I attempted some C ka measurements in my transition element stds and the results are promising. Here's the MAN fit:
(https://smf.probesoftware.com/oldpics/i43.tinypic.com/waj0qc.jpg)
There were some interferences from the light elements (Mg and Al) so I left them out of the MAN bgd calibration. The Cu std also gave a bad interference so that was left out of the bgd fit also. Note that on-peak interferences can be corrected for using the PFE interference correction if the interfering element is also measured. Otherwise just avoid that interference in the MAN calibration curve.
The spot analyses on Fe are here:
ELEM: C Fe SUM
241 -.010 100.439 100.429
242 -.036 99.932 99.896
243 -.019 99.570 99.551
244 .022 100.490 100.511
245 -.019 99.528 99.509
AVER: -.012 99.992 99.979
SDEV: .021 .460 .474
SERR: .010 .206
%RSD: -172.15 .46
PUBL: n.a. 100.000 100.000
%VAR: --- (-.01)
DIFF: --- (-.01)
STDS: 506 526
So 120 PPM +/- 210 PPM is not too bad!
Here is Cr metal using the same calibration curve:
ELEM: C Fe SUM
231 .008 -.007 100.001
232 .019 .055 100.073
233 .017 -.036 99.981
234 -.015 .043 100.029
235 .017 .048 100.065
AVER: .009 .020 100.030
SDEV: .014 .040 .040
SERR: .006 .018
%RSD: 155.63 195.90
PUBL: n.a. .010 100.010
%VAR: --- 104.79
DIFF: --- .010
STDS: 506 526
And here is the Ni std, again using the same calibration curve:
ELEM: C Fe SUM
251 .020 .073 100.093
252 .028 .027 100.055
253 .041 -.006 100.035
254 .000 .007 100.006
255 .031 -.025 100.006
AVER: .024 .015 100.039
SDEV: .015 .038 .037
SERR: .007 .017
%RSD: 64.74 246.91
PUBL: n.a. n.a. 100.000
%VAR: --- ---
DIFF: --- ---
STDS: 506 526
The idea being that one could perform carbon analyses on arbitrary mixtures of alloys.
john
PS These analyses were performed at 10 keV, 30 nA and a 40 sec count time for C ka (on-peak only).
Sensitivity of around 200 PPM is pretty impressive for a diffusion pumped (and cryo-baffle cooled) vacuum system!
First attempt at a 1 pixel high line beam scan using MAN backgrounds for carbon ka showing the effect of the beam intercepting the carbon contamination ring around the initial beam position.
john
Question to all:
is the small initial decrease in carbon concentration on the left side of the graph from the 0.1 % level to zero an indication of the desorption of the "native" carbon layer as the sample heats up during the first 3 or 4 steps (all within 300 or 400 nm (15 to 20 secs) of each other)?
In other words is this what the Aachen lab means when they describe the need for an initial "decontamination time"?
Here's another carbon (and now also nitrogen!) measurement in steel using an MAN background fit for carbon and nitrogen, both without and with a blank correction on a vacuum remelted Fe metal standard (O 310 ppm, N 10 ppm, C assumed 0 PPM).
First here are the MAN fits for carbon (PC2 crystal):
(https://smf.probesoftware.com/oldpics/i44.tinypic.com/v7828k.jpg)
and for nitrogen (PC1 crystal):
(https://smf.probesoftware.com/oldpics/i41.tinypic.com/rrun20.jpg)
and here are the quantitative plots without the blank correction applied to carbon and nitrogen:
(https://smf.probesoftware.com/oldpics/i42.tinypic.com/othirn.jpg)
Note that the nitrogen values fall off the bottom of the scale because without the blank correction, the values are a few hundred PPM negative (and cannot be plotted on a log scale), basically the limit of accuracy for the MAN correction.
And now with the blank correction applied to the MAN bgd correction:
(https://smf.probesoftware.com/oldpics/i41.tinypic.com/2jg3u53.jpg)
Note that the nitrogen values now approach zero (as they should).
This MAN background correction is very similar to the EU ISO method, except that we utilize several standards for the continuum fit model to deal with changes in average atomic number in the sample (up to 7 wt% N and 4 % C which yields an average atomic number of approximately 24.3 as opposed to pure Fe which is of course, around 26).
The Analyze! window Report button generates the following English sentences about the sample:
Compositional analyses were acquired on an electron microprobe (Cameca SX100 (TCP/IP Socket)) equipped with 5 tunable wavelength dispersive spectrometers. Operating conditions were 40 degrees takeoff angle, and a beam energy of 10 keV. The beam current was 50 nA, and the beam diameter was 0 microns.
Elements were acquired using analyzing crystals LIF for Fe ka, Mn ka, Ni ka, Cu ka, LPET for Mo la, Cr ka, LTAP for Al ka, Si ka, PC1 for N ka, and PC2 for C ka.
The standards were Carbon (graphite) for C ka, Aluminum metal for Al ka, Silicon metal for Si ka, Chromium metal for Cr ka, Manganese metal for Mn ka, Iron metal for Fe ka, Nickel metal for Ni ka, Copper metal for Cu ka, Molybedenum metal for Mo la, and AlN ceramic for N ka.
The counting time was 20 seconds for Fe ka, 40 seconds for Mn ka, Ni ka, Cu ka, 60 seconds for C ka, N ka, and 80 seconds for Mo la, Cr ka, Al ka, Si ka. The off peak counting time was 6 seconds for Fe ka, 20 seconds for Mn ka, Ni ka, Cu ka, and 40 seconds for Mo la, Cr ka, Al ka, Si ka. Off Peak correction method was Linear for Mo la, Cr ka, Fe ka, Mn ka, Ni ka, Cu ka, and Exponential for Al ka, Si ka.
The MAN background intensity data was calibrated and continuum absorption corrected for C ka, N ka. See J.J. Donovan and T.N. Tingle, An Improved Mean Atomic Number Correction for Quantitative Microanalysis in Journal of Microscopy, v. 2, 1, p. 1-7, 1996
Unknown and standard intensities were corrected for deadtime. Standard intensities were corrected for standard drift over time.
Interference corrections were applied to C for interference by Cu, and to N for interference by Ni, Cu, and to Fe for interference by Mn, and to Mn for interference by Cr. See J.J. Donovan, D.A. Snyder and M.L. Rivers, An Improved Interference Correction for Trace Element Analysis in Microbeam Analyis, 2: 23-28, 1993
Results are the average of 139 points and detection limits ranged from .005 weight percent for Si ka to .005 weight percent for Al ka to .030 weight percent for C ka to .214 weight percent for Fe ka to .617 weight percent for Cu ka.
Analytical sensitivity (at the 99% confidence level) ranged from .368 percent relative for Al ka to .519 percent relative for Fe ka to 1.813 percent relative for C ka to 36.425 percent relative for N ka to 398.835 percent relative for Ni ka.
The quantitative blank correction was utilized. The exponential or polynomial background fit was utilized. See John J. Donovan, Heather A. Lowers and Brian G. Rusk, Improved electron probe microanalysis of trace elements in quartz, American Mineralogist, 96, 274
Note: it might be that because all MAN (on-peak) intensity measurements receive the same "induced" carbon contamination rate, the contamination effect gets pretty much "normalized" out between them. Therefore I found that one should acquire the MAN standards using the same count time as the unknowns (that is, Unknown Count Factors = 1).
Here are some more carbon beam scan tests using 0.1 um steps to observe the contamination ring smearing effect I've run recently. I still need to do more measurements at higher currents but...
The first attached jpg is carbon ka on a vacuum remelted Fe std using the normal multi-std MAN background correction with multiple metals for the background fit, and with the "blank" correction turned on to improve the accuracy of the MAN fit. Note the large artifact from the carbon contamination ring "smear" as the beam progressively overlaps onto the previous carbon ring.
The next jpg plot is the same carbon beam scan image with the CPQ correction turned on for the MAN assignment of the iron std only. See here:
http://smf.probesoftware.com/index.php?topic=41.msg178#msg178
for a more complete description of the correlated pixel quantification (CPQ) method.
I didn't acquire beam scans on the other MAN std metals which means this is equivalent to the Aachen method of only using Fe for the MAN (on-peak) background measurement for carbon ka. Of course the blank correction needs to be turned off in this case. Otherwise, it's as though one is applying the blank correction twice! In the case of carbon about 300 PPM positive (see normal point analyses below).
The CPQ plot is using the carbon ka on Fe std beam scan for both the unknown image and the also for the CPQ std image so ideally the quant data should zero out. Therefore, this is only a "sanity check" for PFE and CalcImage but still it's nice to see an average very close to zero.
Note the very small 10-20 PPM variance visible in the carbon CPQ plot which I suspect must be due to the differences in the absorption correction from the noise in the iron and nitrogen signals in the beam scan image (carbon ka absorption correction is around 200% in an Fe matrix).
john
Un 2 Fe blank
TakeOff = 40.0 KiloVolt = 10.0 Beam Current = 30.0 Beam Size = 0
(Magnification (analytical) = 40000), Beam Mode = Analog Spot
(Magnification (default) = 400, Magnification (imaging) = 800)
Image Shift (X,Y): .00, .00
Number of Data Lines: 5 Number of 'Good' Data Lines: 4
First/Last Date-Time: 10/24/2013 04:35:53 PM to 10/24/2013 04:42:51 PM
WARNING- Using Blank Trace Correction
Average Total Oxygen: .000 Average Total Weight%: 100.914
Average Calculated Oxygen: .000 Average Atomic Number: 26.004
Average Excess Oxygen: .000 Average Atomic Weight: 55.880
Average ZAF Iteration: 1.00 Average Quant Iterate: 4.00
Un 2 Fe blank, Results in Elemental Weight Percents
ELEM: C Fe N
BGDS: MAN MAN MAN
TIME: 80.00 80.00 80.00
BEAM: 30.04 30.04 30.04
ELEM: C Fe N SUM
46 .019 100.613 -.012 100.620
47 .010 101.488 -.006 101.492
48 .007 100.472 -.013 100.466
49 .009 101.174 -.104 101.078
AVER: .011 100.937 -.034 100.914
SDEV: .005 .477 .047 .465
SERR: .003 .238 .024
%RSD: 47.49 .47 -139.79
STDS: 506 526 604
STKF: .9705 1.0000 .2573
STCT: 1285.92 144.17 14.37
UNKF: .0000 1.0094 -.0002
UNCT: .06 145.52 -.01
UNBG: 4.29 .81 .64
ZCOR: 2.4873 .9999 1.5315
KRAW: .0000 1.0094 -.0009
PKBG: 1.01 181.36 .98
BLNK#: 3 ---- 3
BLNKL: .000000 ---- .000000
BLNKV: -.03305 ---- -.28686
And if anyone is wondering how one might perform such an effective correction for this carbon ring "smearing" effect, provided one acquires a similar image acquisition on the appropriate standard, it really couldn't be easier:
(https://smf.probesoftware.com/oldpics/i43.tinypic.com/24dk4z8.jpg)
Analysis of carbon is a special animal...
The first decision one has to make is whether to coat with another material, because generally one doesn't want to coat with the same element as is being analyzed. Al is often used for carbon analysis, but let's look at the mass absorption coefficients (MACs) for carbon Ka in a list of element absorbers.
Note that Ag is surprisingly good for transmitting C Ka as seen here, especially compared to Al- even though Al is a much lower atomic number:
Al 30861.91
Si 36660.35
P 41012.84
S 47396.64
Cl 50247.13
Ar 45478.05
K 5664.86
Ca 6817.13
Sc 7426.23
Ti 8057.13
V 8800.99
Cr 10542.99
Mn 11419.21
Fe 13834.91
Co 15498.39
Ni 18097.06
Cu 18661.91
Zn 23383.54
Ga 23211.44
Ge 25383.30
As 27750.23
Se 29978.30
Br 32286.59
Kr 32976.59
Rb 35087.66
Sr 34869.63
Y 30797.00
Zr 21516.44
Nb 19127.24
Mo 16244.41
Tc 7981.48
Ru 5518.70
Rh 4635.32
Pd 7552.90
Ag 8178.73
Cd 5759.97
In 6112.51
Sn 6297.19
Sb 6562.40
Te 6627.16
Al, which is often used, is actually a pretty good absorber for C Ka, while Ag is much more transparent to carbon x-rays. So if trace element carbon analysis is required, Ag coating might work well. Also silver oxide is conductive so that helps too!
Or one can leave the sample uncoated if they are conductive to around 1 meg-ohms or less and just connect the samples to ground with painted traces of carbon or Ag paint.
Edit by John: Do not! I repeat do not use Ag coating for carbon measurements. There is a subtle interference from the Ag MG line on the carbon k-alpha peak. Better to use a thin Cr coating.
Hi John,
I thought about using the MAN correction for carbon analyses but I would like to caution you here when you cannot keep the carbon contamination minimal (with an air jet for example). Namely because the amount of carbon contamination on your "blank materials" is not constant but material dependent. This has been reported in the Bastin&Heijligers (1986) paper on the binary carbides and we also see it in our analyses (and may be one reason for the variability seen along your MAN fit). It has been suggested that the thermal conductivity of the material (this is how fast/much it heats up under the beam) is responsible and we are currently trying to test this further. Obviously the beam diameter also factors in and needs to be similar to the unknowns.
Therefore, the MAN standards in the appropriate Z-range also need to have similar thermal properties and should be chosen with caution (for example when using a small number of standards).
Quote from: Anette von der Handt on March 28, 2014, 05:59:33 PM
I thought about using the MAN correction for carbon analyses but I would like to caution you here when you cannot keep the carbon contamination minimal (with an air jet for example). Namely because the amount of carbon contamination on your "blank materials" is not constant but material dependent. This has been reported in the Bastin&Heijligers (1986) paper on the binary carbides and we also see it in our analyses (and may be one reason for the variability seen along your MAN fit). It has been suggested that the thermal conductivity of the material (this is how fast/much it heats up under the beam) is responsible and we are currently trying to test this further. Obviously the beam diameter also factors in and needs to be similar to the unknowns.
Therefore, the MAN standards in the appropriate Z-range also need to have similar thermal properties and should be chosen with caution (for example when using a small number of standards).
Hi Anette,
Yes, exactly. I agree. That is why the MAN standards selected here:
http://smf.probesoftware.com/index.php?topic=48.msg270#msg270
are all metals (with approximately similar thermal conductivities) as is the unknown material (essentially Fe metal). For carbon analyses, I agree this is an important aspect of controlling dependent variables.
Another aspect is this:
http://smf.probesoftware.com/index.php?topic=140.0
As previously mentioned,
http://smf.probesoftware.com/index.php?topic=114.msg3322#msg3322
I decided to acquire some data to see if we could quant carbon using the MAN correction on metals. First I had Julie polish and Ag coat one of our standard blocks with about 15 nm of Ag. I then analyzed C, N, Mo, Fe and Si in pure C, AlN, Mo, Fe and Si (for a range of x-ray energies and average Zs), at 10 keV and 50 nA (for improved sensitivity). I counted 80 seconds on each data point (on-peak MAN only for carbon and nitrogen) and used 16 TDI intervals ( 5 sec each).
First of all, just as shown in the previously acquired data above, the carbon signal shows a small *decrease* over time while all other elements remain stable within precision:
(https://smf.probesoftware.com/oldpics/i62.tinypic.com/33xzv6a.jpg)
Here is nitrogen Ka:
(https://smf.probesoftware.com/oldpics/i57.tinypic.com/2m6x405.jpg)
As for the quant, here you can see that results on a pure Fe std (but acquired separately from the MAN background stds), gives zero within precision for all elements (except for Si at 230 PPM which could be due to the colloidal Si polishing) :
Un 4 iron metal, Results in Elemental Weight Percents
ELEM: C N Mo Fe Si
BGDS: MAN MAN LIN LIN EXP
TIME: 80.00 80.00 60.00 60.00 60.00
BEAM: 49.75 49.75 49.75 49.75 49.75
ELEM: C N Mo Fe Si SUM
47 -.023 -.018 -.025 100.090 .023 100.047
48 .032 .037 .005 100.471 .025 100.570
49 -.010 -.043 -.006 101.030 .021 100.992
50 .005 .006 -.004 100.380 .026 100.414
51 -.013 -.030 -.031 100.456 .021 100.404
AVER: -.002 -.010 -.012 100.486 .023 100.485
SDEV: .021 .032 .015 .341 .002 .342
SERR: .010 .014 .007 .152 .001
%RSD: -1244.94 -331.31 -125.84 .34 10.30
STDS: 506 672 542 526 514
STKF: .9705 .1485 .9901 1.0000 1.0000
STCT: 23361.3 459.1 4349.2 1085.6 73411.0
UNKF: .0000 -.0001 -.0001 1.0048 .0002
UNCT: -.2 -.2 -.5 1090.8 15.0
UNBG: 148.0 13.6 9.2 7.9 134.1
ZCOR: 2.4870 1.5301 1.1661 1.0000 1.1457
KRAW: .0000 -.0004 -.0001 1.0048 .0002
PKBG: 1.00 .99 .95 139.30 1.11
INT%: ---- ---- ---- ---- ----
TDI%: 5.364 -.141 ---- ---- ----
DEV%: .6 3.2 ---- ---- ----
TDIF: LOG-LIN LOG-LIN ---- ---- ----
TDIT: 131.80 134.00 ---- ---- ----
TDII: 148. 13.3 ---- ---- ----
TDIL: 5.00 2.59 ---- ---- ----
I also performed some "fast" beam scans in Probe Image *on the same spot* on pure Fe and will examine that data next week for TDI effects, but in the meantime here is my question: is the small decrease in carbon signal over time, something that is observed on other instruments?
I would have thought that the carbon signal would increase over time, not decrease... so I have to wonder if the cryo baffle (~100 Kelvin) on my instrument is somehow related to this? I would be very grateful for similar measurement results by other labs... thanks!
Again this is a freshly polished Fe metal sample coated with 15 nm Ag and measured at 10 keV and 50 nA. In any case, a standard deviation of around 200 PPM for carbon in 80 seconds isn't bad. That is, with a 5% TDI correction (+/- 0.6%). Which would correspond to an absolute TDI correction of ~800 +/- 100 PPM concentration.
Finally, is this decrease in carbon intensity simply related to the so called "decontamination time", as coined by Stuart Kearns, et al.
http://smf.probesoftware.com/index.php?topic=48.msg161#msg161
Though I would have expected the decontamination effect to apply only to the first 10 seconds or so, at most?
Hi John,
I think its the decontamination effect as you mention. Your not hitting it that hard so its taking a long time to remove the surface carbon. What spot size did you use? I did some tests a while back - but I don't know where the TDI curves are. Anyway I decided at 100 nA 15kV 1um beam I wanted to hit the sample for ca, 30-60 secs before analyzing it. The TDI curves were variable in there behavior but generally over the first 30-60 secs the carbon decreased and then flattened. This was using a LN cold trap.
Have you tried the tdi for a longer time/different power - does it flatten our with time - suggestive of decomtamination?
What's your cryobuffer - is this a cold finger/trap?
It does seem complex as to the regime in which carbon contamination is being removed under the electron beam and when it is being deposited. Removal at high power, directly under the beam, depends on material - and therefore heating of sample, greatly aided by oxygen airjet/ deposition adjacent to the beam as a ring - but also sometimes under the beam.
There is also the issue of beam stability sometimes during the tdi the beam can drift onto the surrounding carbon ring giving a sharp rise.
Ben
Quote from: Ben Buse on September 21, 2015, 06:04:43 AM
I think its the decontamination effect as you mention. Your not hitting it that hard so its taking a long time to remove the surface carbon. What spot size did you use? I did some tests a while back - but I don't know where the TDI curves are. Anyway I decided at 100 nA 15kV 1um beam I wanted to hit the sample for ca, 30-60 secs before analyzing it. The TDI curves were variable in there behavior but generally over the first 30-60 secs the carbon decreased and then flattened. This was using a LN cold trap.
Have you tried the tdi for a longer time/different power - does it flatten our with time - suggestive of decomtamination?
This was 10 keV, 50 nA fully focused. I have to say I'm a little surprised that you think it might take 30-60 seconds to "de-contaminate", but I guess I could try with a less focused beam.
Remember, this is *not* a carbon coated sample, it was freshly polished and coated with 15 nm of silver. Are you sure it takes 30-60 seconds with a fully focused beam at 50 nA to remove 1 to 2 nm of native hydrocarbon?
Quote from: Ben Buse on September 21, 2015, 06:04:43 AM
What's your cryobuffer - is this a cold finger/trap?
It does seem complex as to the regime in which carbon contamination is being removed under the electron beam and when it is being deposited. Removal at high power, directly under the beam, depends on material - and therefore heating of sample, greatly aided by oxygen airjet/ deposition adjacent to the beam as a ring - but also sometimes under the beam.
There is also the issue of beam stability sometimes during the tdi the beam can drift onto the surrounding carbon ring giving a sharp rise.
We have a 100 Kelvin baffle over the diffusion pump, no air jet. I know a carbon ring will form eventually, and when stage scanning, we see the carbon ring being intercepted effect as noted here:
http://smf.probesoftware.com/index.php?topic=114.msg3258#msg3258
Which is what we are trying to *avoid*, by scanning the beam over the analysis area. I'll be posting that data later today, stay tuned!
john
Here are my results (attached below) from performing 10 stage scans each 32 pixels wide (~4000x or ~94 um wide), using carbon Ka (and nitrogen Ka) on pure Fe (freshly polished and coated with 15 nm of Ag), with each scan repeated on the same area.
The 10 stage scans were separately defined in Probe Image (actually pretty easy to do, but I will have Brian implement a TDI option if this method turns out to be as promising as it seems to look), and therefore each scan took about 32 seconds of actual acquisition time and around 45 seconds each with all the software/hardware overhead involved.
Of course I will also need to modify CalcImage so it automatically loads in these replicate TDI scans so it can extrapolate automatically back to zero time, but that shouldn't be too difficult as Probe Image already time stamps each scan at the beginning and end automatically. This should work whether the scan is a line scan or a full frame scan...
The total time on each pixel was about 10 seconds, which is what we would likely utilize for trace carbon analyses although of course this could be increased for improved sensitivity.
I have to say this method might be very useful for trace carbon analyses, not to mention beam sensitive samples! Has anyone heard of this idea previously? Or tried it yourself?
Having done with Phil here earlier today a C Ka TDI count, I have a comment/suggestion.
A TDI acquisition was done (spot mode) so we aren't talking imaging, but rather the philosophy of TDI upon 'sensitive' material, and we did a similar acquisition and it showed a 'linear' (in the log window).
I was suspicious, so said, instead of having a LONNNGGG dwell period of 12 seconds, let's be devils advocates and see if, like in hydrous alkali glasses, whether there is an immediate change in the first second or so...and guess what, the plot now was exponential on the log window.
We were going to suggest that you might want to implement a modified TDI approach, in allowing the user to pick an option of doing a (variable short time, such as one second)) count for the _first_ measurement, which could pick up any sharp change over the first count period.
I know it might be difficult to implement, but you are a smart guy!
Quote from: JohnF on September 21, 2015, 02:58:32 PM
Having done with Phil here earlier today a C Ka TDI count, I have a comment/suggestion.
A TDI acquisition was done (spot mode) so we aren't talking imaging, but rather the philosophy of TDI upon 'sensitive' material, and we did a similar acquisition and it showed a 'linear' (in the log window).
I was suspicious, so said, instead of having a LONNNGGG dwell period of 12 seconds, let's be devils advocates and see if, like in hydrous alkali glasses, whether there is an immediate change in the first second or so...and guess what, the plot now was exponential on the log window.
We were going to suggest that you might want to implement a modified TDI approach, in allowing the user to pick an option of doing a (variable short time, such as one second)) count for the _first_ measurement, which could pick up any sharp change over the first count period.
I know it might be difficult to implement, but you are a smart guy!
Flattery will get you everywhere! :-*
Seriously, this would be pretty difficult to implement. You do know about the option where one can weight the first few TDI measurements?
http://smf.probesoftware.com/index.php?topic=11.msg2520#msg2520
It sort of amounts to the same thing you and Phil are suggesting...
By the way, here are the N Ka data that was acquired at the same time (see attached below). The confidence intervals displayed are for 99%.
Again, by the way, these analyses were performed *without* a blank correction, which would obviously improve these trace measurements.
I would sum up by saying that one can perform excellent trace carbon quantitative analyses using the following methods together:
1. MAN backgrounds
2. Normal matrix corrections
3. Quantitative interference corrections (a la PFE)
4. TDI correction (I generally use asynchronous mode as the new acquisition code doesn't really take any longer than the old synchronous code).
Remember, if you have a suitable standard with a matching matrix, one can alternatively perform a "blank" correction, but not with the interference correction, or you will get a "double" correction on the trace element... though the software will warn you about this!
Hi all
Just necroing an old post. A user has come to me wanting to measure C in scapolite, which can contain up to ~4.5wt% C as an end-member. One can calculate it, but they want to try to measure it directly.
I have a PC2 xtal, but no anticontaminator. My standards are currently carbon coated and I am reticent to repolish and recoat my standard block in something like Al. I guess my question is do you think I will be pushing the proverbial uphill, or if the C is in decent concentration (wt% values) I may be able to get some numbers out of it despite no anticontaminator and my carbon coated standards...?
I will probably give it a go anyway using MAN/TDI and see how the numbers compare to the calculated...
Cheers
Hi Ben,
That is a good amount of carbon, so this shouldn't be too difficult.
No need to strip the carbon coat off your standards. You can specify a different coat for your unknown and your carbon standard in PFE.
As you can see from previous posts in this thread I've tested measuring carbon in pure Fe using a graphite standard for carbon, MAN backgrounds and a TDI correction for carbon contamination.
The benefit of using graphite as a carbon standard is that the carbon coat on the graphite standard doesn't affect the standard intensity (pure carbon is pure carbon). And one can specify no carbon coat for your unknown. Of course if you aren't using a graphite standard for carbon, you could instead just strip the carbon coating off your carbon standard (whatever it is), and coat the carbon standard along with the unknown.
But if the unknown is an insulator you'll have to deal with that. I suggest a thin metal coating. There is a post here on using Ag for coating insulating samples for carbon analysis:
http://smf.probesoftware.com/index.php?topic=48.msg959#msg959
I ended up using a carbon coated graphite standard, and put 15 nm of evaporated Ag on my unknown, which seemed to work well. Remember to perform a TDI correction on both the unknown and the carbon standard as the thermal conductivity affects the carbon contamination rate.
Then there is the issue of peak shape for carbon Ka... Maybe for scapolite you should use a pure siderite (FeCO3) standard as your carbon standard (by assuming carbon concentration by stocihiometry) and coating both the carbon standard and unknown with 15 nm of Ag. You might want to do some wavescans to see how the peak shapes look on your unknown and standard.
I've never tried measuring carbon in a silicate, so I'm interested to hear what you find.
john
Hi John
Excellent, thanks for the tips.
I have some NMNH R2460 Siderite, and some NMNH R6600 Meionite (Carbon end member of scapolite) mounted in a separate block. I might coat them in C, do some wavescans/measurements on them, then polish/plasma clean/recoat them in Ag at the same time as unknowns to see roughly how much it attenuates my C counts. I guess I will probably still end up using the graphite as the C standard though, will have to see. When I have some wavescans/data I will post it.
Couple more questions...do you think 15nm of Ag is overkill? Seems like a lot. Also as per your previous work, I guess I should use 10kV or so to max my C counts?
Cheers
Quote from: BenjaminWade on October 11, 2017, 04:55:08 PM
Hi John
Excellent, thanks for the tips.
I have some NMNH R2460 Siderite, and some NMNH R6600 Meionite (Carbon end member of scapolite) mounted in a separate block. I might coat them in C, do some wavescans/measurements on them, then polish/plasma clean/recoat them in Ag at the same time as unknowns to see roughly how much it attenuates my C counts. I guess I will probably still end up using the graphite as the C standard though, will have to see. When I have some wavescans/data I will post it.
Couple more questions...do you think 15nm of Ag is overkill? Seems like a lot. Also as per your previous work, I guess I should use 10kV or so to max my C counts?
Cheers
Will be very interesting to see. Hopefully you can run a silicate with the scapolite that contains no carbon as a "blank".
Yes, a 15 nm coating might be overkill. 10 keV would be good unless you need high sensitivity Fe measurements.
john
Hmm might be difficult finding a silicate that contains similar major element composition as her scapolite. She might have some albite/plag in her samples which I guess will be somewhat similar containing Si/Al/Ca/Na. I will give it a go as well then. I am hoping that the C in her scapolite isn't so low that it will be too much of an issue...
Yes...well what I didn't mention is that the analysis is further complicated by the fact she wants to run F as well, which I like to do on my PC0 which of course is on the same xtal changer as PC2. So we have run some spots last night at normal 15kV with a full element suite including F on PC0. Totals coming out around 98%, of which I am hoping the remainder is CO2+/-H2O. So the plan is to now setup for C, and rerun as close to the analysed spots as possible with the Ag coat, then using the option in PfE combine the selected samples into new samples so the C goes through all the correct matrix corrections. And its all going to work beautifully and perfectly....
I know it's not quite as good as PC0, but running F Ka on TAP works pretty well especially if you have a large TAP crystal... then you wouldn't have to combine samples.
john
Yes I did think about it, but I stuck with the PC0 as the F content is quite low, usually less than 500ppm or so. So thought I might be better off with the higher count rate on PC0 and less interferences? In hindsight I guess I should have probably tried it out anyway.
Hi John and all
Not sure if it's of any interest, but just thought I would post some of my failures in the last week trying to measure C in scapolite on our SXFive without any form of anticontamination...
Firstly after doing some separate measurements for F we decided that there was little to no F in the scapolite we were looking at, so could ditch F from the package and run C with the rest of the elements to get around the combined analyses problem
(https://smf.probesoftware.com/gallery/318_13_04_20_7_47_38.jpeg)
In regards to the coating, I tried both 5 and 10nm of Ag coat. I get ~1.5 times the counts with a 5nm coat on my scapolite NMNH standard (see wavescans below – ignore the fact that my PC2 xtal is wayyyy out of alignment, that major peak is Oxygen, the carbon peak is the small broader hump to the left of it).
(https://smf.probesoftware.com/gallery/318_13_04_20_7_38_45.jpeg)
The 5nm seemed to work fine with regards to negating charging so I ended up going with that. To get around having to use coating corrections between standards and unknowns, I ended up finding an older Astimex mineral mount laying around so polished the carbon coat off and coated it in Ag at the same time as the unknowns and NMNH scapolite mount. Despite me thinking I polished the life out of the samples, getting the samples to the coater quickly and keeping it all under vacuum, I had real problems with C still being measured in minerals that should have none. Wavescan below shows C peak in standard 501 albite.
(https://smf.probesoftware.com/gallery/318_13_04_20_7_41_36.jpeg)
Some of my measured C in standards are probably interferences, as seen in MAN fit diagram below for C. I am sure some of it is residual carbon coat/adsorbed carbon to the sample surface, which I guess isn't unexpected. If I concentrate on the lower Z end where scapolite will plot (mean Z ~11.6), I can refine the fit to something OK but am not left with many data points (refined MAN fit plot below). I am hoping that these standards left perhaps don't have as much residual carbon on their surface and this correlation is real and mostly due to mean Z. Using the MAN vs traditional, I get ~10% higher carbon values on unknowns so perhaps there is still some other extraneous C contribution.
(https://smf.probesoftware.com/gallery/318_13_04_20_7_44_34.jpeg)
(https://smf.probesoftware.com/gallery/318_13_04_20_7_45_29.jpeg)
Despite this I ended up running a standards and unknowns at 10kV/35nA/10um defocussed beam. Both standards and unknowns had TDI enabled. Standards were collected with traditional background methods, and unknowns both with and without MAN for comparison. TDI plots on unknowns were variable, some spots showing C increase which might be expected, some showing no correlation at all (attached images).
(https://smf.probesoftware.com/gallery/318_13_04_20_7_48_32.jpeg)
(https://smf.probesoftware.com/gallery/318_13_04_20_7_48_53.jpeg)
I ended up using NMNH R6600 Scapolite as my Carbon standard for the unknowns. I tried pure graphite, calcite, dolomite, and all give me extremely high C measurements when treating NMNH R6600 Scapolite as an "unknown" (reported CO2 is 2.5 wt%, returned values range from 7-10wt% when using dolomite/calcite/graphite).
Given the inherent "background" level of carbon, I decided to utilise the blank assignment. For the levels of C I am measuring in my unknowns (<3 wt% C), applying this correction cuts my C concentrations in half, from ~2.5wt% without correction to ~1.2 wt% with. This is due to the quite large "background" contribution and the low amount of counts we are dealing with.
Unfortunately in the unknowns there is no other mineral of similar major element composition to the scapolite, there being only hornblende +/-kspar and biotite in the rock. They have similar C background levels, but are subtlety different assuming related to their mean-Z [K-spar (z avg 11.67/3.69 CPS/nA); hornblende (z avg 13.15/ 4.34 CPS/nA)]. Given scapolite has a mean-Z of ~11.65, the best I could do is use k-spar if I could find it, otherwise I used hornblende. Depending on which mineral you use, there is a ~0.13 wt% difference in the analysed unknowns, so not trivial.
So one question I have is this....I assume my C standard (NMNH R6600 Scapolite) also has this "background" level of carbon on top of its structural carbon? Should I be doing some kind of blank correction for this as well? We aren't talking about a lot of counts to work with here either. An average of 75 analyses of the NMNH R6600 Scapolite returns 7.66 +/- 0.50 CPS/nA, and this is on something that has 2.5 wt% CO2. So not a lot to work with in the first place, and when you look at the "background" levels of C on various other "C-free" standards like albite/plag/sanidine it ranges from 3.0-4.5 CPS/nA. So not really sure how to treat my C standard with regards to blank subtraction at this stage.
So I am at the point where I can get some analyses on scapolite with OK totals and varying C from sample to sample (see Results image of combined samples KM31 and KM22 below), but have no real confidence if those C contents are real or not. What I need is more scapolite standards I think to do more cross checking, it's all a bit circular at the moment.
That is my ramble fest over. I will keep you updated and any suggestions are gratefully received.
(https://smf.probesoftware.com/gallery/318_13_04_20_7_50_21.jpeg)
Cheers
Quote from: BenjaminWade on October 19, 2017, 04:43:38 PM
So one question I have is this....I assume my C standard (NMNH R6600 Scapolite) also has this "background" level of carbon on top of its structural carbon? Should I be doing some kind of blank correction for this as well? We aren't talking about a lot of counts to work with here either. An average of 75 analyses of the NMNH R6600 Scapolite returns 7.66 +/- 0.50 CPS/nA, and this is on something that has 2.5 wt% CO2. So not a lot to work with in the first place, and when you look at the "background" levels of C on various other "C-free" standards like albite/plag/sanidine it ranges from 3.0-4.5 CPS/nA. So not really sure how to treat my C standard with regards to blank subtraction at this stage.
Hi Ben,
I owe you an apology.
Here was my addled brain thinking of analyzing carbon containing minerals, which made me think of carbonates, and that made me think of Ag coating, which people have used to limit beam damage effects when measuring cations in carbonates. But not for measuring carbon!
(https://smf.probesoftware.com/gallery/395_19_10_17_7_34_40.png)
Turns out there is a close interference from an Ag MG (2nd order) emission line right beside the carbon peak... can that be it (above, taller thinner peak on the right) in your wavescan? Remember this x-ray table is uncorrected for refraction, so please plot up the Ag and Cr KLM markers in the wavescan plot. It could be that the Ag MG II is underneath the 2nd order oxygen peak...
I think that is at least part of the reason for your "carbon background". If I remember from my carbon in Fe measurements from last year, I believe I finally used a thin Cr coating on my samples and standards. Cr has a lower MAC than an Al coating and no interferences.
Next time before I suggest a coating material, I'm going to check the darn x-ray database first! :-[
john
PS Did you sputter coat or evaporate? Plasma sputter coating will give a very uneven coating on insulators based on the surface topography and electrostatics. Always use evaporation for an even coating for quantitative work.
I found an old carbon k-alpha scan from 2013 and I think that is when I realized (and subsequently forgot!) that an Ag coating was not going to work for carbon analyses:
(https://smf.probesoftware.com/gallery/395_20_10_17_12_33_17.png)
In fact there is also a small Al (high order) peak in there as well, but far enough away from the carbon peak, so I think one could use an Al coating.
Another lesson here is that one must plot a refraction corrected wavescan plot for these higher order lines because they move around compared to their nominal sin theta positions. You know what I should do is add a checkbox to display the KLM markers *without* the refraction correction just to show the effect.
john
Hi John
No problems! If I was doing due diligence I probably should have checked for any interference of Ag on C anyway. Having said that, looking at my wavescan I don't see any Ag peak like what you document below which is quite large! Was that with 15nm? Perhaps I will have to go back and check again. Its a bit complicated with my PC2 xtal being so far out at the moment, I have to guesstimate where the peaks should be. Unfortunately it will have to wait a few weeks now before I have any more spare time to investigate the C further.
Hmm yes it was Ag coat with sputter coater, which is unfortunately the only option I currently have. Our evaporative coaters only have C at the moment. In our infinite wisdom our old reliable Denton was given away years ago which would have let me do these coats...
Cheers