Gengis,
Problem is, you'll get all the low level dust on the way up and down as well, which is always there.
So dust is always there, surprise surprise!! I know that because many years ago I flew 1-11's in the Gulf and the leading edges of the wings were sandblasted!!
Surely an analysis of what is on any "collector" device would establish what is sand and low level dust and what is volcanic ash. Correlate that with the condition of the engines and we would be able to get an understanding of the safe level of volcanic dust.