How to sum up few spectra

[Nicholas Ritchie]

Or Goldstein 2017 aka SEM-XM 4

I remain somewhat surprised by how much structure comes through in the residual despite the (relatively) poor resolution of the detector.

[sem-geologist]

Oh Goldsetein 2017 – an excellent book without WDS (just proves my point about WDS in Goldstein 2003).

Now I also can't understand why auto reference does so bad. If I use simulation  alien for same detector and simulate LaPO4, it does much better, albeit it simulates it with wider peaks than measured. (same goes for CePO4 and so on). I am a bit mixed up with window of detector, it shows in GUI it as  MOXTEK AP 5, while in file spx at coresponging XML field it is "SLEW AP 3.3" (guess I could use Moxtek AP 3.3 is it the same?). I think I had missed something important like calibration of detector? When I defined detector I had imported spectra, should I do calibration alien on Mn sample probably? Probably it is that as spx does contain no information about real width at Mn DTSA-II used some default values(?), that needs to be measured on real Mn. I am wondering how much mistakes I had conducted, and still I could get reasonable chevkinite composition. I am going to delete all resulting bundled standards and detectors and start a fresh.

[Probeman]

I don't want to get into an argument about WDS vs EDS because I think most reasonable people would agree that both methods have advantages and disadvantages with respect to each other.

Personally I think the use of integrated WDS and EDS provides the most flexible and efficacious approach depending on the particular analytical problem, e.g.,

https://probesoftware.com/smf/index.php?topic=79.msg7818#msg7818

In terms of mere popularity, yes, there will always be more SEM EDS labs compared with WDS EPMA labs. On the other hand, if one compares the number of SEM EDS labs that actually utilize standards with the number of WDS EPMA labs (who *must* use standards), the numbers are much closer I think.

 

[sem-geologist]

Well I have such cases where WDS simple can't cut through .These Gagarinite i.e. that is Na(Ca,REE)2F6 (according to some oldy published data Na is 0.9 apfu, but I guess they just had overlooked the loss of Na). 5µm grains, not much space to put a point with reasonable defocus. Na just gets halved in a few seconds if beam touches that thing, even at 1.5nA (!). With every second the Na is being depleted. TDI? only the first elements, what to do with subsequent ones, and there is plenty to measure? Even for TDI to work reasonably it requires some sufficient statistics (enough counts). If I want better statistics, I need more (nA * s) of beam, but then I loose more of Na, so I would trade accuracy for precision. Accurate Fluorine?, while there is REE and some Ba present in the mineral, it is impossible to intercept the background under the F Ka in a traditional (two point) ways (Maybe MAN would cut through this, I don't know, this is low energy region). Until there are some spots where background can be measured the WDS is nice, but when there is too many elements in the mineral and there is not a single interference-free background spot – WDS is no more fun. Use PHA? Well, if it would be implemented in not half-assed way it could be helpful, but circuits for PHA (AFAIK) on Cameca EPMA's introduces enormous unnecessary energy broadening (yeah I know Goldstein et al times ago explained that broadening originates by processes in proportional... blah blah... what so ever... my physically connected to the raw signal oscilloscope is telling completely something else). How on the earth PHA in 5 volt range can't cleanly separate energy levels which differs by factor of 2 (lets say second order), while cheapo bargain low quality rubbish oscilloscope can distinguish effortlessly few levels of raw signal (in mV) captured in-between pre-amplifier circuitry and spectrometer VME board (before PHA). Then I mean "distinguish" I mean You could easily smuggle elephant in between those clearly different energy levels. I understand that this PHA circuits were "wow" in 80'ies (or as vendors loves this term "a state of art", I would insert "antique") as it had a clear edge over EDS counting, it could be even acceptable in SX100's at end of millennium and had a bit of faster counting than available EDS. But in 21st century EDS SDD's just had took this crown from WDS as EPMA vendors were just putting the same circuits into newest generation e-microprobes. What is maximum pulse throughtput on current WDS on cameca? Goldstein et al 2003 lied – contrary to stated 50kcps, WDS can go actually to a bit over 200kcps (You could do that in 2003 either with SX50 or SX100), but actually it should better not go much over 10kcps, even on a new SXFives, as above that counting rate unaccounted pulse-pileups kicks in, and there is no pileup correction. But what counting speed have to do with PHA? - it is the same bottleneck. The pulses are artificially shaped to live long enough so the "State of antique art" 8bit ADC can digitize amplitude of pulse (actually it is a bit more complicated). For comparison of speeds modern pre-amplifier is able to handle 8 million of pulses per second and such throughput is achievable with most modern EDS while not sacrificing precission

In my opinion to compete with SDD throughput vendors of EPMA should change current counting electronics and replace that with newest generation of preamplifiers which would feed signal directly to fast ADC (40MHz 12bit would be enough), which would feed that digitized signal to modern FPGAs (FPGAs have plenty of DSP cores, so could compute and output amplitude in near real-time). That would bring many improvement for WDS: practically no deadtime (unless using 10µA beam), really functional PHA witch would effortlessly distinguish second order peaks. For spectrometer control there would be left only bias settings – no more gain setting for spectrometer - it would lead to simplified proportional range optimization. No more PHA shifting from intensity level (As DSP's in FPGA would measure not absolute amplitude, but relative to the base level).

But I guess that won't happen, this requires some investments in R&D and we and our pockets are too little to matter. SEM and EDS is more used, so more developed, and used more because being actively developed.

... Oh damn this PHA, I was writing about Gagarinite analytical difficulties.
So on SEM and EDS there is huge difference. On SEM the EDS detectors are close to the sample and so even with lower current (~0.5nA) I get more counts per element than with a bit higher current on EPMA WDS. (at 0.5nA I am getting ~32kcps, so spectral lines are getting around 1k-5kcps).

I agree that combined WDS EDS is good way to go, but that is not a panacea for all problems. There are situations where EDS alone will perform better than WDS, or combining WDS with EDS does not brings anything additional to EDS-alone technique.

[Probeman]

As I stated in my post: "depending on the particular analytical problem".   

Also I hope you realize that if one has a WDS system and an EDS system on their EPMA (or SEM), they can utilize both in an integrated manner, or utilize only WDS or only EDS.  I hope that is that flexible enough for you?   

[sem-geologist]

Also I hope you realize that if one has a WDS system and an EDS system on their EPMA (or SEM)...

As I started to play with DTSA-II I realize this very much. Using both EDS and WDS could improve throughput. Now question is how those well integrate. I had not used that on Cameca Peaksight, as that has very marginal integration, can be used with very simple phases and predefined set of elements. The problem is it have hard-written positions for background (looked so last time I inspected how does it works), there is no documentation how those are integrated (peaksight have two options "fullanalysis" and "deconvolution", but no explanation what it does). I think the way how DTSA-II handles the EDS spectra is the right way (no need to point where is background). And as far as I understand ProbeSoftware has its own implementation. Does ProbeSoftware deconvolution? does it needs to define background regions?

Now I am interested if DTSA-II calculations from k-ratios. I wish there would be possibility to make mixed analyses in DTSA-II. The last question is automation. It is ok to spend lots of time to do methodologically correctly bells and whistles for few (first) analyses of given mineral. But the beauty of EPMA is that when You setup how to measure and do recalculations for defined species/groups of minerals/materials, it can be highly automated for high throughput. I wish epq in DTSA-II would have better documentation. I think as DTSA-II have jython, it should be possible to make some server script (Need to test that there is no timeout for python scripts, as that should run basically forever in opened DTSA-II instance) which would accept k-ratios, spectras and instructions, then do calculations using epq and would return the results (two way communication through platform independent network sockets). I feel so spoiled with modern python environments, that this plain (no tab-completion, no hints) jython environment is so cumbersome to explore, and need to depend on fundamental python functions (like 'dir()', 'help()').