Microdialysis resolution

Lada, M.W., Vickroy, T.W., Kennedy, R.T. (1997). High temporal resolution monitoring of glutamate with aspartate in vivo using microdialysis on-line with capillary electrophoresis with laser-induced fluorescence detection. Anal. Chem. 69, 4560 1565. 

Gonzalez-Mora JT, Fumero B, Mas M. 1991. Mathematical resolution of mixed in vivo voltammetry signals models, equipment, assesment by simultaneous microdialysis sampling. J neurosci methods 231—244. 

RJ Maxwell, E Cohen. Comparison of programmable versus single-wavelength fluorescence for the detection of 3 fluoroquinolone antibacterials isolated from fortified chicken liver using coupled on line microdialysis and HPLC. HRC-J High Resolution Chromatogr 21 241-244, 1998. 

Wang M, Roman GT, Schultz K, Jennings C, Kennedy RT. Improved temporal resolution for in vivo microdialysis by using segmented flow. Analytical Chemistry 2008, 80, 5607-5615. 

Though it was realized very early that microdialysis can serve as an excellent tool for estimating in vivo enzymatic activities (Sharp et al., 1986), this application remained relatively unexplored until recently (Westerink et al., 1990). Microdialysis offers several advantages over existing in vitro techniques because it affords the opportunity for continuous sampling from organs other than blood and because of its high spatial resolution. 

Figure 7 Glutamate changes in the nucleus accumbens after presentation of the predator fox odor 2,5-dihydro-2,4,5-trimethylthiazoline. Data are shown as a percentage of basal levels, and presentation of the fox odor causes glutamate to increase. Microdialysis coupled with CE data (solid circles) are compared with previous studies of microdialysis coupled with HPLC (open triangles). Microdialysis coupled to CE provided information not previously found with HPLC because of increased time resolution.

Another means of sampling is push-pull perfusion (79) using a probe with discrete inlet and outlet tubes. With this method, a small amount of cerebrospinal fluid is pulled directly from the brain through the outlet tube and replaced with artificial cerebrospinal fluid administered via the inlet tube. This approach has greater spatial resolution than microdialysis and because cerebrospinal fluid is collected directly, no concern develops about incomplete recovery. 

By now, chromatographic techniques have been successfully coupled with microdialysis for on-line analysis [70-74]. The advantage of this approach is the amount of data available since several analjdes can be measured almost simultaneously. However, the low time resolution, due to the time necessary for the elution of all the compounds from the chromatographic column, and hence the low analysis frequency, presents a problem. The relatively high cost and the bulkiness of the equipment as well as more particular problems such as loss of sample and carry-over in the injection valve can also negatively affect the analysis [75]. 

A completely satisfactory solution to this problem has not been achieved up to now, but attempts at resolution, also in combination with microdialysis techniques, have been reported. 

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 related issue is that while microdialysis is a continuous process, it is coupled to an analytical separation step that requires discrete sample volumes. Individual samples can be collected off-line with a fraction collector and assayed later (Fig. 3). The temporal resolution is defined by the time interval at which the microdialysis samples are collected. Without the need for further sample cleanup, the temporal resolution for off-line analysis will ultimately be dependent on the perfusion rate and the volume of sample needed for quantitation. If the analytical method does not have sufficient limits of detection, larger sample volumes must be collected, decreasing the temporal resolution of the method. 

Benkestock, K. Edlund, P-O. Roeraade, J. On-Line Microdialysis for Enhanced Resolution and Sensitivity During Electrospray Mass Spectrometry of Non-Covalent Complexes and Competitive Binding Studies, Rapid Commun. Mass Spectrom. 16, 2054-2059 (2002). 

Microdialysis probes come in several sizes and geometries they can be much smaller than ultrafiltration probes because the volume of sample collected does not depend on the membrane surface area (Fig. 3A). Therefore, small microdialysis probes are used when precise spatial resolution is desired. When less spatial resolution is needed and higher recoveries are desirable, longer loop-type probes can be used (Fig. 3B). 

The Kennedy lab has been at the forefront of using rapid capillary electrophoresis to measure neurochemical changes. Optically gated injection of OPA-derivatized amino acids has been achieved in less than 2 s. However, the high salt concentrations of physiological samples, such as cerebral spinal fluid, can reduce EOF, and make separations slower. Still, 10 s temporal resolution for online monitoring of directly sampled and microdialysis samples were obtained. An instrument has been developed in the Kennedy lab for online analysis of microdialysis samples after precolumn... 

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