Someone asked about trace mercury by EDS...
The short answer is that EDS is very insensitive to trace elements in general. Why? Because your detector is seeing all the other photons besides your emission line of interest and hence the pulse processor is spending 99% or more of the time looking at photons that you don't really care about (they might be characteristic lines of other matrix elements or continuum x-rays).
For high Z elements this is particularly bad because the electron interactions are distributed across many shells, meaning that any one emission line of a high Z element is less intense than say the Si Ka emission line which is essentially the only way an electron can interact with that atom.
The latter issue is the same for EDS or WDS, but the first issue (pulse processing) is greatly improved by utilizing a WDS spectrometer. Why? because the detector only sees a narrow portion of the spectrum and hence can spend most of it's time processing photons of interest.
To illustrate this I ran a sample of plastic that was suspected of being contaminated by mercury (it was), but the EDS spectrometer did not pick up the Hg emission lines at all, while the WDS spectrometer quickly found the signal. See PPT attachment below showing the comparison.
So if you can get access to a WDS spectrometer, I would suggest you use that instead of EDS.
I should also point out that probe for EPMA can now perform the quantitative interference correction that I have for WDS elements on EDS elements as well as seen here:
http://smf.probesoftware.com/index.php?topic=482.msg2826#msg2826
This is a little weird, but I wanted to compare running trace elements in zircon on WDS and EDS, but wondered if using the L lines for Th, U and Hf might be better than the M series lines for EDS. Unfortunately our Thermo system does not return the background count rate since it utilizes a filter method for the background correction, so I cannot compare the detection limits and instead can only look at the variance in wt % units.
The conditions for simultaneous acquisition of the WDS and EDS spectra were:
KiloVolt = 20.0 Beam Current = 50.0 Beam Size = 10
and 640 sec on-peak with 640 sec off-peak counting times. Normally I would run WDS elements at 100 or 200 or more nA, but the SDD pulse processor cannot handle much more than that (EDS deadtime was ~30% at 50 nA).
The results on a synthetic zircon (Oak Ridge) using off-peak backgrounds is:
Formula ZrSiO4 is Calculated by Difference from 100%
Un 4 std 257 as unk, Results in Elemental Weight Percents (Off-peak background correction, no blank)
ELEM: Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN
TIME: 640.00 640.00 640.00 640.00 640.00 --- --- ---
BEAM: 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th Hf U P Y O Zr Si SUM
74 .002 .015 -.001 .027 .008 34.936 49.738 15.314 100.040
75 -.002 .015 .004 .028 .011 34.937 49.736 15.313 100.042
76 -.002 .016 .013 .025 .003 34.933 49.736 15.313 100.038
77 -.014 .018 .003 .025 .001 34.937 49.747 15.317 100.034
78 .009 .018 .014 .024 .008 34.928 49.727 15.311 100.040
79 .000 .021 .017 .024 .000 34.929 49.733 15.312 100.037
80 .013 .020 -.002 .025 .005 34.931 49.734 15.313 100.038
81 .005 .022 .003 .025 .006 34.932 49.733 15.312 100.039
82 .008 .019 .008 .025 .006 34.931 49.731 15.312 100.040
83 -.002 .017 .007 .025 .002 34.934 49.739 15.314 100.037
AVER: .002 .018 .007 .025 .005 34.933 49.736 15.313 100.038
SDEV: .008 .003 .006 .001 .003 .003 .006 .002 .002
SERR: .002 .001 .002 .000 .001 .001 .002 .001
%RSD: 431.39 13.98 94.71 4.86 69.39 .01 .01 .01
There is a very small interference from Si ka on Hf Ma:
Interference by Si KA1 at 7.12560 ( 27661.8) ( -1609.2)
Interference by Si KA2 at 7.12970 ( 27677.8) ( -1593.2)
and also a small interference from Zr La on P Ka:
Interference by Zr LA1 at 6.07170 ( 69386.5) ( -986.55)
Since I did not analyze for Si or Zr we cannot perform a proper interference correction, but we can assign the sample as a blank to itself. By assigning the sample as a blank to itself, everything is normalized to zero, but the number we want to look at is the variance in either case. After all elements are assigned as a blank correction (assuming a zero concentration for each) we can see the results here:
Formula ZrSiO4 is Calculated by Difference from 100%
Un 4 std 257 as unk, Results in Elemental Weight Percents (Off-peak background correction, with blank)
ELEM: Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN
TIME: 640.00 640.00 640.00 640.00 640.00 --- --- ---
BEAM: 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th Hf U P Y O Zr Si SUM
74 .000 -.003 -.007 .002 .003 34.918 49.767 15.323 100.002
75 -.004 -.003 -.002 .002 .006 34.918 49.765 15.322 100.003
76 -.004 -.002 .007 .000 -.002 34.914 49.765 15.322 100.000
77 -.016 .000 -.004 .000 -.004 34.918 49.776 15.326 99.996
78 .007 .000 .007 -.001 .004 34.910 49.755 15.319 100.001
79 -.002 .003 .010 -.002 -.005 34.911 49.761 15.321 99.998
80 .011 .001 -.008 -.001 .000 34.913 49.762 15.321 100.000
81 .003 .004 -.004 .000 .001 34.914 49.761 15.321 100.001
82 .006 .001 .002 .000 .001 34.912 49.759 15.320 100.001
83 -.004 -.001 .000 .000 -.003 34.915 49.768 15.323 99.998
AVER: .000 .000 .000 .000 .000 34.914 49.764 15.322 100.000
SDEV: .008 .003 .006 .001 .003 .003 .006 .002 .002
SERR: .002 .001 .002 .000 .001 .001 .002 .001
Note that the variance does not change from the application of a blank correction. Also just for fun I ran the same analyses using the MAN background correction and as expected the variances are smaller (and would have taken half the time since the off-peak measurements are not necessary!):
Formula ZrSiO4 is Calculated by Difference from 100%
Un 4 std 257 as unk, Results in Elemental Weight Percents (MAN background correction, with blank)
ELEM: Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: MAN MAN MAN MAN MAN
TIME: 640.00 640.00 640.00 640.00 640.00 --- --- ---
BEAM: 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th Hf U P Y O Zr Si SUM
74 .004 .002 -.004 .001 -.001 34.915 49.762 15.321 100.002
75 .001 .001 .001 .003 .002 34.916 49.760 15.321 100.005
76 -.003 .002 .001 -.001 -.001 34.914 49.765 15.322 99.999
77 -.012 .000 .001 .000 .002 34.916 49.768 15.323 99.999
78 -.004 -.001 -.003 .000 .005 34.915 49.765 15.322 100.000
79 -.001 .000 .004 -.001 .001 34.913 49.762 15.321 99.999
80 .010 -.001 -.002 -.001 -.001 34.912 49.761 15.321 100.000
81 -.001 -.002 .000 .000 -.001 34.915 49.766 15.323 99.999
82 .004 .000 -.002 .000 -.005 34.914 49.765 15.322 99.999
83 -.004 -.002 -.002 -.001 -.003 34.915 49.769 15.324 99.997
AVER: -.001 .000 .000 .000 .000 34.915 49.764 15.322 100.000
SDEV: .006 .001 .002 .001 .003 .001 .003 .001 .002
SERR: .002 .000 .001 .000 .001 .000 .001 .000
But what about EDS analyses? Next let's compare using the L and M series lines for Th, U and Hf by EDS...
Here are the same analyses but this time looking at the EDS spectra and using M lines for Th, Hf and U:
Un 4 std 257 as unk, Results in Elemental Weight Percents (EDS using M series, without blank correction)
ELEM: Th Hf U P Y Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN EDS EDS EDS EDS EDS
TIME: --- --- --- --- --- 800.00 800.00 800.00 800.00 800.00 --- --- ---
BEAM: --- --- --- --- --- 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th-D Hf-D U-D P-D Y-D Th Hf U P Y O Zr Si SUM
XRAY: (ma) (ma) (ma) (ka) (la) (ma) (ma) (ma) (ka) (la) () () ()
74 --- --- --- --- --- .000 2.913 .072 1.272 1.827 35.458 46.736 14.390 102.668
75 --- --- --- --- --- .000 2.951 .074 1.207 1.826 35.390 46.749 14.394 102.591
76 --- --- --- --- --- .000 2.469 .064 1.131 1.826 35.402 47.032 14.481 102.405
77 --- --- --- --- --- .001 2.494 .085 1.323 1.849 35.572 46.901 14.441 102.666
78 --- --- --- --- --- .000 2.993 .067 1.456 1.855 35.617 46.594 14.346 102.926
79 --- --- --- --- --- .005 2.917 .062 1.398 1.805 35.579 46.685 14.374 102.825
80 --- --- --- --- --- .006 2.898 .079 1.183 1.698 35.384 46.846 14.423 102.517
81 --- --- --- --- --- .000 2.832 .068 1.157 1.741 35.371 46.878 14.434 102.481
82 --- --- --- --- --- .000 2.426 .082 1.133 1.729 35.415 47.092 14.499 102.376
83 --- --- --- --- --- .000 2.418 .069 1.136 1.707 35.424 47.111 14.505 102.371
AVER: --- --- --- --- --- .001 2.731 .072 1.240 1.786 35.461 46.862 14.429 102.583
SDEV: --- --- --- --- --- .002 .244 .008 .118 .061 .092 .176 .054 .190
SERR: --- --- --- --- --- .001 .077 .002 .037 .019 .029 .056 .017
Egads! The Th is good but the other elements have severe overlaps. But let's apply the blank correction to itself as we did before since we can't correct for interferences by Si and Zr since we didn't measure them (well actually we could using the EDS spectra from our standards, but let's not complicate things too much!):
The data looks better now, but as expected the variances are about the same:
Un 4 std 257 as unk, Results in Elemental Weight Percents (EDS using M series, with blank correction)
ELEM: Th Hf U P Y Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN EDS EDS EDS EDS EDS
TIME: --- --- --- --- --- 800.00 800.00 800.00 800.00 800.00 --- --- ---
BEAM: --- --- --- --- --- 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th-D Hf-D U-D P-D Y-D Th Hf U P Y O Zr Si SUM
XRAY: (ma) (ma) (ma) (ka) (la) (ma) (ma) (ma) (ka) (la) () () ()
74 --- --- --- --- --- -.001 .122 -.002 -.017 -.033 34.880 49.729 15.311 99.990
75 --- --- --- --- --- -.001 .160 .000 -.080 -.034 34.815 49.742 15.315 99.916
76 --- --- --- --- --- -.001 -.312 -.010 -.153 -.034 34.828 50.018 15.400 99.735
77 --- --- --- --- --- -.001 -.288 .011 .031 -.012 34.991 49.892 15.362 99.987
78 --- --- --- --- --- -.001 .200 -.007 .159 -.006 35.033 49.592 15.269 100.239
79 --- --- --- --- --- .004 .126 -.012 .104 -.054 34.997 49.680 15.296 100.141
80 --- --- --- --- --- .005 .108 .005 -.103 -.156 34.809 49.835 15.344 99.845
81 --- --- --- --- --- -.001 .043 -.006 -.128 -.116 34.797 49.867 15.354 99.810
82 --- --- --- --- --- -.001 -.354 .008 -.151 -.128 34.841 50.076 15.418 99.708
83 --- --- --- --- --- -.001 -.356 -.004 -.145 -.143 34.851 50.086 15.421 99.710
AVER: --- --- --- --- --- .000 -.055 -.002 -.048 -.072 34.884 49.852 15.349 99.908
SDEV: --- --- --- --- --- .002 .238 .007 .113 .058 .088 .170 .052 .182
SERR: --- --- --- --- --- .001 .075 .002 .036 .018 .028 .054 .017
So let's try again, but this time using the L series lines from our EDS spectra, first without the blank correction:
Un 4 std 257 as unk, Results in Elemental Weight Percents (EDS spectra using L series, without blank correction)
ELEM: Th Hf U P Y Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN EDS EDS EDS EDS EDS
TIME: --- --- --- --- --- 800.00 800.00 800.00 800.00 800.00 --- --- ---
BEAM: --- --- --- --- --- 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th-D Hf-D U-D P-D Y-D Th Hf U P Y O Zr Si SUM
XRAY: (ma) (ma) (ma) (ka) (la) (la) (la) (la) (ka) (la) () () ()
74 --- --- --- --- --- .000 .017 .000 1.237 1.773 35.937 48.257 14.858 102.080
75 --- --- --- --- --- .000 .000 .000 1.173 1.771 35.880 48.299 14.871 101.993
76 --- --- --- --- --- .000 .002 .513 1.105 1.782 35.704 48.071 14.801 101.978
77 --- --- --- --- --- .043 .000 .000 1.291 1.802 35.979 48.203 14.841 102.160
78 --- --- --- --- --- .000 .000 .000 1.415 1.798 36.105 48.165 14.830 102.313
79 --- --- --- --- --- .196 .000 .000 1.360 1.751 36.016 48.118 14.815 102.256
80 --- --- --- --- --- .000 .004 .000 1.150 1.648 35.867 48.369 14.893 101.931
81 --- --- --- --- --- .000 .005 .066 1.126 1.691 35.827 48.326 14.879 101.921
82 --- --- --- --- --- .000 .009 .000 1.107 1.686 35.822 48.370 14.893 101.886
83 --- --- --- --- --- .000 .000 .000 1.110 1.665 35.828 48.383 14.897 101.883
AVER: --- --- --- --- --- .024 .004 .058 1.208 1.737 35.897 48.256 14.858 102.040
SDEV: --- --- --- --- --- .062 .006 .161 .113 .058 .115 .112 .035 .155
SERR: --- --- --- --- --- .020 .002 .051 .036 .018 .036 .035 .011
Much better, though still problems with the P and Y due to severe overlaps in the EDS spectra. And now with the blank correction assigned:
Un 4 std 257 as unk, Results in Elemental Weight Percents (EDS spectra using L series, with blank correction)
ELEM: Th Hf U P Y Th Hf U P Y O Zr Si
TYPE: ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL ANAL FORM FORM FORM
BGDS: LIN LIN LIN LIN LIN EDS EDS EDS EDS EDS
TIME: --- --- --- --- --- 800.00 800.00 800.00 800.00 800.00 --- --- ---
BEAM: --- --- --- --- --- 50.39 50.39 50.39 50.39 50.39 --- --- ---
ELEM: Th-D Hf-D U-D P-D Y-D Th Hf U P Y O Zr Si SUM
XRAY: (ma) (ma) (ma) (ka) (la) (la) (la) (la) (ka) (la) () () ()
74 --- --- --- --- --- -.024 .013 -.058 .014 .015 34.941 49.784 15.328 100.013
75 --- --- --- --- --- -.024 -.004 -.058 -.050 .013 34.885 49.825 15.341 99.928
76 --- --- --- --- --- -.024 -.002 .444 -.117 .024 34.713 49.602 15.272 99.912
77 --- --- --- --- --- .018 -.004 -.058 .067 .044 34.983 49.731 15.312 100.092
78 --- --- --- --- --- -.024 -.004 -.058 .189 .040 35.107 49.692 15.300 100.243
79 --- --- --- --- --- .168 -.004 -.058 .135 -.006 35.019 49.647 15.286 100.187
80 --- --- --- --- --- -.024 .000 -.058 -.071 -.106 34.873 49.893 15.362 99.868
81 --- --- --- --- --- -.024 .001 .007 -.095 -.064 34.834 49.851 15.349 99.858
82 --- --- --- --- --- -.024 .005 -.058 -.113 -.069 34.829 49.893 15.362 99.825
83 --- --- --- --- --- -.024 -.004 -.058 -.110 -.089 34.836 49.905 15.366 99.822
AVER: --- --- --- --- --- -.001 .000 -.001 -.015 -.020 34.902 49.782 15.328 99.975
SDEV: --- --- --- --- --- .061 .005 .158 .111 .056 .113 .109 .034 .152
SERR: --- --- --- --- --- .019 .002 .050 .035 .018 .036 .035 .011
Better than the M series EDS spectra, but not as good as the WDS. So if you thought this would be the result we would obtain right from the start, well you were right! :D
john