This blog will detail the proper maintenance of Gas diffusion membranes. There are various approaches, some labs replace their membrane daily preferring to avoid the issue altogether while others may go weeks or even months using one membrane with no issues.
So, what is the right approach? Unfortunately, there is no definitive answer as there is an extreme level of variance in operating schedules and sample types from lab to lab. However, we can discuss what to look for in a failing membrane and offer some general guidance on maintenance schedules that will cover the needs of a majority of labs.
First, what is the gas diffusion membrane and what purpose does it serve? The actual structural and chemical details are more complex than need to be discussed here but put simply it does what it says on the tin. It is a thin membrane with pores that allow only the analyte gas to diffuse through for analysis while preventing the passage of the matrix and any contaminants that may be present. The most common type are Polypropylene membranes, which are used for Cyanide, and Ammonia/TKN analysis. These utilize a hydrophobic barrier to prevent the matrix from passing through, only allowing the analyte gas through, which is typically achieved by disassociating the desired ion using a strong acid or base. This is especially beneficial as it serves as a type of on-line distillation, reducing or removing the need to distill samples ahead of time.
In general, one can know when it is time to replace the membrane by careful monitoring of the results produced. A degrading membrane will allow less analyte through or will allow it through in an inconsistent manner. If less analyte is passing through the membrane the peak heights in the results will be correspondingly lower, this can also cause quality controls (QCs) to fail, and if the passage is inconsistent the peak shape and reproducibility can be negatively affected. Therefore, if you observe lower peak heights, failing QCs, misshapen peaks, or a lack of reproducibility, one of the first things you should do, after confirming that there are no issues with flow, is change the membrane.
If you would prefer to implement a regular membrane replacement schedule in order to avoid ever experiencing poor results, the exact schedule needed will vary greatly depending on how often the system is run and the contents of the samples. In particular soil samples and mining/industrial matrices are hard on the membrane and will therefore possibly require more frequent maintenance than drinking/wastewater samples.
In general, we recommend most labs start by replacing the membrane weekly, if your results are consistently great you could then move to every two weeks and with continued good results could then move to once a month. Conversely if partway through each week you notice your results begin to degrade but improve after changing the membrane you could shorten to once every two or three days or even daily. It all depends on the needs of your lab and your system.
Other possible points of failure to consider if changing the membrane does not solve your problem, pump tubes, reagents, flowcell, reference electrode, UV coil, HP Coil and heater.
