Shree Narayanan

Two-Port Static Coated Micro Gas Chromatography Column With an Embedded Thermal Conductivity Detector

This paper reports a microfabricated 2×4 cm gas chromatography chip to separate and detect gases in a two-port structure by embedding a microthermal conductivity detector (μTCD) within a separation column. A circular on-chip heater is placed on the backside of the monolithic device enabling temperature programming and consequently faster analysis of the heavier components. A unique process enhanced by reactive ion etching lag (RIE lag) is used to achieve multiple etch depths in silicon and restrict the process flow to just three masks. The silicon substrate contains the separation column, the heater, and the tunnels for the TCD electric feed throughs. A Pyrex wafer containing the TCD elements is anodically bonded to the silicon substrate to seal the structure. Performance of a standalone μTCD fabricated in the same process and integrated in a hybrid fashion is also described. The single-chip design demonstrates successful separation and identification of multi-component gas mixtures with a performance comparable to that obtained through a flame ionization detector connected in series. Further, on-chip temperature programming capability was utilized to elevate the column temperature to 75°C to exhibit analysis in less than a minute.

Fabrication and Characterization of a Suspended TCD Integrated With a Gas Separation Column

This paper reports the design, fabrication, and performance evaluation of a unique high aspect ratio resistor on glass employed in a micro thermal conductivity detector (μTCD). Finite-element simulations demonstrate significant improvements in power dissipation and warm-up time by suspending the resistors in the microchannels. Microfabrication techniques employed here enable dry release of 170 μm-diameter coil-shaped resistors on Borofloat in a two-mask process. The microfluidic channels on silicon wafers are independently processed using deep reactive ion etching and subsequently anodic bonded to the glass substrate harboring the TCD resistors. Experiments reveal that the suspended resistors need only 13 mW to reach 100 °C in less than 6 ms and could yield 200 ppm minimum detection limit. The completed chip containing a 1-m-long, 100- μm-wide column with embedded TCD resistors occupies an area of 2.5 × 2 cm. The column coated with an OV-1 stationary phase using the static coating method successfully separated a mixture of hydrocarbons, while the TCD resistors connected in a Wheatstone configuration were able to generate the chromatogram verified by that produced by a commercial flame ionization detector.

Scout-view assisted interior micro-CT

Micro computed tomography (micro-CT) is a widely-used imaging technique. A challenge of micro-CT is to quantitatively reconstruct a sample larger than the field-of-view (FOV) of the detector. This scenario is characterized by truncated projections and associated image artifacts. However, for such truncated scans, a low resolution scout scan with an increased FOV is frequently acquired so as to position the sample properly. This study shows that the otherwise discarded scout scans can provide sufficient additional information to uniquely and stably reconstruct the interior region of interest. Two interior reconstruction methods are designed to utilize the multi-resolution data without significant computational overhead. While most previous studies used numerically truncated global projections as interior data, this study uses truly hybrid scans where global and interior scans were carried out at different resolutions. Additionally, owing to the lack of standard interior micro-CT phantoms, we designed and fabricated novel interior micro-CT phantoms for this study to provide means of validation for our algorithms. Finally, two characteristic samples from separate studies were scanned to show the effect of our reconstructions. The presented methods show significant improvements over existing reconstruction algorithms.

Experimental studies on few-view reconstruction for high-resolution micro-CT

High-resolution micro-CT offers 3D non-destructive imaging but scan times are prohibitively large in many cases. Advancements in image reconstruction offer great reduction in number of views while maintaining reconstruction accuracy; yet filtered back projection remains the de facto standard. An extensive study of few-view reconstruction using compressed-sensing based iterative techniques is carried out. Also, a novel 3D micro-CT phantom is proposed, and used for analyzing reconstruction accuracy. Numerical tests, and studies on real micro-CT data show that if measurement noise in projections is not extremely high, the number of views may be reduced to 1/8^{th} of the typically acquired view numbers. The study motivates the adoption of advanced reconstruction techniques to allow faster scanning, lower dosage, and reduced data size in high-resolution micro-CT.

Interchannel Mixing Minimization in Semi-Packed Micro Gas Chromatography Columns

Semi-packed columns containing an array of micropillars embedded within an open rectangular column structure are a new class of micro gas chromatography (μGC) columns introduced to provide higher separation efficiency and higher sample capacity. Three different semi-packed column configurations are evaluated with respect to pillar spacing along the flow direction and number of pillars across channel. The efficiencies of semi-packed columns, in terms of height-equivalent-to-a-theoretical-plate (HETP), are compared with two microfabricated open-rectangular columns. According to simulation results, arranging the pillars in a symmetrical configuration with spacing equal to the post dimension can suppress multi-path flows. Experimental results confirm the simulation prediction as the design with 2 μm post spacing (SP1) demonstrates the highest performance among other designs. SP1 is found to have HETP of 0.010 cm (15 000 plates/m) at an optimal velocity of 18 cm/s. An open channel design with comparable channel width yields an HETP of 0.025 cm (6000 plates/m) at an optimal velocity of 45 cm/s. The pressure drop in semi-packed columns is experimentally measured to be ~ 9 kPa which falls within the practical range of microfabricated pumps.

The effect of pillar array in semi-packed micro gas chromatography

Micro pillars within separation columns (SC) offer higher sample capacity but also promote fluidic eddy motion which unlike other applications has a negative effect on gas chromatography (GC). Three different semi-packed column (SPC) configurations have been evaluated with respect to pillar spacing along the flow direction and number of pillars across channel. The efficiencies of SPCs have been compared to 2 MEMS open-rectangular columns (ORCs). Simulation and experimental results demonstrated that SPC can operate at higher efficiency when spacing the pillars such that stagnation regions are created. SPC with 20µm pillar diameter and spacing achieved height-equivalent-to-a-theoretical-plate (HETP) of 0.10mm at 18cm/sec optimal velocity. ORC design with comparable channel width yielded HETP of 0.25mm at 45cm/sec. The SPC presented herein has produced 15,000 plates/m, one of the highest reported to date on coated MEMS SC while requiring only 9kPa back pressure.

Thermostatted micro gas chromatography column with on-chip thermal conductivity detector for elevated temperature separation

We present the design, fabrication and experimental evaluation of the first thermostatted micro gas chromatography (μGC) column with an integrated micro thermal conductivity detector (μTCD). The chip contains on-chip sensors and heater on the backside to provide robust temperature programming capability. Our unique μTCD offers many advantages over previously reported designs including low dead volume, elimination of flow restrictors, and reduction of the number of fluidic interconnects from 4 to 2. The 2cm×4cm chip was fabricated using a 3-mask process. The temperature control capabilities allowed elevated temperature separation without interfering with the detection of the TCD operation. The coated column-detector chip has successfully separated 4-component representative hydrocarbons mixture at a temperature of 75°C in less than 1 minute.

Method to quantify the effect of passivation layer in bio-impedance sensors

This paper investigates the effect of the passivation layer in a bio-impedance sensor. A sensor with 20 sensing sites has been designed, fabricated using a simple two mask process and tested. We have cultured in-vivo MDA-MB231 mammary cells and recorded the impedance from 1kHz to 1MHz. Processing the recorded data brings to light the drawback of the passivation layer which results in a drop in sensitivity of 13%. Simulation results based on parameters extracted from measurements re-affirm the drop in sensitivity. Thus, the passivation layer needs to be provided a special consideration in future design of the sensor as it can modify the response of the sensor.

A high-performance TCD monolithically integrated with a gas separation column

We present the design, fabrication and experimental evaluation of a unique micro gas chromatography (μGC) column with an integrated micro thermal conductivity detector (μTCD). The resistors are dry released to form 170μm diameter coils. The high isolation of the sensing element requires only 13mW and sub-6ms to ramp up to 100°C and in turn an improved sensitivity by an order of magnitude over our previously reported designs. The chip incorporates a 2-port design approach and a device size of 2cm×4cm. Fabricated using a four-mask process, the resistors and microfluidics are independently processed on borofloat and silicon wafers, respectively, and subsequently anodic bonded. The separation column is static coated without damage to the resistors. The coated column-detector chip has successfully separated a representative hydrocarbon mixture in less than 2 minutes.

A micro helium-discharge photoionization detector for gas sensing

This paper reports a 2cm × 1cm easy-to-micromachine helium discharge photoionization detector (μHeDPID) for use in micro gas chromatography by utilizing a lift-off process. This universal detector consumes a miserly 2.5mW for plasma generation and non-destructively photoionizes analyte compounds, thus avoiding fouling of electrodes. The ionized species is detected by a remote electrode connected to a picoammeter. The detector exhibits at least 350pg and 50ppm detection limit for n-octane in air. Despite the detector simplicity, its efficiency is in par with previously reported destructive plasma detectors which are based on more sophisticated spectrometric analysis.