Micro flame ionization detector and micro flame spectrometer

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Abstract

In this paper we present a miniaturized flame ionization detector and flame spectrometer fabricated using conventional micromachining technologies. The main component of both devices is a micro burner unit, which uses minimal oxyhydrogen to produce a stable miniature flame. The oxyhydrogen is generated at low energy consumption by a miniaturized electrolysis cell, which can be operated by battery. Because of the low oxyhydrogen consumption and the minute scale of the burner unit and electrolyzer the oxyhydrogen is generated as-required, rather than stored as in conventional systems. Thus, there is no explosion hazard and the devices are not only made easily portable, but also safe. Furthermore, these systems possess sensitivity and selectivity that is comparable to conventional systems. Concentrations down to 1 ppm have been demonstrated with the micro flame ionization detector and a detection limit in the ppb range appears within reach. The micro flame spectrometer is undergoing initial development, but measurements based on atomic emission spectrometry demonstrate already a detection limit only 100-fold above levels observed in conventional systems.

Introduction

Ordinary flame ionization detectors and flame spectrometers with transparent oxyhydrogen flames are used for years in industry and research, especially by laboratories in medical and environmental protection fields. Most commonly, flame ionization detectors are combined with separation columns and used for gas chromatography analysis. Flame spectrometry is used for an identification and quantitative determination of individual compounds in liquids. Compared with other analytical methods, these systems posses a high sensitivity and selectivity to an enormous range of substances, along with a linear measuring effect. Furthermore, they have a minimum response time that is amenable to online monitoring. A broad field of different applications is conceivable, but the use of the conventional systems has been dropping recently. This downward trend can be explained by the following facts, which found the development of a micro flame ionization detector and flame spectrometer.

The high fuel gas consumption required to maintain a stable oxyhydrogen flame necessitates a large gas supply, which confers a high explosion risk to conventional flame ionization detectors and flame spectrometers. The resulting safety restrictions and large scale of the oxyhydrogen supply make these systems immobile and limits their use to stationary laboratories, which can implement the required safety precautions. In the course of a progressive miniaturization of other conventional analyzing tools to facilitate portability and integration on a chip complex and mobile analytical devices of an easy handling become available nowadays for reasonable prices [1]. Conventional flame ionization detectors and flame spectrometers are no longer competitive with this new generation of miniature analyzing systems in spite of a higher sensitivity and selectivity. Miniaturization would potentially make flame ionization detectors and flame spectrometers not only safer, but also allow their use in a wide variety of applications in which portability and cost, as well as sensitivity, are important factors. Especially the development of a sensitive μTAS for gas chromatography analysis in our department requires a micro flame ionization detector, which can be integrated to a micro separation column [2] on one chip.

In this paper we present a micromachined flame ionization detector and flame spectrometer with capabilities meeting all requirements on modern analyzing systems. A rising use of these micro devices is to be expected.

The main component of the miniaturized flame ionization detector and flame spectrometer is a micro burner unit. We have designed a micro burner head with a miniature nozzle of defined geometry, which produces a stably burning oxyhydrogen flame using a minimum of fuel gas. Unlike conventional burner units, the oxyhydrogen in this micro device is not stored, but is generated as-needed by electrolysis at low energy consumption in a portable miniaturized electrolysis cell. For maximum efficiency, an electrolyzer with a proton exchange membrane serving as a solid electrolyte is used. For the small amount of oxyhydrogen required, the miniature electrolyzer could be battery-operated. Since fuel gas is not stored and there is only little oxyhydrogen in the micro burner unit during operation, there is no explosion risk. Moreover, the low thermal radiation emitted from the miniature flame avoids the need for materials with high thermal stability or for safety restrictions for heat radiation and conduction hazards. Thus, the micro flame ionization detector and flame spectrometer are safe and easy to handle. Furthermore, they have an unlimited mobility, operate at low cost, and consume a minimum about of sample.

Section snippets

Measuring principles: flame ionization detector and atomic emission flame spectrometer

Flame ionization detectors are used for quantification of volatile organic compounds in gaseous samples [3], [4]. The measuring effect is based on an ionization of organic substances burned in an oxyhydrogen flame. A high electric field between an upper annular electrode (anode), which surrounds the flame, and a substrate electrode (cathode) accelerates the ions, generated in the flame, to the electrodes (Fig. 1). An ionization current proportional to the number of present carbon atoms is

Construction of the micro burner unit

The micro burner unit of both analytical micro systems Fig. 1, Fig. 2 is a sandwich construction consisting of a pyrex cover, a silicon substrate in the middle, and a pyrex base. All components are structured separately and joined together using standard microsystem technology manufacturing methods.

The silicon substrate, at 520-μm thickness, is the main component of the micro burner unit, as it contains the micro nozzle for the oxyhydrogen and a concentric annular jet nozzle for the sample gas.

Combustion characteristic, flame stability and flow properties

Both the flame ionization detector and the flame spectrometer require a stable flame of sufficient size and intensity to guarantee reproducible and strong detector signals. In order to optimize the micro burner flame, the nozzle diameter of the micro burner unit and the oxyhydrogen flow rate were investigated for their effect on the size and stability of the miniature flame. The addition of isopropanol to the flame turns it blue, making the flame easy to visually inspect.

One main condition for

Construction of the micro flame ionization detector and results

To construct the micro flame ionization detector, an electrode system is incorporated into the micro burner unit. The substrate electrode (Fig. 11) is comprised of a thin inert gold coating, deposited on the pyrex cover by sputtering through a shadow mask. For the upper annular electrode, a hollow silica cylinder is used as an insulating carrier substrate. Half of the hollow cylinder is coated on the inside with a thin layer of gold by sputtering through shadow mask (Fig. 12). Finally, the

The miniaturized oxyhydrogen flame for atomic emission spectrometry

The micro atomic emission flame spectrometer, consisting of the micro burner unit (Fig. 15), an optical micro spectrometer system [9], [10] and a piezo-driven micro atomizer for liquid samples [11], [12], is still only in the initial stages of development. To show more generally that the miniature oxyhydrogen flames can be used for atomic emission spectrometry, the micro burner unit was assembled with a conventional pneumatic atomizer and a conventional spectrometer system. The sensitivity and

Oxyhydrogen supply: miniaturized electrolysis cell

The low fuel gas consumption of the micro burner unit to maintain a stable oxyhydrogen flame enables the use of a miniaturized bipolar electrolysis cell [13] or a miniaturized PEM-Electrolyzer3 (Fig. 18) to generate the required oxyhydrogen by electrolysis [14]. Because of the more convenient design and higher efficiency of operation, the PEM-Electrolyzer, in which a proton exchange membrane serves as a solid electrolyte, is used as

Acknowledgements

The research project “Flame Ionization detector and Flame Spectrometer in Microsystem Technology” is financially supported by the Deutsche Forschungsgesellschaft.

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