Neil Bostrom

Low-power microwave-generated helium microplasma for molecular and atomic spectrometry

Atmospheric pressure microplasmas are a promising technology for low-power optical emission spectroscopy for chemical detection. In this work, we examine a microstrip split-ring resonator (MSRR) discharge operating at 1.8 GHz in helium as an excitation source. The source can sustain a plasma with as little as 0.2 W of microwave power, and can be operated continuously with no electrode damage. With a compact 156 mm focal length spectrometer system, we determined detection limits on the order of 1 ppm for methane, n-butane, carbon dioxide, and hydrogen sulfide with plasma powers of both 0.3 and 1.0 W. With appropriate choice of the monitored emission lines, the detection system is robust to small additions of air. We also demonstrate the applicability of the MSRR as a sensor for gas chromatography.

Ultrasonic Spectroscopy of Asphaltene Aggregation

High-Q high-resolution ultrasonic spectroscopy is used to detect the onset of aggregation in asphaltene solutions and micelle formation with standard surfactants. This technique allows determination of the speed of sound in solution to a few parts in a million. Aggregation is accompanied by a change in compressibility enabling this ultrasonic technique to determine concentrations of aggregate formation. The ability to detect the critical micelle concentration (CMC) for different standard surfactants with CMCs varying over two orders of magnitude establishes high-Q ultrasonics as a sensitive probe. Asphaltene in toluene is shown to have a critical nanoaggregate concentration (CNAC) of ∼100 mg/l which is a much lower concentration than previous reports using other techniques. The strong tendency of asphaltenes to aggregate explains why asphaltene “molecular” weights determined by vapor pressure osmometry are always well in excess of accurate asphaltene molecular weights. Findings herein are consistent with the Yen model with the restriction that the asphaltene molecules are relatively small having mean molecular weights of 750 g/mole. This restriction on molecular structure enables identification of key dynamics of asphaltene behavior, thereby considerably extending the Yen model. The Yen model consists of a hierarchy of aggregation for asphaltene solutions.1 Different hierarchies of aggregation correspond to different energies of interaction. There has been considerable uncertainty as to what concentrations correspond to what aggregation. Reports utilizing surface tension measurements2 and microcalorimetry3 indicate that primary aggregation or critical micelle concentration occurs at the grams per liter concentration of asphaltenes in toluene (the presence of dispersed water in toluene affects this result4). These concentrations seem rather high and there is a question as to whether these techniques have requisite sensitivity and applicability for detecting the primary aggregation of asphaltenes. The uncertainties regarding aggregation vs. concentration are so great as to cast doubt on the Yen model itself. Exacerbating this situation is that