Microdialysis sampling detection limits

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. 

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. 

Kennedy s group used microdialysis sampling to monitor extracellular dopamine concentration in the brains of rats. The dialysate, mixed online with 6 mM naphthalene-2,3-dicarboxaldehye and 10 mM potassium cyanide in a reaction capillary, was periodically analyzed by MEKC at 90-s intervals [60]. The MEKC system consisted of a 10 p,m ID, 369 p,m OD, and 16 cm (14.5 cm to the detector) capillary, 30 mM phosphate buffer (pH 7.4) containing 6.5 mM SDS and 2 mM 2-hydoxypropyl-)3-CD, with LIE detection using the 413-nm line of a 14-mW diode-pumped laser, and an electric field of 850 V/cm. The detection limit for dopamine was 2 nM when sampling by microdialysis. The separation capability of the developed method is illustrated in Figure 3.10 where... 

By coupling microdialysis to capillary electrophoresis, much shorter sampling times can be used, since the separating step can be very fast. As mentioned in Chapter 6, injection volumes are in the nanoliter range. This allows not only short sampling times but also a low perfusate flow rate that vhll give better detection limits (Section 9.6.1). 

CEEC with methyl-O-P-cyclodextrin as the modifier was used to monitor the pharmacokinetics of (-) and (+) isoproterenol by I.V. microdialysis sampling. Using a combination of peak stacking and electrochemical detection, limits of detection of 0.6 ng/mL were achieved. Isoproterenol has a very short half-life. A flow rate of 1 pL/min was used for the microdialysis sampling. It was possible to analyze the 1 pL samples and monitor the pharmacokinetics of the isoproterenol enantiomers with a temporal resolution of one minute (Figure 14). 

There are two main reasons for entering the realm of micro- and nano-LC to obtain necessary sensitivity or if only the smallest quantities of samples are available. The main fields of application are currently proteomics, pharmacokinetics, metabolism studies, microdialysis, and, increasingly, environmental analysis. Flow rates in micro- (2-50 pL min ) and nano-LC (200-2000 nL min ) place high demands on the HPLC system and the user. Continuous optimization with regard to robustness, sensitivity, detection limit, and resolution tends to be a feature of any application. 

Over the last decade, direct detection of ACh with LC—MS/MS systems employing an electrospray-ionization (ESI) source have resulted in superior limits of detection, increased reliability, and improved temporal resolution with microdialysis sampling. Like monoamines, ACh is a small, polar, hydrophilic compound that is not readily retained on RP columns. In MS analysis, the selection criteria of LC systems. 

The flow rates of the microdialysis experiment are such that samples of 1-10 J,L are typically obtained. At typical perfusion rates, the perfusate is not at equilibrium with the extracellular fluid. As such, the concentration of sample in the dialysate is some fraction of that in the surrounding tissue. This is termed the extraction efficiency and is a function of the delivery or recovery of the probe. Not only are the sample volumes small but also the concentration in the dialysate may be low, typically ranging from 1 pM to 1 lM. Because the recovery is typically less than 100%, the limit of detection of the method should be lower than the lowest concentration expected in the dialysate. This presents a tremendous challenge for the analyst. 

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