A portable micro-FID has been developed that utilized a diffusion flame encased in a micro-fabricated SOI structure. The micro-flame was fueled by a portable electrolyzer that provided the hydrogen and oxygen flow rates necessary for the device operation. The micro-FID was able to detect organic analytes with sensitivity that competes with the one of large-scale devices.
A first major accomplishment was the encasement of a stable diffusion flame inside a quartz-silicon-quartz sandwich structure. Several micro-burner configurations were tested by varying the angle between the oxidizer and the hydrogen gas of silicon channels. The final design employed a diffusion flame that was encapsulated inside a 750 µm thick silicon channel. When the streams of oxidizer and hydrogen met at a 150° angle, a single folded flame structure with low flame strain was obtained that minimized analyte loss.
The effect of channel geometry on flame structure was explored using the ANSYS FLUENT computational fluid dynamics (CFD) software package in order to compute the reactive flow in a two-dimensional geometry that simulated the micro-burner channels. Hydroxyl radical (OH) mass fraction distributions were computed as an indicator of the high-temperature zone of the flame. Also, the two-dimensional flow-field was computed in order to determine the strain rate on the non-premixed flamelet, which was used in order to rationalize flame structure. It was determined from the simulation that an angle of convergence between the channels around 150°-160° provided the least analyte loss as well as relatively low strain rates.
In order to verify this experimentally, two different channel configurations, a folded flame channel design (150° angle) and a counter-flow channel design (180° angle) were tested and compared. Methane gas was injected in the hydrogen stream and the resulting ion current was measured. The folded flame channel design produced about 34 times stronger signals compared to the counter-flow channel design. The micro-FID that was based on this configuration had a linear response.
In order to improve signal strength and reduce fuel consumption rate, an oxygen stream was added to the air stream and the overall channel width size was reduced to 50%. Additionally, reducing the dead volume of the injection port and introducing anchor points to the channels improved the overall device performance. These modifications allowed reducing the overall fuel consumption and improved the sensitivity by a factor of about three. In order to increase the sensitivity of the device, not only was the signal strength improved, but also efforts were made to reduce the noise. To this end, channels were fabricated with silicon-on-insulator wafers and the entire micro-FID was tested inside a Faraday cage. These modifications suppressed the noise by a factor of 43 and improved the overall sensitivity and the minimum detection level. Furthermore, it enabled the instrument to detect a mixture of 17 different gas compounds.
The fuel flow rate was varied in order to determine the flow conditions that optimized flame stability and resulted in increased signal-to-noise ratio. Results showed that the highest signal-to-noise ratio was achieved with an air flow of 45 ml/min, a hydrogen flow of 26 ml/min, and an oxygen flow of 13 ml/min. Finally, the saturation voltage of the electrodes was determined in order to avoid unnecessary charge to the battery during operation and it was shown to be very closely equal to 100 V
