I have looked at the Palmer Balston unit and think it is kind of expensive for what you get and does not work in many applications.
One of the sponsors of this forum, Trace Analytics has subject matter expertise in this area. I've noticed that the sponsors even participate in the forum now and again and it may not be too difficult to pose your question directly to them.
No offense intended to Trace, but they extremely expensive.
what is the use of compressed air at the contact point ,why don,t you use nitrogen which is inert in nature .
if you using compressed air, than test for micribiological tpc & yeast mold.oil & grease.
Most auditors treat nitrogen as a compressed air and want to see it tested as well.
There was a great thread here, but can't find it with a testing procedure. It used white cloth to test for oil and water and then collecting samples with either agar or in bags with sponges. Some digging might turn it up.
But the above method works well and is a lot cheaper....relatively speaking.