Further development of analytical methods for cristobalite using infrared spectroscopy and X-ray diffraction

Status:

completed 12/2023

Aims:

The determination of silicogenic dust components is regularly carried out at workplaces. With regard to cristobalite, however, non-systematic differences emerge repeatedly when comparing the results of established infrared spectroscopic and X-ray diffraction evaluations: While validation using ideal reference materials shows very good correlations, real samples of cristobalite from technical applications sometimes show significant differences in quantification. The technical extraction of cristobalite is carried out through the annealing of quartz, which produces not only cristobalite but also portions of amorphous SiO2 phases in the products. This is assumed to be a major cause for the differences. The investigations were intended to clarify the relationship between the proportion of amorphous components and the determination of cristobalite, and whether other previously unknown aspects are relevant. The aim of the research was to be able to quantify the interfering influences and thus optimize the existing analytical methods.

Activities/Methods:

Following proven methods for removing amorphised surfaces from quartz and cristobalite, tests were carried out with different intensities of treatment of cristobalite samples (various technical cristobalites and burnt diatomaceous earths) with caustic soda. The strength of the caustic solution, the temperature and the duration of treatment were varied. The tests were used to determine the appropriate parameters of this treatment for real cristobalite samples. The samples from the test series were analysed using both infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The analyses were supplemented by phase-contrast microscopic and scanning electron microscopic examinations. A strategy was derived from the tests as to how valid results can be achieved with the established analytical methods individually or in combination when analysing workplace dusts with regard to cristobalite. Aspects that may speak in favor of modifying or changing the existing methods were identified.

Results:

Based on the results of the tests with caustic soda treatment, the discrepancies between the analytical determination of cristobalite in dust samples by XRD and FTIR, which have been noticeable up to now, are largely due to a high proportion of amorphous silicas. However, these cannot be completely separated from the crystalline phase by treatment from typical cristobalites used in work areas. Moreover, since the cristobalite is also increasingly dissolved with more extensive treatment, this method is not suitable for workplace samples. However, it was possible to derive a procedure for the established analytical methods that allows the cristobalite to be quantified with as little interference and as reproducible as possible. The essential aspect is that the side band at 620 cm-1 of the FTIR spectroscopy should be used for this purpose (peak area or peak height of the band). In the case of interference due to cross-sensitivities, in the unfavorable case that the less intense secondary reflection at 36.7 °2θ is not quantifiable (peak area), the height of the main reflection at 25.58 °2θ should be used. Although the half-value width of the peaks in the XRD analysis tends to show/have smaller values for all samples due to the treatment, it does not approach the optimum assumed to be ideal, and shows different values depending on the sample.

As a result, it is not possible to make a well-founded statement on the basis of the half-value width alone as to how large the amorphous fraction contained in the sample is.

Caustic soda treatment is not recommended for routine samples, partly because working with 10-molar caustic soda is critical for routine work safety and treatment durations of more than one week are not practical. For the purification of cristobalite samples to be used as in-house standards however, the treatment is helpful.

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