a mole is a unit that you use 6.02^10^23 atoms.
your equation gives you the answer in moles. You multiple it to see how many atoms you have in total. its like how many red and green balls there are in a box not counting trace gasses. each atom weighs different
It is just a standard chosen to base equations around, like the kilogram, but with number of atoms instead of mass.
there are trace gases and probobly small amounts of isotopes but thats usually irrelevant. You will also have some dust particles, fungal spores and maybe water aerosol and stuff, but again it sounds irrelevant towards your work.
The pump will probably have lubricant and also it must shed some of its body as it works because there is friction. Unless you are running a chromotograph or other very sensitive instruments these won't typically be issues.
There is industrial standards for 'clean dry air' , they focus on humidity but you should have mechanical filters too. These standards will illuminate you on what might actually be in your air.
Very nasty air might destroy pneumatic tools and cause clogging, humidity may cause corrosion, and you can possibly get cavitation like effects happen that can wear down air motors if they get condensate in them because they spin very fast.
Another 'typical' effect where you care about air quality would be for using compressed air in a plasma cutter. You can ready industrial literature all about wear and tare on plasma cutter heads and cut quality being effected by the quality of the air that you supply them with. If you pull air from a bad location you might get other contaminates like stuff being emitted by soil, radon, etc. For instance if you have a poorly maintained nasty ass compressor room that is added as an after thought and not finished well you might end up with funky air. I have seen this before, but it would only really effect sensitive things which would typically be well filtered anyway.
I mean like the kind of place where you wonder why the fuck they don't pressure wash it and add a dehumidifier. Certain installations may spit semen like foamy discharges out of the air lines (not kidding). Air intake location matters too, like don't put your intake near by a cooling tower, sprinklered lawn, etc. Common sense shit.