In general, the shorter the time elapsing between the collection and the
analysis of a sample the more reliable will be the analytical results.
Under many conditions analyses made in the field are to be commended, as
data so obtained are frequently preferable to data obtained in a distant
laboratory after the composition of the water has changed.
The time that may be allowed to elapse between the collection of a
sample and the beginning of its analysis cannot be stated definitely. It
depends on the character of the sample, the examinations to be made, and
other conditions. The following are suggested as fairly reasonable
maximum limits.
_Physical and chemical analysis._
Ground waters 72 hours
Fairly pure surface waters 48 "
Polluted surface waters 12 "
Sewage effluents 6 "
Raw sewages 6 "
_Microscopical examination._
Ground waters 72 hours
Fairly pure surface waters 24 "
Waters containing fragile organisms Immediate examination
_Bacteriological examination._