Terminal and Intermediate Combustion Products Observed from 2.0 to 5.0 μm in Flame/Furnace Infrared Emission Spectrometry

Abstract

A computer-controlled, dispersive, scanning spectrometer with a wavelength range from 1 to 15 μm is described and used to study the flame/furnace infrared emission (FIRE) spectra of combustion products formed in a small analyte/air flame and in an electrically heated furnace (570°C), operated with and without a column of heated hopcalite (370°C). When lead selenide was used as the detector, the emission spectra of the combustion products of pentane, benzene, dichloromethane, and methanol could be measured over the wavelength range from 2 to 5 μm. In addition to discrete emission bands from terminal combustion products such as CO2, H2O, and HCl, discrete emission from CO (4.6–4.9 μm) and continuum emission associated with soot formation were also observed under oxygen-limited combustion of benzene, dichloromethane, and possibly pentane. Bands centered at approximately 3.3 μm (3030 cm−1) and 3.5 μm (2857 cm−1) were observed in several spectra and attributed to C-H stretching in intermediate combustion products, with the 3.5- μm band being assigned to the stretching of the carbonyl hydrogen of aldehydes (formaldehyde, in particular). On the basis of these results, the “anomalous emission” observed at 3.8 μm in previous studies employing electrothermal excitation is attributed either to the presence of formaldehyde or to the formation of particulate carbon, which are both associated with conditions favoring the incomplete combustion of hydrocarbons.