Thus for the first time a simple method for the detection of the total fluorine content (ionic and covalent combined fluorine) exists. Time-consuming and error-prone upstream pyrohydrolysis or derivatisation for water-based media become obsolete. Even fluorine in only partially hydrolised, yet water-based combinations, such as in sodium monofluorphosphate (MEP), a caries prophylactic active component in toothpaste, is detected completely using the method described. This simplifies and shortens the quality control during production for the detection of the total fluorine content in toothpaste. Other potential applications can be found not least by the detection of the total fluorine after the digestion of solid samples.
REFERENCES
1. Kolditz, L. Anorganische Chemie [Inorganic chemistry]. Berlin: Deutscher Verlag der Wissenschaften, 1980, p.492.
2. Quentin, Karl-Ernst. Trinkwasser – Untersuchung und Beurteilung von Trink- und Schwimmbadwasser [Examination and evaluation of drinking water and swimming pool water]. Heidelberg: Springer-Verlag,1988.
3. Becker, Harald. Wasser – Analyse, Bewertung, Reinigung [Water analysis, evaluation, purification]. Darmstadt: GIT Verlag,1985. ISBN 3-921956-48-X.
4. Fluorose [fluorisis].
http://de.wikipedia.org/wiki/Fluorose: s.n. Figure 3. HR-CS AAS contrAA® 700 (Analytik Jena)
Method optimisation of the GaF molecular absorption
The temperature/time program for the analytical use of molecular absorption consists of 3 phases: Drying, ashing and molecule formation. During the drying and ashing steps any advance losses of the analyte in the form of volatile HF must be prevented by optimising the drying and pyrolysis temperature and using an efficient modifier. The purpose of the molecule formation phase is to generate the desired biatomic molecule by adding an appropriate molecule generation reagent. By selecting an optimal temperature a sufficient large number of these molecules must be formed. However, the temperature must not be chosen too high to prevent too early a decomposition into its atomic components.
To form Ga monofluoride a 10g/L Ga standard (from SCP Science) in 4% HNO3 is used as molecule formation reagent. The best analytical results could be achieved when the graphite furnace with integrated PIN platform used was permanently coated with Zr before its analytical use. To stabilise the analyte and the Ga during ashing, a Pd/ Zr modifier (0.1% Pd, 20mg/L Zr) was brought into a thermally active form together with the molecule formation reagent at 1100°C before each sample injection. Under these conditions an ideal ashing temperature of 550°C was identified. NaAc and Ru-III-nitosyl nitrate were used as modifier to reduce advance analyte losses by formation of volatile HF. Under these conditions 1550°C was identified as the ideal molecule formation temperature.
The results of the examined samples are shown in Table 1 and display a very good correspondence to the certified values, the parallel determinations using ISE and the average manufacturer information in the case of the mineral water.
SUMMARY
A simple, fast, fully automatable and robust method for the detection of fluorine in drinking water is presented. The method is based on the measurement of molecular absorption of GaF at a wavelength of 211.248 nm with a commercially available HR-CS AAS in graphite furnace technology. As molecule formation reagent a 10 g/L Ga solution is used. The best results were achieved with a permanently Zr coated PIN platform tube. To stabilise the analyte and to prevent advance losses various modifiers are used. Using a special thermal pre-treatment of the Pd/ Zr modifier and a reagent at 1100°C fluorine can be pyrolysed up to 550°C and is available at 1550°C for an efficient molecule formation. Using this optimised method a detection limit of 0.26µg/L F was identified which is clearly greater than that of all other currently available methods. With the basic principle of molecular absorption (MAS) analogue to atomic absorption (AAS) the detection is very robust. Limitations and disadvantages of the common methods IC (sample throughput, limitation to water-based medium, non- particular samples) and ISE (limited pH range, defined ionic strength, salt content) are not a factor. Another benefit of this method is that the range of application can also be extended to biological matrixes, such as urine, serum and blood, without difficulty.
5. TrinkwV 2001. Verordnung über die Qualität von Wasser für den menschlichen Gebrauch (Trinkwasserverordnung) [Ordinance about the quality of water for human use] . 2001.
6. Dionex. Application Note 140. Fast Analysis of Anions in Drinking water by Ion Chromatography.
7. Tokalioglu, S, Kartal, S. and Sahin, U. Determination of Fluoride in Various Samples and some Infusions Using a Fluoride Selective Electrode. Turk. J. Chem. 2004, Vol. 28, pp. 203-211
8. Dittrich, K., Shkinev, V. M. and Spivakov, B. V. Molecurar absorption spectrometry (MAS) by electrothermal evaporation in a graphite furnace-XIII: Determination of traces of fluoride by MAS of AlF after liquid-liquid extraction of fluoride with triphenylantimony(V) dihydroxide. Talanta. 1985, Vol. 32, pp. 1019-1022.
9. Dittrich, K. Prog.Anal. Atom. Spectrosc. 1980, Vol.3, p.209.
10. Tsunoda, Kin-Ichi, Haraguchi, Hiroki und Fuwa, Keilchiro. Studies on the occurence of atoms and molecules of aluminum, gallium, indium and their monohalogenides in an electrothermal carbon furnace. Spectrochimica Acta. 1980, Bd. 35, S. 715-729.
11. Welz, Bernhard, et al. Determination of phosphourus, sulfur and the halogens using high.temperature molecular absorption spectrometry in flames and furnaces – A review. Analytica Chimica Acta. June 2009, Vol.647, p.137-148.
12. Heitmann, Uwe und Becker-Ross, Helmut. Atomabsorptions-Spektrometrie mit einem Kontinuumstrahler (CS-AAS) [Atomic absorption sepctrometry using a continuous radiation source (CS-AAS)]. GIT Laborfachzeitschrift. 2001, 7, S. 728-731.
13. Gleisner, Heike, et al. Die AAS wird neu definiert – Atomabsorptionsspektrometrie mit nur einer Strahlungsquelle [AAS redefined - atomic absorption sepctrometry using a single radiation source]. LABO. April 2004, S. 64-67.
14. Schlemmer, Gerhard und Gleisner, Heike. Leistungsfähigkeit der Untergrundkorrektur [Capability of background correction]. Chemie. 04 2008, S. 34-39.
Table 1. Sample results, *VF= dilution factor, ** information value, *** concentration information from the manufacturer catalogue Sample ION-915 Hamilton-20
Figure 4. F calibration curve using GaF molecular absorption at a wavelength of 211.248nm: 2 – 10µg/L F, 20µL sample injection
Calibration
Under the ideal conditions established a calibration was carried out in the range of 2 - 10 µg/L F (Figure 4). On the basis of this calibration and the triple standard deviation from 11 repetitions of the blank calibration value a detection limit of 0.26µg/L F was determined. This detection limit for fluorine is approx. one decade better than with IC or ISE.
To test the developed method 3 drinking water samples, one mineral water and two certified reference materials were examined. Because the fluoride content to be expected was way beyond the calculated detection limit, another calibration was carried out in the range of 10 - 50 µg/L F.
TW Bad Berka TW Tiefengruben TW Sachsenhausen Mineral water QC standard 4:
VF* Concentration in µg/L F
1 10 5 5 5 5 41,0 ± 2,0 424 ± 21 132 ± 8,7 146 ± 8,8 237 ± 11 148 ± 8,2 38.7 ± 1.14 0.4 40µg/L F QC standard 2: (96,7%) 20.4 ± 1,03 3.7 20µg/L F (102%) RSDin % 2,4 2,5 4,3 2,8 1,1 1.5 0.14 0.15 0.25 0.16**
ISE concentration in mg/L F
Certified
concentration in mg/L F
0,03**/0,048*** 0,42 ± 0,078
Spectroscopy Focus
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