I'm looking for information from ion selective electrode (ISE) users specifically, but others can chime in on this as well. I know that SM says it is unnecessary for this analysis. However, 40 CFR says that you have to distill unless you have comparability data on hand while also saying to consult the analytical method in regards to the need for distillation. The question becomes, do you distill your samples when using ISE for ammonia analysis? For either case, is this something that you have just done, do you have guidance from your state or other program area, or is there something else that drives your decision? Thanks in advance for the input.

I've posted this on LinkedIn as well. You don't need to respond in both places.

Thanks for the feedback Chuck. What group required the comparison study? Do they allow the same provisions for colorimetric analysis as well?

As one who has seen non-distilled samples give strange results, for industrial samples we always have the lab distill.  For PURE municipal, there has to be a study to show it makes no difference.  Of course if you are doing the color method, distillation has to be done if any color is present.

So like anything, it is site dependent.  We have even found distilling the samples and comparing ISE with the color method can give sometimes wacky results probably due to poor distillation or other species present that distill.

Hi David, I'm in a "PURE" municipal wtp/stp lab (CT).  We use ISE only, and conducted a study for I'm guessing the DEP assessor (it was before my time that they distilled).  I have previously worked in a contract lab with both routine (muni) and contract samples where we distilled 100% and ran colorimetric (Lachat).  As long as the matrix warrants it, it seems like there is less introduction of error without distillation and the ability to achieve more reproducible results.