Application of the time-of-flight mass spectrometer to monitor time-dependent behaviour following pulsed laser heating of solids

Abstract

Because of its very fast scanning capability, the time-of-flight mass spectrometer (TOFMS) would appear to be an ideal instrument for real-time monitoring of fast, transient chemical processes. Realisation of this objective has been handicapped by the so-called “statistics” problem and the difficulty of recording the data. Instrumental developments to overcome these difficulties are described with reference to work concerned with TOFMS monitoring of pulsed laser heating of solids. Early experiments at NASA showed that the power density of the incident irradiation on a sample has a significant influence on the composition of the vapour generated by laser pyrolysis. In general, the higher the power density the lower the molecular weights of the species detected and the greater the number of laser produced ions. Thus it is important to simulate the heating conditions of interest when determining the vaporisation characteristics of a particular material. The Salford laser pyrolysis-TOFMS technique has been used to monitor benzene evolution from a number of flame-retarded PVC samples containing different plasticisers. The information obtained has been interpreted in relation to the smoke-generating behaviour of these samples. This indicates that plasticisers which favour the evolution of benzene rather than higher aromatics probably produce less smoke in a conflagration. Preliminary studies on flame-retarded cotton fabrics are also reported. Although initially concerned with cellulose pyrolysis, the main direction of the NASA work has been vaporisation behaviour of candidate thermal protection materials for space vehicles. Laser pyrolysis of various forms of carbon and graphite generated multiple carbon ions up to C + 19 with C + 10 and C + 11 being the most intense. The neutral vapours were dominated by C, C 2 and particularly C 3. Silicon carbide fibres yielded negligible carbon species but mostly SiC 2, Si 2C and Si at 10–60 kW cm −2. Laser pyrolysis of WO 3 was the only case in which dissociation with clustering was observed.