Improving accuracy of water lead test

Home water tests for lead have an accuracy of about 15 ppb. If I first do a gentle/slow boil for one hour (hot plate under a glass beaker of water) to reduce 2000 mL to 20 mL, will I then effectively be able to detect 0.15 ppb?


My goal is really just 1 ppb.


My main concern now is that any lead might adhere to the glass
(though I think these home tests have a reagent which helps
dissolve lead in case of this) so, if this is truly a problem,
is there a better beaker material or boiling method to use?


Of course, if improvement were this easy, I would expect the
test kits to already be recommending it, so I am probably making
some mistake. Please explain it.


