Sample preconcentration porous membrane

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]. 

The first microfluidic chip with a porous membrane for the preconcentration of a biological sample was presented by Khandurina et al. in 1999. In this experiment, they demonstrated preconcentration of DNA samples. The channels were etched into a glass substrate and bonded to a cover plate using a silicate solution as an adhesive. The chip layout used in this experiment is presented in Figure 50.30. 

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... 

Depending on the type of membrane used, dialysis may be either passive or active (Donnan type). Passive dialysis employs a neutral porous membrane with a fine pore structure. This type of dialysis is used primarily to remove sample matrix materials such as proteins and/or fiber or other solids [9, 10]. Donnan dialysis can do the same, but can also perform preconcentration of the ions in a sample. In Donnan dialysis, an ion-exchange membrane is used to exchange sample ions for receiving solution ions [11-14]. Each type of dialysis is described below. 

SPME is a fast, simple, and green sample preparation technique that can easily combine the process of sample preconcentration and GC or HPLC determination. However, in some cases, it still has difficulty in the fabrication of SPME fibers. In 2006, Lee s group reported a novel extraction and preconcentration technique termed micro-solid-phase extraction (p-SPE), based on the packing of sorbent material in a sealed porous polypropylene membrane envelope. Recently, Lee s group reported... 

The heart of the SLM extraction procedure is the SLM unit, in which analyte extraction, preconcentration, and sample cleanup take place in one single step. The design of the SLM unit can be engineered according to the intended use of the unit. For instance, if an SLM unit is needed for an automated and flowing sample extraction, it can be manufactured from two blocks of polytetra-fluoroethylene (PTFE), poly vinylidine difluoride (PVDF), or titanium. However, for nonautomated, nonflowing SLM extraction, a short piece of porous HF membrane is used. 

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