Using thin films of adsorbents to preconcentrate in gaseous or liquid phases


Preconcentration of samples and interference elimination are critical steps in producing a micro total analysis system (TAS). A micro-sized preconcentrator may have greater preconcentration efficiency than a macrosystem, sometimes even more so. The adsorption parameter is a key factor for many of these procedures, but its application is not completely understood yet. Sensor development could be greatly aided by an application of adsorbent films similar to this. Sandia Labs recently released a small analyzer. This system could analyze liquids or gases by using chromatography 4 or electrophoresis 5. In this device, preconcentration is one of the fundamental steps, which collects and concentrates samples. In order to desorb the substance, an adsorbent film was applied to a silicon nitride membrane. The detection was also performed by means of a surface acoustic wave (SAW) sensor array. In addition to chemiresistor sensors, adsorbent films can be used. A chemical resistor and a slant angle wave sensor have been tested for VOC (volatile organic compounds) detection, but not for TCE (trichloroethylene), which required further concentration..

To analyze VOCs, which are harmful to the environment, gas chromatography and conventional pre-concentrators 11 are often used. In a preconcentrator, an adsorbent product is contained in a stainless steel pipe. The microtrap 12-14 and the miniaturized system 15 are new types of sorbent traps. It is possible to use a microconcentrator as an injector for gas chromatography by applying a thin polymeric layer during the adsorption process. Using a multistage micro preconcentrator, researchers have detected volatile organic compounds in indoor environments. They are approximately 200 mm in diameter, and 16-20 mm in surface area, graphitized carbon granules arranged roughly spherically. There is a possibility of detection as low as 25 ppb because the adsorbent material has a large surface area. A planar geometries 21 is usually used in micro- and small-sized systems to avoid desorption due to difficult heating and cooling22.

In addition to voltammetry 23 and capillary electrophoresis24, which are versatile tools for detecting anything from ions25 to proteins26, you can also use adsorption on liquid phases. Solid phase extraction can also be performed with adsorbent films, which have been miniaturized. In addition to chromatographic columns, adsorbent films can be used to miniaturize adsorbent films. Analyses of semivolatile organic compounds in aqueous matrix have been carried out by liquid chromatography and membrane extraction. Both methods require microchannels.

Microchannels have been created by using an ink jet system 28 that uses non-conventional methods. Using ordinary machining tools and sealing without heat, non-planar microchannels 29 were made in some cases. It was possible to make an inexpensive structure with interchangeable parts that could be easily disassembled at any time. This structure did not have to be bonded at high temperatures in order to be used for dry or wet deposition. It looked like a screw since it had a microthread and a cylinder. Supplies and outlets were made from stainless steel capillaries, and sealing was accomplished with silicone glue only.

Microchannels can concentrate VOCs from nitrogen by utilizing only capillary effects. Avoiding capillary effects can be accomplished by using cavities that promote primarily adsorption. A cavity functions similarly to a sensor30.

In our prior work, plasma reactors were used to create two adsorbent films using 2-propanol and hexamethyldisilazane (HMDS) as reaction agents. 2-propanol31 may be plasma polymerized into polar films to obtain selective membranes impermeable to water and permeable to organic compounds. It was also interesting to note that once polar films were obtained, they could be deposited on hydrophobic substrates after a discontinuous layer had been formed. The island’s discontinuous nature enabled an increase in adsorption surface area. 

HMDS plasma films 32 are capable of selectively permeating alcohol in aqueous solutions and have other unique properties as well. 

As well as HMDS-coated non-planar microchannels, cavities and HMDS-coated non-planar microchannels showed good adsorption properties for organic compounds according to a variety of substrates 39.

Therefore, the objectives of this research are twofold: 1) to test the feasibility of a plasma polymerized 2-propanol adsorbent film that uses different frequencies and geometries from the conventional 40 KHz planar reactor used in previous research; and 2) to investigate how microchannels and cavities are best used to preconcentrate organic compounds in gaseous or liquid phases.

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