Spectroscopy Focus Fluorine Detection in Drinking Water Using HR-CS AAS


Due to its high electro negativity the element fluorine is the most reactive non-metal and thus does not occur in elementary, but only in combined form. Fluorine is the most widespread halogen. Its share in the earth's crust is approx. 0.08%. It occurs in large


quantities in apatite Ca5(PO4)3(OH,F) and in fluorite CaF2 as well as in the almost exhausted cryolite Na3AlF6 [1]. It is therefore no surprise that fluoride is also found in almost all water


bodies - although the fluoride concentration can differ greatly by water type and the geogenic conditions. Seawater contains more than 1mg/L fluoride, rivers and lakes approx. 0.05 - 0.5mg/L, whereas in ground water values above 0.5mg/L are relatively rare. However, in deepwater, especially in sources from hydrothermal deposits, significantly higher fluoride content can also be found, e.g. in geysers more than 20mg/L. Mainly responsible for the fluoride content are the pH value, temperature, solubility conditions and geological preconditions [2, 3].


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The methods range from classical gravimetry and volumetry to photometry and electrochemical titration.


Author Details:


Heike Gleisner, Analytik Jena AG, 07745 Jena, Germany. www.analytik-jena.com


There are two sides to the effects of fluoride on human health. On the one hand it is essential for the human organism, because the fluoride ingested with food is a condition for the mineralisation of the apatite in bones and teeth. In this respect a corresponding fluoride content in the drinking water as the most important food is also of great importance with regard to an adequate caries prophylaxis. On the other hand too high a daily absorption of fluoride in all the ingested food can have fatal consequences. Tooth and bone fluorosis result if the daily total fluorine absorption exceeds approximately 20mg F/day [4].


The control of the fluoride concentration in our foodstuffs is therefore of major importance. Drinking water is the number 1 food and thus subject to a particularly intensive control. Fluoride has been categorised as a substance causing health disorders after a given concentration. The limit defined for fluoride in the drinking water regulations is 1.5mg/L [5].


MEASURING METHODS FOR FLUORIDE DETECTION


The detection of fluorine as non-metal has been sufficiently described in the literature. The methods range from classical gravimetry and volumetry to photometry and electrochemical titration. The methods for detecting the fluoride concentration in water dominating today are the ion chromatography (IC) [6] and the use of ion-selective electrodes (ISE) [7]. Both detection methods have in common that they only respond to ionic dissolved fluoride. Organic or covalent combined fluorine is not detected, so that these detection methods can only be used for purely water-based matrixes.


SPECTROSCOPIC METHODS


Spectroscopic methods, such as ICP-OES, for the detection of fluorine are not practicable because of the very high ionisation potential of 17.42 eV and the resonance lines of the fluorine thus being below 100nm. For a similar reason, classic AAS can also not be used to determine fluorine. A suitable alternative is the detection of fluorine using molecular absorption spectrometry (MAS). First investigations were carried out by Dittrich [8,9] and Tusunda [10], roughly at the same time. Due to the relatively moderate resolution of the spectrometers used at the time and a limited background correction, this method did not succeed in the detection of fluorine.


With the development of the High-Resolution Continuum Source AAS (HR-CS AAS) and the commercial availability of these AAS devices with the contrAA 300 and contrAA 700 from Analytik Jena the conditions have now been provided to use this method successfully in the detection of fluorine.


Below a simple, fast and robust method for the detection of fluorine in drinking water using HR-CS AAS is described and its practical applicability tested in different drinking water samples and a reference material.


FLUORINE DETECTION BY MOLECULAR ABSORPTION WITH HR-CS AAS


In recent years, based on the availability of HR-CS AAS, various methods for the detection of non-metals such as P, S, F, Cl, Br, J by molecular absorption in combination with an AAS were published. An article by Welz et.al. [11] providing an overview summarised the work.


The analytical use of molecular absorption spectrometry for the detection of fluorine is based on the formation of stable monofluorides (AlF, GaF, InF, CaF). These biatomic molecules can just like atoms absorb defined energy from a continually emitting spectral radiation source, resulting in the generation of molecular absorption spectra. The molecular absorption spectra correspond to the molecular transitions between the different molecule states.


Figure 2. Molecular absorption spectrum of GaF by wavelength resolution in the region of 211.248nm, injection of 10ng F


Figure 1. Molecular absorption spectrum of AlF by wavelength resolution in the region of 227.47nm, injection of 10ng F


A difference is made between electron excitation, oscillation and rotation transitions. The number of possible transitions is greater than for atoms, which means that the molecular absorption spectra have more lines than atomic absorption spectra. The line width of various molecular absorption lines is roughly equal to that of atomic absorption lines and can thus be resolved and used for analysis in the HR-CS AAS.


Figures 1 and 2 show the molecular absorption spectra of the most sensitive AlF and GaF molecule lines. All subsequent investigations were carried out on the molecular absorption line of GaF at a wavelength of 211.248nm due to its greater sensitivity and better resolved molecular lines.


EXPERIMENTAL


Instrumentation All measurements were carried out with a HR-CS AAS contrAA®


furnace technology.


This is an atomic absorption spectrometer with a Xe short arc lamp as radiation source [12,13]. The Xe lamp emits a continuous spectrum in the range of 185 - 900nm. This provides every wavelength required for analytical use. This is a condition for the analytical use of molecular absorptions of any wavelength. As atomisers a flame and a transversely heated graphite tube furnace are available in two separate rooms. The high resolution spectrometer consists of a prism upstream monochromator and an echelle grating to guarantee a resolution of 2pm at a wavelength of 200nm. As detector an CCD array is used guaranteeing a simultaneous and powerful background correction and providing additional information about the examined analysis line through the simultaneous evaluation of 200 detector pixels [14].


700 (Figure 3) from Analytik Jena using graphite


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