Roger V. Lloyd

Unexpected Photolysis of the Sunscreen Octinoxate in the Presence of the Sunscreen Avobenzone

Abstract A major concern raised about photostability studies of sunscreen products is that the photodegradation of sunscreens does not readily translate into changes in product performance. This study examines the correlation between photochemical degradation of sunscreen agents and changes in protection provided by sunscreen films. Films of a commercial sunscreen product containing avobenzone, oxybenzone and octinoxate were irradiated using a fluorescent UV-A phototherapy lamp with additional UV-B blocking filter. Periodically, during irradiation the transmittances of the films were measured and samples collected for chemical analysis of the sunscreen agents using high-performance liquid chromatography techniques. The results show that UV-induced changes in UV transmittance of sunscreen films correlate with changes in concentration of sunscreen agents. In a parallel experiment, we also irradiated a thin film of the same product in the cavity of an electron spin resonance (ESR) spectrometer. We report the concomitant photolysis of avobenzone and octinoxate that predominates over expected E/Z photoisomerization and that irradiation of a film of this product produced free radicals detected by ESR spectroscopy that persisted even after exposure had ended.

Mn(II) partitioning between calcium and magnesium sites in studies of dolomite origin

Abstract In an Electron Spin Resonance (ESR) study of over 30 dolomites, representing a variety of probable sedimentary origins, we observed a Mg-site to Ca-site ratio in the Mn distribution that suggests two groups of dolomite. In one group the dolomites have a Mn distribution ratio near 5.0 and an excess of Ca; in the other group the dolomites are near-stoichiometric but have a Mn distribution ratio that varies from near 5 to near 70. We suggest that these two groups reflect different origins for the dolomites. The wide variation in the manganese distribution ratio for the near-stoichiometric dolomites supplies a parameter against which other variables can be plotted in studies of dolomite origin.

Mineralogy and Mn geochemistry of laboratory-synthesized dolomite

Abstract Dolomite was synthesized in hydrothermal bombs to assess influences on Mn partitioning into the Mg and Ca sites. Calcite was added to magnesium chloride solutions doped with 0, 10, 20, or 50 ppm Mn. Runs were made for 8 to 241 hr at 192 and 224°C. The calcium content of dolomite increased to 60–64% after 75 hr at 192°C and reverted to stoichiometry thereafter. Increases in the Mn content of the starter solution progressively retarded formation of dolomite. For dolomites formed without added Mn, or formed in less than 40 hr with 10 ppm added Mn, the ESR signals were too broad to permit determination of Mn site distribution ratios. Measurable ratios of Mn in the Mg site to Mn in the Ca site vary as a linear function of time, from 0.3 to 7 for samples prepared at 192°C and from 3 to 16 for those prepared at 224°C. Stoichiometric dolomites have higher Mn partitioning ratios than nonstoichiometric dolomites, as observed in natural systems. Dolomites formed in supratidal/sabkha, early groundwater, and Dorag conditions may differ in their Mn signature from those formed during burial diagenesis.

Relationship between paleotemperatures of metamorphic dolomites and ESR determined Mn(II) partitioning ratios

Abstract An Electron Spin Resonance (ESR) study of the distribution of Mn(II) between Mg sites and Ca sites in 28 contact metamorphic dolomite samples suggests that the distribution is temperature sensitive. The data can be modelled by alternative interpretations. In the first a polynomial equation approximates the trend of the data suggesting an exponential relation with Mn(II) ratios decreasing to a stable value of 10 with increasing temperature (to near 900°C). The second suggests two groups of dolomites. One group, close to the intrusion (

The Role of Iron and Mg/Ca Ratio in Dolomite Synthesis at 192°C

The reaction path that dolomite follows in its formation, approach to stoichiometry, and approach to order differs in the presence or absence of iron. Dolomite was synthesized in hydrothermal bombs using eight starter combinations heated at 192°C for eight time periods. When iron was present dolomite became the dominant mineral within two days, whereas it took five days to produce similar amounts of dolomite without iron. Near-stoichiometric composition (51%-52% Ca) was reached in three days when iron was present, versus six days in iron-free experiments. Further, a better cation-ordered dolomite formed earlier in iron-containing experiments. The absence of a significant difference in the results of experiments using

The influence of ferrous and ferric iron on the Mn+2 partitioning ratio and ESR signal of synthetic dolomite

Fe^{+2} vs. Fe^{+3}

Polarized emission from doublet state organic free radicals

suggests that the influence of iron was exerted in solution rather than in the solid phase and that oxidation state exerts relatively little influence on the formation of dolomite. More