Use of variable p(H) interface to a mass spectrometer for the measurement of dissolved volatile compounds.

A wide variety of dissolved chemicals can be measured continuously with some specificity by directly interfacing a relatively inexpensive mass spectrometer to a solution. Specifically, we describe the construction and initial use of a membrane interace to a mass spectrometer which allows the continuous measurement of the concentration of dissolved volatile compounds in buffered aqueous solution. In addition to volatile compounds that do not dissociate appreciably at p(H) 7 (e.g., ethanol, ethylene glycol), volatile acids and bases, such as acetic acid or ammonia, can be measured with the interface is operated within the range 1</=p(H)</=12. We also discuss the sensitivity of the system to variations in flow rate and describe a satisfactory method for providing sufficiently constant flow rates.