Silver plating is always a nuisance to keep in mind. Happens with glass stopcock burets, too. It's important to keep things rinsed out. I assume the reason to go with such elaborate tools as hypodermic needles is the hope that they will give you extreme precision (??). It sounds more like a set-up for spurious precision. I know that sounds like a harsh buzz-kill, but if there's even a little bit of gunk in a hypodermic, the changes in rate of delivery and the turbulence will seriously throw off your results. K.I.S.S.