Gas diffusion modules are used in autoanalyzers whether segmented flow analysis (SFA), sometimes referred to as continuous flow analysis (CFA), or flow injection analysis (FIA).
The purpose of the gas diffusion module is to function as an in-line cleanup eliminating particulates, potential ionic interferences and chemical complexes. The target analyte permeates through the membrane into a separate flow path going on to further analysis while all the other components of the stream go off to waste. Without separation these interferences can cause problems with both amperometric and photometric flowcells by disrupting the electronic potential of the target molecule, causing precipitation in the colorimetric reagents, or simply creating problems with large particulates.
Originally Alpkem used a gas diffusion module that had a fairly long path, but the width of the path was deep and narrow and utilized a very easy latching lever device to secure the membrane between the diffusion module pieces. OI Analytical purchased Alpkem just as an amperometric cyanide method was being approved by the EPA; this method originally utilized the same gas diffusion membrane module it had used for years in many types of colorimetric methods. During that period it became obvious that greater sensitivity was required if analysis below 10ppb cyanide was going to be possible. Therefore a new module was developed with a shallow but wider surface area path that increased sensitivity by at least 30% or more. At that time, a new acrylic module was produced that incorporated the wider and shallower path development, but there were two major differences, the latching device and the material it was made of. OI Analytical changed the part that secured down the membrane holding assembly to three screws and a plate configuration instead of a simple latch. They also changed the material it was constructed with to acrylic instead of polysulfone.
The screws created many problems over the years such as rusting, difficulty in quickly securing the membrane between the module pieces without tearing, and stripped screw holes. There were also problems at the nut inlets but it turns out it was not related to chemical issues of the acrylic material but how the holes were drilled. Acrylic requires a special drill bit that if not used creates strain on the material thus making the hole not perfectly round. This problem was obvious since often the softer nuts would strip when being screwed in, which is when OI switched to the harder PEEK nut. Instead of solving the drilling issue, using PEEK nuts put great strain on the material thus making it susceptible to cracking. If the holes are drilled out with a special drill bit this issue does not occur, especially if a softer material is used for the nuts like polypropylene (which is what Alpkem always used).
Here at EZkem, we have produced a gas diffusion module that utilizes the wide path, the simple to use latch device, and acrylic. We have used these modules on every type of chemical from acids to bases and have seen absolutely no cracking. The incidence of membrane tearing is solved as well as any leaking. The best result is of course the greater level of sensitivity that the module produces as well. The tubing nuts screw in easily and do not cause strain on the material or destroy the nut itself. For further information on this topic check out our blog post about flanged fittings, which eliminates the cracking issue and helps with flow dynamics as well. We hope you will consider our new module on your flow analyzer, thank you.