There are a number of applicable ASTM STandards on this subject.
You should also look at ANSI/ASME PTC 19.11-1997 Steam and Water Sampling, Conditioning and Analysis in the Power Cycle.
Also ASTM D1066-06 Standard Practice for Sampling Steam should be followed.
Collecting representative steam and water samples is difficult and making sure they are conditioned properly is equally important.
Contact Waters Equipment Company or Sentry Equipment Company They can point you in the right direction.