Scavenging of Hydrocarbon Radicals from Flames with Dimethyl Disulfide. I. Characterisation and Discussion of the Method and the Scavenging Process

Abstract

The method of scavenging free radicals sampled through a rarefied free‐jet from stationary low‐pressure high‐temperature reaction systems (flames, pyrolysis) in the condensed phase of dimethyl disulfide (DMDS) has been studied systematically. For this purpose the flat open condensing device originally used has been replaced by a condensation trap consisting of two stainless‐steel hemispheres cooled with liquid nitrogen. The scavenging products were separated and identified by GC/MS. The condensing efficiencies of the scavenger DMDS and condensable substances from the reaction system were so determined as to make the scavenging R+CH3SSCH3RSCH3+CH3S a quantitative method of radical analysis. It is shown that with a sufficient excess of DMDS the scavenging reaction takes place to 90‐100% for hydrocarbon radicals with two or more C atoms, with a slight interference from addition reaction of radicals to condensable unsaturated products from the reaction system. The type of reactions with mono‐ and divalent radicals and carbenes in the solid DMDS matrix are investigated. Consecutive reactions during warming‐up are also studied and discussed. It is stated that this scavenging method is well suited for the analysis of all hydrocarbon radicals heavier than ethyl, provided that the scavenging products can be separated and identified, which might become a problem with heavy polycyclic aromatic radicals.

Mass Spectrometry of Free Radicals

“…Homann and co-workers − introduced a radical scavenging method for the identification of hydrocarbon radicals by supersonic nozzle-freezing combined with a reaction with dimethyl disulfide on a liquid nitrogen cooled trap followed by subsequent characterization using GC/MS after warming-up. This method has been applied to radicals generated from electrical discharges or flames and shown to allow a quantitative analysis of the effluent content.…”

“…475 Among the above radicals, the hydroxyl and hydroperoxo radicals are particularly well-documented in kinetic studies often using mass spectrometry as a complementary tool, even if MS has been particularly useful for the determination of quantities such as in particular, IEs [476][477][478][479][480][481] and EAs. [482][483][484]308,310 Homann and co-workers [485][486][487] introduced a radical scavenging method for the identification of hydrocarbon radicals by supersonic nozzle-freezing combined with a reaction with dimethyl disulfide on a liquid nitrogen cooled trap followed by subsequent characterization using GC/MS after warming-up. This method has been applied to radicals generated from electrical discharges or flames and shown to allow a quantitative analysis of the effluent content.…”

“…486 The original trapping device was later modified to improve the quantification of the radical species. 487 Large hydrocarbon radicals, usually difficult to detect by the only optical methods have been characterized by this method of radical scavenging with dimethyl disulfide. Further sampling of acetylene/oxygen flames 488 lead to the characterization of large C x H y radicals (x ) 4), as well as H • , CH 3 • , C 4 H 3 • , and C 6 H 5 • radicals scavenged as monomethylthio compounds.…”

Effect of Metal Doping on the Initial Pyrolysis Chemistry of Cellulose Chars

“…All of the volatilzed and ablated materials from the pyrolysis event is quenched on the cold surface of the collection cup, and any radicals are neutralized and trapped as phenylsulfide adducts. Traditionally, dimethyldisulfide has been used as a radical trap in flame work, , but its high volatility precluded its use in solid-state trapping. The result is a snapshot of the evolving gas plume allowing us to separate solid-phase charring chemistry from the gas-phase reactivity of the pyrolyzate plume.…”