1. A system comprising: means for automatically evaporating a predetermined volume of liquid from a sample solution having a known vapor pressure versus temperature characteristic curve, said system solutions, including, an enclosure means; supply means for delivering at least part of said predetermined volume of sample solution to said enclosure means; and control means for evaporating said supply solution from said enclosure means at a predetermined rate by automatically controlling temperature in said enclosure means to maintain the pressure and temperature in said enclosure means substantially on said known vapor pressure versus temperature characteristic curve.