Author Topic: Trace element blank correction  (Read 5942 times)

Brian Joy

  • Professor
  • ****
  • Posts: 131
Trace element blank correction
« on: March 12, 2014, 11:57:33 am »
Hi John,

I have a couple of questions regarding implementation of your trace element blank correction, as presented in American Mineralogist, v. 96, p. 274-282:

If I write for the general case for element A,

C(A)_unk = C(A)_std * (I(A)_unk/I(A)_std) * ([ZAF](A)_unk/[ZAF](A)_std),

then do I substitute I(A)_corr from equation 5 (below) for I(A)_unk in the above expression when applying the blank correction?  Or am I not looking at it correctly?

I(A)_corr = I(A)_unk - I(A)_std * ((C(A)_meas - C(A)_level) / C(A)_std ) * ([ZAF](A)_std/[ZAF](A)_unk),

where each I is a net intensity, C(A)_level is the known concentration of element A in the blank, and C(A)_meas is the measured concentration of element A in the blank.

Second, how exactly is C(A)_meas determined?  Is it done in the simplest possible way, with background interpolated linearly between offsets on either side of the peak?  Does it really matter how it's done as long as it's done exactly the same way on the blank as on the unknown?

I'd like to apply the blank correction to analysis of tens to hundreds ppm Au in pyrite, which is a fairly intractable case if using a conventional approach, even if non-linear background models are considered.  It appears that the tail of S Ka and/or its satellites cause undulations in the spectrum in the vicinity of both Au Ma and Au Mb.  The undulations are difficult to model as a function of spectrometer position, and so the blank correction appears to be the best approach.

I've attached a set of wavelength scans on PET showing Au-bearing pyrite in red and Au-free pyrite in green.  I should have set the PHA window a little narrower to better suppress Fe Ka(3), but the scans still clearly show the undulations around both Au Ma and Au Mb.

Thanks,

Brian
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Probeman

  • Emeritus
  • *****
  • Posts: 1672
  • Never sleeps...
    • John Donovan
Re: Trace element blank correction
« Reply #1 on: March 12, 2014, 01:35:28 pm »
Hi Brian,
Yes, this Au in FeS2 example is nasty for background fitting, but pyrite is also (usually) a simple matrix where a pure "blank" standard may be obtained, so is therefore a perfect candidate for using the blank correction!

I'll start by describing it in my software, since this is a very simple process and good for the overview of the details explained later:

1. Acquire your unknown sample as usual (using off-peak or even MAN backgrounds as discussed here):

http://probesoftware.com/smf/index.php?topic=29.msg237#msg237

2. Then yes, acquire another unknown using the *exact same* conditions (PHA settings, count time, beam current etc.) on a standard suitable for the blank correction. Note that this "blank" standard should be a material that is ideally exactly the same composition as your unknown, but with a known (zero) concentration of the element of interest. This known concentration is usually easiest if it is say a pure synthetic and therefore zero due to purity, but the blank "level" as it is called can also be non-zero, so long as it is accurately known!  Our Ti in SiO2 synthetic has 1.42 PPM Ti (which could usually be rounded to zero!). The Spectrosil silica described here:

http://probesoftware.com/smf/index.php?topic=130.msg606#msg606

is even purer, but is not crystalline, so I'm not sure how that might effect its use as a blank std for quartz...

3. Finally perform a normal quant analysis on the blank standard and see how far off you are from the zero or non-zero concentration that you are expecting. This is your accuracy check.

So let's say, your FeS2 blank standard contains zero PPM of Au (could be checked by ICP-MS for example), but when you measure it on your EPMA (using off-peak or MAN modeling), you obtain -40 PPM (yes, negative 40 PPM, though the artifact could be positive as well, though it is usually negative due to secondary Bragg diffraction (Ti Ka in SiO2) or self absorption in the sample, which I believe is the case for Au Ma in FeS2).

For example, see the Ti in quartz example here:

http://probesoftware.com/smf/index.php?topic=29.msg387#msg387

So, let's see what we have for our blank correction equation now remembering that we are only making an adjustment to the unknown intensity,  I(A)_unk in your first equation:

What we basically want to do is to calculate the intensity represented by the blank accuracy characterization, so that it can be added to the matrix iteration to improve accuracy (and be reflected in the matrix correction if it is not a small adjustment!). That is, because the blank correction is being performed with a full matrix correction (just like the interference correction), the "blank" value could be a minor or even major element!  It all comes out in the iteration!

So, you'll want to ratio the blank concentration error, I(A)_blank_meas - I(A)_blank_level, to obtain the intensity error (to put it another way), as seen here:

I(A)_unk_corr = I(A)_unk - I(A)_std * (I(A)_blank_meas - I(A)_blank_level)/ C(A)_std * (ZAF_std/ZAF_unk)

where I(A)_blank_meas is the concentration we actually measure in the blank standard (negative 40 ppm in the pure Fes2 in the above example) and I(A)_blank_level is the concentration that we know is there from purity or ICP-MS or other considerations (ideally zero for a pure FeS2).

Now that we have the corrected I(A)_unk_corr value, that is simply plugged into the matrix correction using the general case (your first equation)...

However, since you aren't using Probe for EPMA (yet!), you won't be easily able to do that, so you could just do a one step subtraction since the matrix correction will not change significantly for a 40 PPM correction in the matrix.

Bottom line, you could skip the quant aspect completely and just subtract the measured from the expected from your unknown measurements, that is, add 40 PPM to all analyses using the same example above!

I think this is a perfect example for the blank correction, and particularly if you decide to use MAN backgrounds, to avoid the off-peak interferences, as described in this abstract:

http://probesoftware.com/smf/index.php?topic=29.msg706#msg706
« Last Edit: March 12, 2014, 06:42:14 pm by Probeman »
The only stupid question is the one not asked!

Probeman

  • Emeritus
  • *****
  • Posts: 1672
  • Never sleeps...
    • John Donovan
Re: Trace element blank correction
« Reply #2 on: March 13, 2014, 11:08:04 am »
Note that in the above equations all concentrations should be in weight fraction.
john
The only stupid question is the one not asked!

sckuehn

  • Post Doc
  • ***
  • Posts: 20
Re: Trace element blank correction - for glasses?
« Reply #3 on: May 21, 2014, 06:23:26 am »
I'm thinking of trying the blank correction with a new glass trace element routine, and I have a couple of questions:

 (1) Is there a reason why a sample that has been run as a standard can't be selected in PFE for use as the blank correction sample?

 (2) How closely does the blank correction sample have to match the unknowns in practice? 

Quartz and pyrite are both much simpler matrices than glasses. In my case, I am looking for something appropriate to rhyolitic to dacitic glasses. A blank with a reasonable mean rhyolite-dacite composition might look something like 74% SiO2, 13.5% Al2O3, 2.5% FeO, 3% CaO, 4% Na2O, and 4% K2O with everything else at least under 50 ppm. I am not aware of a synthetic glass that looks like this, so I am wondering if something else might work. Besides pure SiO2, the closest that I currently have is a synthetic K-feldspar glass synthesized by Corning that I obtained from Harvey Belkin at the USGS Reston probe lab (USGS code GFOR). This is 64.7% SiO2, 18% Al2O3, and 16.5% K2O with a little Na2O, FeO, and H2O. The trace element content is unknown, though. Alternatively, NIST 612 might work for a couple of the elements that I am interested in, but it does have a rather high Na content.

John Donovan

  • Administrator
  • Emeritus
  • *****
  • Posts: 2259
  • Other duties as assigned...
    • Probe Software
Re: Trace element blank correction - for glasses?
« Reply #4 on: May 22, 2014, 05:13:21 pm »
(1) Is there a reason why a sample that has been run as a standard can't be selected in PFE for use as the blank correction sample?

It is just to ensure that the blank sample is run under the exact same conditions as your unknowns. It matters!

(2) How closely does the blank correction sample have to match the unknowns in practice? 

The blank correction is designed for simple matrices where a sample with the same matrix but with a zero concentration (or where a non-zero concentration of the trace element is known) is utilized to provide an accuracy reference.

Having said that, I have experimented with using the blank correction on a complex glass as a sort of "accuracy" adjustment. This is possible because the code re-calculates the concentration offset into a quantitative intensity and applies that correction during the matrix iteration.

So for example, I used the blank correction to get better accuracy for measuring oxygen in glass for determination of water (Nash, et al.). Here is an example:

Un   17 Withers-N5, Results in Elemental Weight Percents
 
ELEM:       Na       K      Cl      Ba       F      Ti      Fe      Mn      Ca      Si      Al      Mg       O       H
TYPE:     ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    ANAL    SPEC
BGDS:      MAN     LIN     LIN     LIN     LIN     LOW     MAN     LIN     MAN     MAN     MAN     MAN     EXP
TIME:    60.00   20.00   10.00   20.00   40.00   10.00   40.00   10.00   20.00   20.00   20.00   60.00  120.00
BEAM:     9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98    9.98

ELEM:       Na       K      Cl      Ba       F      Ti      Fe      Mn      Ca      Si      Al      Mg       O       H   SUM
   574   2.780   3.536    .205    .030    .086    .108   3.136    .060    .162  32.680   5.404    .012  50.078    .665  98.943
   575   3.072   3.557    .207    .017    .084    .124   3.120    .079    .148  32.990   5.365    .007  49.939    .594  99.303
   576   2.846   3.314    .180    .032    .100    .137   3.131    .083    .129  32.914   5.316    .000  49.919    .624  98.727
   577   2.925   3.550    .258    .041    .111    .098   3.165    .058    .118  32.858   5.437   -.001  49.985    .621  99.224
   578   3.011   3.502    .258   -.014    .083    .072   3.173    .081    .116  32.715   5.364    .009  49.812    .626  98.810
   579   3.043   3.561    .239   -.079    .070    .124   3.191    .083    .130  32.812   5.428    .006  50.012    .623  99.244
   580   2.842   3.531    .223    .016    .089    .101   3.125    .055    .132  32.799   5.335   -.001  49.969    .644  98.859
   581   3.179   3.459    .248   -.019    .121    .118   3.100    .050    .152  33.024   5.362    .006  50.053    .605  99.455
   582   2.924   3.505    .164   -.025    .080    .134   3.197    .051    .120  33.124   5.375    .009  49.790    .562  99.009
   583   2.890   3.436    .218   -.062    .033    .147   3.165    .031    .143  32.919   5.388    .003  49.908    .609  98.830
   584   2.952   3.381    .213    .004    .033    .124   3.124    .064    .133  33.068   5.325    .004  49.852    .588  98.865
   585   2.699   3.608    .191    .029    .071    .121   3.150    .061    .161  33.000   5.380    .006  50.223    .641  99.341

AVER:    2.930   3.495    .217   -.003    .080    .117   3.148    .063    .137  32.909   5.373    .005  49.962    .617  99.051
SDEV:     .133    .084    .030    .038    .026    .020    .030    .016    .016    .139    .038    .004    .122    .028    .248
SERR:     .038    .024    .009    .011    .008    .006    .009    .005    .005    .040    .011    .001    .035    .008
%RSD:     4.54    2.40   13.87-1523.47   33.10   17.21     .97   25.48   11.67     .42     .70   80.01     .24    4.47
STDS:      336     374     285     835     835      22     395      25     358     162     336      12      12       0

STKF:    .0735   .1132   .0601   .7430   .1715   .5546   .6779   .7341   .1693   .2018   .1331   .4737   .2328   .0000
STCT:   2447.9  2423.7   839.3  8520.6  2398.7  6097.6 14136.8 13590.2  2247.6 34290.6 23223.7 24410.3  8335.9      .0

UNKF:    .0154   .0303   .0017   .0000   .0002   .0010   .0261   .0005   .0012   .2688   .0412   .0000   .2315   .0000
UNCT:    513.0   648.1    24.2     -.2     2.7    10.8   545.2     9.5    16.2 45673.8  7190.2     1.7  8290.2      .0
UNBG:      9.9    12.6     5.0    29.0     3.8     5.8    23.4    16.1     4.4   134.1   103.2    15.2    53.7      .0

ZCOR:   1.9017  1.1548  1.2500  1.3725  4.0763  1.1972  1.2041  1.2234  1.1196  1.2241  1.3036  1.4987  2.1582   .0000
KRAW:    .2096   .2674   .0289   .0000   .0011   .0018   .0386   .0007   .0072  1.3320   .3096   .0001   .9945   .0000
PKBG:    52.69   52.56    6.39    1.00    1.77    3.03   24.32    1.61    4.71  341.58   70.64    1.11  156.16     .00
INT%:     ----    ----    ----   -3.81    ----    -.02    ----    ----    ----    ----    ----    ----    ----    ----
APF:      ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----   1.031    ----

TDI%:   79.930    .454    ----    ----    ----    ----    ----    ----    ----   -.870    ----    ----  -3.030    ----
DEV%:      2.7     2.4    ----    ----    ----    ----    ----    ----    ----      .3    ----    ----      .3    ----
TDIF:   QUADRA  LINEAR    ----    ----    ----    ----    ----    ----    ----  LINEAR    ----    ----  LINEAR    ----
TDIT:    72.58   30.67    ----    ----    ----    ----    ----    ----    ----   31.00    ----    ----  129.42    ----
TDII:     522.    660.    ----    ----    ----    ----    ----    ----    ----  45860.    ----    ----   8327.    ----
BLNK#:    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----      19    ----
BLNKL:    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ---- 43.5580    ----
BLNKV:    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ---- 44.9622    ----



Here is a white paper on TDI effects when measuring Na, Si and O in hydrous glasses (method of B. Nash, et al.):
http://epmalab.uoregon.edu/reports/Withers%20hydrous%20glass.pdf

You can try with a complex glass for traces but I can't predict how helpful it will be for this situation. But please see this post for the technical details:

http://probesoftware.com/smf/index.php?topic=204.msg919#msg919
« Last Edit: May 22, 2014, 08:56:54 pm by John Donovan »
John J. Donovan, Pres. 
(541) 343-3400

"Not Absolutely Certain, Yet Reliable"

gmorgan@ou.edu

  • Post Doc
  • ***
  • Posts: 20
Blank Correction
« Reply #5 on: September 25, 2015, 07:11:25 am »
Greetings all,

This is not an entirely new topic, but perhaps a new take on an old one.

I have been experimenting with using the blank correction to remove fictive boron from analyses of B-absent glass that results from internal fluorescence of the B4C interlayer of the 200-ansgtrom LSM device, and it has produced a question about how the correction actually works or could work. The main problem with this kind of analysis is that at HV greater than 3-5 kV (which I need for other elements, and am already using two beam conditions) the B Ka intensity is extremely weak. Using a B-free granitic glass for the blank correction, which yields about 0.2 cps/nA at B K-alpha, I get great results for glasses I know to be B-absent (~0.02-0.07 wt% B2O3, relative to a calculated MDL of ~0.3 wt%), a spot-on match for an E-glass I know to contain 5.5 wt% B2O3, and very reasonable concentrations and totals for anhydrous glasses with 1-2 wt% B2O3.

The issue I have is that if the blank correction is applied to my Pyrex standard it yields a value significantly lower than its known value. This tells me that the blank correction works simply by subtracting the concentration of the blank sample from every analysis it is applied to, which isn't a problem for its intended application in trace element analysis using hard x-ray emissions; after all, it isn't intended to be applied to the standard, and subtracting a few ppm from a standard in which the element is of interest is a major element makes no difference to the result. But in the case of boron, the 0.2 cps/nA of the blank can translate to 1-2 wt% fictive B2O3 (a Pyrex with near 13 wt% yields only about 1.4 cps/nA at 15 kV - I told you, the intensity is weak).

So although the blank correction apparently yields good results for low-boron glasses, its application to the standard yields a poor result(1-2 wt% low in B2O3). Could, or does, the blank correction simply work such that the intensity corresponding to the bank represents zero concentration and then the intensity corresponding to the standard (or standard intensity minus the intensity of the blank) is then used to calculate the cps/na/wt% used for calculating concentration?

In other words, rather than subtracting a concentration, subtract the intensity and then calculate composition? If the intensity of the blank were defined as zero, the process would be much more like scaling image intensities via histogram stretching (changing the "display range"). I wonder if such a method wouldn't better linearize the intensity-concentration relationship, and could be applied to all analyses (including standards and standards analyzed as unknowns).
« Last Edit: September 25, 2015, 10:24:21 am by John Donovan »

John Donovan

  • Administrator
  • Emeritus
  • *****
  • Posts: 2259
  • Other duties as assigned...
    • Probe Software
Re: Blank Correction
« Reply #6 on: September 25, 2015, 10:22:27 am »
Greetings all,

This is not an entirely new topic, but perhaps a new take on an old one.

I have been experimenting with using the blank correction to remove fictive boron from analyses of B-absent glass that results from internal fluorescence of the B4C interlayer of the 200-ansgtrom LSM device, and it has produced a question about how the correction actually works or could work. The main problem with this kind of analysis is that at HV greater than 3-5 kV (which I need for other elements, and am already using two beam conditions) the B Ka intensity is extremely weak. Using a B-free granitic glass for the blank correction, which yields about 0.2 cps/nA at B K-alpha, I get great results for glasses I know to be B-absent (~0.02-0.07 wt% B2O3, relative to a calculated MDL of ~0.3 wt%), a spot-on match for an E-glass I know to contain 5.5 wt% B2O3, and very reasonable concentrations and totals for anhydrous glasses with 1-2 wt% B2O3.

The issue I have is that if the blank correction is applied to my Pyrex standard it yields a value significantly lower than its known value. This tells me that the blank correction works simply by subtracting the concentration of the blank sample from every analysis it is applied to, which isn't a problem for its intended application in trace element analysis using hard x-ray emissions; after all, it isn't intended to be applied to the standard, and subtracting a few ppm from a standard in which the element is of interest is a major element makes no difference to the result. But in the case of boron, the 0.2 cps/nA of the blank can translate to 1-2 wt% fictive B2O3 (a Pyrex with near 13 wt% yields only about 1.4 cps/nA at 15 kV - I told you, the intensity is weak).

So although the blank correction apparently yields good results for low-boron glasses, its application to the standard yields a poor result(1-2 wt% low in B2O3). Could, or does, the blank correction simply work such that the intensity corresponding to the bank represents zero concentration and then the intensity corresponding to the standard (or standard intensity minus the intensity of the blank) is then used to calculate the cps/na/wt% used for calculating concentration?

In other words, rather than subtracting a concentration, subtract the intensity and then calculate composition? If the intensity of the blank were defined as zero, the process would be much more like scaling image intensities via histogram stretching (changing the "display range"). I wonder if such a method wouldn't better linearize the intensity-concentration relationship, and could be applied to all analyses (including standards and standards analyzed as unknowns).

Hi George,
You could have saved yourself a little typing if you'd read the blank correction paper first...    :(

In fact the blank correction in Probe for EPMA does subtract the corrected intensity from the measurement as described in the paper.  Here's how: the concentration difference between the "known" blank standard value, and what you actually measure on that standard, is calculated as a k-ratio intensity and then subtracted during the matrix iteration for unknown samples.  See the link here:

http://probesoftware.com/smf/index.php?topic=29.0

Even so, the blank correction should only be applied when the blank standard and the unknown are very similar in matrix.  In the case of boron measurement, the situation is even more critical due to the huge absorption corrections present.

Try reading the original paper

http://probesoftware.com/Ti%20in%20Quartz,%20Am.%20Min.%20Donovan,%202011.pdf

Then try reading the above posts in this topic and also try reading the boron analysis topic here:

http://probesoftware.com/smf/index.php?topic=248.0

What you are attempting to do is possible I suspect (though I've never tried analyzing silicate glasses for boron!), but I'm pretty sure it's not going to be easy!   

But you've come to the right place to ask...   John Fournelle I know has looked at boron...
« Last Edit: September 26, 2015, 11:01:32 am by John Donovan »
John J. Donovan, Pres. 
(541) 343-3400

"Not Absolutely Certain, Yet Reliable"

John Donovan

  • Administrator
  • Emeritus
  • *****
  • Posts: 2259
  • Other duties as assigned...
    • Probe Software
Re: Trace element blank correction
« Reply #7 on: September 25, 2015, 11:04:27 am »
Let me discuss an example that I think will help.

There are so many way one can "die" using the blank correction, but here is one out of many: I'm analyzing a trace element in a mineral or glass, and the standard I decided to use for making the blank correction is almost exactly the same as my unknown, but in fact has a spectral interference from a minor element on the trace element I am attempting to measure. An interference that my actual unknown matrix does not have...

So my blank registers a significantly higher blank "value" (measurement) than the expected blank "level" (known) and therefore when applied to my actual unknown, over corrects the measured intensity resulting in a negative k-ratio.

Remember the standard used for the blank correction should be as exactly the same matrix as the unknown as possible, but with a zero or known non-zero concentration of the element one is trying to measure.  It will not always be possible to obtain a proper blank standard for many sorts of compositions.
« Last Edit: September 25, 2015, 11:07:58 am by John Donovan »
John J. Donovan, Pres. 
(541) 343-3400

"Not Absolutely Certain, Yet Reliable"

Probeman

  • Emeritus
  • *****
  • Posts: 1672
  • Never sleeps...
    • John Donovan
Re: Blank Correction
« Reply #8 on: September 25, 2015, 05:40:43 pm »
So although the blank correction apparently yields good results for low-boron glasses, its application to the standard yields a poor result(1-2 wt% low in B2O3). Could, or does, the blank correction simply work such that the intensity corresponding to the bank represents zero concentration and then the intensity corresponding to the standard (or standard intensity minus the intensity of the blank) is then used to calculate the cps/na/wt% used for calculating concentration?

Hi George,
If you are asking above if the blank correction can be based on, not only a zero concentration standard, but also on a *non zero* concentration in the blank standard, the answer is : yes!

See the "Blank Level" field circled here:



This is discussed in the post linked to here:

http://probesoftware.com/smf/index.php?topic=29.msg387#msg387

john
« Last Edit: September 27, 2015, 05:46:15 pm by Probeman »
The only stupid question is the one not asked!

gmorgan@ou.edu

  • Post Doc
  • ***
  • Posts: 20
Re: Trace element blank correction
« Reply #9 on: September 28, 2015, 09:57:23 am »
Okay, so I see that the blank correction does an intensity subtraction to the samples. But again, it does not remove the blank intensity from the intensity of the primary standard to yield a new, blank subtracted, intensity for it: i.e., such that I(Std)corrected = I(Std)original - I(blank). If this procedure were done - in other words also blank correcting the standard intensity - then Analyzing the standard with the blank correction would still yield the correct concentration for the standard, which the present procedure does not do.

As an example, imagine using a Pyrex standard with 12.8 wt% B2O3 for boron intensity, and that intensity acquisition and analysis of a B-free glass yields fictive 1.5 wt% B2O3. Using the B-free glass as the "blank" during Analysis removes the fictive boron from the samples just fine, but if the Pyrex standard is Analyzed using the blank correction it now yields 11.3 wt% B2O3.

So I get that this is a somewhat different kettle of fish than what the blank correction was intended for, and that such concerns are unimportant for analyzing most heavier trace elements at very low concentrations. I am grateful to see that, as presently applied, the blank correction does seem to yield a good result for glasses of low to intermediate boron content, but just find it a bit disconcerting that the blank correction seems to yield correct results for everything BUT the primary standard. Given such poor counting statistics for boron, I worry about the accuracy of concentration values between the detection limit and that of the primary standard - just how linear is the intensity-concentration relationship? Without being able to reproduce the standard concentration when using the blank correction I am left somewhat unable to evaluate this.

UofO EPMA Lab

  • Professor
  • ****
  • Posts: 38
    • CAMCOR MicroAnalytical Facility
Re: Trace element blank correction
« Reply #10 on: September 28, 2015, 10:52:43 am »
Okay, so I see that the blank correction does an intensity subtraction to the samples. But again, it does not remove the blank intensity from the intensity of the primary standard to yield a new, blank subtracted, intensity for it: i.e., such that I(Std)corrected = I(Std)original - I(blank). If this procedure were done - in other words also blank correcting the standard intensity - then Analyzing the standard with the blank correction would still yield the correct concentration for the standard, which the present procedure does not do.

As an example, imagine using a Pyrex standard with 12.8 wt% B2O3 for boron intensity, and that intensity acquisition and analysis of a B-free glass yields fictive 1.5 wt% B2O3. Using the B-free glass as the "blank" during Analysis removes the fictive boron from the samples just fine, but if the Pyrex standard is Analyzed using the blank correction it now yields 11.3 wt% B2O3.

So I get that this is a somewhat different kettle of fish than what the blank correction was intended for, and that such concerns are unimportant for analyzing most heavier trace elements at very low concentrations. I am grateful to see that, as presently applied, the blank correction does seem to yield a good result for glasses of low to intermediate boron content, but just find it a bit disconcerting that the blank correction seems to yield correct results for everything BUT the primary standard. Given such poor counting statistics for boron, I worry about the accuracy of concentration values between the detection limit and that of the primary standard - just how linear is the intensity-concentration relationship? Without being able to reproduce the standard concentration when using the blank correction I am left somewhat unable to evaluate this.

Hi George,
Yes, the blank correction is intended only for correction of unknown samples.

This is because the specimen (standard) used for the blank correction must be run at exactly the same conditions, etc as the unknown to be blank corrected.  Allowing only unknown samples for the blank correction helps to ensure this.

What you are attempting to do, is not only a "different kettle of fish", but I think you are "barking up the wrong tree"!   I think you need to first look at all the other light element issues for example, background fitting, area peak factors and MACs.  Quant boron is not for the "faint of heart".  Here are some links to some magnesium boride analyses I performed, maybe this will help:

http://probesoftware.com/smf/index.php?topic=248.0

http://probesoftware.com/smf/index.php?topic=536.0

As you say, the blank correction is designed for situations where one wants to first, check if they can measure zero (or a known non-zero) in a standard material that is exactly like their unknown, but contains a zero (*or known non-zero) concentration.  Then subsequently (and optionally) apply that observed offset to correct for actual unknowns.

So I think you've got "lots of other fish to fry" with the light element backgrounds, APFs, and MACs, not to mention beam sensitivity and sample damage...

In any case, just run your standard as an unknown and then you can see how good a job the blank correction does on it, right?
john
UofO MicroAnalytical Facility

Probeman

  • Emeritus
  • *****
  • Posts: 1672
  • Never sleeps...
    • John Donovan
Re: Trace element blank correction
« Reply #11 on: September 28, 2015, 05:23:42 pm »
Hi George,
It just occurred to me what might be very helpful to you for your boron analyses.  Assuming that you can obtain a set of boron silicate glass standards with a similar matrix to your unknown boro-silicate glasses and covering the range of boron concentrations you have in your unknowns...

That is try the "multi-standard" calibration curve option as described here:

http://probesoftware.com/smf/index.php?topic=461.msg2528#msg2528

By the way, this calibration curve method is what many Japanese investigators utilize for measuring trace carbon in steel.  Some European steel companies utilize a different method for background determination of carbon, that is they measure a "background" on pure Fe metal and then simply subtract that intensity from both the carbon standard and the Fe unknown.  It's essentially a very crude version of my MAN background method, albeit only using a single standard and no continuum absorption correction! 

Now, if you can't obtain a suitable set of standard boro-silicate glass standards, you'll have to go "full monty" in PFE with fancy background corrections, APFs, MACs, etc.

Note however, that you can acquire either MAN or off-peak measurements for this multi-standard calibration curve method.  If you utilize the off-peak measurements for boron Ka, you can also take advantage of the zero point fit option as described here:

http://probesoftware.com/smf/index.php?topic=461.msg2531#msg2531

john
« Last Edit: September 29, 2015, 10:20:20 am by Probeman »
The only stupid question is the one not asked!

Mike Matthews

  • Professor
  • ****
  • Posts: 88
Re: Trace element blank correction
« Reply #12 on: September 29, 2015, 01:20:48 pm »
Hi John,

If your calibration curve reference materials are similar to your Unknown are background measurements needed at all? The carbon in steel is a good example - the bulk is essentially the same for the CC RM's and the sample with just the trace C level changing. What's going to make the background change?

Probeman

  • Emeritus
  • *****
  • Posts: 1672
  • Never sleeps...
    • John Donovan
Re: Trace element blank correction
« Reply #13 on: September 29, 2015, 03:25:31 pm »
Hi John,

If your calibration curve reference materials are similar to your Unknown are background measurements needed at all? The carbon in steel is a good example - the bulk is essentially the same for the CC RM's and the sample with just the trace C level changing. What's going to make the background change?

Hi Mike,
That is exactly right.   It shouldn't matter.

But I allow the user to utilize background corrected intensities for the multi-standard calibration curve so one can optionally include a 0,0 point for fitting trace elements.  In other words, if one is utilizing background corrected intensities for the calibration curve method, a zero concentration *should* yield a zero intensity!

But as you say, on-peak only (MAN) acquisitions should also be fine when using the multi-standard calibration curve method.
john
« Last Edit: September 30, 2015, 08:57:54 am by Probeman »
The only stupid question is the one not asked!

gmorgan@ou.edu

  • Post Doc
  • ***
  • Posts: 20
Re: Trace element blank correction
« Reply #14 on: September 30, 2015, 10:41:28 am »
Hey John,

With the calibration curve method for boron, the only thing that can be measured at one time would be boron, correct? That is, unless all elements are analyzed by calibration curve? I mean, can boron be quantified with a calibration curve while other components are quantified using off-peak intensities from single standards? I am measuring compositional gradients in crystal-glass systems, where the glass represents silicate liquid quenched at particular stages of crystal growth, to look at chemical systematics (long-range diffusion, etc.). So measuring only boron is not really an option (yes, the matrices are similar, but composition does vary with position in each experiment - and that is what we are evaluating).

Much of the problem with measuring boron is due to the internal fluorescence from the Mo-BC4 LSM device. In the old days I could minimize this by acquiring and overlaying broad range WDS scans from different standards to select the peak position and background offsets that minimize or eliminate that fluorescence in the glasses, but I haven't found a convenient way to perform near full range scans and overlay them with PFE as was easily available with my previous automation system. In the PFE documentation I only see how to perform wavescans for individual elements based on their setups in the Elements menu, and to plot them one at a time.  I presume it should be possible, and that's one thing I want to work on with Gareth during his next visit.