a 1-mm-i.d. tube, the surface area available for immobilization is on the order of 30 cm2 per meter of length. This compares with hundreds of square meters per gram of inorganic support material and, as mentioned above, the amount of enzyme immobi lized is related to the available surface area. To circumvent this problem, a number of methods to produce organic or inorganic annuli have been report ed. Unfortunately, the presence of the annulus thickens the stagnant layer at the wall of the tube, requiring greater diffusion of the product to re-enter the fluid stream and resulting in greater dispersion of the product. Nevertheless, a large number of suc cessful demonstrations of open-tubu lar reactors have been reported. Dispersion in packed-bed IMERs is analogous to the dispersion seen in chromatographic columns. Ap proaches to minimizing dispersion in clude improved packing structure, de creased support particle diameter, and optimization of the linear fluid veloci ty. At present, typical particle diame ters in IMERs are 37-74 μηι, and packing techniques are archaic. In our experience, there are other factors that limit the utility of decreasing the particle size and achieving lower dis persion, although the theoretical basis and techniques are well established by
modern high-performance liquid chro matography (HPLC). Most of the work done on the enzyme kinetics in IMERs has assumed a continuous in troduction of substrate, when in actu ality a sample for analysis is intro duced as a pulse. Adachi and his co workers (18) have studied the elution profile for an impulse injection for both a single enzyme and consecutive first-order reactions. The most impor tant characteristic of the reactor in minimizing dispersion was the ratio of the time required to diffuse into the support to the time required to be eluted from the column. When this parameter approaches one, very broad and asymmetric peaks were observed. Fortunately, for the conditions nor mally used in analytical reactors (100-μηι particles, 0.5 X 5-cm reactor), the peaks should be symmetrical and relatively narrow. In essence, we have decoupled the dispersion and reaction rate processes. We have also shown that if the reaction is not totally first order, "kinetic" broadening can occur. Recently a third type of reactor has been described in which solid beads are packed single file in a tube whose inside diameter is one to five times the diameter of the beads. Reijn, Poppe, and van der Linden have studied dis persion in the single-bead STRING reactor (SBSR) (19). The authors con
clude that there is a residence timereaction rate component and a flow rate-reactor length-bead diameter component to the dispersion. The re actor can be optimized using either one of the two components, and the authors conclude that they are inde pendent of each other. Using nonreactive compounds, they conclude that the SBSR has similar dispersion, re agent consumption, and residence time as a more conventional packedbed reactor. Gnanasekaran and Mottola have reported on an immobilized penicillinase SBSR, but no compari son to a packed-bed system was in cluded (20). Because the issue of com parable enzyme loading cannot be eas ily addressed, it is unclear what the role of the SBSR in immobilized-enzyme technology will be. The possibility that the enzyme it self can cause peak broadening has re cently been described. Because the substrate must bind to a specific site on the enzyme to facilitate the reac tion and the product(s) must then be released, it is not inconceivable that the enzyme could influence the peak shape. Slow release of product, for ex ample, would be analogous to a kinetically slow dissociation from a binding site in adsorption chromatography and would result in asymmetric, broad peaks. This phenomenon has been ob-
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Stopped Flow Rapid Scan Spectrophotometer • Minimum dead time — 500 Msec • 16 spectra measured every 1 msec sequentially • S i m p l e and robust mixing system without syringe • Fluorescence, T-jump and flash accessories. CIRCLE 2 ON READER SERVICE CARD 526 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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