Microdialysis interfaces

Liu, C. L. and Verma, S. S., Direct coupling of ionic high performance liquid chromatography with electrospray ionization mass spectrometry utilizing a microdialysis junction interface, ]. Chromatogr. A, 835, 93, 1999. 

Lada, M.W., Kennedy, R.T. (1996). Quantitative, in vivo monitoring of primary amines in rat caudate nucleus using microdialysis coupled by a flow-gated interface to capillary electrophoresis with laser-induced fluorescence detection. Anal. Chem. 68, 2790-2797. 

Both microdialysis and ultrafiltration collection obtain analytes from a sample in the reverse direction regardless of how a normal hemodialysis membrane is used. In hemodialysis, the blood is passed through the inner fiber lumen and filtrate is then collected on the outside of the hollow fiber. When these fibers are used as microdialysis or ultrafiltration devices for collection of samples, the outside of the fiber is interfaced with the sample and the analyte is collected into the inner fiber lumen of the hollow fiber. This is important particularly for the asymmetric membranes that have their large porous support layer on the outside facing the tissue sample. 

All the current efforts directed at miniaturization of the analytical techniques are addressing the problem of interfacing the miniaturized probes used in microdialysis with suitable instrumentation, so that smaller sample volumes can be subjected to appropriate analyses, thus increasing the time resolution of the overall systems. 

A microdialysis probe could help to overcome some of these problems by acting as a protective interface between the bioreactor and the biosensing system, at the same time allowing on-line detection of the selected analyte. In fact, the microdialysis probe can be easily sterilized, thus avoiding problems of contamination. Membranes impermeable to bacteria would also act as a sterile barrier between the interior of the bioreactor and the analysis... 

There are other techniques, however, including microbatch crystallization, where the protein and precipitant are just mixed at the final supersaturation concentration. Free interface diffusion is similar to microbatch but the two components have to diffuse toward each other the concentrations of both protein and precipitant therefore vary with distance from the original interface. In microdialysis, the precipitant solution is allowed to equilibrate with the protein solution through a semipermeable membrane, which permits passage of the precipitant but not the protein (Figure 7). Of these techniques, the first two also lend themselves to automation. 

Figure 8 General experimental setup for the on-line analysis of drugs in the blood of a live rat. The dialysate from a microdialysis device implanted in the jugular vein of a rat is allowed to flow into the mass spectrometer at the CF— FAB interface.

Fig. 9.7.18). The external sample source was left to float electrically (no grounding necessary), which was to make a more compatible interface with real external process sampling equipment such as a microdialysis probe head for bioreactor sampling. In addition, a gated injection method was introduced to accommodate this conhguration. In this design, the buffer and sample streams flow towards each other in orthogonal directions in the microchannels, then meet at the cross in a fashion similar to Fig. 9.7.7.

Severs, J. C. and Smith, R. D., Characterization of the microdialysis junction interface for capillary electrophoresis/microelectrospray ionization mass spectrometry. Anal. Chem., 69, 2154, 1997. 

One last variant of membrane-based LLE will be mentioned briefly. Microdialysis corresponds to a miniaturized version of dialysis techniques routinely used to remove small ions and molecules from biological extracts. At this time it is clear that there are many obstacles to be overcome before routine use of this approach in trace quantitative analysis can he established (Torto 2001). An example of successful application of the method to a specialized biological problem is the determination of basal acetylchohne in micro-dialysate from the brain striatum of awake and freely moving rats, in which the miocrodialysis prohe was interfaced to HPLC with tandem mass spectrometric detection (Zhu 2000). 

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