RESUMO
To disentangle the factors controlling the rates of accelerated reactions in droplets, we used mass spectrometry to study the Katritzky transamination in levitated Leidenfrost droplets of different yet constant volumes over a range of concentrations while holding concentration constant by adding back the evaporated solvent. The set of concentration and droplet volume data indicates that the reaction rate in the surface region is much higher than that in the interior. These same effects of concentration and volume were also seen in bulk solutions. Three pyrylium reagents with different surface activity showed differences in transamination reactivity. The conclusion is drawn that reactions with surface-active reactants are subject to greater acceleration, as seen particularly at lower concentrations in systems of higher surface-to-volume ratios. These results highlight the key role that air-solution interfaces play in Katritzky reaction acceleration. They are also consistent with the view that reaction-increased rate constant is at least in part due to limited solvation of reagents at the interface.
RESUMO
The Katritzky reaction in bulk solution at room temperature is accelerated significantly by the surface of a glass container compared to a plastic container. Remarkably, the reaction rate is increased by more than two orders of magnitude upon the addition of glass particles with the rate increasing linearly with increasing amounts of glass. A similar phenomenon is observed when glass particles are added to levitated droplets, where large acceleration factors are seen. Evidence shows that glass acts as a "green" heterogeneous catalyst: it participates as a base in the deprotonation step and is recovered unchanged from the reaction mixture. Reaction acceleration at two separate interfaces is recognized in this study: i)â air/solution phase acceleration, as is well known in microdroplets; ii)â solid/solution phase, where such acceleration appears to be a new phenomenon.
RESUMO
Human scalp hair samples were collected and used to assess exposure to toxic elements and essential elements in the state of North Carolina, USA using accelerated microwave assisted acid digestion and inductively coupled plasma optical emission spectroscopy (ICP-OES). The figures-of-merit of the ICP-OES were appropriate for elemental analysis in scalp hair with detection limits as low as 0.0001 mg/L for Cd, good linearity (R2 > 0.9978), and percent recoveries that ranged from 96 to 106% for laboratory-fortified-blanks and 88-112% for sample spike recovery study. The concentrations of essential elements in scalp hair were larger than those of toxic elements, with Ca having the highest average concentration (3080 µg/g, s = 14,500, n = 194). Some of the maximum concentrations observed for As (65 µg/g), Ni (331 µg/g), Cd (2.96 µg/g), and Cr (84.6 µg/g) in individual samples were concerning, however. Samples were statistically analyzed to determine the influence of race, gender, smoking habits, or age on the elemental concentrations in scalp hair. Higher concentrations of essential elements were observed in the scalp hair of Caucasians, females, and non-smokers, and the differences were often significant at a 90% confidence level. Several pairs of essential elements, for example Ca-K, Ca-Mg, and Ca-Zn, were strongly correlated in Caucasian hair but uncorrelated in African-American hair. Similarly, essential elements were strongly correlated in female hair but weakly correlated in male hair. Toxic element pairs (As-Cd, As-Se, Pb-As, and Se-Cd) were strongly correlated in the hair of smokers but uncorrelated in that of non-smokers, suggesting that cigarette smoke is a common source of toxic elements in humans.
