Electron Spin Resonance. Part Two: A Diagnostic Method in the Environmental Sciences

Article excerpt

ABSTRACT

A review is presented of some of the ways in which electron spin resonance (ESR) spectroscopy may be useful to investigate systems of relevance to the environmental sciences. Specifically considered are: quantititave ESR, photocatalysis for pollution control; sorption and mobility of molecules in zeolites; free radicals produced by mechanical action and by shock waves from explosives; measurement of peroxyl radicals and nitrate radicals in air; determination of particulate matter, polyaromatic hydrocarbons (PAH), soot and black carbon in air; estimation of nitrate and nitrite in vegetables and fruit; lipid-peroxidation by solid particles (silica, asbestos, coal dust); ESR of soils and other biogenic substances: formation of soil organic matter, carbon capture and sequestration (CCS) and no-till farming; detection of reactive oxygen species in the photosynthetic apparatus of higher plants under light stress; molecular mobility and intracellular glasses in seeds and pollen; molecular mobility in dry cotton; characterisation of the surface of carbon black used for chromatography; ESR dating for archaeology and determining seawater levels; measurement of the quality of tea-leaves by ESR; green-catalysts and catalytic media; studies of petroleum (crude oil); fuels; methane hydrate; fuel cells; photovoltaics; source rocks; kerogen; carbonaceous chondrites to find an ESR-based marker for extraterrestrial origin; samples from the Moon taken on the Apollo 11 and Apollo 12 missions to understand space-weathering; ESR studies of organic matter in regard to oil and gas formation in the North Sea; solvation by ionic liquids as green solvents, ESR in food and nutraceutical research.

Keywords: quantititave ESR, photo-catalysis, zeolites, explosives, peroxyl radicals, nitrate radicals, particulate matter, PAH, asbestos, coal dust, photosynthesis, fuel cells, photovoltaics, kerogen, carbonaceous chondrites, nutraceutical research

1. Introduction

The present review is a companion to a related survey published in a previous issue of Science Progress entitled, "Electron spin resonance: a diagnostic method in the biomedical sciences (1) ", and extends its coverage into the application of ESR to the many and various aspects of the environmental sciences. In reality, there is an element of overlap and certain topics could legitimately have been included in either review, since various environmental factors do indeed influence human health and hence are of relevance to the biomedical sciences. Therefore, while trying to avoid too much duplication, I reiterate the following essentials of the method per se, of which a more detailed coverage may be found in the first review (1). Electron spin resonance (ESR)--also known as electron paramagnetic resonance (EPR)--tends to receive far less coverage than its relative, nuclear magnetic resonance (NMR). This is partly because NMR spectrometers and their uses are more ubiquitous, and furthermore, unless there is a specialist in the subject on the staff, ESR receives only scant mention in university science courses. In principle, NMR spectra may be recorded from dozens of different nuclei, whereas obtaining an ESR spectrum requires some of the sample molecules to contain one or more unpaired electrons, which might appear to be a curiosity feature. Nonetheless, here lies the real crux of ESR, since the method is completely specific for unpaired electrons, which are frequently formed in materials that have encountered one or more of a range of important energetic conditions, for which a signature is supplied in the form of the consequent ESR spectrum. The unpaired electron-bearing sites are usually termed "damage centres", "defects" or "trapped electrons" by physicists, geologists, archeologists and environmental scientists, but are normally referred to by chemists and biologists as "free radicals", whose detailed molecular structures may be revealed from the spectral parameter of hyperfine splitting, where it is observed, and that of the g-factor. …