Blank Correction

EPMA Probe

Blank Correction

The blank correction can be used to ensure absolute accuracy when measuring trace element levels close to zero provided a suitable sample exists that is similar to the unknown matrix but contain none of the measured element or a known trace level of the measured element.

 

Simply acquire an unknown sample on the blank calibration sample, for example when measuring Ti in quartz, one could measure Ti in a synthetic quartz that contains no Ti. From the Standard Assignment dialog, the user simply selects the measured blank calibration sample (using the same conditions if possible) and sets the blank level to zero (the default) in this case and the program will automatically apply the blank correction when the analysis is performed.

 

The correction is performed in the matrix iteration by calculating the corresponding intensity based on the measured blank offset and this data is used to adjust the background correction. In this way the P/B statistics and k-ratios are always self-consistent.

 

This correction is necessary because of analyzing crystal artifacts which produce systematic errors in the measurement of peak intensities as seen in the following graphs which contain scans on both SiO2 and TiO2 at the Ti Ka peak position. Note that spectrometers 2 and 3 show “holes” in the background which will affect the accuracy of trace element analysis. The blank correction will correct for this systematic error.

 

The following analysis is of a standard synthetic quartz with a Ti concentration of zero. The blank correction has been applied to itself to demonstrate the validity of the correction by calculating the intensity offset from the measured concentrations and calculating the resulting concentrations (which should be zero). Note the fact that the P/B values are very close to unity.

 

Un    4 SiO2 synthetic

TakeOff = 40.0  KiloVolt = 20.0  Beam Current = 100.  Beam Size =   20

(Magnification (analytical) =   8000),        Beam Mode = Analog  Spot

(Magnification (default) =      600, Magnification (imaging) =    100)

Number of Data Lines:  12             Number of 'Good' Data Lines:  12

First/Last Date-Time: 05/31/2007 04:41:29 PM to 05/31/2007 06:12:21 PM

WARNING- Using Blank Trace Correction

WARNING- Using Alternating On and Off Peak Acquisition

WARNING- Using Polynomial Alpha-Factor Matrix Corrections

 

Average Total Oxygen:       53.257     Average Total Weight%:  100.000

Average Calculated Oxygen:  53.257     Average Atomic Number:   10.805

Average Excess Oxygen:        .000     Average Atomic Weight:   20.029

Average BET Iteration:        2.00     Average Quant Iterate:     3.00

 

Oxygen Calculated by Cation Stoichiometry and Included in the Matrix Correction

WARNING- Duplicate analyzed elements are present in the sample matrix!!

Use Aggregate Intensity option or Disable Quant feature for accurate matrix correction.

 

Results in Elemental Weight Percents

 

ELEM:       Ti      Ti      Ti      Ti      Ti      Si       O

TYPE:     ANAL    ANAL    ANAL    ANAL    ANAL    SPEC    CALC

BGDS:      LIN     LIN     LIN     LIN     LIN

TIME:   200.00  200.00  200.00  200.00  200.00

 

ELEM:       Ti      Ti      Ti      Ti      Ti      Si       O   SUM 

XRAY:     (ka)    (ka)    (ka)    (ka)    (ka)      ()      ()

   421  .00007  .00028 -.00001  .00074 -.00022 46.7430 53.2576 100.001

   422 -.00011 -.00009  .00010 -.00039  .00001 46.7430 53.2567 99.9992

   423  .00006  .00002  .00019  .00011  .00055 46.7430 53.2576 100.002

   424  .00033 -.00045 -.00007 -.00106 -.00013 46.7430 53.2561 99.9977

   425  .00019  .00021 -.00013  .00078 -.00004 46.7430 53.2577 100.002

   426  .00000  .00013 -.00008  .00015 -.00008 46.7430 53.2571 100.000

   427 -.00023  .00029  .00005  .00076 -.00061 46.7430 53.2572 100.000

   428 -.00034 -.00002  .00015 -.00047 -.00015 46.7430 53.2564 99.9986

   429 -.00023 -.00003 -.00001 -.00009  .00022 46.7430 53.2569 99.9998

   430 -.00010  .00002 -.00011 -.00003  .00045 46.7430 53.2572 100.000

   431  .00016 -.00007 -.00009 -.00035  .00037 46.7430 53.2570 100.000

   432  .00019 -.00029  .00002 -.00016 -.00037 46.7430 53.2566 99.9990

 

AVER:   .00000  .00000  .00000  .00000  .00000 46.7430 53.2570 100.000     <- blank corrected

SDEV:   .00020  .00022  .00011  .00056  .00034  .00000  .00049

SERR:   .00006  .00006  .00003  .00016  .00010  .00000  .00014

%RSD:     ----    ----    ----    ----    ----      .0      .0

STDS:       22      22      22      22      22       0       0

 

STBE:   3.4651  3.4651  3.4651  3.4651  3.4651   .0000   .0000

STCT:  57993.6 22162.3 69644.5  3194.7 20247.7      .0      .0

 

UNBE:   1.1935  1.1935  1.1935  1.1935  1.1935   .0000   .0000

UNCT:       .0      .0      .0      .0      .0      .0      .0             <- blank corrected

UNBG:     81.1    30.8   105.1     2.4    27.7      .0      .0

KRAW:   .00000  .00000  .00000  .00000  .00000  .00000  .00000

PKBG:  1.00002 1.00004 1.00000 1.00079 1.00007  .00000  .00000             <- blank corrected

BLNK#:       4       4       4       4       4    ----    ----             <- blank assignment

BLNKL: .000000 .000000 .000000 .000000 .000000    ----    ----             <- blank level

BLNKV: -.00043 -.00009 -.00053 .000039 .000704    ----    ----             <- blank measurement

 

The calculation is performed by calculating the intensity correction for the unknown sample from the quantitative measurement of the blank sample and subtracting that intensity from the measured intensity in the matrix iteration using the following expression:

 

 

Where:                   [ZAF]S is the ZAF correction for the primary standard

          [ZAF]U is the ZAF correction for the unknown (iterated)

          IS        is the measured standard intensity in the primary standard

          CP       is the measured concentration of the element in the blank sample

          CL       is the blank level concentration of the element in the blank sample (usually zero)

          CS       is the concentration of the element in the primary standard