Microdialysis perfusion flow rate

Since the first reports on microdialysis in living animals, there have been efforts to estimate true (absolute) extracellular concentrations of recovered substances (ZetterstrOm et al., 1983 Tossman et al., 1986). Microdialysis sampling, however, is a dynamic process, and because of a relatively high liquid flow and small membrane area, it does not lead to the complete equilibration of concentrations in the two compartments. Rather, under steady state conditions, only a fraction of any total concentration is recovered. This recovery is referred to as relative or concentration recovery, as opposed to the diffusion flux expressed as absolute or mass recovery. The dependence of recovery on the perfusion flow rate is illustrated in Figure 6.2. As seen, relative recovery will exponentially decrease with increasing flow as the samples become more... 

A number of variables, such as the perfusion flow rate, the membrane surface area and geometry, the MWCO of the membrane, the diffusion characteristics of the collected analyte, the composition of the perfusion medium, and the temperature, influence recovery parameters. When the microdialysis is carried out in vivo, the recovery can also be affected by some tissue properties, including tissue tortuosity, the extracellular space volume, the tissue blood fluid and the tissue metabolism of the substance. 

In microdialysis, the recovery of analyte from the sample depends on a number of factors, including the chemistry of the analyte, temperature, perfusion rate, membrane surface area, membrane characteristics, and the nature of the sample (including its fluid volume percentage and whether it is in motion). MiCTodialysis is typically done at low perfusion flow rates (0.5 to 2 /d/min). As flow rate increases, relative recovery decreases, but absolute recovery increases. Relative recoveries from small membrane probes are in the 1-20% range. SubstantiaUy higher recoveries in the 50-80% range can be obtained with longer loop-type probes. 

There have only been a few reports of coupling microdialysis to microchip-based separation systems. Ideally, the microchip system should allow the injection of discrete sample plugs from a continuously flowing stream of dialysate without disturbing the separation element of the analysis. This allows maximal temporal resolution and limits the effect of perfusion flow rate on system performance. 

External microdialysis probes fabricated in-house were used in these experiments. Initial studies found that the addition of ethylenediamine tetraacetic acid (EDTA) to the derivatizing reagent was necessary to prevent divalent cations (Mg + and Ca +) present in the cerebral spinal fluid from decreasing the EOE. Using the device, separations could be carried out at 1.8 min intervals however, the effective temporal resolution was estimated to be between 2 and 4 min due to delay times attributed to the dead volume in the system. Increases in perfusion flow rates led to a decreased delay time but reduced recovery through the probe was observed. 

In order to conduct microdialysis experiments, several other components are required. Syringe pumps are often used to control the perfusate flow rate. The pump has to be able to deliver flow rates precisely in the microliter per minute range. Tubing is needed to connect between the probe and the pump which drives the perfusion flow and, in some cases, between the probe and a sample collector as well. The total dead volume of tubing should also be maintained as small as possible to have better time resolution. The perfusion fluid is a medium resembling the composition of extracellular fluid with minimal or zero concentration of the molecules of interest. Dial-ysate exiting from the outlet of the microdialysis probe is usually collected in a vial for later analysis. It is also possible to coimect the outlet directly to an analysis instrument without using a collector, which is usually preferred, if possible, for its convenience and usually faster analysis results. 

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). 

As the relative recovery will never reach 100%, the dialysate concentrations are only a fraction of the true concentration of the analyte of interest in the surrounding fluid. Before using a microdialysis probe for continuous sampling or monitoring, the trae concentration of the analyte of interest in the surrounding environment and the recoveries at certain perfusate flow rates have to be obtained. There are many different methods for calibration, which are discussed in the following [1]. 

One of the most important issues to consider when making microdialysis measurements is the recovery of the analyte from the dialysis probe and the numerous factors that can influence recovery. The factor that the experimenter has the most control over is perfusion flow rate, which can regulate percent recovery, sample volume, and throughput capabilities related to the temporal resolution of the method. Employing a low perfusion flow rate (<1 pl/min) results in enhanced relative recovery but a concurrent decrease in absolute recovery. Figure 20.3(b). Relative recovery is the concentration of the analyte in the dialysate sample divided by the concentration in the sample media [6]. Absolute recovery is defined as the mass of analyte transport... 

In vivo microdialysis is based on the principle of dialysis, the process whereby concentration gradients drive the movement of small molecules and water through a semipermeable membrane. In vivo microdialysis involves the insertion of a small semipermeable membrane into a specific region of a living animal, such as the brain. The assembly that contains this semipermeable membrane is called a probe, which is composed of an inlet and an outlet compartment surrounded by a semipermeable membrane (see O Figure 9-1). Using a microinfusion pump set at a low flow rate (0.2-3 /rL/min), an aqueous solution known as the perfusate is pumped into the inlet compartment of the microdialysis probe. Ideally, the... 

Calibration is necessary to allow correlation between collected dialysis concentrations to external sample concentrations surrounding the microdialysis probe. Extraction efficiency (EE) is used to relate the dialysis concentration to the sample concentration. The steady-state EE equation is shown in equation (6.1), where Coutiet is the analyte concentration exiting the microdialysis probe, Ci iet is the analyte concentration entering the microdialysis probe, CtiSSue> is the analyte tissue concentration far away from the probe, Qd is the perfusion fluid flow rate and Rd, Rm, Re, and Rt are a series of mass transport resistances for the dialysate, membrane, external... 

Compared to basic research microdialysis sampling devices, those used for clinical studies are much longer with typical membrane lengths between 10 and 30 mm. Flow rates of the perfusion fluid through these devices are also much lower (0.3 pL/min) than typically applied in basic research studies. This combination results in high EE% values for low molecular weight and hydrophilic analytes such as glucose. 

In practice, a microdialysis probe is implanted into the tissue with tubing connecting it to external components a perfusion pump and a fraction collector. The probe is perfused continually at low flow rate (0.1-10 /uL/min) with an artificial physiological solution. As the perfusate emerges from the probe, fractions are collected and samples of each fraction analyzed. 

The perfusion rate of the probe is typically 0.5 to 5.0 lL/min. At this flow rate there is no net transport of liquid across the membrane. One view of microdialysis is that the probe is like a blood vessel in which mass transport of compounds in and out of the probe is a function of the concentration gradient. If the concentration of a compound is higher inside the probe than in the extracellular fluid some fraction will be delivered to the extracellular fluid. This is termed... 

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. 

Several characteristics of chromatography impact on the microdialysis experiment. The chromatography process inherently dilutes the sample. If we assume that a typical microdialysis experiment will involve a perfusion rate of 1 lL/min, with sampling for 5 min, 5 4L of sample will be obtained for the assay. A typical analytical column (15 cm X 4.6 mm) with a mobile-phase flow rate of 1 mL/min may have a peak width of 30 sec and would correspond to a 500-... 

In microdialysis, when the perfusion solution is pumped through the fiber capillary, there is ideally no change in volume of the perfusate and no fluid removed from the tissue. This requires a negligible pressure gradient across the membrane. The perfusate must be isosmotic with the tissue. If either the membrane capillary or the outlet capillary is very long, the back pressure at the membrane can become sufficient for it to leak. This can be minimized by using low flow rates and membranes that have relatively low molecular weight cutoffs. 

In ultrafiltration, analyte molecules are basically carried along with the flow of water and electrolytes. The factors determining recovery in ultrafiltration are membrane characteristics, temperatme, and chemistry of the analyte. Unlike microdialysis, recovery is not dependent on flow rate, membrane surface area, or probe size. Recovery tends to be higher than for dialysis, since there is no perfusion medium to dilute the collected analyte. Ultrafiltration recovery rates are typically in the 90-100% range. This high recovery rate simplifies determination of in vivo analyte concentrations. Table I illustrates some in vitro recoveries obtainable with ultrafiltration probes. 

Concentric Microdialysis Probe When a microdialysis probe with perfusion fluid flowing at a flow rate of is placed in a bath of analyte of concentration Cr, the microdialysis recovery, also known as dialysate extraction fraction, is described by a balance of the diffusion of the analyte across the microdialysis membrane into the perfusion fluid with the convective flux due to the perfusion flow ... 

This is of particular interest for determination of drugs. Most drugs bind to proteins to a certain extent (up to 99%) and only the unbound fraction is biologically active. In such cases, microdialysis gives an estimate of realistic free concentration. Factors such as flow rate of the perfusate, diameter and length of the membrane, molecular mass cutoff, and membrane composition have influence on microdialysis. 

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