Transcript Electron probe microanalysis EPMA

Electron probe microanalysis
EPMA
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%
wt%
ppm
100 1000000
10 100000
1
10000
0.1
1000
0.01
100
0.001
10
0.0001
1
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 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.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 - 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:
• the first* uses the Student t test values:
CMDL
a
CS
21/ 2 (t 1
n1 )SC

1/ 2
N S  NSB
n
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):
(3.29) a
CMDL 
(nTP  PB )1/ 2
where n=number of measurements, T=seconds per measurement, P=pure
element count 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:
• 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-1
1. 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-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 s 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-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 kratio 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 interest
KCamdl
unk
bkgctsunk
ZAFCa
Ca
st d
 3

C
st d
st d
Ca
(pkcts bkgcts)Ca
ZAFCa
and multiply by 3 (for 3 sigma, 99% confidence) and the ZAF of each and
then by the composition C of the standard.The mdl will be in whatever
units C is in.
Deriving the MDL equation-4
ZAFCaol 1.1087
ZAFCastd 1.080
MDL
ol
Ca
90 1.1087
3

 38.6  .017 wt%Ca
66250 1.080
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
Good totals
Excellent
stoichometry
These are the single line
detection limits, calculated
with Ziebold’s equation
(Goldstein, eq 9.85, p. 500)
MDL in olivine - average
For a homogeneous sample, it is “legal” to
add together all the counts, which gives
greater precision and a lower detection
limit, e.g. 110 ppm here for Ca at 99% ci.
This is a handy chart that shows what kind
of counting time would be required, under
the same analytical conditions, to achieve a
lower detection limit. For example, to get a
mdl of 25 ppm, you’d need to count for ~5
minutes on the Ca peak and then
background (for each of 10 spots). The
current analysis here was a little over 1
minute per spot (thus, about 12 minutes for
a mdl of 110 ppm. For 25 ppm, it would
add an additional 100 minutes to the
analysis time.
“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
detection, for mineral exploration
research.Utilizing extreme operating
conditions (50 kV, 475 nA, 10 minute
counts) they have achieved mdl’s below
5 ppm for some elements.
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.
They utilize a “figure of merit” of
P2/B as a measure of how to achieve
lower mdl (the higher the P2/B ).
From our first principles derivation,
we can see that the P comes from the
standard, the B from the unknown.
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 E0 as feasible (concern: increased
penetration/range)
• Longer count times
• Correctly determine background locations
• Correct for unavoidable on-peak interferences within 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 La1
peaks fall close
to potential
backgrounds for
Pb Ma.
Monazite (Ce,La,REE,Th)PO4 has been used for age
dating, using U, Th and Pb concentrations.
Backgrounds ... holes
Au La
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).
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).
(Research of Norlene Emerson.)
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 Iron
C Ka
Fe Ka
(Research of Jun Park, Carl Loper and John Fournelle.)
Pb Ma
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
Zircon grain A
BSE
BSE
(2 Ma, 2500 km3, normal d18O)
U wt%
Th wt%
Th and U Ma (PET)
• 18 keV, 400 nA
• 94 points
• 10 elements
• 11 mm spacing
• 50 sec on peak + 50
sec on bkgs = 8
hours total time
• mdl = 130 ppm
(.013 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 secondary trace level standard 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