Preconcentrators, microfabricated

Preconcentration is another commonly encountered sample pretreatment method that has been successfully integrated onto a CE chip. Ramsey and coworkers incorporated a porous membrane structure into a microfabricated injection valve, enabling electrokinetic concentration of DNA samples using homogeneous buffer conditions [5]. Sample preconcentration in nonhomogeneous buffer systems — a technique known as sample stacking —has also been achieved on-chip [6]. 

Foote and coworkers [120] developed a microfabricated system with the ability to electrophoretically preconcentrate fluorescently labeled proteins prior to their separation (see Fig. 6). The authors were able to preconcentrate the proteins using a porous silica membrane situated between adjacent microchannels that allowed for the passage of buffer ions, but excluded larger migrating molecules, such as proteins. Preconcentration factors of 600-fold were achieved using this on-chip format followed by an electrophoretic separation of proteins with SDS-PAGE. Using this chip, fluorescently labeled ovalbumin was detected at concentrations as low as 100 fmol by a combination of field-amplified injection and preconcentration at the membrane prior to microchip electrophoresis. 

For volatiles and semivolatiles with vapor pressures too low to be detected by direct IMS introduction methods, preconcentration techniqnes are often anployed. The most common approach is to pack a glass tube or stainless steel column with an adsorbing material such as Tenax or Carbosieve. Other methods, such as micro-fabricated adsorbents, have also been proposed, bnt these microfabricated concentrators are relatively new, with limited field exposnre. Nevertheless, for handheld and mini-IMS instruments, these preconcentration devices appear promising, offering rapid concentration and desorption with low power. ... 

As illustrated by the examples given in Table 3, the application of labs-on-chips to real samples is still limited. This is partly due to the fact that the analytical assay is only the final step of the whole procedure, which includes sample pretreatment protocols such as filtration, analyte cleanup, or analyte preconcentration. However, also the integration of corresponding microfabricated elements is described. Filtration was achieved by porous membranes or arrays of thin channels preventing particulates to enter the analytical device. Analyte preconcentration in combination with removal of other sample constituents is achieved by solid-phase extraction modules, which are either capillaries or beads coated with a suitable adsorbent, such as a Cl 8 phase originating from coating with octa-decyltrimethoxysilane, from which the analyte is... 

Microfabricated versions of the multibed preconcentrator shown in Figure 5.10 are under development. Examples are shown in the photomicrographs of Figure 5.29. The deep-etched silicon structures hold beads of the adsorbent materials. After sample collection, the silicon structures are resistively heated to inject the sample into a microfabricated column. These MEMS preconcentrators are designed to quantitatively collect 30-50 volatile target compounds from 250-500-cm air samples. No performance data are available at this time. 

FIGURE 5.29 Photomicrographs of microfabricated preconcentrator using deep-etched silicon slats or posts to hold carbon beads and to resistively heat beads for sample injection. 

The dual-column ensemble will consist of two 3.0-m-long microfabricated columns, one using a nonpolar dimethyl polysiloxane stationary phase (see Figure 5.27) and the other using a moderately polar trifluoropropylmethyl polysiloxane stationary phase. The column ensemble can be operated in the stop-flow mode by the use of a valve between the preconcentrator and the column junction point. Detection is provided by an array of microfabricated chemiresistor sensors, each having a different selectivity. The pattern of responses from the varions sensors will be nsed for vapor recognition and the quantitative analysis of overlapping peaks. 

---