The interaction between the dispensing mechanism and the membrane affects line width (10). Dispensing mechanisms can be categorized as contact and noncontact. In contact systems, a flexible tip is placed in contact with the membrane. The reagent solution is dispensed through the tip as the membrane is pulled underneath or as the tip is dragged across the membrane; the engineering of the dispenser dictates whether the tip or the membrane moves. In either case, contact of the tip with the membrane introduces the opportunity to damage the surface of the membrane. If the downward pressure of the tip is too great, the tip will emboss a groove in the membrane. Although the groove may not affect distribution of the capture reagent, it introduces a discontinuity in the sample stream that can physically entrap detector particles when a sample is run. For systems using latex detector particles, grooves as shallow as a few microns can be problematic.
The other application mechanisms are categorized as noncontact and involve the dispensing of a liquid stream or aerosol onto the membrane. The initial width of the line is dictated by the diameter of the liquid stream or aerosol when it contacts the membrane. For liquid-dispensing systems, a gap height that is too large will cause breaks in the fluid stream that appear as gaps in the line on the membrane. If the gap is too small, there is a risk that the tip will scrape into the nitrocellulose. For aerosol dispensers, the tip normally is too high to come into contact with the membrane. The major consideration for the gap height is how high the tip should be, because the aerosol spreads out as it moves farther from the tip.
Regardless of the type of dispenser used, variation in material thickness must be taken into account. Engineering of the dispensing equipment allows the gap between the dispenser tip and platform to be fixed (Fig. 2). The thickness of the materials between the tip and platform, however, can vary. Mounting a backed membrane on an adhesive card results in a composite of up to four layers: membrane, polyester film, adhesive, and plastic card. Variation in one or more of these layers will affect the consistency of the final gap height between the membrane and tip. Additional variation will arise if the materials are not laminated together smoothly or if the plastic card does not lie flat on the platform. If at all possible, thickness specifications (means and ranges) should be obtained for all of the materials, especially when changing vendors.
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