Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD

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XRF Calibration Curves with Synthetic Standards (2 replies)

6 years ago
(unknown) 6 years ago

According to our procedures, for a long period of time, we have been preparing synthetic calibration standards in our Lab to develop applications for XRF analysis of various matrix samples. Here the wet chemical analysis facilities are also available for backing up the XRF system in terms of parallel, partial as well as cross check analysis of materials to be required. 

Generally users install XRF system to cater analytical needs in terms of obtaining rapid, accurate and reproducible analytical results of material of interest. From collection of sample to output analytical result so many factors are involved that increase uncertainty in analytical results. The materials require to be tested is first sampled, prepared and finally analyzed using XRF Analyzer. During entire process errors are introduced that need to be estimated carefully and minimized them to a globally accepted value. Accurate and reproducible measurement of samples on XRF is an important stage of QC cycle. In addition to other factors calibration curves of elements of interest is also an important factor that is responsible for producing good analytical results.
Principally enough number of CRM should be available to calibrate the application but it is almost impossible to get all the CRM in similar matrix to cover the entire range of calibration curve. To overcome this problem the methodology of producing synthetic standard has been adopted for the last several decades.
For instance we have to develop calibration curves to cover broader range for our iron-base application. For doing so we have to define elements/oxides require to be analyzed with their ranges and also get available the appropriate iron-base CRMs and pure chemicals contain 99.99 % element/oxides of interest to cover the defined ranges. Now work out as follows;

  1. Prepare a sheet containing the certified values in ascending order of elements/oxides of available CRMs from the certificates.
  2. Also prepare a table containing calculated mass proportions of available CRMs and Pure chemicals along with their compositional values of elements/oxides. These mass portions should be calculated on the percentage basis of total mass of sample.
  3. Taking account all certified values of elements/oxides determine the target values for which synthetic standards require to be prepared. After doing this home work one can easily decide how to cover the required values of elements/oxides with the synthetic standards.
  4. As an example take calibration curve of SiO2 for iron-base application the curve range is set 0 - 30% but we have not enough CRM to cover this range. In such case Pure chemical containing 99.99 % SiO2 can be used with iron base CRM having high %age of SiO2. If dilution ratio is 1:10 it means the total weight of mass portions of CRM and additive materials should be 1 gram.
  5. From the table choose the calculated mass proportions and their corresponding values of pure chemical and CRM to be used for preparation of synthetic standard containing 30% SiO2. In such case mass portion of pure chemical is 0.19 gram and 0.81 gram is for CRM.

Example: Description Certified value of SiO2 Mass portion(gram) Calculated value of SiO2

CRM 13.69 % 0.810 11.09
Pure Chemical 99.99 % 0.190 19.00
Total 30.09

How to calculate:

( Certified value X Mass portion ) / Total weight
( 13.69 X 0.81 ) = 11.09 ( Here total weight is 1 gram so neglected )
( 99.99 X 0.19 ) = 19.00
Total = 30.09

1. Weight of sample must be accurate.
2. Chemical used for preparation of synthetic std. must be anhydrous.
3. The standards are selected in a way that they are free from line-overlaps.
4. Use only appropriate flux material with bead releasing agent. 

6 years ago
(unknown) 6 years ago

Good points. Synthetic standards when made this way are primary standards which is different than the CRM's typically used for calibration which are secondary standards and not easily traceable. This approach works best when preparing samples as fused beads. This minimizes the mineralogical effects that can cause issues when using synthetic standards for pressed pellets.

6 years ago
(unknown) 6 years ago

Valid points put across by Mr. Ali...the last note mentions that the Fused bead method had to be used...PANalytical's WROXI works in this fashion, all are synthetically prepared. In one installation, we added CRMs for Rutile / Similitude to extrapolate the calibration range, it works wonderful, other fusion machine mfrs also nowadays supports with Universal Calibration modules.

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