- The Micro-ESR an innovation in sensor design determines free radicals
- DPPH dissolved in toluene exhibits a characteristic set
- The g-factor of the peroxy radical signal increases slightly
- The specificity of ESR implies that no compounds other than free radicals
Free radicals are increasingly reactive chemical species that govern several fundamental chemical processes in nature, most notably combustion and oxidation. Till now direct measurement of the composition and concentration of free radicals has signified a challenge for chemists due to the complexity and expense of the necessary equipment. The Micro-ESR an innovation in sensor design determines free radicals with a low-cost, compact and ruggedized device.
New low-cost applications are enabled by the spectrometer such as online measurement of lubricant breakdown in engines and machinery, online airborne particulates monitoring in diesel engine exhaust and even spin immunoassay medical diagnostics.
Initial experiments for verifying the spectrometer operation were conducted with DPPH, a dye containing a stable free radical, dissolved in toluene solvent. DPPH dissolved in toluene exhibits a characteristic set of resonant peaks that correspond to hyperfine splitting.
Engine oil from a Honda gasoline engine was studied. As the antioxidant package in the oil is depleted by oxidation, the intensity of the peroxy radical (RO2•) signal increases steadily from zero. This is the induction period. Also, the g-factor of the peroxy radical signal increases slightly as the hydrocarbon chains are broken down.
When the oil is approaching the end of its useful life, the intensity of the peroxy radical signal increases dramatically and failure is imminent,
This spectrometer determines intrinsic chemical properties of the oil (concentration of chemical constituents), while other approaches measure physical properties of the oil (such as dielectric constant, viscosity, electrical impedance) and then relate that data to underlying chemical changes in the oil.
Another benefit is that the spectrometer gives an absolute reading of the condition of the oil. New oil has a null spectrum i.e there are no free radicals, carbon or other contaminants present in the oil. The presence of any ESR spectrum clearly shows that contamination is present in the oil.
In addition, the g-factor as in Equation 1 of each free radical is only weakly dependent on temperature. because free radicals in oil can be uniquely identified by their g-factors, this enables the user to easily identify any ESR signals with absolute confidence at any operating temperature. The specificity of ESR implies that no compounds other than free radicals or transition metal ions will produce a signal. The technique, therefore, does not exhibit crossfactors commonly seen with other sensors.
This application of the spectrometer offers the end-user a new and cost-effective method to ensure compliance with environmental standards. It also enables the user to identify when an engine is running too rich, which severely reduces fuel efficiency.
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