CALCZAF and correction of interference by subsidiary and higher order lines?

[JakubHaifler]

Hello,
I would like to perform an interference correction of Pb MB (and maybe also Pb MA) intensity for interfering lines of other elements. I know, that the topic (associated with monazite and xenotime dating) is quite extensively discussed in this forum. However, I face some additional issues.

First, our EPMA lab is not, unfortunately, equipped with PfE software. Thus, I would like to perform the correction after Donovan, J. J., Snyder, D. A., & Rivers, M. L. (1993). An improved interference correction for trace element analysis. Microbeam Analysis, 2, 23-28., manually. I wish to use CALCZAF for calculating the ZAF correction factors. My question is, whether CALCZAF can be used for calculating ZAF’s of interfering lines in cases, in which subsidiary and higher order lines (MAC’s of which are not given in the MAC tables) rather than the major interfere. I can see, that such correction is implemented in the full version of PfE, the approach is described here:

https://probesoftware.com/smf/index.php?topic=69.msg257#msg257

In the example shown in this referred topic, overlap of Y LG3 on Pb Ma is corrected using YPO4 interference standard. How the matrix correction factor for this Y LG3 line is calculated in PfE?  I noticed some discussions on Monte-Carlo-simulated alpha factors, but I am somewhat confused and lost, and I do not understand, whether this is the way how the correction of e.g. Y LG3 on Pb Ma is done in PfE. Can this be done in CALCZAF, generally, for any interfering line? And is there some relation between ZAF of let’s say 1st order line and 2nd order line (e.g. TiKb (II), CeLa(II))?

The second issue is not so important for the question but explains my motivation. We measure (and wish to correct) Pb concentrations in complex HFSE (Ti-,Zr-,Nb-), REE, U+Th -bearing minerals rather than in monazite or xenotime. In addition to the well documented Th-, Ce-, and Y- interfering lines, Pb MA is strongly overlapped by NbLb2,3,4 lines and Pb MB may be slightly overlapped by Nb Lc line and a shoulder of a strong TiKb(II) line in our material. So, I need to calculate the corrections not only for the lines involved in mnz/xtm analysis, but also for the lines of the mentioned additional elements.

By the way, I can see that CALCZAF can calculate ZAF using tabulated MAC’s by several approaches, but the X-Phi routine of Merlet used by PeakSight of our CAMECA SX100 is not implemented. Is there some special reason? I have not found any mention of that in this forum. 

Thank you very much for your reply and help,
Jakub Haifler

[Probeman]

Hi Jakub,
As you know the idea of the quantitative spectral interference correction is to correct for the differences in the matrix correction between the standard used for the interference correction and the sample being measured.  Since the standard used for the interference correction must contain a known amount of the interfering element, *and* none of the interfered element, *nor* any other elements interfering with the element of interest, they are likely to be compositionally different. In fact, since we generally prefer an interference standard which contains a high concentration of the interfering element (for best precision), this is usually the case. Though not always!

But there's a trick we use in the spectral interference correction code to make the matrix correction of these obscure interfering emission lines much easier.

Because any photons from any interfering emission lines are being detected by the same spectrometer (which is already tuned to one of the major lines of interest that we are measuring), we can make an assumption (at least for the case of first order emission lines causing the interference), that the photon energy is the same as the emission line that is being interfered with.

Therefore we might assume that we can apply the same matrix correction (for the line being measured), to the line causing the interference.  However, this assumption breaks down if spectral interference is caused by a higher order reflection. In these cases, the photon energy of the interfering line is at least twice the energy of the line being observed, so in these cases we simply skip the matrix correction and just utilize the concentration ratios without a matrix correction, as suggested in our original paper. The justification being that a high energy emission line has a small matrix correction, and is therefore close to unity.

You can see that (if you had PfE), by running the software in "Verbose Mode" and looking at the output as shown in this post:

https://probesoftware.com/smf/index.php?topic=626.msg8020#msg8020

The other assumption is that the fluorescence effects are the same for the measured line and the line causing the interference in the interference standard and the sample.  For K edge fluorescences this is going to a reasonable assumption, but it starts to break down for L and M edges. So this assumption is at least approximately the case, but to be absolutely rigorous, we should perform the full matrix correction of each interfering emission line, which as you point out, would be a lot of work!

As you know, CalcZAF only calculates matrix corrections for 12 different emission lines (Ka, Kb, La, Lb, Ma, Mb, Ln, Lg, Lv, Ll, Mg, Mz), so for those really obscure lines, this is going to be difficult.  For the example you mentioned of Y LG3 interfering with Pb Ma, the Y LG3 line is a first order reflection, so you are good to go with the assumption that the matrix correction for Y LG3 is approximately the same as for Pb Ma.

In fact, we have not observed cases where the simplifying assumptions made above caused any problems, but we would be interested in learning of such cases if you come across any.
john

[JakubHaifler]

Dear John Donovan,
thank you very much for your reply. I should have thought of the physics more, now it seems to be so clear. It is nice to realize, that neither the photons passing through the matrix and being detected at PbMA position nor those being absorbed, take care of their origin; instead, their behaviour depends on their energy!

Please, could you write me, if there is some special reason for the fact that X-Phi algorithm of Merlet is not implemented in CALCZAF?

By the way: due to numerous and relatively strong spectral interferences of Nb, Ce, La, Th, Ti, Zr, Y, U... in the surroundings of PbMA and PbMB, my measured Pb is highly probably undervalued, as the continuum is buried below the noticeable characteristic spectra of the interfering elements in the whole region. I wonder about trying to correct the intensity not only at the peak position, but also at the “background positions”. The idea is to measure: (I) Peak of Pb line+continuum+Interferences at the position of the line; (II) Bgr1 with minor interferences; (III) Bgr2 with minor interferences; (IV) manually correct Bgr1 and Bgr2 for interferences; (V) calculate Bgr at peak position from (IV); (VI) correct peak of Pb for interferences and subtract Bgr yielded in (V), (VII) calculate concentration of Pb using CALCZAF. Has anyone attempted to perform such a complicated measurement this way?

Thank you very much.
Best regards, Jakub Haifler

[Julien]

Hi Jakub,

Sorry to be crude, but you are looking for extreme high complications and almost impossible things to do without huge errors. Forgive me this stupid analogy, but it is "as if you were trying to make an over-easy egg while starting with a bunch of scrambled eggs...".

If I understand you correctly, you would like to correct a **wrong** background measurement (i.e., not a true background as there is an influence from a neigbouring peak) by an interference correction??? Even if you could do so, you would certainly add a LOT of uncertainty, along with a high inaccuracy. The problem is here multiple. First, your "interference" measurement on the background will be subject to a high precision error as this interference will be (hopefully!) high. Second, and most important, considering that you apparently do not have a way to perform a proper interference correction with the improved Donovan method, you will have to run an iterative background correction MANUALLY (even Probe for EPMA does not offer such an insane option as "background interference correction" - a background MUST be interference-free). Even if you could extract the ZAF factor using CalcZAF (which I'm not sure you can do easily in your case), you will certainly have to do that 4-5 times for ONE interference on ONE measurement, and then repeat this 4-5 times until the results converge. If you add 5 interferences on the peak and more interferences on the background, you are looking at a 100+ iteration loop (the interference correction has to be done iteratively WITHIN the iterative ZAF correction)! And we are still speaking of ONE measurement! This is insane!

To cut short: if you want to have an accurate trace element analysis, you MUST first find a background free interference, which I know is difficult in REE-rich materials and when analyzing low (trace) amount. This is why Mike Williams, Mike Jercinovic, John Donovan and myself developed the multipoint background correction. A paper is coming out in the February issue of Microscopy and Microanalysis (to come out soon...). If you send me a private email at julien.allaz [-at-] erdw.ethz.ch, I can send you a copy of the pre-print version). It is of utmost importance to have adequate background (again WITHOUT any interference, even a small one!) as even a small error on the background will easily yield an inaccuracy of 50-100 ppm on your measurement.

I would highly recommend you to read carefully Jercinovic et al (2012) DOI:10.1088/1757-899X/32/1/012012 and reference therein for details on accurate Pb measurement in REE phosphate.

To come back to your specific question about Y Lg2,3, this is only applicable to Pb Ma measurement. If you consider Pb Mb, then you "only" have to deal with the Ce La (n=2) interference (plus any other additional interference from the additional elements you mentioned that are not present in monazite: Ti. Nb, etc.). Pb Mb is NOT interfered by any significant Th lines. Not sure about Nb, Ti, Zr...

I think you should really consider finding a better software, or if you use Cameca's Peak Sight, then re-consider your background position and find at least two suitable backgrounds, then apply an exponential background correction (because the background is NOT linear, and it is likely you will have to select positions far away from the peak). It won't be a "best" solution, but definitely better than trying to correct a background for an interference... What are you currently using (instrument model, software?)?

I cannot answer your question about the X-Phi algorithm of Merlet, John will be better at answering, but if you want to emulate what Cameca is doing, you can simply choose the PAP or XPP model - to my knowledge, it is rigorously what the original Cameca software employs.

At the end you should also ask you this question: is the EPMA the right tool? Cannot you get good data using for instance LA-ICP-MS? Maybe you could also explain us a bit better what you are analyzing and for what...

Best,

Julien

[John Donovan]

Please, could you write me, if there is some special reason for the fact that X-Phi algorithm of Merlet is not implemented in CALCZAF?

Thank you very much.
Best regards, Jakub Haifler

Hi Jakub,
It's a good question. But there are already 10 matrix corrections in CalcZAF/Probe for EPMA, of which several give excellent results. Frankly I think that we have bigger problems in EPMA with beam sensitive samples, peak shape issues, background measurements and interference corrections, particularly the last two as you correctly point out.

I read about Merlet's X-Phi algorithm at the time he presented it, but he did not want to share his source code and in any case I no longer know how to reach him now that he retired.  But to be honest I'm not sure it is any improvement over the Armstrong/Bastin, PAP and XPP models so it would be a very low priority for me. 

My personal take on matrix corrections is that I would like to implement a rigorous "fundamental" matrix correction without any approximations.  That is the purpose of this option here:



Both Philippe Pinard and Xavier Llovet think this is the way forward, but both are pretty busy, though I would be pleased to implement anything they come up with.

[Brian Joy]

I read about Merlet's X-Phi algorithm at the time he presented it, but he did not want to share his source code and in any case I no longer know how to reach him now that he retired.  But to be honest I'm not sure it is any improvement over the Armstrong/Bastin, PAP and XPP models so it would be a very low priority for me. 

Personally, I think that Merlet’s model should be fully documented and preserved in CalcZAF since it is not fully presented in the literature (the atomic number correction is missing).  X-Phi was one of the last major models to emerge (during the period 1992-1995), and so Merlet had the benefit of being able to address shortcomings of previous models.  As with PAP, his model was designed to produce accurate phi(rho*z) curves.  I’d be happy to do the programming.

[John Donovan]

How would you deal with the missing atomic number correction?

[Brian Joy]

How would you deal with the missing atomic number correction?

I guess I'd need to plead with Merlet or one of his colleagues to make it available.

[JakubHaifler]

Hi Jakub,
It's a good question. But there are already 10 matrix corrections in CalcZAF/Probe for EPMA, of which several give excellent results. Frankly I think that we have bigger problems in EPMA with beam sensitive samples, peak shape issues, background measurements and interference corrections, particularly the last two as you correctly point out.

I read about Merlet's X-Phi algorithm at the time he presented it, but he did not want to share his source code and in any case I no longer know how to reach him now that he retired.  But to be honest I'm not sure it is any improvement over the Armstrong/Bastin, PAP and XPP models so it would be a very low priority for me. 

Dear John Donovan, thank you very much for your answer.
I did not want to assign you a homework to build a code with X-Phi correction. I was only interested in the reason, because this algorithm is used in the default Cameca software, at least in the version we have. So, to know, if there are any reasons against the algorithm of Merlet. So, it is not possible to run the same correction algorithm in both, CALCZAF and PeakSight; but it is no problem. Now, I understand the reason.
Best regards, Jakub Haifler. 

[JakubHaifler]

Hi John,
I would like to ask you if you could be so kind as to make one thing possible in CALCZAF. There are several MAC datasets, but only one of them (FFAST) having values for additional lines, such as e.g. CeLg. The user is allowed to edit MAC’s in any of the datasets. I can insert MAC’s for the CeLg into the USER MAC database or even into some other. However, if I want to define a composition and calculate ZAF for the emission, CALCZAF want me to choose FFAST even though there are values for the additional line in the dataset of interest.



Maybe someone wishes to ask, where do the MAC’s for the CeLg come from and why do I need such an uncommon calculation.

(1)   We can find in the X-ray database, that energy of CeLg is very similar to that of GdLa (6.0511 vs 6.0561 keV). If we plot MAC’s (from FFAST) for both with increasing atomic number of the absorber, we can see they show (almost) identical values. (Here, I want to note, that some pairs with similar energies other than GdLa+CeLg may differ in the position of an absorption edge = the overlap may be disrupted at the edge). Plot of GdLa MAC’s from MAC30 database shows some shift towards higher values. So, I adjusted the CeLg-pattern from the FFAST database to be compatible with GdLa from MAC30 database as an approximation, because I do not want to use FFAST for my calculations.



(2)   Why to do this? I have been calculating some interference corrections and testing some situations, where spectral overlaps are relatively strong. In some cases the interference standard (with high concentration of the interfering element) can give apparent concentration of e.g. 7 wt.% of the interfered element. So, for the calculation of ZAF in the interference standard, I guess, there could be significant matrix effect of the interfered element although its concentration in the matrix is zero. So, my idea was to calculate the ZAF directly for the (additional), interfering line rather than for the interfered line in such situations. So that the effect of interference standard matrix is not affected by the elevated total and the fake 7 wt.% of the interfered element.     

So please, is it somehow possible to enable CALCZAF to run calculations with the additional lines + other than FFAST database selected?

Thank you very much. Best regards, Jakub Haifler
 

[John Donovan]

So please, is it somehow possible to enable CALCZAF to run calculations with the additional lines + other than FFAST database selected?

Thank you very much. Best regards, Jakub Haifler

Hi Jakub,
I am sorry for the warning message you are receiving.  When Philipp Poeml and I added MACs for the "additional lines" (e.g., Ce Lg, etc.) to the FFAST table, I was concerned that people would try to utilize the other (historical) MAC tables for these lines. But of course you are correct. If someone is creating their own table of MACs using the USERMAC table, they should be able to.

I am at the QMA 2019 meeting this week, but if you can wait until next week I will change the code to allow one to utilize the USERMAC table for these "additional lines".
john

[John Donovan]

Hi Jakub,
I was just talking to Philipp Poeml about your interference calculation efforts. One thing Philipp mentioned was that we should add the Penelope and Pouchou MAC tables to CalcZAF. So we will work on doing that as soon as we get home from the conference.

[John Donovan]

Hi John,
I would like to ask you if you could be so kind as to make one thing possible in CALCZAF. There are several MAC datasets, but only one of them (FFAST) having values for additional lines, such as e.g. CeLg. The user is allowed to edit MAC’s in any of the datasets. I can insert MAC’s for the CeLg into the USER MAC database or even into some other. However, if I want to define a composition and calculate ZAF for the emission, CALCZAF want me to choose FFAST even though there are values for the additional line in the dataset of interest.

Hi Jakub,
I made some changes to the code in CalcZAF that should allow you to utilize the "additional" x-ray lines, e.g., Ce Lg in the USERMAC MAC data file for your calculations.

Be aware that MACs for the traditional x-ray lines (Ka, Kb, La, Lb, Ma, Mb) are stored in the USERMAC.DAT file (imported from the USERMAC.TXT file), while MACs for the "additional" x-ray lines are stored in the USERMAC2.DAT files (imported from the USERMAC2.TXT file).

I tested it a bit and it seems to work, but please let me know what you find.  You can update using the Help | Update CalcZAF menu.
john

[JakubHaifler]

Hi Jakub,
I made some changes to the code in CalcZAF that should allow you to utilize the "additional" x-ray lines, e.g., Ce Lg in the USERMAC MAC data file for your calculations.

Hi John.

I hope you and other participants enjoyed the meeting. Thank you very much for changing the code. Meanwhile, I played with the FFAST database and changed my MATLAB script, which does interference corrections in cooperation with CALCZAF.

Now it can do the corrections by mean of calculating intensities of the interfering lines during the iteration. It seems to work. Maybe, I will have some benitoite measurements with Ti/Ba overlaps this week, so we will see. I am not sure, whether this modified approach could provide some improvement to the correction. But the approach from 1993 could be somewhat limited, where large fluorescence occurs or maybe in case of higher order lines, as noted in the article and in this thread above…

Best regards, Jakub Haifler

[John Donovan]

Hi Jakub,
Excellent work.

Yes, I think you are exactly correct. The 1993 work was an approximation (as are all science models!), but at the time it was the best we could come up with, given our limited resources.

A proper treatment of the matrix correction for the interfering line is exactly what we should be doing, and it sounds as though you have done that. Would you be interested in writing this up with me and submit it for publication?  I would be pleased to be a trailing author if you are interested.

I think it would be worth comparing the two methods and see exactly how large an effect we see from an accurate treatment of the matrix correction for the interfering line. I think it would be most interesting to examine interference matrix corrections where, as you suggest, there are large differences in the fluorescence correction, or situations involving higher order interfering lines where the assumption of utilizing the matrix effects from the interfered line are not correct.

Here's a nasty scenario: As Ka interfering with Pb La. Just to lay it out for everyone, try measuring Pb in a GaAs standard and one will obtain a total of over 200%?  This is because the As Ka emission is much stronger than the Pb La emission. Yet, the 1993 model gets the right answer.  Why? I think it helps to utilize the ratio of concentrations as opposed to earlier models which simply ratioed the intensities. Here is an measurement of Pb in GaAs, *without* an interference correction:

St  662 Set   2 GaAs (synthetic)
TakeOff = 40.0  KiloVolt = 20.0  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

Alpha Products, #88458
99.999%, poly-crystalline
Electronic grade
Number of Data Lines:   3             Number of 'Good' Data Lines:   3
First/Last Date-Time: 05/25/2015 05:47:00 PM to 05/25/2015 05:52:13 PM
WARNING- Using Exponential Off-Peak correction for pb la
WARNING- Using Exponential Off-Peak correction for as ka

Average Total Oxygen:         .000     Average Total Weight%:  202.169
Average Calculated Oxygen:    .000     Average Atomic Number:   58.790
Average Excess Oxygen:        .000     Average Atomic Weight:  110.890
Average ZAF Iteration:        3.00     Average Quant Iterate:     2.00

St  662 Set   2 GaAs (synthetic), Results in Elemental Weight Percents
 
ELEM:        S      Pb      As      Ga
TYPE:     ANAL    ANAL    ANAL    SPEC
BGDS:      LIN     EXP     EXP
TIME:    80.00   80.00   80.00     ---
BEAM:    29.88   29.88   29.88     ---

ELEM:        S      Pb      As      Ga   SUM 
   327    .008 108.541  45.651  48.200 202.400
   328    .006 108.165  45.468  48.200 201.839
   329    .019 108.424  45.626  48.200 202.269

AVER:     .011 108.377  45.582  48.200 202.169
SDEV:     .007    .193    .099    .000    .293
SERR:     .004    .111    .057    .000
%RSD:    62.70     .18     .22     .00

PUBL:     n.a.    n.a.  51.800  48.200 100.000
%VAR:      ---     ---(-12.00)     .00
DIFF:      ---     --- (-6.22)    .000
STDS:      730     731     662     ---

STKF:    .4736   .8044   .4976     ---
STCT:   232.45  301.58   92.02     ---

UNKF:    .0001   .9489   .4976     ---
UNCT:      .04  355.75   92.02     ---
UNBG:      .28    9.69    2.42     ---

ZCOR:   1.3539  1.1422   .9160     ---
KRAW:    .0002  1.1796  1.0000     ---
PKBG:     1.15   37.73   39.11     ---

Note the 108 wt% Pb when it should be zero!  Think about it: the interference of As Ka on Pb La is larger than the As concentration.  And here is the same analysis *with* the 1993 interference correction:

St  662 Set   2 GaAs (synthetic)
TakeOff = 40.0  KiloVolt = 20.0  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

Alpha Products, #88458
99.999%, poly-crystalline
Electronic grade
Number of Data Lines:   3             Number of 'Good' Data Lines:   3
First/Last Date-Time: 05/25/2015 05:47:00 PM to 05/25/2015 05:52:13 PM
WARNING- Using Exponential Off-Peak correction for pb la
WARNING- Using Exponential Off-Peak correction for as ka

Average Total Oxygen:         .000     Average Total Weight%:  100.037
Average Calculated Oxygen:    .000     Average Atomic Number:   32.057
Average Excess Oxygen:        .000     Average Atomic Weight:   72.330
Average ZAF Iteration:        2.33     Average Quant Iterate:    62.00

St  662 Set   2 GaAs (synthetic), Results in Elemental Weight Percents
 
ELEM:        S      Pb      As      Ga
TYPE:     ANAL    ANAL    ANAL    SPEC
BGDS:      LIN     EXP     EXP
TIME:    80.00   80.00   80.00     ---
BEAM:    29.88   29.88   29.88     ---

ELEM:        S      Pb      As      Ga   SUM 
   327    .010    .003  51.880  48.200 100.092
   328    .008    .205  51.588  48.200 100.001
   329    .024   -.066  51.859  48.200 100.017

AVER:     .014    .047  51.776  48.200 100.037
SDEV:     .009    .141    .163    .000    .049
SERR:     .005    .081    .094    .000
%RSD:    62.74  296.94     .31     .00

PUBL:     n.a.    n.a.  51.800  48.200 100.000
%VAR:      ---     ---  (-.05)     .00
DIFF:      ---     ---  (-.02)    .000
STDS:      730     731     662     ---

STKF:    .4736   .8044   .4976     ---
STCT:   232.45  301.58   92.02     ---

UNKF:    .0001   .0004   .4974     ---
UNCT:      .04     .14   91.99     ---
UNBG:      .28    9.69    2.42     ---

ZCOR:   1.6780  1.3089  1.0409     ---
KRAW:    .0002   .0005   .9996     ---
PKBG:     1.15    1.01   39.10     ---
INT%:     ----  -99.96    -.04     ---

Note the extremely large number of iterations to converge to a result. Yet, an analysis of PbSiO3 (alamosite) doesn't fare as well:

St  386 Set   3 Alamosite (PbSiO3)
TakeOff = 40.0  KiloVolt = 20.0  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

Tsumeb, South West Africa
From Mineralogical Research, CA
(assumed stoichiometric)
Number of Data Lines:   3             Number of 'Good' Data Lines:   3
First/Last Date-Time: 05/25/2015 05:30:18 PM to 05/25/2015 05:35:33 PM
WARNING- Using Exponential Off-Peak correction for pb la
WARNING- Using Exponential Off-Peak correction for as ka

Average Total Oxygen:         .000     Average Total Weight%:   97.865
Average Calculated Oxygen:    .000     Average Atomic Number:   61.054
Average Excess Oxygen:        .000     Average Atomic Weight:   55.122
Average ZAF Iteration:        4.00     Average Quant Iterate:    50.33

St  386 Set   3 Alamosite (PbSiO3), Results in Elemental Weight Percents
 
ELEM:        S      Pb      As      Si       O
TYPE:     ANAL    ANAL    ANAL    SPEC    SPEC
BGDS:      LIN     EXP     EXP
TIME:    80.00   80.00   80.00     ---     ---
BEAM:    29.88   29.88   29.88     ---     ---

ELEM:        S      Pb      As      Si       O   SUM 
   321   -.014  69.418   2.154   9.910  16.939  98.407
   322   -.013  68.820   2.343   9.910  16.939  97.999
   323   -.004  67.306   3.037   9.910  16.939  97.189

AVER:    -.010  68.515   2.511   9.910  16.939  97.865
SDEV:     .006   1.088    .465    .000    .000    .620
SERR:     .003    .628    .268    .000    .000
%RSD:   -56.86    1.59   18.51     .00     .00

PUBL:     n.a.  73.151    n.a.   9.910  16.939 100.000
%VAR:      ---   -6.34     ---     .00     .00
DIFF:      ---  -4.636     ---    .000    .000
STDS:      730     731     662     ---     ---

STKF:    .4736   .8044   .4976     ---     ---
STCT:   232.38  301.58   92.02     ---     ---

UNKF:   -.0001   .5768   .0265     ---     ---
UNCT:     -.04  216.27    4.89     ---     ---
UNBG:      .60   14.64    3.37     ---     ---

ZCOR:   1.1884  1.1878   .9490     ---     ---
KRAW:   -.0002   .7171   .0532     ---     ---
PKBG:      .93   15.77    2.45     ---     ---
INT%:     ----   -8.07  -88.74     ---     ---

Note that the interference quantification iterated over 50 times per point, yet was still unable to converge to the correct (zero) concentration for As. Maybe a more accurate matrix correction for the interfering line would help? If you're interested in writing this up, let's continue off-line. Please feel free to contact me at donovan@oregon.edu
john