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Determination of fluorine by total reflection X-ray fluorescence spectrometry
Spectrochimica Acta Part B 65 (2010) 287–290
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Determination of fluorine by total reflection X-ray fluorescence spectrometry☆ G. Tarsoly, M. Óvári ⁎, Gy. Záray Dept. of Analytical Chemistry, Eötvös University, P.O. Box 32, H-1518, Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 2 November 2009 Accepted 24 February 2010 Available online 10 March 2010 Keywords: TXRF Fluorine Detection limit Linearity range Interference effects
a b s t r a c t There is a growing interest in determination of low Z elements, i.e. carbon to phosphorus, in various samples. Total reflection X-ray fluorescence spectrometry (TXRF) has been already established as a suitable trace element analytical method with low sample demand and quite good quantification limits. Recently, the determinable element range was extended towards Z = 6 (carbon). In this study, the analytical performance of the total reflection X-ray fluorescence spectrometry for determination of fluorine was investigated applying a spectrometer equipped with Cr-anode X-ray tube, multilayer monochromator, vacuum chamber, and a silicon drift detector (SDD) with ultra thin window was used. The detection limit for fluorine was found to be 5 mg L− 1 (equivalent to 10 ng absolute) in aqueous matrix. The linear range of the fluorine determination is between 15 and 500 mg L− 1, within this range the precision is below 10%. The matrix effects of the other halogens (chlorine, bromine and iodine), and sulfate were also investigated. It has been established that the upper allowed concentration limit of the above interfering elements is 100, 200, 50 and 100 mg L− 1 for Cl, Br, I and sulfate, respectively. Moreover, the role of the pre-siliconization of the quartz carrier plate was investigated. It was found, that the presence of the silicone results in poorer analytical performance, which can be explained by the thicker sample residue and stronger self-absorption of the fluorescent radiation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The halogens play an important role in nature; they occur in the lithosphere both as main and trace components. Their presence is indispensable in living organisms; however, they can be harmful at elevated concentration. Thus, the control of their input is an important task. In vertebrates, fluoride is a constituent of the skeleton and teeth, but its essentiality is not confirmed. Fluorine occurs in nature exclusively at the oxidation stage of −1 (fluoride). There are two fluoridebearing biominerals in vertebrates: fluorite (CaF2) and fluorapatite (Ca5(PO4)3F). Fluoride plays an important role in avoiding of dental caries, since fluorapatite is much less soluble in acids than hydroxilapatite (Ca5(PO4)3OH). A part of fluoride is stored in hair and nails. On the other hand, 2–3 mg L− 1 fluoride in drinking water can cause fluorosis, the tooth enamel building will be hampered and the bones become more fragile. Above 4 mg L− 1 the teeth become black and will be lost. Fluorosis is irreversible. The current WHO recommendation is maximally 1.5 mg L− 1, but according to some sources this limit should be decreased in the tropical regions to 0.7 or 0.5 mg L− 1, since the water consumption is higher there [1]. At a
☆ This paper was presented at the Colloquium Spectroscopicum Internationale XXXVI, held in Budapest, Hungary, August 30–September 3, 2009 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: + 36 1 372 2500x1216; fax: + 36 1 372 2608. E-mail address: [email protected] (M. Óvári). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.02.019
concentration of 50 mg L− 1, fluoride can be harmful after administered once. This can be explained by the precipitation formation with calcium. Fluoride and the other haloids can easily be determined by ion chromatography, where the haloid ions will be separated well, even if using organic solvents (e.g. acetone, isopropanole etc.) [2]. A detection limit of 0.005 mg L− 1 can be achieved [3]. For the determination of fluoride in toothpaste a derivatization– gas chromatography method was reported in [4]. The detection limit was calculated as 3σ and amounted to 6 mg kg− 1. The most widespread method for the determination of dissolved fluoride in aqueous samples is the use of ion selective electrodes. The electrode contains a single crystal membrane made of LaF3 which is doped with Eu(II) in order to increase the electric conductivity [5]. The analytical performance of the ion selective electrodes is limited mostly by the solubility of LaF3 resulting in typical detection limit of about 0.02 mg L− 1. Since the determination is strongly influenced by the pH and ionic strength of the sample, a so called total ionic strength adjusting buffer has to be given to all standards and samples. For the continuous monitoring of fluoride in surface or technological water a flow-cell system was developed [6]. The method does not require the addition of total ionic strength adjusting buffer; however, the detection limit amounted to only 0.3 mg L− 1. One of the most powerful tools of the modern instrumental analysis is the inductively coupled plasma mass spectrometry (ICP-MS). Since the ionization efficiency for fluorine in the Ar-ICP is only 0.0009%, it cannot be determined as positive ion in the mass spectrometer.
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Recently, the negative ion formation of atoms with high electron affinity was observed and on this basis the negative ion ICP-MS developed [7]. Although the detection limit of fluorine is sufficiently low (0.11 µg L− 1), this method requires special instrumentation and cannot be used widely. The determination of fluorine using X-ray fluorescence (XRF) is not common, because of the very low fluorescence yield for the elements with atomic number (Z) less than 15, and the absorption of the fluorescent radiation in the beam pathway. Using the conventional wavelength dispersive XRF technique, for determination of fluorine in solid samples and a detection limit of 120 µg g− 1 was achieved [8]. The above mentioned problems can be partly solved if the XRF spectrometer is specially designed for the low Z (Z b 15) elements. A very important factor influencing the fluorescence yield is the energy of the primary X-ray beam. With decreasing energy the fluorescence yield can be enhanced considerably [9]. Conducting the entire beam pathway in vacuum, the absorption in the air can be avoided. Furthermore, the most frequently used Be entrance window of the detector has a very low transmittance for F-Kα, but this window can be replaced by a special polymer foil [10,11]. The first total reflection X-ray fluorescence (TXRF) spectrometer for the determination of the low Z elements was developed in Atominstiute, Vienna [11]. This equipment is already available under the name Wobistrax. Although the range of detectable elements was extended down to carbon (Z = 6), this spectrometer has not been used for fluorine determination yet. In the present work the most important analytical figures of merit of the fluorine determination in water using the Wobistrax TXRF spectrometer are demonstrated. The investigated parameters were: detection limit, linearity range, sample amount, precision of the measurement, and the matrix effect caused by chlorine, bromine, iodine and sulfate. 2. Experimental
stable at about neutral pH, since the acidification would convert fluoride to hydrofluoric acid, which is volatile and can damage the sample support made of quartz or silicon, and (iii) its characteristic line has to be free from spectral interferences. Among the elements having K-lines in the above mentioned range, only vanadium in the oxidation stage of +5 can fulfill these requirements. Thus, vanadium as VO− 3 can be used. 2.2. Reagents For the preparation of standard solutions, analytical grade crystalline NaF was used (Reanal, Budapest, Hungary). Vanadium internal standard stock solution of 1000 mg L− 1 was prepared from analytical grade solid NH4VO3 (Reanal, Budapest, Hungary). The 5000 mg L− 1 single stock solutions of the interfering components (Cl, Br, I, and sulphate) were prepared from analytical grade crystalline KCl, KBr, KIO3 and Na2SO4·10H2O, respectively. For preparation of these solutions, distilled and deionized water (Milli-Q Plus, Millipore, Molsheim, France) was used thoroughly. 2.3. Instrumentation The measurements were carried out using a Wobistrax low Z spectrometer (Atominstitut, Vienna, Austria) described in [11] (Fig. 1). The operating conditions were as follow: Cr-anode X-ray tube at 30 kV, 30 mA, Ni/C multilayer monochromator set to 5.4 keV, KETEK silicon drift detector with 10 mm2 active area and ultra thin polymer window; electron trap around the window in order to decrease the spectral background from Auger- and photoelectrons, 500 s live time. The concentration of the internal standard was 24 mg L− 1 in the end volume of all solutions. The samples were pipetted onto quartz supports. All measurements were performed 12 times, whereas each of the three triplicate supports were measured in four different positions (always rotated by 90°).
2.1. Internal standardization 2.4. Investigation of the analytical figures of merit The choice of an appropriate internal standard is a critical point in the TXRF analysis, especially in the low Z range. There are several requirements to be fulfilled: (i) the internal standard element has to have a characteristic line below 5.4 keV (Cr-Kα), (ii) it should be
In order to investigate the effect of siliconization on the analytical signal of fluorine 2 μL of standard solutions was dropped onto the quartz carrier plates with and without siliconization.
Fig. 1. The schematic structure of the Wobistrax TXRF spectrometer.
G. Tarsoly et al. / Spectrochimica Acta Part B 65 (2010) 287–290
289
Table 1 Limit of detection (LOD) of F in presence of different interfering components with concentration of 100 mg L− 1.
LOD/mg L− 1
Pure water
Cl
Br
I
Sulfate
5
20
20
35
35
For the investigation of the influence of sample volume (practically the size of the sample spot) on the analytical signal 0.5 μL, 1 μL, 2 μL, 4 μL, and 6 μL standard solutions with a concentration of 50 mg L− 1 F were pipetted onto the supports. The detection limit was determined both in pure water with just V internal standard, and in presence of the interfering elements. In this latter case the elements were added separately at an F concentration of 100 mg L− 1. The detection limit was calculated as 3σ of the blank. The linearity range was checked using 25, 50, 100, 200, 500, and 1000 mg L− 1 F standard solutions. The interference effects of Cl, Br, I, and sulfate were investigated in the concentration range of 50 to 1000 mg L− 1 for each interfering component separately. The F concentration was 50 mg L− 1 in all samples. 3. Results and discussion 3.1. Influence of hydrophobization
Fig. 2. a Calibration curve of F with siliconized supports. R2 = 0.999. Error bars represent the standard deviation of 12 replicates. b Calibration curve of F with non-siliconized supports. R2 = 0.933. Error bars represent the standard deviation of 12 replicates.
Usually, the sample supports in the TXRF practice are always hydrophobized by a silicone solution prior to use, in order to inhibit the spreading of the sample spot and getting out of the detector active area. On the other hand, in case of low Z measurements, extreme care must be taken to keep the sample thickness possibly below 1 μm. Therefore, the usefulness of the siliconization has to be checked. For this propose, calibration curves were taken both with and without silicone pre-treatment, as described in Chapter 2.4. As it is shown in Fig. 2a–b, the sensitivity for F is 8.5 times higher in the case of non-siliconized supports (within the linear range), which is caused by the considerably thinner sample spots, resulting in lower selfabsorption. Moreover, the linearity range is much broader, the upper concentration limit was 500 mg L− 1, against 200 mg L− 1 with hydrophobized supports. On the basis of these observations, it was decided to omit the siliconization of the supports in the further experimental work. 3.2. Influence of the sample amount The sample spot thickness can be controlled in two ways. Either the sample dilution or the sample volume has to be optimized. Since the detector in the spectrometer has a quite narrow entrance window 10 mm2, the influence of the sample amount had to be investigated by all means. In Fig. 3 it can be seen, that the sample volume does not
Fig. 3. Influence of the sample volume onto the relative intensity applying non-siliconized quartz carrier plates. Error bars represent the standard deviation of 4 replicates.
Fig. 4. Spectrum of a sample containing 50 mg L− 1 F and 50 mg L− 1 sulfate.
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influence considerably the measured relative intensity of F. On the other hand, the relative standard deviation (RSD) varies between 2.5 and 8.9% (n = 12), having its minimum at 2 μL. At both edges of the investigated range the RSD amounts to more than 8%. This can be explained by the facts, that (i) at lower volume the count rate is lower, and the statistics of the peak fitting is worse, (ii) at higher volume a considerable part of the sample spot is outside the area which can be seen by the detector, and incidental inhomogeneities in the spots can disturb the precision of the determination. 3.3. Detection limit of fluorine Table 1 contains the detection limit data of F, both in pure water and in the presence of the investigated interfering elements, each at a concentration of 100 mg L− 1. It can be established that the detection limit expressed in concentration is relatively high compared with e.g. the ion selective electrode, and will be considerably impaired by the interfering elements. On the other hand, it should be considered that the TXRF method requires only 2 μL of sample, and the absolute detection limit amounts to 10 to 70 ng F. This can be favorable in the case when the available sample volume is very low, e.g. saliva samples from the interdental space. 3.4. Linearity range As already mentioned in Chapter 3.1, the upper concentration limit is 500 mg L− 1. Considering a quantification limit of about 15 mg L− 1 (equal to 10σ), the linearity range covers about 1.5 orders of magnitude. 3.5. Interference effects 3.5.1. Chlorine Chlorine is the most abundant accompanying element of fluorine. It has an escape peak at 0.88 keV, which might have interfered with the FKα peak at 0.68 keV. Fortunately, the detector could resolve the two peaks completely down to baseline. On the other hand, the increasing concentration of Cl had a negative influence onto the intensity and RSD of the F signal. In the presence of 100 mg L− 1 Cl, the RSD and signal depression were 16 and 5%, respectively. Increasing the Cl concentration further to 200 mg L− 1, the F peak cannot be detected any more. In conclusion, the upper limit of Cl concentration is 100 mg L− 1. As mentioned in Chapter 3.3, the detection limit increased to 20 mg L− 1. 3.5.2. Bromine Bromine has theoretically no interference with either F or V. The BrLα peak (1.48 keV) does not cause any spectral interference. Similar to Cl, the spectral background will be considerably higher with increasing Br concentration, but its extent is lower. At a Br concentration of 200 mg L− 1, the RSD of the F signal amounts to 25%. Both F and V signals are depressed by about 20%, but the calculated relative intensity remains practically constant. In conclusion, the upper limit of Br concentration is 200 mg L− 1. As mentioned in Chapter 3.3, the detection limit increased to 20 mg L− 1. 3.5.3. Iodine Iodine has a number of L-series peaks in the range around the V-Kα line (4.95 keV). The presence of iodine has a very strong depressing influence onto the F signal, which can be explained by the strong absorption of the fluorescent X-rays. Although in an extra test solution containing 100 mg L− 1 F and 1000 mg L− 1 I, the F peak could be fitted with an RSD of 45%, is was absolutely impossible to find the V peak. The spectral interference on V facilitates the peak fitting up to an I concentration of 200 mg L− 1, but the RSD amounts to 68% there. The RSD of the relative intensity as IF/IV at iodine concentrations less than 50 mg L− 1 remains below 20%.
In conclusion, the upper limit of I concentration is 50 mg L− 1. The detection limit increased to 35 mg L− 1 F. 3.5.4. Sulfate The presence of sulfate causes very strong spectral interference onto the F-Kα peak, since there are two escape peaks of S at 0.57 and 0.72 keV, and the detector is not able to resolve the three peaks. Especially the more intensive S-Kα-escape peak has a strong influence; at 50 mg L− 1 sulfate it has almost 50% intensity of the F-Kα peak (Fig. 4). This observation can be an explanation for the considerable increase of the RSD of F signal, although the net F signal intensity did not change systematically within the investigated concentration range up to 1000 mg L− 1 sulfate. In conclusion, the upper limit of sulfate concentration is 100 mg L− 1. The detection limit increased to 35 mg L− 1 F. 4. Conclusions Generally, it can be established that fluorine determination can be performed by TXRF using a specially designed spectrometer for low Z elements. However, the analytical performance should be improved, especially the detection limit is not enough low for analysis of drinking water. Moreover, there are serious spectral interferences caused by Cl, Br, I and sulfate, which are common constituents in various sample types, having typically higher concentration than F. To overcome this problem, a matrix separation method can be recommended. The method could be used for the determination of F in airborne dust samples in aluminum smelters (cryolite). A further possible application can be the determination of F in saliva samples from the interdental space. Since the Wobistrax spectrometer will be available soon with detectors of broader entrance window (now maximally 30 mm2, later 100 mm2), the sample volume can be increased resulting in lower detection limits. Henceforward, it is projected to reach the concentration range of drinking and mineral waters, and to carry out the determination in real matrices. Acknowledgment The authors are grateful for the financial support of the EU in framework of a Marie Curie European Reintegration Fellowship (MERG-CT-2004-513463). References [1] C. Reimanna, D. Banks, Setting action levels for drinking water: are we protecting our health or our economy (or our backs!)? Sci. Total Environ. 332 (2004) 13–21. [2] L.E. Vanatta, Ion-chromatographic determination of seven common anions in electronic-grade, water-miscible solvents, J. Chromatogr. A 1213 (2008) 70–76. [3] S. Jeyakumar, V. Vaibhavi, V. Raut, K.L. Ramakumar, Simultaneous determination of trace amounts of borate, chloride and fluoride in nuclear fuels employing ion chromatography (IC) after their extraction by pyrohydrolysis, Talanta 76 (2008) 1246–1251. [4] G. Wejnerowska, A. Karczmarek, J. Gaca, Determination of fluoride in toothpaste using headspace solid-phase microextraction and gas chromatography–flame ionization detection, J. Chromatogr. A 1150 (2007) 173–177. [5] K. Burger, Principles of the Analytical Chemistry — Chemical and Instrumental Analysis (in Hungarian), Alliter Publishing, Budapest, 2002. [6] J.R. Santos, R.A.S. Lapa, J.L.F.C. Lima, Development of a tubular fluoride potentiometric detector for flow analysis. Evaluation and analytical applications, Anal. Chim. Acta 583 (2007) 429–436. [7] A.A. Pupysheva, V.T. Surikov, Application of negative ions in inductively coupledplasma-mass spectrometry, Spectrochim. Acta Part B 59 (2004) 1021–1031. [8] S.M. Hasany, F. Rashid, A. Rashid, H. Rehman, Determination of fluorine in solids down to 120 μg/g by wavelength dispersive X-ray fluorescence spectrometry, J. Radioanal. Nucl. Chem. 148 (1991) 211–216. [9] J.W. Robinson, Handbook of Spectroscopy, Vol. 1, CRC, New York, 1979. [10] Official homepage of Canberra Inc. http://www.canberra.com/products/524.asp. [11] C. Streli, P. Wobrauschek, G. Pepponi, N. Zoeger, A new total reflection X-ray fluorescence vacuum chamber with sample changer analysis using a silicon drift detector for chemical analysis, Spectrochim. Acta Part B 59 (2004) 1199–1203.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Determination of fluorine by total reflection X-ray fluorescence spectrometry☆ G. Tarsoly, M. Óvári ⁎, Gy. Záray Dept. of Analytical Chemistry, Eötvös University, P.O. Box 32, H-1518, Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 2 November 2009 Accepted 24 February 2010 Available online 10 March 2010 Keywords: TXRF Fluorine Detection limit Linearity range Interference effects
a b s t r a c t There is a growing interest in determination of low Z elements, i.e. carbon to phosphorus, in various samples. Total reflection X-ray fluorescence spectrometry (TXRF) has been already established as a suitable trace element analytical method with low sample demand and quite good quantification limits. Recently, the determinable element range was extended towards Z = 6 (carbon). In this study, the analytical performance of the total reflection X-ray fluorescence spectrometry for determination of fluorine was investigated applying a spectrometer equipped with Cr-anode X-ray tube, multilayer monochromator, vacuum chamber, and a silicon drift detector (SDD) with ultra thin window was used. The detection limit for fluorine was found to be 5 mg L− 1 (equivalent to 10 ng absolute) in aqueous matrix. The linear range of the fluorine determination is between 15 and 500 mg L− 1, within this range the precision is below 10%. The matrix effects of the other halogens (chlorine, bromine and iodine), and sulfate were also investigated. It has been established that the upper allowed concentration limit of the above interfering elements is 100, 200, 50 and 100 mg L− 1 for Cl, Br, I and sulfate, respectively. Moreover, the role of the pre-siliconization of the quartz carrier plate was investigated. It was found, that the presence of the silicone results in poorer analytical performance, which can be explained by the thicker sample residue and stronger self-absorption of the fluorescent radiation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The halogens play an important role in nature; they occur in the lithosphere both as main and trace components. Their presence is indispensable in living organisms; however, they can be harmful at elevated concentration. Thus, the control of their input is an important task. In vertebrates, fluoride is a constituent of the skeleton and teeth, but its essentiality is not confirmed. Fluorine occurs in nature exclusively at the oxidation stage of −1 (fluoride). There are two fluoridebearing biominerals in vertebrates: fluorite (CaF2) and fluorapatite (Ca5(PO4)3F). Fluoride plays an important role in avoiding of dental caries, since fluorapatite is much less soluble in acids than hydroxilapatite (Ca5(PO4)3OH). A part of fluoride is stored in hair and nails. On the other hand, 2–3 mg L− 1 fluoride in drinking water can cause fluorosis, the tooth enamel building will be hampered and the bones become more fragile. Above 4 mg L− 1 the teeth become black and will be lost. Fluorosis is irreversible. The current WHO recommendation is maximally 1.5 mg L− 1, but according to some sources this limit should be decreased in the tropical regions to 0.7 or 0.5 mg L− 1, since the water consumption is higher there [1]. At a
☆ This paper was presented at the Colloquium Spectroscopicum Internationale XXXVI, held in Budapest, Hungary, August 30–September 3, 2009 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: + 36 1 372 2500x1216; fax: + 36 1 372 2608. E-mail address: [email protected] (M. Óvári). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.02.019
concentration of 50 mg L− 1, fluoride can be harmful after administered once. This can be explained by the precipitation formation with calcium. Fluoride and the other haloids can easily be determined by ion chromatography, where the haloid ions will be separated well, even if using organic solvents (e.g. acetone, isopropanole etc.) [2]. A detection limit of 0.005 mg L− 1 can be achieved [3]. For the determination of fluoride in toothpaste a derivatization– gas chromatography method was reported in [4]. The detection limit was calculated as 3σ and amounted to 6 mg kg− 1. The most widespread method for the determination of dissolved fluoride in aqueous samples is the use of ion selective electrodes. The electrode contains a single crystal membrane made of LaF3 which is doped with Eu(II) in order to increase the electric conductivity [5]. The analytical performance of the ion selective electrodes is limited mostly by the solubility of LaF3 resulting in typical detection limit of about 0.02 mg L− 1. Since the determination is strongly influenced by the pH and ionic strength of the sample, a so called total ionic strength adjusting buffer has to be given to all standards and samples. For the continuous monitoring of fluoride in surface or technological water a flow-cell system was developed [6]. The method does not require the addition of total ionic strength adjusting buffer; however, the detection limit amounted to only 0.3 mg L− 1. One of the most powerful tools of the modern instrumental analysis is the inductively coupled plasma mass spectrometry (ICP-MS). Since the ionization efficiency for fluorine in the Ar-ICP is only 0.0009%, it cannot be determined as positive ion in the mass spectrometer.
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Recently, the negative ion formation of atoms with high electron affinity was observed and on this basis the negative ion ICP-MS developed [7]. Although the detection limit of fluorine is sufficiently low (0.11 µg L− 1), this method requires special instrumentation and cannot be used widely. The determination of fluorine using X-ray fluorescence (XRF) is not common, because of the very low fluorescence yield for the elements with atomic number (Z) less than 15, and the absorption of the fluorescent radiation in the beam pathway. Using the conventional wavelength dispersive XRF technique, for determination of fluorine in solid samples and a detection limit of 120 µg g− 1 was achieved [8]. The above mentioned problems can be partly solved if the XRF spectrometer is specially designed for the low Z (Z b 15) elements. A very important factor influencing the fluorescence yield is the energy of the primary X-ray beam. With decreasing energy the fluorescence yield can be enhanced considerably [9]. Conducting the entire beam pathway in vacuum, the absorption in the air can be avoided. Furthermore, the most frequently used Be entrance window of the detector has a very low transmittance for F-Kα, but this window can be replaced by a special polymer foil [10,11]. The first total reflection X-ray fluorescence (TXRF) spectrometer for the determination of the low Z elements was developed in Atominstiute, Vienna [11]. This equipment is already available under the name Wobistrax. Although the range of detectable elements was extended down to carbon (Z = 6), this spectrometer has not been used for fluorine determination yet. In the present work the most important analytical figures of merit of the fluorine determination in water using the Wobistrax TXRF spectrometer are demonstrated. The investigated parameters were: detection limit, linearity range, sample amount, precision of the measurement, and the matrix effect caused by chlorine, bromine, iodine and sulfate. 2. Experimental
stable at about neutral pH, since the acidification would convert fluoride to hydrofluoric acid, which is volatile and can damage the sample support made of quartz or silicon, and (iii) its characteristic line has to be free from spectral interferences. Among the elements having K-lines in the above mentioned range, only vanadium in the oxidation stage of +5 can fulfill these requirements. Thus, vanadium as VO− 3 can be used. 2.2. Reagents For the preparation of standard solutions, analytical grade crystalline NaF was used (Reanal, Budapest, Hungary). Vanadium internal standard stock solution of 1000 mg L− 1 was prepared from analytical grade solid NH4VO3 (Reanal, Budapest, Hungary). The 5000 mg L− 1 single stock solutions of the interfering components (Cl, Br, I, and sulphate) were prepared from analytical grade crystalline KCl, KBr, KIO3 and Na2SO4·10H2O, respectively. For preparation of these solutions, distilled and deionized water (Milli-Q Plus, Millipore, Molsheim, France) was used thoroughly. 2.3. Instrumentation The measurements were carried out using a Wobistrax low Z spectrometer (Atominstitut, Vienna, Austria) described in [11] (Fig. 1). The operating conditions were as follow: Cr-anode X-ray tube at 30 kV, 30 mA, Ni/C multilayer monochromator set to 5.4 keV, KETEK silicon drift detector with 10 mm2 active area and ultra thin polymer window; electron trap around the window in order to decrease the spectral background from Auger- and photoelectrons, 500 s live time. The concentration of the internal standard was 24 mg L− 1 in the end volume of all solutions. The samples were pipetted onto quartz supports. All measurements were performed 12 times, whereas each of the three triplicate supports were measured in four different positions (always rotated by 90°).
2.1. Internal standardization 2.4. Investigation of the analytical figures of merit The choice of an appropriate internal standard is a critical point in the TXRF analysis, especially in the low Z range. There are several requirements to be fulfilled: (i) the internal standard element has to have a characteristic line below 5.4 keV (Cr-Kα), (ii) it should be
In order to investigate the effect of siliconization on the analytical signal of fluorine 2 μL of standard solutions was dropped onto the quartz carrier plates with and without siliconization.
Fig. 1. The schematic structure of the Wobistrax TXRF spectrometer.
G. Tarsoly et al. / Spectrochimica Acta Part B 65 (2010) 287–290
289
Table 1 Limit of detection (LOD) of F in presence of different interfering components with concentration of 100 mg L− 1.
LOD/mg L− 1
Pure water
Cl
Br
I
Sulfate
5
20
20
35
35
For the investigation of the influence of sample volume (practically the size of the sample spot) on the analytical signal 0.5 μL, 1 μL, 2 μL, 4 μL, and 6 μL standard solutions with a concentration of 50 mg L− 1 F were pipetted onto the supports. The detection limit was determined both in pure water with just V internal standard, and in presence of the interfering elements. In this latter case the elements were added separately at an F concentration of 100 mg L− 1. The detection limit was calculated as 3σ of the blank. The linearity range was checked using 25, 50, 100, 200, 500, and 1000 mg L− 1 F standard solutions. The interference effects of Cl, Br, I, and sulfate were investigated in the concentration range of 50 to 1000 mg L− 1 for each interfering component separately. The F concentration was 50 mg L− 1 in all samples. 3. Results and discussion 3.1. Influence of hydrophobization
Fig. 2. a Calibration curve of F with siliconized supports. R2 = 0.999. Error bars represent the standard deviation of 12 replicates. b Calibration curve of F with non-siliconized supports. R2 = 0.933. Error bars represent the standard deviation of 12 replicates.
Usually, the sample supports in the TXRF practice are always hydrophobized by a silicone solution prior to use, in order to inhibit the spreading of the sample spot and getting out of the detector active area. On the other hand, in case of low Z measurements, extreme care must be taken to keep the sample thickness possibly below 1 μm. Therefore, the usefulness of the siliconization has to be checked. For this propose, calibration curves were taken both with and without silicone pre-treatment, as described in Chapter 2.4. As it is shown in Fig. 2a–b, the sensitivity for F is 8.5 times higher in the case of non-siliconized supports (within the linear range), which is caused by the considerably thinner sample spots, resulting in lower selfabsorption. Moreover, the linearity range is much broader, the upper concentration limit was 500 mg L− 1, against 200 mg L− 1 with hydrophobized supports. On the basis of these observations, it was decided to omit the siliconization of the supports in the further experimental work. 3.2. Influence of the sample amount The sample spot thickness can be controlled in two ways. Either the sample dilution or the sample volume has to be optimized. Since the detector in the spectrometer has a quite narrow entrance window 10 mm2, the influence of the sample amount had to be investigated by all means. In Fig. 3 it can be seen, that the sample volume does not
Fig. 3. Influence of the sample volume onto the relative intensity applying non-siliconized quartz carrier plates. Error bars represent the standard deviation of 4 replicates.
Fig. 4. Spectrum of a sample containing 50 mg L− 1 F and 50 mg L− 1 sulfate.
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influence considerably the measured relative intensity of F. On the other hand, the relative standard deviation (RSD) varies between 2.5 and 8.9% (n = 12), having its minimum at 2 μL. At both edges of the investigated range the RSD amounts to more than 8%. This can be explained by the facts, that (i) at lower volume the count rate is lower, and the statistics of the peak fitting is worse, (ii) at higher volume a considerable part of the sample spot is outside the area which can be seen by the detector, and incidental inhomogeneities in the spots can disturb the precision of the determination. 3.3. Detection limit of fluorine Table 1 contains the detection limit data of F, both in pure water and in the presence of the investigated interfering elements, each at a concentration of 100 mg L− 1. It can be established that the detection limit expressed in concentration is relatively high compared with e.g. the ion selective electrode, and will be considerably impaired by the interfering elements. On the other hand, it should be considered that the TXRF method requires only 2 μL of sample, and the absolute detection limit amounts to 10 to 70 ng F. This can be favorable in the case when the available sample volume is very low, e.g. saliva samples from the interdental space. 3.4. Linearity range As already mentioned in Chapter 3.1, the upper concentration limit is 500 mg L− 1. Considering a quantification limit of about 15 mg L− 1 (equal to 10σ), the linearity range covers about 1.5 orders of magnitude. 3.5. Interference effects 3.5.1. Chlorine Chlorine is the most abundant accompanying element of fluorine. It has an escape peak at 0.88 keV, which might have interfered with the FKα peak at 0.68 keV. Fortunately, the detector could resolve the two peaks completely down to baseline. On the other hand, the increasing concentration of Cl had a negative influence onto the intensity and RSD of the F signal. In the presence of 100 mg L− 1 Cl, the RSD and signal depression were 16 and 5%, respectively. Increasing the Cl concentration further to 200 mg L− 1, the F peak cannot be detected any more. In conclusion, the upper limit of Cl concentration is 100 mg L− 1. As mentioned in Chapter 3.3, the detection limit increased to 20 mg L− 1. 3.5.2. Bromine Bromine has theoretically no interference with either F or V. The BrLα peak (1.48 keV) does not cause any spectral interference. Similar to Cl, the spectral background will be considerably higher with increasing Br concentration, but its extent is lower. At a Br concentration of 200 mg L− 1, the RSD of the F signal amounts to 25%. Both F and V signals are depressed by about 20%, but the calculated relative intensity remains practically constant. In conclusion, the upper limit of Br concentration is 200 mg L− 1. As mentioned in Chapter 3.3, the detection limit increased to 20 mg L− 1. 3.5.3. Iodine Iodine has a number of L-series peaks in the range around the V-Kα line (4.95 keV). The presence of iodine has a very strong depressing influence onto the F signal, which can be explained by the strong absorption of the fluorescent X-rays. Although in an extra test solution containing 100 mg L− 1 F and 1000 mg L− 1 I, the F peak could be fitted with an RSD of 45%, is was absolutely impossible to find the V peak. The spectral interference on V facilitates the peak fitting up to an I concentration of 200 mg L− 1, but the RSD amounts to 68% there. The RSD of the relative intensity as IF/IV at iodine concentrations less than 50 mg L− 1 remains below 20%.
In conclusion, the upper limit of I concentration is 50 mg L− 1. The detection limit increased to 35 mg L− 1 F. 3.5.4. Sulfate The presence of sulfate causes very strong spectral interference onto the F-Kα peak, since there are two escape peaks of S at 0.57 and 0.72 keV, and the detector is not able to resolve the three peaks. Especially the more intensive S-Kα-escape peak has a strong influence; at 50 mg L− 1 sulfate it has almost 50% intensity of the F-Kα peak (Fig. 4). This observation can be an explanation for the considerable increase of the RSD of F signal, although the net F signal intensity did not change systematically within the investigated concentration range up to 1000 mg L− 1 sulfate. In conclusion, the upper limit of sulfate concentration is 100 mg L− 1. The detection limit increased to 35 mg L− 1 F. 4. Conclusions Generally, it can be established that fluorine determination can be performed by TXRF using a specially designed spectrometer for low Z elements. However, the analytical performance should be improved, especially the detection limit is not enough low for analysis of drinking water. Moreover, there are serious spectral interferences caused by Cl, Br, I and sulfate, which are common constituents in various sample types, having typically higher concentration than F. To overcome this problem, a matrix separation method can be recommended. The method could be used for the determination of F in airborne dust samples in aluminum smelters (cryolite). A further possible application can be the determination of F in saliva samples from the interdental space. Since the Wobistrax spectrometer will be available soon with detectors of broader entrance window (now maximally 30 mm2, later 100 mm2), the sample volume can be increased resulting in lower detection limits. Henceforward, it is projected to reach the concentration range of drinking and mineral waters, and to carry out the determination in real matrices. Acknowledgment The authors are grateful for the financial support of the EU in framework of a Marie Curie European Reintegration Fellowship (MERG-CT-2004-513463). References [1] C. Reimanna, D. Banks, Setting action levels for drinking water: are we protecting our health or our economy (or our backs!)? Sci. Total Environ. 332 (2004) 13–21. [2] L.E. Vanatta, Ion-chromatographic determination of seven common anions in electronic-grade, water-miscible solvents, J. Chromatogr. A 1213 (2008) 70–76. [3] S. Jeyakumar, V. Vaibhavi, V. Raut, K.L. Ramakumar, Simultaneous determination of trace amounts of borate, chloride and fluoride in nuclear fuels employing ion chromatography (IC) after their extraction by pyrohydrolysis, Talanta 76 (2008) 1246–1251. [4] G. Wejnerowska, A. Karczmarek, J. Gaca, Determination of fluoride in toothpaste using headspace solid-phase microextraction and gas chromatography–flame ionization detection, J. Chromatogr. A 1150 (2007) 173–177. [5] K. Burger, Principles of the Analytical Chemistry — Chemical and Instrumental Analysis (in Hungarian), Alliter Publishing, Budapest, 2002. [6] J.R. Santos, R.A.S. Lapa, J.L.F.C. Lima, Development of a tubular fluoride potentiometric detector for flow analysis. Evaluation and analytical applications, Anal. Chim. Acta 583 (2007) 429–436. [7] A.A. Pupysheva, V.T. Surikov, Application of negative ions in inductively coupledplasma-mass spectrometry, Spectrochim. Acta Part B 59 (2004) 1021–1031. [8] S.M. Hasany, F. Rashid, A. Rashid, H. Rehman, Determination of fluorine in solids down to 120 μg/g by wavelength dispersive X-ray fluorescence spectrometry, J. Radioanal. Nucl. Chem. 148 (1991) 211–216. [9] J.W. Robinson, Handbook of Spectroscopy, Vol. 1, CRC, New York, 1979. [10] Official homepage of Canberra Inc. http://www.canberra.com/products/524.asp. [11] C. Streli, P. Wobrauschek, G. Pepponi, N. Zoeger, A new total reflection X-ray fluorescence vacuum chamber with sample changer analysis using a silicon drift detector for chemical analysis, Spectrochim. Acta Part B 59 (2004) 1199–1203.