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Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes
Journal Pre-proof Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes
Zuzanna Kowalkiewicz, Włodzimierz Urbaniak PII:
S0584-8547(19)30222-8
DOI:
https://doi.org/10.1016/j.sab.2019.105736
Reference:
SAB 105736
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date:
9 May 2019
Revised date:
19 November 2019
Accepted date:
19 November 2019
Please cite this article as: Z. Kowalkiewicz and W. Urbaniak, Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes, Spectrochimica Acta Part B: Atomic Spectroscopy(2018), https://doi.org/10.1016/j.sab.2019.105736
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© 2018 Published by Elsevier.
Journal Pre-proof Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes
Zuzanna Kowalkiewicz1# , Włodzimierz Urbaniak1 1
Adam Mickiewicz University in Poznań, ul. Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland #
corresponding author [email protected]
Abstract
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Total reflection X-ray fluorescence spectrometry (TXRF) is known as a state-of-theart and fast-growing analytical technique. The spectral range of TXRF spectrometry encompasses elements from sodium to uranium. The object of this study was to develop, optimize, and validate the analytical method for fluorine in a form of calcium fluoride determination in fluoride fluxes by total reflection X-ray fluorescence spectrometry. The sample was lixiviated in acetic acid, then, the sample was filtered and the sediment and the filter were ignited at 650 ºC. A portion of dried and ignited and weighed sample was placed in a plastic centrifuge tube. To this tube Triton X-100, Ga as internal standard and polyvinyl alcohol were added. The accuracy of the element determinations was estimated by comparison between obtained and certified values of the elements determined in the Certified Reference Materials of fluorite (fluorspar). The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 . The variation coefficient that characterized the reproducibility of the measurement equals 16.94%. In addition to this, it was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. The results of this work demonstrate the acceptable and fully satisfactory validation criteria such as limit of detection and quantification, accuracy, and precision.
keywords: total reflection X-ray fluorescence spectrometry; TXRF; fluorite, fluorine
1. Introduction Versatile analytical techniques have been developed to determine fluorine concentration in solid samples. Potentiometry (a fluoride ion-selective electrode (FISE)), ultraviolet–visible spectrometry (UV-VIS), ion-exchange chromatography and neutron activation analysis are the most popular techniques dedicated to quantitative analyses of fluorine. In practice, determination of fluorine in the form of inorganic salts which are 1
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practically insoluble in water (e.g. CaF 2 ) in solid samples poses a difficult analytical challenge. Fluoride fluxes are used in the metallurgical and ceramic industry. In the metallurgical industry the fluxes are used to weld slag and smelt steel. In ceramic industry the fluxes fulfil two crucial functions. Firstly, fluoride fluxes have ability to remove moistness from ceramic products that influences porosity, plasticity, and shrinkage of these products. Secondly, fluoride fluxes lower smelting temperature that leads to a reduction of production’s costs. The composition of fluoride fluxes varies substantially in the amount of CaF 2 (40% – 80% (m/m)) and other components such as silica, inorganic salts (e.g. CaCO 3 ), and metal oxides (e.g. MgO, Al2 O3 ). A slight change in the flux composition causes significant changes in the flux properties, therefore it is crucial to confirm the properties of the fluoride fluxes before their industrial application [1]. Determination of fluorine in solid samples, in particular in fluoride fluxes, causes a number of additional analytical difficulties caused by the presence of interferents such as cations (Ca2+, Mg2+, Ba2+, Al3+, Fe2+, Fe3+) and anions (sulphates, phosphates). These interferents hamper the determination with the use of FISE, UV-VIS and AAS spectrometry. There are a few methods of fluoride determination in fluoride fluxes such as the Berzelius method, the Foote method, Yeager method [2, 3], the Polish standard no. PN-70 H04132 [4] and Zheng & Jian-Ping [4] method. Those methods employ indirect fluorine determination in the fluxes, where the concentration of calcium fluoride in fluxes is calculated based on a constant molar ratio of calcium and fluorine in calcium fluoride. The Berzelius method, initially dedicated to determining fluoride in rocks, involves fusing a sample with basic carbonates, removing silicon and aluminium by precipitation with ammonium and zinc carbonate. Then chromates and phosphates are precipitated with a silver(I) nitrate solution. The fluoride content is calculated based on the mass of the precipitated CaF2 sediment [2,3]. Whereas the Foote method encompasses pyrohydrolysis of a sample with NaOH and HCl and indirect calcium titration of the obtained solution. In contrast, in the work of Yeager and his colleagues [3], fluoride fluxes were fused with basic fluxes (sodium carbonate/sodium tetraborate) and then digested with nitric acid(V). Fluoride content was measured using FISE at the pH range 8.0 – 9.0. However, according to procedure by the Polish standard, the sample is leached with acetic acid to remove all calcium salts except calcium fluoride. Then, the sample is filtered and the precipitate is dissolved in boric and hydrochloric acid to remove fluorine and isolate insoluble silicic acid. The final determination is complexometric titration of calcium in the filtrate using disodium edetate. A completely different analytical approach was followed by the Zheng and Jian-Ping method [5]. According to the analytical procedure, the sample is fused with Li2 B4 O 7 . The calcium content in the sample is then determined by X-ray fluorescence and the carbon content as calcium carbonate by infrared spectroscopy. The total calcium content of the sample is obtained based on measurements made with XRF spectroscopy. The carbon content 2
Journal Pre-proof of the sample allows to calculate the calcium content found as calcium carbonate (assuming that CaCO 3 is the only carbon source in the sample). Based on the difference between the total calcium content of the sample (obtained by XRF analysis) and the calcium content present as calcium carbonate, the calcium content of the sample derived solely from CaF 2 can be calculated. However, the results of this work cannot be directly translated into the possibility of determining the fluoride content in the form of calcium fluoride in fluoride fluxes, because the authors of this work conducted tests on samples containing only CaCO 3 and CaF2 . In addition, the method proposed by Zheng and Jian-Ping [5] requires the use of two analytical methods for the determination of one sample, which causes an increase in the cost of performing the determinations.
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These above-discussed methods employ the decomposition of samples prior to their final analysis that poses the risk of analyte loss and sample contamination. This approach leads to changes in the sample composition as well as unsatisfactory accuracy and precision of measurements [3]. Additionally, the main drawback of the above-mentioned methods is the time-consuming and labour-consuming process of sample pre-treatment. Besides, the Yeager’s method is limited to measuring samples containing less than 63% (m/m) of calcium fluoride in a sample because of the difficulty of selecting a masking agent for samples having a higher content of CaF2 that results in the instability of the working apparatus. Owing to the variability of sample matrices, it is impossible to select a universal masking agent.
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Total reflection X-ray fluorescence (TXRF) spectrometry is known as a state-of-the-art and fast-growing analytical technique [5]. TXRF spectrometry works on the principle of emission of characteristic fluorescence radiation afterwards the radiation of sample elements. The TXRF spectrometry is used to versatile types of environmental and industrial samples due to (1) minute sample amounts, (2) no matrix effects, (3) simplified sample preparation, (4) low detection limit, and (5) the capability of simultaneous multielement determination [7]. The spectral range of TXRF spectrometry encompasses elements from sodium to uranium. To the best of our knowledge, commercially available TXRF spectrometers have not been used to determine fluorine yet, because the element is out of its spectral range. This research is also a first attempt to develop an indirect method of fluorine determination by TXRF spectroscopy. The object of this study was to (1) develop and optimize the analytical method for fluorine in a form of calcium fluoride determination in fluoride fluxes by total reflection X-ray fluorescence spectrometry, and (2) validate this method.
2. Materials and methods 2.1 Reagents and materials 3
Journal Pre-proof Method validation and optimisation were carried out against Certified Reference Material (CRM) (fluorite no. GBW 07253 (fluorspar) purchased from Brammer Standard Company, Inc) Reference Material (RM) (two synthetic fluorite fluxes: U1 and U2) and calcium stock solution (ICP standard, 1 g Ca ∙ ml-1 , matrix: 2 – 3% HNO 3 , Merck Millipore®). High purity water obtained from Chemsolve® was used for dilution of solutions and samples. The commercial non-ionic surfactant Triton X-100® (Sigma Aldrich®) was used to form sample suspension. For sample cleaning, the procedure elaborated by E. K. Towett and his coworkers [6] was applied. All analyses were performed in triplicate.
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2.2 Sample preparation method A 1.00 g portion of flux sample was weighed on an analytical balance and placed in a 50 ml flat bottom plastic centrifuge tube. The sample was lixiviated in 30 ml 1.74 M acetic acid for 60 minutes. Then, the sample was filtered and the sediment and the filter were ignited at 650 ºC. A 20.00 mg portion of dried and ignited sample was weighed on an analytical balance and placed in a 15 ml plastic centrifuge tube. The 5 ml of 1% Triton X-100, 1 µg · l-1 of Ga as internal standard, and 20 µl 0.3% polyvinyl alcohol (PVA) acting as a film former were added to the sample. Later, the sample was homogenized by a vortex for minimum 60 seconds. In the end of homogenization, the speed of homogenization was lowered and the 5 µl aliquot was pipetted on a sapphire sample carrier. The sample carrier was dried by a heat plate and loaded to the apparatus. The measurement was performed for 200 s (live time).
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2.3 Instrumentation The S2 PICOFOX spectrometer (Bruker AXS) was used in this research. The apparatus is equipped with an air-cooled molybdenum X-ray tube (50 W, 1 mA), a Peltiercooled Xflash® silicon drift detector with 10 mm2 active area, and a multilayer monochromator (17.5 keV). The analysis of the X-ray spectra was performed by Spectra 7.0 Bruker Nano GmbH. In order to determine both calcium and silicon, sapphire sample carriers with a diameter of 30 mm and a thickness of 3 mm were used to hold samples during measurements. 2.4 Calculation The concentration of an element is calculated according to the user manual [7]. Statistical differences between data were verified on the basis of a t-Studying tests, U MannWhitney test, and F-Snedecor test. The correlation between variabilities was performed by a Person correlation. The detection limit (DL) was computed according to 3 σ criterion (Equation 1): DL=
√
(1)
4
Journal Pre-proof where Ci is concentration of the element, N is an area of the fluorescence peak in counts, and Nbg is background area subjacent the fluorescence peak. The limit of quantification (LOQ) was taken as triple the previously determined detection limit. The repeatability of the analytical procedure was determined as the standard deviation of six independent measurements of the samples at the concentration level corresponding to the analyte concentration in real samples. The samples constitute six different samples of 1.00 g CRM prepared independently.
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The value of coefficient of variability was also calculated and the repeatability limit (r) was determined as 2.8 times of a standard deviation of series of measurements.
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3. Results
Particle size
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3.1 Procedure optimisation
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Sample’s particle size is a key factor to meet the assumptions of TXRF spectrometry. Sample’s particle size must not be larger than 20 µm [5]. Fluoride flux’s samples and CRM were ground before the analysis with an agate mortar. The histogram of sample’s particle shows that particles size is from 0 µm to 10 µm (Fig. 1). In addition to this, 99% of all particles was smaller than 10 µm and is in agreement with the theoretical thickness imposed by Klockenkämper and Von Bohlen [7].
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3.1.2 Sample volume and measurement time The measurement time is an important parameter to be tested. 20 mg of calcium fluoride standard was prepared according to the procedure described in 2.2 and it was measured for 100 s, 200 s, 300 s, 400 s, 500 s, 600 s, 700 s, 800 s, and 900 s. The ratio of analyte to internal standard area for samples measured for 200 – 900 s slightly varies between 0.559 and 0.575. The optimal measurement time was chosen 200 seconds, because this is the highest amount of this ratio whereas the measurement time is the lowest. The volume of sample suspension can also significantly influence TXRF analyses. The aliquots of 5 µl and 10 µl (5 µl pipetted and dried twice) of the samples were deposited onto sample carriers. The quality of the analysis can be improved, for instance, by using smaller sample volumes in order to avoid movement of the droplet on the carrier [10]. On the basis of the highest ratio of analyte to internal standard area, the optimal volume of sample pipetted on a sample carrier is 10 µl. 3.1.3
Sonication effect
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Sonification can improve the sample homogeneity. Usually, samples are homogenized by a vortex mixer before analyses. A few papers demonstrated that sonification can be also a useful tool to increase samples homogeneity in the case of soil samples [8]. Therefore, the influence of sonification on the sample homogeneity was tested. The influence was verified as the difference between results precision (expressed as standard deviation) of non-homogenized and the homogenized real sample and the certified reference material. The internal standard was added to homogenized samples before homogenization. Then, the obtained values of standard deviations were compared for each set of results using the FSnedecor test. The null hypothesis was the statement that standard deviations of results obtained by different methods of sample preparation do not differ in a statistically significant way, while an alternative hypothesis was the statement that standard deviations of results obtained by different sample preparation methods are statistically significant. The p value was below the confidence interval (F< Fcritic), therefore the null hypothesis has failed to be rejected. It denotes that homogenization does not have statistically significant influence on the analysis precision. Moreover, the difference between particle size of homogenized and non-homogenized samples was tested. Sample aliquots submitted to sonification were taken at a middle depth in a vial. It was demonstrated that there is a statistically significant difference between particle size of a pair of homogenized and non-homogenized sample and a pair of homogenized and non-homogenized certificated reference material (Fig. 2). It means that homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
Linearity, range and detection and quantification limit
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3.2 Procedure evaluation
The linearity was determined using a calibration curve based on measurements of analyte concentrate from 0.15 mg to 0.40 mg that corresponds to 29% – 78% CaF2 (0.14 mg – 0.38 mg F) in a fluorine flux sample (Fig. 3) according to the approach presented at Borgese and her co-workers [12] (exemplary tests results: Shapiro-Wilk test: p=0.27; Dixion test: Q 1 =0.14, Q 3 =0.07, Q crit =0.53; Hartley test: Fmax = 1.47, Fcrit =2.46; the lack of fit for linear and quadratic regressions: F= 3.18; Fcrit =3.2). The linearity coefficient (R) is ≥ 0.9951 for fluorine and the relationship between the content of the analyte and the detector signal is described by the equation y = 5.1847x – 0.0255. The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 .
3.2.2
Repeatability and accuracy
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The repeatability of the analytical procedure equals 0.6175 g · F g-1 and the coefficient of variation is 16.94%. In order to explain the reason for the not entirely satisfactory repeatability of the analytical procedure, the effect of matrix on the TXRF results was assessed. For this reason, 125 μg of selenium in solution (in the form of a Se (NO 3 )4 ) was added to samples containing calcium in CaF2 form to assess how repeatability will change if the analyte and internal standard are liquid. The repeatability of the analytical procedure was one order of magnitude lower and equals 0.05 g · Se g-1 and the coefficient of variation is 0.01%. It was noticed that a lower precision of results was observed when the analyte was a solid and the internal standard was a liquid and the opposite effect – the higher precision of results was noted when the analyle and internal standard were liquid.
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The results’ differences could be caused by two factors. Firstly, the sample suspension could be inhomogeneous therefore sample pipetting may lead to the unrepresentativeness of sample. Secondly, the sample aliquot deposited on the disc may have inhomogeneous distribution that also reduces the precision of the analytical procedure [13].
2 √u2(x i
u2(x
(2)
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|xi – xref |
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The accuracy of the analytical procedure was determined by the certified reference material. The value of the discrepancy between the CRM and the TXRF results equals 0.01 g ·g-1 and there are no statistically significant differences in these results. After determining the measurement uncertainty, the accuracy of the analytical procedure was again checked using the following inequality (Equation 2) [14].
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where: xi is an analysis result (g ∙ g-1 ), xref – an analysis of CRM (g ∙ g-1 ), u( x i is an uncertainty of the analysis result (g ∙ g-1 ), u(x 1
is an uncertainty of the reference result (g ∙ g-
). When the above inequality is fulfilled, then the obtained results are in agreement with the reference result. Table 1 shows the obtained results are in accordance with the reference result.
3.2.3 Uncertainty There is some scientific research on the uncertainty of the TXRF results and its determining factors [15,16]. The total uncertainty consists of two components: the variance, characterizing the reproducibility of the measurement of an analytical signal from one sample, and the variance, characterizing the stability of the sample preparation conditions from the same specimen, including the addition of the internal standard [15]. The measurement uncertainty was presented as a result ± an expanded uncertainty (U). The expanded uncertainty was calculated on the basis of Equation 3. 7
Journal Pre-proof U = k uc(y (3) where U is the expanded uncertainty, k is a covered factor (k = 2), uc(y is the combined standard uncertainty. The below-presented mathematical model of measurement procedure is dedicated the fluorine determination in a form of CaF 2 according to the procedure (Equation 4). CF(CaF2 =
i SIS IS
m IS 2 MF
Si m MCa
(4)
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where CF(Ca F2 is the content of fluorine in a form of CaF2 (g ∙ g-1 ), N i is the area of analyte peak, SIS is the detector sensitivity for the internal standard, mIS is the mass of internal standard (g), MF is the molar mass of fluorine (g · mol-1 ), NIS is the area of internal standard peak, Si is the detector sensitivity for the analyte, m is the sample mass (g), MCa is the molar mass of calcium (g · mol-1 ).
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Combined uncertainties were calculated by propagation individual uncertainty components on the basis of The Guide for Uncertainty in Measurements with the use of Statistica Sofware® [14]. The combined uncertainty was obtained by the below Equation.
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u( CF(Ca F
2
CF(Ca F2 u i i
)
2
(
uSIS SIS
)
2
(
umIS m IS
)
2
(
uMF MF
)
u IS
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√(
2
(
IS
)
2
u
( Si ) Si
= 2
u
( m) m
2
(
uMCa MCa
)
2
u2(P
(5)
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The relative contributions of the Equation 6 are presented in the Table 2. Substituting the values from the Table 2 into Formula (7), the result and its expanded uncertainty equal to:
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CF(Ca F2 ± U = 0.426 ± 0.012 g · g-1
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The parameters contributing to the measurement of uncertainty are embedded in the Ishikawa diagram (Fig. 4). The dominant factor affecting the measurement uncertainty is the area of the analyte peak (75%). The area of the internal standard peak has a three-fold smaller influence on the uncertainty of measurement. Other parameters do not influence the measurement uncertainty. Those results are in agreement with Floor and his team’s results [16]. In their work, their attention was drawn to the fact that it is impossible to easily determine the factors affecting the reproducibility of the spatial distribution of sample onto a disk using the theoretical model. At the same time, this parameter has the greatest impact on the uncertainty of measurement results. In the studies of Cherkashin and his co-workers [15] on the determination of Rb, Sr, Cs, Ba and Pb in feldspars, it was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. Identical observations were recorded in the results of this work, where the influence of only two factors affecting the uncertainty of the measurement results was indicated the area of analyte peak (75%) and area of internal standard peak (25%)). 8
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3.3 Comparison of other analytical procedures
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There is little scientific research on the determination of fluorine in geological samples and fluoride fluxes. Due to the lack of data, the validation parameters were compared with the values of these parameters determined by other analytical procedures for the determination of fluorine in geological samples and fluoride fluxes (Table 3). It is not possible to fully compare the validation parameters, because the authors of the other methods provide only two validation parameters – a limit of detection and precision. The detection limit for the validated analytical procedure is the lowest among the other fluorine determination methods presented in Table 3. Instead of the fact that the validated method does not characterize the lowest precision, its value is satisfactory comparing it to the values obtained in TXRF measurement of light element. In the case of a total time of sample analysis, it can be concluded that the determination takes less time than the titration and takes longer than analysis using potentiometry or XRF technique.
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3.4 Application of the procedure The application of the validated procedure was the determination of fluorine content in the form of calcium fluoride in two fluoride fluxes obtained as a result of an unpublished work (U1 and U2). The samples were prepared for analysis in accordance with the procedure described in 2.2. The composition of fluoride fluxes is embedded in Table 4. The content of calcium fluoride in the U1 and U2 flux were 58.39% ± 1.64% and 46.25% ± 0.01%, respectively. Additionally, determined the content of calcium fluoride in U1 flux was determined by an independent laboratory (owning Cemex’s company) with the use of XRD technique and equals 59.8% ± 3.00% (the value of measurement uncertainty was estimated on the basis the date from the laboratory). Those results show a good agreement for elements determined. The values of the discrepancies between the TXRF and the XRD results are less than the permissible standard deviation σr.
4 Conclusion In this work, the new method has been devised for indirect fluorine in a form of calcium fluoride determination in fluoride fluxes by TXRF spectrometry. It was shown that the preparation of the suspensions with the Triton X-100 solution and the fluoride flux after lixiviation with acetic acid is a simple fast procedure that is a simple, rapid and reliable tool for analysing fluoride fluxes, especially in cases when analyses have to be carried out in continuous production conditions. The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 . The variation coefficient that characterized the 9
Journal Pre-proof reproducibility of the measurement equals 16.94%. The lower precision of results can be affected by the fact that the internal standard and the analyte were liquid and solid, respectively. The method shows a good accuracy that was proven with CRM and the independent laboratory. There are no statistically significant differences between the results measured in the CRM and results of sample measured by the new method and the independent laboratory. The value of the discrepancy between the CRM and the TXRF results equals 0.01 g · g-1 .
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Moreover, in this study, it was demonstrated that there is a statistically significant difference between particle size of a pair of homogenized and non-homogenized sample and a pair of homogenized and non-homogenized certificated reference material. It means that homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
Acknowledgements
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It was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. The dominant factor affecting the measurement uncertainty is the area of the analyte peak (75%). The area of the internal standard peak has a three-fold smaller influence on the uncertainty of measurement. Other parameters do not influence the measurement uncertainty.
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The work was financially supported by the project no. UDA-POIG.01.04.00-30012/10-00 founded under the Operational Programme Innovative Economy, Measure 1.4 and the project no. 309/2015/Wn15/MN-XN-03/D funded under ЕЕA Financial Mechanism 2009–2014.
6 References 1. Wroblewski K., Fields J., Fraley J., Werner R., Rudoler S., 2009. Quick determination of total fluoride in electroslag refining fluxes. International Symposium on Liquid Metal Processing and Casting 2009. 303–307. 2. Jeffery P., 1981. Chemical Methods of Rock Analysis: Fluorine. International series of monographs in analytical chemistry. Pergamon Series in Analytical Chemistry, 36, 165– 174. DOI: 0.1016/B978-0-08-023806-7.50025-7. 3. Yeager J., Miller M., Ramanujachary K., 2006. Determination of total fluoride content in electroslag refining fluxes using a fluoride ion-selective electrode. Ind. Eng. Chem. Res., 45, 4525–4529. DOI: 10.1021/ie060128a. 4. Polish Standard no. PN-61 H11105. Metallurgic fluorite, 1961. 5. Zheng S., Jian-ping M., 2008. Determination of calcium fluoride in fluorite with X-ray fluorescence spectrometry and infrared absorption. Metall Anal. 28, 8, 73–75. 10
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6. West M., Ellis E., Potts P., Streli C., Vanhoofe C, Wobrauschek P., 2014. Atomic Spectrometry Update – a review of advances in X-ray fluorescence spectrometry. J. Anal. At. Spectrom., 2014, 29, 1516. DOI: 10.1039/c4ja90038c. 7. Bruker Nano GmbH, 2007. S2 PICOFOX user manual. Berlin: Bruker AXS Microanalysis GmbH. 8. Towett E., Shepherd K., Cadisch G., 2013. Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF). Sci. Total Environ., 463–464, 374–338. DOI: http://dx.doi.org/10.1016/j.scitotenv.2013.05.068. 9. Klockenkämper R., von Bohlen A., 1989. Determination of the critical thickness and the sensitivity for thin-film analysis by total reflection X-ray fluorescence spectrometry. Spectrochim. Acta B, 44, 5, 1989, Pages 461-469. DOI: 10.1016/05848547(89)80051- 5. 10. Riano S., Regad M., Binnemans K., Hoogerstraete T., 2016. Practical guidelines for best practice on Total Reflection X-ray Fluorescence spectroscopy: Analysis of aqueous solutions. Spectrochim. Acta Part B. 124, 109-115. DOI: 10.1016/j.sab.2016.09.00. 11. Álvarez-Vázquez M., Bendicho C., Prego R., 2014. Ultrasonic slurry sampling combined with total reflection X- ray spectrometry for multi-elemental analysis of coastal sediments in a ria system. Microchem. J., 112, 172–180. DOI: http://dx.doi.org/10.1016/j.microc.2013.09.026. 12. Borgese L., Dalipi R., Riboldi A., Bilo F., Zacco A., Federici S., Bettinelli M., Bontempi E., Depero L., 2018, Comprehensive approach to the validation of the standard method for total reflection X-ray fluorescence analysis of water. Talanta, 1;181:165-171. DOI: 10.1016/j.talanta.2017.12.087. 13. Stosnach H., 2005. Environmental trace-element analysis using a benchtop total reflection X-ray fluorescence spectrometer. Anal Sci. 2005 Jul;21(7):873-6. DOI: 10.1154/1.1913723. 14. EURACHEM: Quantifying Uncertainty in Analytical Measurement. 3rd edition, 2012. https://www.eurachem.org/images/stories/Guides/pdf/QUAM2012_P1.pdf 15. Cherkashina T., Panteev S., Finkelshtein A., Makagon V., 2013. Determination of Rb, Sr, Cs, Ba, and Pb in K-feldspars in small sample amounts by total reflection X-ray fluorescence. X-Ray Spectrom., 42, 207–212. DOI: 10.1002/xrs.2469. 16. Floor G., Queralt I., Hidalgo M., Marguí E., 2015. Measurement uncertainty in Total Reflection X- ray Fluorescence. Spectrochim. Acta Part B. 111, 30–37. DOI: 10.1016/j.sab.2015.06.015.
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Figure 1. The histogram of fluoride fluxes particle size and CRM ground with an agate mortar
Figure 2. The difference between standard deviation of homogenised and non-homogenised samples and certified reference material (U Mann-Whitney test p < 0,00)
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Figure 3. The linearity between the detector signal and analyte’s content in a calibration curve
Figure 4. Ishikawa diagram for the TXRF measurement procedure
Table 1. Accuracy assessed with regard of results uncertainty with the use of inequality (4) (the detailed description of symbols was presented in the Equation 2)
g ∙ g -1 ) 0.006
|
g ∙ g -1 ) 0.10
| 0.10
13
√ 0.20
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Table 2. The parameters contributing to the measurement of uncertainty influencing combined uncertainties The value of parameter x
Combined standard uncertainty u(x)
Type of uncer tainty
Type of distribution
4.95 · 105
6.00 · 103
A
t-Student
8.53 · 105
6.01 · 103
A
t-Student
1.08 · 10-1
1.28 · 10-6
A
t-Student
0.93 · 10-1
1.61 · 10-4
A
t-Student
Cp (g)
5.00 · 10-8
B
triangular
Pp (g)
1.00 · 10-6
A
t-Student
1.25 · 10-1
2.89 · 10-8 1.00 ∙ 10-5
B
Pw (mg)
1.00 ∙ 10-5
A
R (mg)
1.00 ∙ 10-5
B
P (g · g-1)
Analytical method and analytical technique ion-selective potentiometry spectrometry XRF titration (Foote’s method titration (the Polish standard method P − 1 H−111 TXRF spectrometry (the validated method)
5.00 · 10-8
rectangular
B
4.00 · 10-3
40.08
3.27 · 10-2
Sample pretreatment
rectangular
B
rectangular
A
t-Student
Limit of determination (mg F)
mg
S u(SI IS
)2
)2 SIS uCp ( ) √ u2 (Pw u(mIS 2 ( ) mIS uSe ( ) √3 u2 (Pw) u ( ) √3 u(m 2 ( ) m u( M F √3 ( )2 MF
f
t-Student
Pr
18.99
al
(g · mol -1)
1.41 · 10-10
rn
(g · mol -1)
2.05 ∙ 10-2
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m (g)
rectangular
e-
S e (mg)
IS u(Si
(
oo
(g)
Combined standard uncertainty u( i ( )2 I u( IS ( )2 (
pr
Parameter
∙ kg -1 d w)
u (MCa √3 ) 2 ( MCa u2
The value of combined standard uncertainty 1.47 · 10-4 4.96 · 10-5 1.40 · 10-10 3.02 · 10-10 2.89 · 10-8 1.00 · 10-12 5.33 · 10-14 5.77 · 10-6 1.00 · 10-10 5.77 · 10-6 4.76 · 10-17 2.31 · 10-18
3.32 · 10-9 1.07 · 10-3
Precision expressed as standard deviation (% )
Time of analysis (h)
Reference
fuse with NaOH
1.5 ∙ 10-3
3
2–3
≈4
Yeager et al. 2006 An et al. 2012
pelletising
1.62
812
no data
≈ 0.25
pyrohydrolysis with NaOH and HCl
no data
no data
2–3
< 10
Yeager et al. 2006
no data
no data
no data
≈3
PN-70 H04132 1970
1.04 ∙ 10-3
4.2 ∙ 10-3
6.5
≈8
this work
lixiviation CH3 COOH
Table 3. The comparison among methods of fluorine determination in geological samples and fluoride fluxes
14
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Table 4. The composition of fluoride fluxes
Substrates
Ingredients (% ) SiO2
CaF2
others
36
K, Fe, S < 0.1
32
Mg, K, Fe, S < 0.1
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technical calcium carbonate (U1) 58.39 ± 1.64 26% H2 SiF6 distiller waste* (U2) 46.25 ± 0.01 26% H2 SiF6 * a by-product of the Solvay process
15
Journal Pre-proof Zuzanna Kowalkiewicz: Conceptualization, Validation, Methodology, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing, Project administration, Visualization U b n k:
Conceptualization,
Supervision,
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z
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16
Resources,
Funding
Journal Pre-proof A methodology for determination of fluorine in a form of calcium fluoride in fluoride fluxes is proposed.
Homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
The uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry.
The acceptable and fully satisfactory validation criteria such as limit of detection and quantification, accuracy, and precision.
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17
Zuzanna Kowalkiewicz, Włodzimierz Urbaniak PII:
S0584-8547(19)30222-8
DOI:
https://doi.org/10.1016/j.sab.2019.105736
Reference:
SAB 105736
To appear in:
Spectrochimica Acta Part B: Atomic Spectroscopy
Received date:
9 May 2019
Revised date:
19 November 2019
Accepted date:
19 November 2019
Please cite this article as: Z. Kowalkiewicz and W. Urbaniak, Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes, Spectrochimica Acta Part B: Atomic Spectroscopy(2018), https://doi.org/10.1016/j.sab.2019.105736
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2018 Published by Elsevier.
Journal Pre-proof Determination of fluorine by total reflection X-ray fluorescence in fluoride fluxes
Zuzanna Kowalkiewicz1# , Włodzimierz Urbaniak1 1
Adam Mickiewicz University in Poznań, ul. Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland #
corresponding author [email protected]
Abstract
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Total reflection X-ray fluorescence spectrometry (TXRF) is known as a state-of-theart and fast-growing analytical technique. The spectral range of TXRF spectrometry encompasses elements from sodium to uranium. The object of this study was to develop, optimize, and validate the analytical method for fluorine in a form of calcium fluoride determination in fluoride fluxes by total reflection X-ray fluorescence spectrometry. The sample was lixiviated in acetic acid, then, the sample was filtered and the sediment and the filter were ignited at 650 ºC. A portion of dried and ignited and weighed sample was placed in a plastic centrifuge tube. To this tube Triton X-100, Ga as internal standard and polyvinyl alcohol were added. The accuracy of the element determinations was estimated by comparison between obtained and certified values of the elements determined in the Certified Reference Materials of fluorite (fluorspar). The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 . The variation coefficient that characterized the reproducibility of the measurement equals 16.94%. In addition to this, it was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. The results of this work demonstrate the acceptable and fully satisfactory validation criteria such as limit of detection and quantification, accuracy, and precision.
keywords: total reflection X-ray fluorescence spectrometry; TXRF; fluorite, fluorine
1. Introduction Versatile analytical techniques have been developed to determine fluorine concentration in solid samples. Potentiometry (a fluoride ion-selective electrode (FISE)), ultraviolet–visible spectrometry (UV-VIS), ion-exchange chromatography and neutron activation analysis are the most popular techniques dedicated to quantitative analyses of fluorine. In practice, determination of fluorine in the form of inorganic salts which are 1
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practically insoluble in water (e.g. CaF 2 ) in solid samples poses a difficult analytical challenge. Fluoride fluxes are used in the metallurgical and ceramic industry. In the metallurgical industry the fluxes are used to weld slag and smelt steel. In ceramic industry the fluxes fulfil two crucial functions. Firstly, fluoride fluxes have ability to remove moistness from ceramic products that influences porosity, plasticity, and shrinkage of these products. Secondly, fluoride fluxes lower smelting temperature that leads to a reduction of production’s costs. The composition of fluoride fluxes varies substantially in the amount of CaF 2 (40% – 80% (m/m)) and other components such as silica, inorganic salts (e.g. CaCO 3 ), and metal oxides (e.g. MgO, Al2 O3 ). A slight change in the flux composition causes significant changes in the flux properties, therefore it is crucial to confirm the properties of the fluoride fluxes before their industrial application [1]. Determination of fluorine in solid samples, in particular in fluoride fluxes, causes a number of additional analytical difficulties caused by the presence of interferents such as cations (Ca2+, Mg2+, Ba2+, Al3+, Fe2+, Fe3+) and anions (sulphates, phosphates). These interferents hamper the determination with the use of FISE, UV-VIS and AAS spectrometry. There are a few methods of fluoride determination in fluoride fluxes such as the Berzelius method, the Foote method, Yeager method [2, 3], the Polish standard no. PN-70 H04132 [4] and Zheng & Jian-Ping [4] method. Those methods employ indirect fluorine determination in the fluxes, where the concentration of calcium fluoride in fluxes is calculated based on a constant molar ratio of calcium and fluorine in calcium fluoride. The Berzelius method, initially dedicated to determining fluoride in rocks, involves fusing a sample with basic carbonates, removing silicon and aluminium by precipitation with ammonium and zinc carbonate. Then chromates and phosphates are precipitated with a silver(I) nitrate solution. The fluoride content is calculated based on the mass of the precipitated CaF2 sediment [2,3]. Whereas the Foote method encompasses pyrohydrolysis of a sample with NaOH and HCl and indirect calcium titration of the obtained solution. In contrast, in the work of Yeager and his colleagues [3], fluoride fluxes were fused with basic fluxes (sodium carbonate/sodium tetraborate) and then digested with nitric acid(V). Fluoride content was measured using FISE at the pH range 8.0 – 9.0. However, according to procedure by the Polish standard, the sample is leached with acetic acid to remove all calcium salts except calcium fluoride. Then, the sample is filtered and the precipitate is dissolved in boric and hydrochloric acid to remove fluorine and isolate insoluble silicic acid. The final determination is complexometric titration of calcium in the filtrate using disodium edetate. A completely different analytical approach was followed by the Zheng and Jian-Ping method [5]. According to the analytical procedure, the sample is fused with Li2 B4 O 7 . The calcium content in the sample is then determined by X-ray fluorescence and the carbon content as calcium carbonate by infrared spectroscopy. The total calcium content of the sample is obtained based on measurements made with XRF spectroscopy. The carbon content 2
Journal Pre-proof of the sample allows to calculate the calcium content found as calcium carbonate (assuming that CaCO 3 is the only carbon source in the sample). Based on the difference between the total calcium content of the sample (obtained by XRF analysis) and the calcium content present as calcium carbonate, the calcium content of the sample derived solely from CaF 2 can be calculated. However, the results of this work cannot be directly translated into the possibility of determining the fluoride content in the form of calcium fluoride in fluoride fluxes, because the authors of this work conducted tests on samples containing only CaCO 3 and CaF2 . In addition, the method proposed by Zheng and Jian-Ping [5] requires the use of two analytical methods for the determination of one sample, which causes an increase in the cost of performing the determinations.
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These above-discussed methods employ the decomposition of samples prior to their final analysis that poses the risk of analyte loss and sample contamination. This approach leads to changes in the sample composition as well as unsatisfactory accuracy and precision of measurements [3]. Additionally, the main drawback of the above-mentioned methods is the time-consuming and labour-consuming process of sample pre-treatment. Besides, the Yeager’s method is limited to measuring samples containing less than 63% (m/m) of calcium fluoride in a sample because of the difficulty of selecting a masking agent for samples having a higher content of CaF2 that results in the instability of the working apparatus. Owing to the variability of sample matrices, it is impossible to select a universal masking agent.
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Total reflection X-ray fluorescence (TXRF) spectrometry is known as a state-of-the-art and fast-growing analytical technique [5]. TXRF spectrometry works on the principle of emission of characteristic fluorescence radiation afterwards the radiation of sample elements. The TXRF spectrometry is used to versatile types of environmental and industrial samples due to (1) minute sample amounts, (2) no matrix effects, (3) simplified sample preparation, (4) low detection limit, and (5) the capability of simultaneous multielement determination [7]. The spectral range of TXRF spectrometry encompasses elements from sodium to uranium. To the best of our knowledge, commercially available TXRF spectrometers have not been used to determine fluorine yet, because the element is out of its spectral range. This research is also a first attempt to develop an indirect method of fluorine determination by TXRF spectroscopy. The object of this study was to (1) develop and optimize the analytical method for fluorine in a form of calcium fluoride determination in fluoride fluxes by total reflection X-ray fluorescence spectrometry, and (2) validate this method.
2. Materials and methods 2.1 Reagents and materials 3
Journal Pre-proof Method validation and optimisation were carried out against Certified Reference Material (CRM) (fluorite no. GBW 07253 (fluorspar) purchased from Brammer Standard Company, Inc) Reference Material (RM) (two synthetic fluorite fluxes: U1 and U2) and calcium stock solution (ICP standard, 1 g Ca ∙ ml-1 , matrix: 2 – 3% HNO 3 , Merck Millipore®). High purity water obtained from Chemsolve® was used for dilution of solutions and samples. The commercial non-ionic surfactant Triton X-100® (Sigma Aldrich®) was used to form sample suspension. For sample cleaning, the procedure elaborated by E. K. Towett and his coworkers [6] was applied. All analyses were performed in triplicate.
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2.2 Sample preparation method A 1.00 g portion of flux sample was weighed on an analytical balance and placed in a 50 ml flat bottom plastic centrifuge tube. The sample was lixiviated in 30 ml 1.74 M acetic acid for 60 minutes. Then, the sample was filtered and the sediment and the filter were ignited at 650 ºC. A 20.00 mg portion of dried and ignited sample was weighed on an analytical balance and placed in a 15 ml plastic centrifuge tube. The 5 ml of 1% Triton X-100, 1 µg · l-1 of Ga as internal standard, and 20 µl 0.3% polyvinyl alcohol (PVA) acting as a film former were added to the sample. Later, the sample was homogenized by a vortex for minimum 60 seconds. In the end of homogenization, the speed of homogenization was lowered and the 5 µl aliquot was pipetted on a sapphire sample carrier. The sample carrier was dried by a heat plate and loaded to the apparatus. The measurement was performed for 200 s (live time).
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2.3 Instrumentation The S2 PICOFOX spectrometer (Bruker AXS) was used in this research. The apparatus is equipped with an air-cooled molybdenum X-ray tube (50 W, 1 mA), a Peltiercooled Xflash® silicon drift detector with 10 mm2 active area, and a multilayer monochromator (17.5 keV). The analysis of the X-ray spectra was performed by Spectra 7.0 Bruker Nano GmbH. In order to determine both calcium and silicon, sapphire sample carriers with a diameter of 30 mm and a thickness of 3 mm were used to hold samples during measurements. 2.4 Calculation The concentration of an element is calculated according to the user manual [7]. Statistical differences between data were verified on the basis of a t-Studying tests, U MannWhitney test, and F-Snedecor test. The correlation between variabilities was performed by a Person correlation. The detection limit (DL) was computed according to 3 σ criterion (Equation 1): DL=
√
(1)
4
Journal Pre-proof where Ci is concentration of the element, N is an area of the fluorescence peak in counts, and Nbg is background area subjacent the fluorescence peak. The limit of quantification (LOQ) was taken as triple the previously determined detection limit. The repeatability of the analytical procedure was determined as the standard deviation of six independent measurements of the samples at the concentration level corresponding to the analyte concentration in real samples. The samples constitute six different samples of 1.00 g CRM prepared independently.
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The value of coefficient of variability was also calculated and the repeatability limit (r) was determined as 2.8 times of a standard deviation of series of measurements.
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3. Results
Particle size
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3.1 Procedure optimisation
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Sample’s particle size is a key factor to meet the assumptions of TXRF spectrometry. Sample’s particle size must not be larger than 20 µm [5]. Fluoride flux’s samples and CRM were ground before the analysis with an agate mortar. The histogram of sample’s particle shows that particles size is from 0 µm to 10 µm (Fig. 1). In addition to this, 99% of all particles was smaller than 10 µm and is in agreement with the theoretical thickness imposed by Klockenkämper and Von Bohlen [7].
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3.1.2 Sample volume and measurement time The measurement time is an important parameter to be tested. 20 mg of calcium fluoride standard was prepared according to the procedure described in 2.2 and it was measured for 100 s, 200 s, 300 s, 400 s, 500 s, 600 s, 700 s, 800 s, and 900 s. The ratio of analyte to internal standard area for samples measured for 200 – 900 s slightly varies between 0.559 and 0.575. The optimal measurement time was chosen 200 seconds, because this is the highest amount of this ratio whereas the measurement time is the lowest. The volume of sample suspension can also significantly influence TXRF analyses. The aliquots of 5 µl and 10 µl (5 µl pipetted and dried twice) of the samples were deposited onto sample carriers. The quality of the analysis can be improved, for instance, by using smaller sample volumes in order to avoid movement of the droplet on the carrier [10]. On the basis of the highest ratio of analyte to internal standard area, the optimal volume of sample pipetted on a sample carrier is 10 µl. 3.1.3
Sonication effect
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Sonification can improve the sample homogeneity. Usually, samples are homogenized by a vortex mixer before analyses. A few papers demonstrated that sonification can be also a useful tool to increase samples homogeneity in the case of soil samples [8]. Therefore, the influence of sonification on the sample homogeneity was tested. The influence was verified as the difference between results precision (expressed as standard deviation) of non-homogenized and the homogenized real sample and the certified reference material. The internal standard was added to homogenized samples before homogenization. Then, the obtained values of standard deviations were compared for each set of results using the FSnedecor test. The null hypothesis was the statement that standard deviations of results obtained by different methods of sample preparation do not differ in a statistically significant way, while an alternative hypothesis was the statement that standard deviations of results obtained by different sample preparation methods are statistically significant. The p value was below the confidence interval (F< Fcritic), therefore the null hypothesis has failed to be rejected. It denotes that homogenization does not have statistically significant influence on the analysis precision. Moreover, the difference between particle size of homogenized and non-homogenized samples was tested. Sample aliquots submitted to sonification were taken at a middle depth in a vial. It was demonstrated that there is a statistically significant difference between particle size of a pair of homogenized and non-homogenized sample and a pair of homogenized and non-homogenized certificated reference material (Fig. 2). It means that homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
Linearity, range and detection and quantification limit
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3.2 Procedure evaluation
The linearity was determined using a calibration curve based on measurements of analyte concentrate from 0.15 mg to 0.40 mg that corresponds to 29% – 78% CaF2 (0.14 mg – 0.38 mg F) in a fluorine flux sample (Fig. 3) according to the approach presented at Borgese and her co-workers [12] (exemplary tests results: Shapiro-Wilk test: p=0.27; Dixion test: Q 1 =0.14, Q 3 =0.07, Q crit =0.53; Hartley test: Fmax = 1.47, Fcrit =2.46; the lack of fit for linear and quadratic regressions: F= 3.18; Fcrit =3.2). The linearity coefficient (R) is ≥ 0.9951 for fluorine and the relationship between the content of the analyte and the detector signal is described by the equation y = 5.1847x – 0.0255. The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 .
3.2.2
Repeatability and accuracy
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The repeatability of the analytical procedure equals 0.6175 g · F g-1 and the coefficient of variation is 16.94%. In order to explain the reason for the not entirely satisfactory repeatability of the analytical procedure, the effect of matrix on the TXRF results was assessed. For this reason, 125 μg of selenium in solution (in the form of a Se (NO 3 )4 ) was added to samples containing calcium in CaF2 form to assess how repeatability will change if the analyte and internal standard are liquid. The repeatability of the analytical procedure was one order of magnitude lower and equals 0.05 g · Se g-1 and the coefficient of variation is 0.01%. It was noticed that a lower precision of results was observed when the analyte was a solid and the internal standard was a liquid and the opposite effect – the higher precision of results was noted when the analyle and internal standard were liquid.
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The results’ differences could be caused by two factors. Firstly, the sample suspension could be inhomogeneous therefore sample pipetting may lead to the unrepresentativeness of sample. Secondly, the sample aliquot deposited on the disc may have inhomogeneous distribution that also reduces the precision of the analytical procedure [13].
2 √u2(x i
u2(x
(2)
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|xi – xref |
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The accuracy of the analytical procedure was determined by the certified reference material. The value of the discrepancy between the CRM and the TXRF results equals 0.01 g ·g-1 and there are no statistically significant differences in these results. After determining the measurement uncertainty, the accuracy of the analytical procedure was again checked using the following inequality (Equation 2) [14].
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where: xi is an analysis result (g ∙ g-1 ), xref – an analysis of CRM (g ∙ g-1 ), u( x i is an uncertainty of the analysis result (g ∙ g-1 ), u(x 1
is an uncertainty of the reference result (g ∙ g-
). When the above inequality is fulfilled, then the obtained results are in agreement with the reference result. Table 1 shows the obtained results are in accordance with the reference result.
3.2.3 Uncertainty There is some scientific research on the uncertainty of the TXRF results and its determining factors [15,16]. The total uncertainty consists of two components: the variance, characterizing the reproducibility of the measurement of an analytical signal from one sample, and the variance, characterizing the stability of the sample preparation conditions from the same specimen, including the addition of the internal standard [15]. The measurement uncertainty was presented as a result ± an expanded uncertainty (U). The expanded uncertainty was calculated on the basis of Equation 3. 7
Journal Pre-proof U = k uc(y (3) where U is the expanded uncertainty, k is a covered factor (k = 2), uc(y is the combined standard uncertainty. The below-presented mathematical model of measurement procedure is dedicated the fluorine determination in a form of CaF 2 according to the procedure (Equation 4). CF(CaF2 =
i SIS IS
m IS 2 MF
Si m MCa
(4)
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where CF(Ca F2 is the content of fluorine in a form of CaF2 (g ∙ g-1 ), N i is the area of analyte peak, SIS is the detector sensitivity for the internal standard, mIS is the mass of internal standard (g), MF is the molar mass of fluorine (g · mol-1 ), NIS is the area of internal standard peak, Si is the detector sensitivity for the analyte, m is the sample mass (g), MCa is the molar mass of calcium (g · mol-1 ).
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Combined uncertainties were calculated by propagation individual uncertainty components on the basis of The Guide for Uncertainty in Measurements with the use of Statistica Sofware® [14]. The combined uncertainty was obtained by the below Equation.
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u( CF(Ca F
2
CF(Ca F2 u i i
)
2
(
uSIS SIS
)
2
(
umIS m IS
)
2
(
uMF MF
)
u IS
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√(
2
(
IS
)
2
u
( Si ) Si
= 2
u
( m) m
2
(
uMCa MCa
)
2
u2(P
(5)
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The relative contributions of the Equation 6 are presented in the Table 2. Substituting the values from the Table 2 into Formula (7), the result and its expanded uncertainty equal to:
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CF(Ca F2 ± U = 0.426 ± 0.012 g · g-1
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The parameters contributing to the measurement of uncertainty are embedded in the Ishikawa diagram (Fig. 4). The dominant factor affecting the measurement uncertainty is the area of the analyte peak (75%). The area of the internal standard peak has a three-fold smaller influence on the uncertainty of measurement. Other parameters do not influence the measurement uncertainty. Those results are in agreement with Floor and his team’s results [16]. In their work, their attention was drawn to the fact that it is impossible to easily determine the factors affecting the reproducibility of the spatial distribution of sample onto a disk using the theoretical model. At the same time, this parameter has the greatest impact on the uncertainty of measurement results. In the studies of Cherkashin and his co-workers [15] on the determination of Rb, Sr, Cs, Ba and Pb in feldspars, it was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. Identical observations were recorded in the results of this work, where the influence of only two factors affecting the uncertainty of the measurement results was indicated the area of analyte peak (75%) and area of internal standard peak (25%)). 8
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3.3 Comparison of other analytical procedures
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There is little scientific research on the determination of fluorine in geological samples and fluoride fluxes. Due to the lack of data, the validation parameters were compared with the values of these parameters determined by other analytical procedures for the determination of fluorine in geological samples and fluoride fluxes (Table 3). It is not possible to fully compare the validation parameters, because the authors of the other methods provide only two validation parameters – a limit of detection and precision. The detection limit for the validated analytical procedure is the lowest among the other fluorine determination methods presented in Table 3. Instead of the fact that the validated method does not characterize the lowest precision, its value is satisfactory comparing it to the values obtained in TXRF measurement of light element. In the case of a total time of sample analysis, it can be concluded that the determination takes less time than the titration and takes longer than analysis using potentiometry or XRF technique.
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3.4 Application of the procedure The application of the validated procedure was the determination of fluorine content in the form of calcium fluoride in two fluoride fluxes obtained as a result of an unpublished work (U1 and U2). The samples were prepared for analysis in accordance with the procedure described in 2.2. The composition of fluoride fluxes is embedded in Table 4. The content of calcium fluoride in the U1 and U2 flux were 58.39% ± 1.64% and 46.25% ± 0.01%, respectively. Additionally, determined the content of calcium fluoride in U1 flux was determined by an independent laboratory (owning Cemex’s company) with the use of XRD technique and equals 59.8% ± 3.00% (the value of measurement uncertainty was estimated on the basis the date from the laboratory). Those results show a good agreement for elements determined. The values of the discrepancies between the TXRF and the XRD results are less than the permissible standard deviation σr.
4 Conclusion In this work, the new method has been devised for indirect fluorine in a form of calcium fluoride determination in fluoride fluxes by TXRF spectrometry. It was shown that the preparation of the suspensions with the Triton X-100 solution and the fluoride flux after lixiviation with acetic acid is a simple fast procedure that is a simple, rapid and reliable tool for analysing fluoride fluxes, especially in cases when analyses have to be carried out in continuous production conditions. The limit of detection for TXRF measurement is 4.26 µg F · dm-3 whereas the limit of quantification equals 12.78 µg F · dm-3 . The variation coefficient that characterized the 9
Journal Pre-proof reproducibility of the measurement equals 16.94%. The lower precision of results can be affected by the fact that the internal standard and the analyte were liquid and solid, respectively. The method shows a good accuracy that was proven with CRM and the independent laboratory. There are no statistically significant differences between the results measured in the CRM and results of sample measured by the new method and the independent laboratory. The value of the discrepancy between the CRM and the TXRF results equals 0.01 g · g-1 .
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Moreover, in this study, it was demonstrated that there is a statistically significant difference between particle size of a pair of homogenized and non-homogenized sample and a pair of homogenized and non-homogenized certificated reference material. It means that homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
Acknowledgements
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It was found that the uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry. The dominant factor affecting the measurement uncertainty is the area of the analyte peak (75%). The area of the internal standard peak has a three-fold smaller influence on the uncertainty of measurement. Other parameters do not influence the measurement uncertainty.
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The work was financially supported by the project no. UDA-POIG.01.04.00-30012/10-00 founded under the Operational Programme Innovative Economy, Measure 1.4 and the project no. 309/2015/Wn15/MN-XN-03/D funded under ЕЕA Financial Mechanism 2009–2014.
6 References 1. Wroblewski K., Fields J., Fraley J., Werner R., Rudoler S., 2009. Quick determination of total fluoride in electroslag refining fluxes. International Symposium on Liquid Metal Processing and Casting 2009. 303–307. 2. Jeffery P., 1981. Chemical Methods of Rock Analysis: Fluorine. International series of monographs in analytical chemistry. Pergamon Series in Analytical Chemistry, 36, 165– 174. DOI: 0.1016/B978-0-08-023806-7.50025-7. 3. Yeager J., Miller M., Ramanujachary K., 2006. Determination of total fluoride content in electroslag refining fluxes using a fluoride ion-selective electrode. Ind. Eng. Chem. Res., 45, 4525–4529. DOI: 10.1021/ie060128a. 4. Polish Standard no. PN-61 H11105. Metallurgic fluorite, 1961. 5. Zheng S., Jian-ping M., 2008. Determination of calcium fluoride in fluorite with X-ray fluorescence spectrometry and infrared absorption. Metall Anal. 28, 8, 73–75. 10
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6. West M., Ellis E., Potts P., Streli C., Vanhoofe C, Wobrauschek P., 2014. Atomic Spectrometry Update – a review of advances in X-ray fluorescence spectrometry. J. Anal. At. Spectrom., 2014, 29, 1516. DOI: 10.1039/c4ja90038c. 7. Bruker Nano GmbH, 2007. S2 PICOFOX user manual. Berlin: Bruker AXS Microanalysis GmbH. 8. Towett E., Shepherd K., Cadisch G., 2013. Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF). Sci. Total Environ., 463–464, 374–338. DOI: http://dx.doi.org/10.1016/j.scitotenv.2013.05.068. 9. Klockenkämper R., von Bohlen A., 1989. Determination of the critical thickness and the sensitivity for thin-film analysis by total reflection X-ray fluorescence spectrometry. Spectrochim. Acta B, 44, 5, 1989, Pages 461-469. DOI: 10.1016/05848547(89)80051- 5. 10. Riano S., Regad M., Binnemans K., Hoogerstraete T., 2016. Practical guidelines for best practice on Total Reflection X-ray Fluorescence spectroscopy: Analysis of aqueous solutions. Spectrochim. Acta Part B. 124, 109-115. DOI: 10.1016/j.sab.2016.09.00. 11. Álvarez-Vázquez M., Bendicho C., Prego R., 2014. Ultrasonic slurry sampling combined with total reflection X- ray spectrometry for multi-elemental analysis of coastal sediments in a ria system. Microchem. J., 112, 172–180. DOI: http://dx.doi.org/10.1016/j.microc.2013.09.026. 12. Borgese L., Dalipi R., Riboldi A., Bilo F., Zacco A., Federici S., Bettinelli M., Bontempi E., Depero L., 2018, Comprehensive approach to the validation of the standard method for total reflection X-ray fluorescence analysis of water. Talanta, 1;181:165-171. DOI: 10.1016/j.talanta.2017.12.087. 13. Stosnach H., 2005. Environmental trace-element analysis using a benchtop total reflection X-ray fluorescence spectrometer. Anal Sci. 2005 Jul;21(7):873-6. DOI: 10.1154/1.1913723. 14. EURACHEM: Quantifying Uncertainty in Analytical Measurement. 3rd edition, 2012. https://www.eurachem.org/images/stories/Guides/pdf/QUAM2012_P1.pdf 15. Cherkashina T., Panteev S., Finkelshtein A., Makagon V., 2013. Determination of Rb, Sr, Cs, Ba, and Pb in K-feldspars in small sample amounts by total reflection X-ray fluorescence. X-Ray Spectrom., 42, 207–212. DOI: 10.1002/xrs.2469. 16. Floor G., Queralt I., Hidalgo M., Marguí E., 2015. Measurement uncertainty in Total Reflection X- ray Fluorescence. Spectrochim. Acta Part B. 111, 30–37. DOI: 10.1016/j.sab.2015.06.015.
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Figure 1. The histogram of fluoride fluxes particle size and CRM ground with an agate mortar
Figure 2. The difference between standard deviation of homogenised and non-homogenised samples and certified reference material (U Mann-Whitney test p < 0,00)
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Figure 3. The linearity between the detector signal and analyte’s content in a calibration curve
Figure 4. Ishikawa diagram for the TXRF measurement procedure
Table 1. Accuracy assessed with regard of results uncertainty with the use of inequality (4) (the detailed description of symbols was presented in the Equation 2)
g ∙ g -1 ) 0.006
|
g ∙ g -1 ) 0.10
| 0.10
13
√ 0.20
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Table 2. The parameters contributing to the measurement of uncertainty influencing combined uncertainties The value of parameter x
Combined standard uncertainty u(x)
Type of uncer tainty
Type of distribution
4.95 · 105
6.00 · 103
A
t-Student
8.53 · 105
6.01 · 103
A
t-Student
1.08 · 10-1
1.28 · 10-6
A
t-Student
0.93 · 10-1
1.61 · 10-4
A
t-Student
Cp (g)
5.00 · 10-8
B
triangular
Pp (g)
1.00 · 10-6
A
t-Student
1.25 · 10-1
2.89 · 10-8 1.00 ∙ 10-5
B
Pw (mg)
1.00 ∙ 10-5
A
R (mg)
1.00 ∙ 10-5
B
P (g · g-1)
Analytical method and analytical technique ion-selective potentiometry spectrometry XRF titration (Foote’s method titration (the Polish standard method P − 1 H−111 TXRF spectrometry (the validated method)
5.00 · 10-8
rectangular
B
4.00 · 10-3
40.08
3.27 · 10-2
Sample pretreatment
rectangular
B
rectangular
A
t-Student
Limit of determination (mg F)
mg
S u(SI IS
)2
)2 SIS uCp ( ) √ u2 (Pw u(mIS 2 ( ) mIS uSe ( ) √3 u2 (Pw) u ( ) √3 u(m 2 ( ) m u( M F √3 ( )2 MF
f
t-Student
Pr
18.99
al
(g · mol -1)
1.41 · 10-10
rn
(g · mol -1)
2.05 ∙ 10-2
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m (g)
rectangular
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S e (mg)
IS u(Si
(
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(g)
Combined standard uncertainty u( i ( )2 I u( IS ( )2 (
pr
Parameter
∙ kg -1 d w)
u (MCa √3 ) 2 ( MCa u2
The value of combined standard uncertainty 1.47 · 10-4 4.96 · 10-5 1.40 · 10-10 3.02 · 10-10 2.89 · 10-8 1.00 · 10-12 5.33 · 10-14 5.77 · 10-6 1.00 · 10-10 5.77 · 10-6 4.76 · 10-17 2.31 · 10-18
3.32 · 10-9 1.07 · 10-3
Precision expressed as standard deviation (% )
Time of analysis (h)
Reference
fuse with NaOH
1.5 ∙ 10-3
3
2–3
≈4
Yeager et al. 2006 An et al. 2012
pelletising
1.62
812
no data
≈ 0.25
pyrohydrolysis with NaOH and HCl
no data
no data
2–3
< 10
Yeager et al. 2006
no data
no data
no data
≈3
PN-70 H04132 1970
1.04 ∙ 10-3
4.2 ∙ 10-3
6.5
≈8
this work
lixiviation CH3 COOH
Table 3. The comparison among methods of fluorine determination in geological samples and fluoride fluxes
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Table 4. The composition of fluoride fluxes
Substrates
Ingredients (% ) SiO2
CaF2
others
36
K, Fe, S < 0.1
32
Mg, K, Fe, S < 0.1
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technical calcium carbonate (U1) 58.39 ± 1.64 26% H2 SiF6 distiller waste* (U2) 46.25 ± 0.01 26% H2 SiF6 * a by-product of the Solvay process
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Journal Pre-proof Zuzanna Kowalkiewicz: Conceptualization, Validation, Methodology, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing, Project administration, Visualization U b n k:
Conceptualization,
Supervision,
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Resources,
Funding
Journal Pre-proof A methodology for determination of fluorine in a form of calcium fluoride in fluoride fluxes is proposed.
Homogenization helps to diminish particle size, but it is not effective to homogenize samples and improve standard deviation of results.
The uncertainty of results associated with the sample preparation process for TXRF measurement is negligible, compared to the measurement uncertainty at the measurement stage using TXRF spectrometry.
The acceptable and fully satisfactory validation criteria such as limit of detection and quantification, accuracy, and precision.
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