Richard Sacks

High-performance temperature-programmed microfabricated gas chromatography columns

This paper reports the first development of high-performance, silicon-glass micro-gas chromatography (/spl mu/GC) columns having integrated heaters and temperature sensors for temperature programming, and integrated pressure sensors for flow control. These 3-m long, 150-/spl mu/m wide and 250-/spl mu/m deep columns, integrated on a 3.3 cm square die, were fabricated using a silicon-on-glass dissolved wafer process. Demonstrating the contributions to heat dissipation from conduction, convection, and radiation with and without packaging, it is shown that using a 7.5-mm high atmospheric pressure package reduces power consumption to about 650 mW at 100/spl deg/C, while vacuum packaging reduces the steady-state power requirements to less than 100 mW. Under vacuum conditions, 600 mW is needed for a temperature-programming rate of 40/spl deg/C/min. The 2300 ppm//spl deg/C TCR of the temperature sensors and the 50 fF/kPa sensitivity of the pressure sensors satisfy the requirements needed to achieve reproducible separations in a /spl mu/GC system. Using these columns, highly resolved 20-component separations were obtained with analysis times that are a factor of two faster than isothermal responses.

High-Speed MEMS-Based Gas Chromatography

This paper reports microfabricated silicon-glass separation columns for high-speed micro gas chromatography (muGC) systems. The microfabricated columns are integrated with resistive heaters and temperatures sensors and capacitive pressure sensors to allow temperature and pressure programming and flow control and to achieve reproducible separations in a muGC system. These 25-cm-long, 150-mum-wide, and 250-mum-deep columns are fabricated on a 1.2-cm square die using a silicon-on-glass dissolved wafer process. Programmed with temperature ramps of 10 degC/s, the low-mass columns separate eleven-component gaseous mixtures in less than 10 s, including alkanes from C5 to C16 and simulants for C-4, TNT, sarin, and mustard gas. When used in arrayed architectures, these MEMS columns should allow high-speed analysis without sacrificing separation efficiency

A Low-Power Pressure- and Temperature-Programmable Micro Gas Chromatography Column

This paper presents the theory, fabrication, and experimental results for a high-performance low-power micro gas chromatography column. The suspended-dielectric 1-m-long column is split into two sections, permitting independent pressure and temperature programming. Integrated column heaters have a mean resistance of 16.8 kOmega and a temperature coefficient of resistance of 431 ppm/degC. The suspended column requires 11 mW to raise its temperature by 100degC in vacuum (1 mtorr). The column separates ten volatile organic compounds in 52 s and four chemical warfare agent simulants and an explosive simulant in 60 s.

An Integrated Micro-Analytical System for Complex Vapor Mixtures

A micro gas chromatograph (muGC) capable of quantitatively analyzing the components of complex vapor mixtures at trace concentrations is described. The muGC features a micro- preconcentrator/focuser (muPCF), dual-column pressure- and temperature-programmed separation module, and an integrated array of nanoparticle-coated chemiresistors. The latest design modifications and performance data are presented. Highlights include a 4-min separation of a 30-component mixture with a 3-m DRIE Si/glass microcolumn, a 14-sec separation of an 11-component mixture on a 25-cm microcolumn, a complete multi-vapor analysis from a hybrid microsystem that combines analytical, rf- wireless, and microcontroller modules, and a rapid analysis driven by a 4-stage peristaltic micropump.

Direct determination of selected metals in refractory powder micro samples with exploding thin film excitation

Abstract The direct determination of metallic elements in powder micro samples is explored. Emphasis is placed on refractory materials prepared as alcoholic suspensions of their powders. Silver thin films are vacuum deposited on polypropylene strips and on polycarbonate membrane filters. Based on a parametric study, explosions conducted at 700 torr in Ar(60%)O 2 (40%) using 4 kV, 180 J discharges are most satisfactory. Particle size studies suggest complete vaporization of particles smaller than ∼10 μm independent of their boiling points. High circuit inductance and capacitance as well as O 2 added to the plasma support gas promote vaporization of larger particles. Particles which pass through a 5 μm mesh sieve can be analyzed with aqueous solution standards. Analytical curves and precision data are presented for Zr, V, Mo and Ni. Interparticle concomitant effects are not significant. Feasibility studies are presented for the direct analysis of airborne particles collected on metallized membrane filters.

Saturable Inductor Based Controlled Waveform Excitation Source for Exploding Thin Film Atomic Spectroscopy

An adjustable waveform excitation source for exploding thin film atomic spectroscopy is described. Waveform control is achieved through a saturable inductor having control windings and placed in series with the underdamped discharge tank circuit. This results in a high-current pulse for efficient sample atomization followed by a low-current measurement interval. When this is combined with gated integration, ∼15-fold increases in analysis line/background intensity ratios are observed for V and Mn. With additional inductance added to the circuit, the current and duration of both the atomization pulse and measurement interval can be adjusted independently. Other features of the excitation source include a plasma shunt capacitor, which results in more reproducible dielectric breakdown of the plasma support gas, and a variety of high-current series switches for initiating the discharge. Triggered spark gap, gravity operated spark gap, silicon-controlled rectifier, and ignitron switches are compared with respect to resistance and switching time jitter. Examples of discharge current and voltage waveforms and preliminary analytical data are presented.

Magnetically Tailored, Atmospheric-Pressure Plasmas for Atomic Spectroscopy

Three magnetic field-plasma configurations are used to study the interaction of external magnetic fields with analytically useful, atmospheric-pressure plasmas. A magnetic field normal to the electric field in the plasma produced by the capacitive discharge vaporization of a thin Ag film is used to obtain an ExB drift motion of the plasma. Photographs show that this drift motion can drastically alter the size, shape, and location of the plasma. The same plasma-generation technique is combined with a nonuniform magnetic field to obtain an adiabatic magnetic-mirror ion trap. Finally, the cylindrically symmetric plasma produced by the capacitive discharge vaporization of a thin metal wire or a bundle of graphite fibers is combined with an axial magnetic field to obtain a theta pinch of the plasma. In all cases, the plasma current is used to generate the magnetic field in a large air-core inductor surrounding the plasma. Radiative and electrical properties of these magnetically modified plasmas will be presented.

Radiative and Electrical Properties of Exploding Silver Wires

The radiative properties of silver wires exploded by capacitive discharge are studied using time-integrated and time-resolved spectroscopy. Discharge current measurements are obtained from a calibrated current shunt and are correlated with time-resolved spectra to obtain a model for the current conduction processes during wire explosions. Two basic explosion mechanisms are described, both of which begin with the conversion of the wire from a conducting solid to a rapidly expanding dielectric cylinder of metal vapor. This results in a rapid reduction of discharge current. Current then may resume in one of two ways. In He at atmospheric and reduced pressure and in air at reduced pressure dielectric breakdown of the gas surrounding the wire occurs with current conduction peripheral to the expanding metal vapor cylinder. In air at atmospheric pressure, dielectric breakdown occurs through the metal vapor. This results in current conduction along the vapor cylinder axis. The advantages of peripheral current conduction for the analysis of metals electroplated on the surface of silver wires are discussed. Parametric studies with Cd-plated silver wires indicate that the intensity of the background continuum can be reduced by nearly 2 orders of magnitude with little effect on Cd line intensities by reducing the pressure from 730 Torr to 50 Torr in He where peripheral current conduction occurs.

Exploding wire excitation for trace analysis of Hg, Cd, Pb and Ni using electrodeposition for preconcentration

Abstract The use of exploding silver wires for spectrochemical excitation is described. The apparatus used in these studies is discussed along with the electrical and radiative properties of exploding silver wires. The use of controlled-potential electrodeposition onto silver wires for sample introduction is considered. Experimental parameters for electrodeposition and exploding-wire excitation are presented. Cd, Ni, Hg and Pb are considered for analysis and show absolute detection limits of 10 ng, 10 ng, 30 ng and 15 ng, respectively. These amounts of material on the wire represent relative detection limits of 0.001 ppm, 0.002 ppm, 0.003 ppm and 0.001 ppm respectively in the analytical solutions. The exploding wire excitation method exhibits minimal matrix dependency and good reproducibility. Percent relative standard deviations for Cd, Ni, Hg and Pb are ±17.3%, ±16.6%, ±26.5% and ±12.4%, respectively. Sample introduction by controlled-potential electrodeposition offers not only a convenient means of preconcentrating trace metals from solution but also the possibility of selectively plating one element in the presence of a large excess of other elements in solution.

Instrumentation and strategies for high-speed gas chromatography

Short lengths of capillary column operated at unusually high carrier-gas velocities are used to separate relatively simple mixtures on a timescale of 20 s or less. Two inlet systems are described which can generate the narrow injection bands needed for these elution times. A cryofocusing inlet using a vacuum-pump-operated gas flow direction control is used for compounds generally less volatile than n-pentane. A low-friction gas valve inlet combined with porous-layer open-tubular columns is used for volatile compounds of low molecular mass. Strategies for high-speed separations including high-speed backflush, backflush with recycle, vacuum-outlet operation and selective detection are described. These techniques are used to increase the operational flexibility and the amount of information that can be obtained using high-speed gas chromatography.