Gary W. Rice

Highly Stable Surface Functionalization of Microgas Chromatography Columns Using Layer-by-Layer Self-Assembly of Silica Nanoparticles

A controllable and high-yield surface functionalization of silicon microchannels using layer-by-layer (LbL) self-assembly of SiO2 nanoparticles (SNPs) is presented. The application of SNPs (45 nm average diameter) coating as a stationary phase for chromatographic separation is also demonstrated with surface functionalization using chloroalkylsilanes. This method facilitates a simple, low-cost, and parallel processing scheme that also provides homogeneous and stable nanoparticle-based stationary phases with ease of control over the coating thickness. The SNP-functionalized microfabricated columns with either single capillary channels (1 m long, 150 μm wide, 240 μm deep) or very narrow multicapillary channels (25 cm long, 30 μm wide, 240 μm deep, 16 parallel channels) successfully separated a multicomponent gas mixture with a wide range of boiling points with high reproducibility.

Improved performance of micro-fabricated preconcentrators using silica nanoparticles as a surface template

A new approach of enhancing the adsorption capability of the widely used polymer adsorbent Tenax TA poly(2,6-diphenylene oxide) through its deposition on a nano-structured template is reported. The modified Tenax TA-coated silica nanoparticles (SNP) are incorporated as an adsorbent bed in silicon based micro-thermal preconcentrator (μTPC) chips with an array of square microposts embedded inside the cavity and sealed with a Pyrex cover. The interior surface of the chip is first modified by depositing SNP using a layer-by-layer self-assembly technique followed by coating with Tenax TA. The adsorption capacity of the SNP-Tenax TA μTPC is enhanced by as much as a factor of three compared to the one coated solely with thin film Tenax TA for the compounds tested. The increased adsorption ability of the Tenax TA is attributed to the higher surface area provided by the underlying porous SNP coating and the pores between SNPs affecting the morphology of deposited Tenax TA film by bringing nano-scale features into the polymer. In addition, the adsorption ability of the SNP coating as a pseudo-selective inorganic adsorption bed for polar compounds was also observed. The modified Tenax TA-coated SNP μTPC is a promising development toward integrated micro-gas chromatography systems.

Direct Quantitation of PCB Congeners Using a Helium Discharge Detector and Internal Standard Techniques

Individual PCB congeners have been quantitated at ppm levels, with an average error of ±3.2%, with the use of a helium discharge detector (HDD) for element-selective detection of Cl emission. Chlorinated internal standards of known concentrations were added to each solution determined to establish the relative peak areas per unit concentration of Cl present. No detector precalibration or response factor formulations were required, since the detector response is based solely on the moles of Cl present. The same methodology was utilized to determine the % Cl in Aroclor samples without prior identification of the PCB congeners present.

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.

Form of Dietary Methylmercury does not Affect Total Mercury Accumulation in the Tissues of Zebra Finch

Exposure to mercury in humans, other mammals, and birds is primarily dietary, with mercury in the methylated form and bound to cysteine in the tissues of prey items. Yet dosing studies are generally carried out using methylmercury chloride. Here we tested whether the accumulation of total mercury in zebra finch blood, egg, muscle, liver, kidney or brain differed depending on whether dietary mercury was complexed with chloride or cysteine. We found no effect of form of mercury on tissue accumulation. Some previous studies have found lower accumulation of mercury in tissues of animals fed complexed mercury. Much remains to be understood about what happens to ingested mercury once it enters the intestines, but our results suggest that dietary studies using methylmercury chloride in birds will produce similar tissue accumulation levels to those using methylmercury cysteine.

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.

Trace organics monitoring in drinking water using Tenax-coated fibers

Silica fibers were coated with 3 µm-thick Tenax TA films and investigated for potential use as a sorptive material in solid-phase microextraction (SPME). The performance of these fibers was evaluated as an effective tool for water contamination analysis of various simulated pollutants and VOCs using static headspace extraction. We show that Tenax TA fibers have high thermal stability as well as stability in organic solvents and acidic/basic solutions. Results indicate the high performance of Tenax TA coated fibers in terms of adsorption capacity, saturation time, and limits of detection. In-fiber reproducibility and fiber-to-fiber reproducibility was evaluated and changes in deposition method was made to enhance performance. High temperature desorption was explored at 400°C in which the Tenax TA fiber was determined to be capable of extracting high molecular weight compounds similar to common water pollutants not currently extractable using conventional SPME fibers.