There are two primary forms of the element fluorine that we measure: inorganic fluoride and organic (covalent) fluorine. When deciding the analytical method, it is important to distinguish which is desired. Some preparation methods convert covalent fluorine into fluoride, thereby providing a "total fluorine" result.
A mass of sample is weighed for each analysis. The objective is to afford a final solution concentration that is within the calibration range of the method. To assist the technician in deciding what mass of sample to take, it is helpful for the client to provide a best-guess estimate of the fluorine concentration in the sample.
We have four primary sample preparation techniques for the measurement of the element fluorine: dissolution in Ionic Strength Adjuster (ISA) buffer (G35A), extraction with ISA buffer (G40), combustion (G54), and pyrohydrolysis (E9-3B). Three of the preparation techniques are used to measure inorganic fluoride. Only one measures organic fluorine.
- Dissolution (G35A) is for samples that are aqueous and/or are water soluble. The sample is dissolved in a known volume of ISA. This is a prep method for measuring inorganic fluoride only.
- Extraction (G40) is for samples that are not aqueous and/or are not water soluble. For this preparation method, the sample and extraction solvent are present as a heterogenous mixture. The insoluble sample is filtered or decanted off before proceeding with the analysis. This is a prep method for measuring inorganic fluoride only.
- Combustion (G54) is for samples containing significant carbon content, e.g. pharmaceuticals, plastic, wood, rubber, textiles, petroleum products, etc. This method indicates combustion of the sample in a Schoniger Tube. It is the only prep method for measuring organic fluorine and the only prep method that measures both inorganic fluoride and organic fluorine. This method cannot be used for aqueous samples. Water doesn’t burn, even in the pure oxygen atmosphere of a Schoniger Tube. Giving the sample a good steaming will not release organic fluorine.
- Pyrohydrolysis (E9-3B) is for samples containing little to no water or carbon content and typically are not water soluble, e.g. sand, dirt, ceramic, metals, some metal oxides. This method heats the sample in a tube furnace to 1100ºC in the presence of wet oxygen. Combustion products are conveyed to an absorber solution by the oxygen. This is a prep method for measuring inorganic fluoride only. This technique should not be used for a sample containing significant water or carbon content as in either case it will yield low biased results.
Once prepared, the analysis solution is analyzed by a fluoride ion-specific electrode (ISE) that is calibrated externally with fluoride standards. There are two calibrations that may be used, general and trace.
Summaries of the individual analysis methods are linked below. Please click on them to learn more.
Limit of Quantitation
Unless determined otherwise, the limit of quantitation of these methods is considered the concentration of the lowest calibration standard. The concentration of that solution for the general calibration is 0.5 µg/mL; for the trace calibration it is 0.1 µg/mL.
The weight/weight concentration of fluorine or fluoride in the sample is reported. The result may be reported on the dried basis or the as-received basis, depending on instructions from the client regarding sample drying.
The preparation and analysis methods offered by Galbraith Laboratories for fluoride are tailored to provide accurate results for a wide-range of sample matrices.