501

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