Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43 424 EFFECT OF CALIBRATION SPECIMEN PREPARATION TECHNIQUES ON NARROW RANGE X-RAY FLUORESCENCE CALIBRATION ACCURACY ABSTRACT Scott H. Nettles Construction Technology Laboratories, Inc. 5420 Old Orchard Road Skokie, IL 60077- IO30 The accuracy of quantitative X-ray fluorescence analysis is dependent on a set of calibration standards. There are two approaches for producing borate glass calibration standards: (1) use certified Standard Reference Materials, (SRMs) or (2) make calibration standards from highpurity reagent grade chemicals. There are advantages and disadvantages to each method of standard fabrication. SRMs are easier to use but you must rely on the certification data and evaluate the standard deviations of the measurements. Making standards from high-purity reagent grade chemicals allows you to customize your concentration ranges to fit your specific needs, but having to weigh multiple reagents increases the chance for errors. To compare the two approaches, a set of NIST (National Institute of Standards and Technology) Portland cement standards were made and a set of calibration standards mirroring the NIST SRMs were made using high-purity reagent grade materials. The calibrations thus obtained were compared for standard error of estimate of the baseline intercepts. INTRODUCTION Construction Technology Laboratories, Inc. (CTL) has worked closely with NIST to analyze cement SRMs for certification since the 1970s. Recently, it was necessary to renew several SRMs including one ordinary Portland cement and two calcium aluminate, (CA) cements. Combinations of the two standard preparation methods were used to analyze these cements. The ordinary Portland cement was analyzed using existing NIST standards for calibration. The calcium aluminate cements were analyzed against standards made from high-purity reagents because no series of SRMs existed for calibration purposes. The question of which method was more accurate became a concern while doing this work. The main concern when using SRMs is that if there are any errors in the certification data it will cause errors in the calibration and final results of unknowns. The goal of this research was to evaluate the two methods for standard preparation to determine which produced more accurate results. SPECIMEN PREPARATION There are two methods for producing standards from high-purity reagents. (synthetic standards) One method used by Staats is to make large batches (20-30 grams) of the mixes and try to homogenize them using blending operations. (i.e. ring and puck mill, ball mill, etc.) The second method is to weigh each reagent separately for each fused disc. This eliminates homogeneity concerns, but increases potential errors associated with weighing O.lmg of sample for trace elements. The second approach of fabrication was used in this research.
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Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43 425 The standard CTL cement XFW analysis reports twelve elements (expressed as oxides): SiOz, A1203, FezOj, CaO, MgO, SOS, Na20, K20, TiOz, PzO4, MnzOa, and SrO. To create calibration curves that included all twelve of these elements the following high-purity reagents were used: SiOz, A1203, FezOa, CaCO3, CazP207, CaSO4, MgCOa, Na2S04, K2SO4, TiO2, MnO2, and SrO. All twelve reagents were ignited at 950 C and cooled in a dessicator containing MgC104 (magnesium perchlorate) before being weighted. Cements are usually fused using lithium tetraborate (LizB407) because it is relatively inexpensive and it fuses cements easily. Because some high-purity reagents are more difficult to fuse (particularly SiO2 and Al203) a mixture containing 67% lithium tetraborate and 33% lithium metaborate (LiB02) was used for fusing the mixtures. The LiB02 has better solubility for acidic elements like SiO2 and A1203. Also the temperature at which this flux melts is lower then lithium tetraborate alone decreasing the amount of loss due to volatilization during fusion. There are eight Portland cement standards available from NIST (SRMs 1880, 1881, 1884, 1885, 1886, 1887, 1888, and 1889). Eight synthetic standards were made from high-purity reagents to mirror the concentrations of the NIST standards as closely as possible. A Claisse Fluxy (3 position flame unit)(available form SPEX CertiPrep, Inc. 203 Norcross Ave. Metuchen, NJ 08840) using propane gas as fuel was used to fuse all discs. The fusion time was three minutes and the flux to sample ratio was two to one. Flux fusion temperature was approximately 950 C measured using a type S thermocouple placed directly into the fusion mass. Accurate measurements were impossible to obtain because the crucibles had to be stationary to make measurements and thus it is reasonable to assume the temperature is somewhat lower during rotation. The purity of the high-purity reagents also needed to be checked. Early experiments produced poor results for calcium. The pure calcium carbonate was certified 99.9965% pure (metals basis) by the manufacturer. XRF and wet chemical analysis of the CaC@ showed that there was 0.3% SO3 in the reagent. The manufacturer was contacted about the problem, and they told us that they only checked for metals in the sample and never checked for sulfur. It is important for anyone attempting to create a calibration curve from high-purity reagents to check the purity of the reagents. Staats investigated reagent purity in his research2, but he only checked for metal contamination in calcium carbonate. Several methods could be used but XRF scans of fused discs of the reagents proved most useful. RESULTS The two calibration curves were very similar for most elements. The standard error of estimate (SEE) was generally better for major constituents with the synthetic standard curve, but the SEE was generally better for the trace elements with the SRM calibration. (Table 1.) This is probably due to errors associated with weighing the trace elements. The SEE is a standard deviation of the absolute and relative differences and the smaller it is the better the fit of the results.
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43 426 Table 1 Comparisons of Standard Error of Estimate Analyte Al Ca Fe K Mg Mn Na P S Si Sr Ti SRM Curve SEE Synthetic Curve SEE 0.0579 0.0559 0.3666 0.2982 0.0478 0.0414 0.0085 0.0113 0.0579 0.0207 0.0036 0.0125 0.0111 0.0075 0.0106 0.005 0.033 0.0343 0.2042 0.1116 0.0041 0.0058 0.0085 0.0149 The biggest difference between calibration curves was with phosphorus. It was the only element where the slopes of the calibration curves appeared to be different. (Figure 1) The slope of the curve was 2.3679 for the synthetic curve and 3.2171 for the SFM curve. Work in our lab has raised question about the accuracy of the SRh4 phosphorus values, but because the concentration range is so small any errors would have a small effect on values obtained from using them to calibrate. Phosphorus in low concentrations also poses little if any problem to the overall cement chemistry. Some have suggested the use of interelement corrections be applied for the element however it seems unlikely given the information supplied here. The calibration curves for each of the other eleven elements had almost identical slopes for each element with differences in SEE. (See for example Silicon and Sulfur, Figures 2 and 3) o 8 Figure 1. Concentration I vs. Intensity for Phosphorus f 3 0.6 y E 0.5 V.= h 0.4 to $ 0.3 I 0.2 Of 0 0.05 0.1 0.15 0.2 0.25 0.3 Concentration l Synthetic SRM Linear (Synthetic) -!!! - Linear (SRM)
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43 427 Figure 2. Concentration vs. Intensity for Silicon 4.3 4.2 4.1 4 3.9 3.8 3.7 3.6 3.5 19.7 20.2 20.7 21.2 21.7 22.2 22.7 23.2 Concentration l Synthetic m SRM Linear (Synthetic) - - - Linear (SRM) Figure 3. Concentration vs. Intensity for Sulfur 11 G f Y -8 >r.= 10 i----: 7 0 0 0 z ii 6 5 4 1.5 2 2.5 4 4.5 5 l Synthetic q SRM Linear (Synthetic) - - - Linear (SRM) 1 The eight SRM cement standards were analyzed using the calibration created with synthetic standards. The results met ASTM C-l 14 accuracy requirements3 for ten of the twelve analytes.
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43 428 Silicon and potassium did not meet C-l 14 accuracy requirements, but the calibration did not take reagent impurity into account. Potassium is the most volatile and we may not be able to use this one in the fabrication process unless we are confident that the losses are insignificant to the accuracy of the calibration. Several questions remain to be answered before the synthetic standard method can be employed to analyze cement samples with accuracy comparable to using SRMs for calibration. The loss on fusion needs to be determined, to make sure that none of the alkalies or sulfur is lost during the standard fusion. The entire synthetic standard fabrication needs to be repeated to make sure the results are reproducible. The biggest problems with fabricating synthetic standards are errors associated with weighing the trace elements. The research indicates that once these problems are solved the synthetic standard method will generate acceptable and possibly more accurate calibration sets than using SRMs. Most schooled professionals agree that the same set of standards should not be used for calibration and verification of the calibration. Since this would mean a substantially greater number of SRMs be fabricated for calibration (16 vs. 8 now available for ordinary Portland cements) this technique can surely be beneficial to any analyst. Certified SRMs, however can only be utilized for pressed powder techniques and for qualification under ASTM Cl 14 and is traditionally the best alternative to the labor-intensive production of using reagent grade materials for standards. REFERENCES r Staats, Gotthard; Synthetic macro-reference samples for the calibration of instruments in inorganic bulk analysis, Fresenius Z Anal Chem. 1989,334; 326-330. 2 Staats, Gotthard; On the role of pure oxide materials for primary calibration and validation in inorganic bulk analysis. Fresenius ZAnaZ Chem.1990,336; 132-135. 3 Standard test methods for Chemical Analysis of Hydraulic Cement. Annual Book of ASTM Standards, Vol. 04.01. C 114-97a.