Miniature gas chromatograph

Any device sent into outer space must have certain characteristics. It must be able to withstand the stress of propulsion into space. It must also be able to use power efficiently. And it must be able to operate under thermal extremes. It is also very favorable for such devices to be as small and lightweight as possible. Several space microdevices have been created that meet these criteria. Miniature gas chromatographic ionization detectors, ion mobility spectrometers, x-ray diffraction devices, and fluorescence instruments are all in various stages of development. Each of these devices plays an important role in exobiology, the science of extraterrestrial environments that may support life. 

Looking into the past, the first microfluidic technology was developed in the early 1950s when efforts to dispense small amounts of liquids in the nanoliter and sub-nanoliter range were made, providing the basics of today s ink-jet technology [4]. In terms of fluid propulsion within microchannels with sub-millimeter cross sections, the year 1979 set a milestone when a miniaturized gas chromatograph (GC) was realized on a silicon (Si) wafer... 

Terry, S. C. and Jerman, J. H., A Miniature Gas-chromatograph Utilized in a Portable Gas-analysis System, Abstracts of Papers of the American Chemical Society 185 (MAR), 123-Anyl, 1983. 

The great advances in this area over the last twenty years have shown already interesting results. One of the most appealing examples is the miniaturized gas chromatograph presented by Angell et al.[4]. This subject, being one of the main topics of the workshop, will be extensively discussed by scientists better qualified to speak about this subject than this author. 

The advanced IC fabrication techniques have opened up a new window to fabricate nhniaturized devices, and are called micromachining It has permitted the fabrication of miniaturized gas chromatographic systems for the use in space laboratories Subsequent efforts have extended the technique to the field of chemical measurement, allowing plate-type sensors, such as ISFET and thin-layer electrodes, which handle a sample outside of the device, to be produced by photolithographic techniques... 

Initially, the main goal of miniaturization was to enhance the analytical performance of devices rather than to reduce their size. However, it was found that such tools had the advantage of reducing the consumption of sample and reagents, for example, the smaller consumption of carrier and mobile phases in separation systems (the first analytical system to be miniaturized was the gas chromatograph). Research in this area focused on the development of components such as micropumps, microvalves, and chemical sensors. 

Kristenson, E.M., E.G.J. Haverkate, C.J. Slooten, L. Ramos, R.J.J. Vreuls, and U.A.Th. Brinkman. 2001. Miniaturized automated matrix solid-phase dispersion extraction of pesticides in fruit followed by gas chromatographic-mass spectrometric analysis. J. Chromatogr. A 917 277-286. 

The Miniature automatic Continuous Agent Monitoring System (MiniCAMS ) utilizes an automated near-real-time gas chromatograph. An air sample is drawn through a preconcentrator loop filled with an... 

Saito, Y., Ueta, I., Ogawa, M., Jinno, K. Simultaneous derivatization/preconcentration of volatile aldehydes with a miniaturized fiber-packed sample preparation device designed for gas chromatographic analysis. Anal. Bioanal. Chem. 386, 725-732 (2006)... 

The future of SPE is quite exciting and will continue to involve miniaturization of SPE methods and more on-line use of both liquid and gas chromatography. As instruments such as GC/MS and HPLC/MS become more sensitive, smaller sample sizes may be used and on-line methods will become routine. Sampling handling will be minimized with automated systems that fit directly on the gas chromatograph, so that analysis proceeds directly from the water sample or biological sample to the instrument. 

Figure 2.19 Portable gas chromatograph. A lightweight (6.6kg), battery operated, isothermal GC. This miniature analytical engine using a capillary column and a photoionization detector is conceived for the analysis of gas and other volatile compounds (VOCs) (reproduced courtesy of Photovac). Below is an example of chromatogram obtained with such an instrument.

Solvent removal through evaporation is feasible only for compounds of limited volatility. Most compounds of biological interest as well as their more volatile derivatives qualify for this, provided that volatile solvents are used (hexane, dichlo-romethane, ether, carbon disulfide, etc.). While concentrations down to 50-100 /xl are feasible with miniature concentric vials, the volumes of samples injected into a gas chromatograph should be no more than a few microliters. In cases of the trace determinations, this is not a satisfactory practice, as it would be more sensible to use a greater sample aliquot. As discussed below, several remedies to this general problem have been suggested. 

Realisation of the first approach - trapping and offline analysis - employs a miniature gas-sampling tube packed with a mixture of Tenax (molecular sieve) and Carbopak (activated charcoal) absorbent material. Such tubes are routinely used for environmental monitoring of hazardous industrial atmospheres whereby operators during the normal course of their duties carry a small tube (about the size of a pen) clipped to their clothes. A pump may be used to draw gas through the tube at a controlled rate and, at the end of the work period, the tube is sealed and sent for analysis. Heating the sorbent tube drives off the trapped material into a gas chromatograph for separation and quantification. 

The first miniaturized analytical device reported was a gas chromatographic analyzer fabricated from silicon in 1979. The work showed the separation of a simple mixture of compounds in only a few seconds. While unperfected, this was an example unlike anything else reported in the literature at that time, and while the scientific community did not immediately grasp the achievements of Terry and coworkers, they did see a glimpse of the future provided in this work with respect to the value of miniaturizing analytical methods. Multiple papers followed this initial work in miniaturization over the next 12 years however, it was not until the concept of a total analysis system (TAS) was first proposed by Manz et al. in 1990 that the concept of miniaturization began to have a significant impact. 

Eiceman, G.A. Tadjikov, B. Krylov, E. Nazarov, E.G. Miller, R.A. Westbrook, J. Funk, P., Miniature radio-frequency mobility analyzer as a gas chromatographic detector for oxygen-containing volatile organic compounds, pheromones and other insect attractants, J. Chromatogr. A 2001, 917, 205-217. 

Adding a separation dimension in tandem with ion mobility spectrometry improves the separation power of a miniaturized device, in terms of increasing the number of peaks that it can separate. Multicapillary coluirms and gas chromatograph had been coupled at the front end of miniaturized ion mobility spectrometers to pre-separate otherwise co-eluting mobility peaks. The added dimension is ideal for separating analytes with close mobilities and also for separating complex samples. 

The change of temperature under the isocratic, as well as programing, conditions is often used to tune separation selectivity in gas chromatographic separations. Temperature has not been very actively utilized in HPLC, mainly because of reported stability problems of the most commonly used stationary phases. However, more interest in the application of temperature for retention control has come nowadays because of the trend of miniaturization in chromatography and the availability of temperature-stable stationary phases.""... 

Hence, all compounds having a conductivity less than the carrier gas will be detected by the TCD, which is a universal concentration-sensitive detector. The TCD is nondestructive, and may be used for preparative separations. The detector has however low sensitivity, and the minimum detectable (M D) mass is about 10 ng even using He or H2 as carrier gas. Other detector characteristics can be found in Table 2.4. The TCD is commonly used for determination of light and permanent gases in packed or PLOT columns. The TCD is well suited for portable gas chromatographs because it is easily miniaturized and does not require extra gases. 

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