Microdialysis limitations

One of the advantages of CE, which is also a disadvantage, is the very small volume needed for analysis. Typical injection volumes are 1-10 nL. This ability to inject small volumes has been shown to be useful in the analysis of single cells and microdialysis samples. Although the concentration-based detection limits of LCEC and CEEC are similar, detection limits based on actual mass analyzed are much lower for CEEC. Typical mass limits of detection for an electroactive compound are in the attomole range. 

Mannio and Cosio (78) described a sensitive, specific, and rapid chromatographic procedure to determine BA and SA in different food products. Benzoic acid and SA were extracted from foods by a microdialysis probe connected online to an HPLC column that allows separation of BA and SA. Detection was done at 228 and 260 nm for BA and SA, respectively. The procedure was linear from 1 to 80 ppm, with a detection limit of 1 ppm for SA and 2 ppm for BA. The assay was successfully applied to soft drinks, fruit juices, and dairy products (cheese, yogurt, and cream). 

The use of the HPLC technique with a programmable fluorescence detector was described for FLU analysis (201). This method was compared with on-line microdialysis used as the cleanup step for a chicken liver sample. After the on-line microdialysis sample cleanup, the resultant HPLC chromatograms were free of background interference, enabling the programmable detector to optimize the quantitation of the three analytes in a single run. The limit of quantitation was 1 yug/ml. 

A characteristic that should presently be improved is the limited linear range of the biosensors which could cause some problem in the cases of hyperglycemic levels. Studies are in progress to solve this problem by finding a suitable microdialysis probe which would be able to recover the subcutaneous glucose in the desired range of concentrations. 

A system underpinned by commercially made screen-printed electrochemical cells was described by Palmisano et al. [19]. The cells were converted into biosensors for lactate in milk and yoghurt by addition of an electrochemically polymerised barrier to interference and a layer composed of lactate oxidase, glutaraldehyde and BSA. These steps appeared to have been carried out by hand. As there was no outer diffusion-limiting membrane, the linear range of the sensors was quite small (0-0.7 mM). They were incorporated into a FIA with a microdialysis unit based on a planar membrane and a buffer reservoir (earlier work used a microdialysis fibre with a platinum electrode [29]. The concentration of lactate was determined in various milks (0.27-1.64 mM), and in raw milk (c. 0.5-0.9 mM) left to degrade on the laboratory bench. The recovery of the microdialysis unit, 2.6%, implied that the sensor had an ability to return measurable currents for very low concentrations of lactate. A further implication is that the electro-polymerised layer was very effective at preventing interference. 

De Groote L, Olivier B, Westenberg HG. Extracellular serotonin in the prefrontal cortex is limited through terminal 5-HT(lB) autoreceptors a microdialysis study in knockout mice. Psychopharmacology (Berl) 2002 162 419-424. 

Zhou and Johnston [55] reported protein characterization by capillary isoelectric focussing (CIEF) on-hne coupled to RPLC-MS. Direct coupling of CIEF to ESl-MS is limited by interferences by the ampholytes. Inserting RPLC in-between can help removing these interferences. CIEF is performed in combination with a microdialysis membrane-based cathodic cell to remove the ampholyte and to collect protein fractions by stop-and-go CIEF prior to transfer to a 5><0.3-pm-ID C,8 trapping colunm and RPLC separation on a 50><0.3-pm-ID C4 column. The separation is performed using an acetonitrile-water gradient (0.1% acetic acid). ESI-MS is performed on a quadrupole-TOF hybrid (Q-TOF) instrument. 

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