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Application Notes
The mineral fluorspar (or fluorite) is the most important source of fluorine. The purity (calcium fluoride content) of the mineral will determine its eventual use in the steel, aluminium or chemical industries. The highest grade is used to produce hydrogen fluoride which is a precursor to many important compounds including pharmaceuticals and polymers, e.g. polytetrafluoroethylene (PTFE), known by the commercial name of Teflon. It is therefore important to determine the fluorine content of fluorspar for commercial and quality control purposes.
Fig. 1 — NMR calibration for calcium fluoride in fluorspar
Conventional wet chemical methods of calcium fluoride (CaF₂) determination include measurement of fluoride by ion selective electrode or ion chromatography, and calcium by complexometric titrimetry (ASTM E815). All of these methods are time-consuming operations, require skilled chemists and involve the use of potentially hazardous chemicals which require disposal, all of which contribute to the cost of the analysis.
The MQC+ benchtop Nuclear Magnetic Resonance (NMR) analyser offers a simple, non-destructive and rapid method for measurement of fluorine in fluorspar which may be used for routine analysis in a production environment without any requirement for additional chemicals and specialist operator training.
The analytical technique is based on direct measurement of the Nuclear Magnetic Resonance (NMR) signal of fluorine-19 which has 100% natural abundance. It is assumed that the measured NMR response is linearly proportional to the CaF₂ content of a material. The acquired NMR signal is normalised by the sample mass and then the CaF₂ content (weight-%) is calculated using an appropriate calibration curve.
It is possible to calibrate the MQC+ using only two samples if the reference values are known to be accurate. However, initially it is recommended that the instrument is calibrated by 3–6, preferably more, standards with known CaF₂ contents evenly spread over the range of interest. NMR is a comparative technique therefore cannot be more accurate than the reference technique against which it is being compared; error is reduced by analysing more reference samples.
A tared sample tube is filled to a given height with the powder sample, then weighed prior to NMR analysis. Measurement time is 80 seconds per sample.
Table 1 shows a comparison of the predicted NMR values against the given reference values. Deviations can be accounted for by error in the reference values as well as fluorine from sources other than calcium fluoride contributing to the NMR signal.
Table 2 shows that the repeatability (or precision) of the NMR measurement of the same sample is excellent.
| Sample ID | Given %CaF₂ (wt.-%) | Predicted %CaF₂ (wt.-%) | Difference %CaF₂ (wt.-%) |
| FG1 | 81.9 | 83.5 | +1.6 |
| FG2 | 66.0 | 66.3 | +0.3 |
| FG3 | 11.3 | 12.6 | +1.3 |
| FG4 | 26.8 | 24.2 | -2.6 |
| FG5 | 47.7 | 45.1 | -2.6 |
| FG6 | 11.6 | 11.6 | +0.0 |
| FG7 | 10.7 | 12.5 | +1.8 |
| FG8 | 43.8 | 44.1 | +0.3 |
Table 1: NMR results predicted for calibration samples
| Repeat | Measured CaF₂ content, % |
| 1 | 37.1 |
| 2 | 37.0 |
| 3 | 37.3 |
| 4 | 37.1 |
| 5 | 37.2 |
| 6 | 37.0 |
| 7 | 37.0 |
| 8 | 37.1 |
| 9 | 37.2 |
| 10 | 37.0 |
| Mean % | 37.1 |
| Std. Dev. % | 0.1 |
Oxford Instruments offers a package especially tailored to the measurement of fluorine in fluorspar.
Optional items are:
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