Part I : Kinetic d a t a for t h e s t a t i c system s i l a n e pyrolysis ( f r o m 640-703 K , 60-400 torr) are presented. For conversion from 3-304. first-order kinetics a r e obtained, with silane loss rates equal to half the hydrogen formation rates. At conversions greater than 4 0 4 , rate inhibition attributable to the back reaction of hydrogen with silylene occurs. Overall reaction rates are not surface sensitive. but disilane and trisilane yield maxima under some conditions are. A nonchain mechanism capable of describing quantitatively all stages of the silane pyrolysis is proposed. Post 1.05; initiation is both homogeneous (gas phase) and heterogeneous (on the walls), and reaction intermediates are silylenes and disilenes. Free radicals are not involved a t any stage of the reaction. Rate data a t high conversions and with added hydrogen provide kinetics for the addition of silylene to hydrogen [reaction (-1iI relative to its addition to silane [reaction (2)l: k , / k , = 1 0~0 6 5 x e 3 2 0 " c~i ' H T . With E2 = 1300 cal, this gives a highpressure activation energy for silylene insertion into hydrogen of E -, = 8200 cal.Part 11: An analysis is made of each rate constant of the silane mechanism and the modeling results are compared with experimental results. Agreement is excellent. It is concluded that the dominant sink reaction for silylene intermediates is 1,2-H, elitnination from disilane (followed by SiYHl polymerization and wall deposition). The model is in accord with slow isomerization between disilene and silylsilylene and near exclusive 1,2-H2 elimination from S i L H G .It is also concluded t h a t disilene is about 10 kcal/mol more stable than silylsilylene and that the activation energy for isomerization of silylsilylene to disilene is greater than 26 kcal/;nol.
A safe, reliable, and efficient method is described for automated acid decomposition of biological samples, agricultural food products, and environmental material under high temperature and pressure. A high temperature/pressure asher that provides temperatures up to 320°C and pressures up to 100 bars (1450 psi) is used to heat 5 mL of ultrapure acid 4 h for complete sample decomposition. The unit is microprocessor-controlled with up to 5 programmable temperature levels and ramp time intervals. These levels and intervals are displayed on a CRT screen. Both temperature and pressure program set points and actual readings inside the autoclave can be monitored by an analyst as the decomposition program progresses. Biological samples and agricultural food products of 1.0 g each are weighed into 70 mL closed quartz vessels for acid digestion. The digested samples are analyzed by using an inductively coupled argon plasma (ICP) emission spectrometer for aluminum, boron, calcium, iron, lead, magnesium, manganese, phosphorus, potassium, sodium, strontium, sulfur, and zinc. Environmental material of 0.2 g each is weighed into 20 mL glassy carbon vessels for digestion and ICP analysis for calcium, iron, lead, magnesium, manganese, nickel, phosphorus, potassium, sodium, strontium, sulfur, titanium, and zinc. The average recovery for 31 elemental standards evaluated by digestion in the high temperature/ pressure asher provided an average recovery of 98.7%.
A method is described for collection and preparation of mainstream smoke condensate from cigarettes for determination of trace metal constituents. A sample of mainstream smoke was collected from 300 cigarettes in a final volume of 25 mL. Extremely clean laboratory conditions during sample preparation were essential to avoid contamination and improve precision and accuracy for trace element determination. The clean laboratories maintained a positive pressure of HEPA-filtered air with constant temperature and humidity control. Mainstream cigarette smoke was collected by electrostatic precipitation (EP) on a modified rotary smoking machine located in a wooden CORESTA cabinet. The EP unit was equipped with a high tension generator supplying 17.5 kV to a tungsten electrode followed by 2 secondary acid traps. Samples were collected under 2 smoking regimens: FTC (35 mL puff volumes, one puff every 60 s, and puff duration 2 s) and “Extreme” (75 mL puff volumes, one puff every 35 s, puff duration 2 s). Condensate was extracted from collection tubes with methanol into Teflon microwave digestion vessels and evaporated in an environmental evaporation chamber (EEC). The EEC provided an ultraclean environment for sample evaporation and predigestion steps that eliminated most of the sample matrix. The remainder was decomposed in a programmable closed vessel microwave digestion system. Digested sample solutions were analyzed by graphite furnace atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry for aluminum, arsenic, cadmium, chromium, nickel, and silicon.
The cool plasma asher (CPA) consists of a high-frequency generator and a quartz sample vessel equipped with a cooling finger that prevents loss of volatile elements. After sample decomposition within an O2–Ar–F plasma, the ashing residues and the elements condensed on the surface of the vessel or cooling finger are dissolved by refluxing in 1–5 mL of double-distilled acid. The sample solutions are analyzed for elemental content by inductively coupled plasma-atomic emission spectrometry (ICP–AES). The recovery values for 42 elements (Ag, Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Eu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Pd, Pt, Rb, S, Sb, Se, Sn, Sr, Te, Ti, V, Y, Yb, Zn, Zr, La, Au, and Sc) are documented after cool plasma ashing of elemental spectrometric standards. In addition, NIST Standard Reference Materials consisting of botanical and biological samples are ashed by CPA, and results are reported for 23 elements (Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Pb, S, Se, Sr, V, and Zn) analyzed by ICP-AES. This method achieves good recoveries for many elements while allowing decomposition of difficult sample matrixes without acid, at a temperature slightly above 100°C. We investigated several ashing facilitators to improve ashing efficiency. This paper describes improved ashing conditions due to sample agitation, gas mixtures, Teflon balls, and a Teflon vessel. The time required to ash 1.0 g of botanical sample in the CPA was reduced from 80 h with no ashing aids to 3 h with maximum ashing aids. The optimum plasma ashing conditions for 1.0 g of sample was 6 h at a high-frequency power of 30 W with a 1 h acid reflux to dissolve sample ash. Because reflux acid in the final sample volume was minimal, trace elemerits were concentrated and blank contamination was extremely low.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.