We present designs for a portable trace gas sampler, based on membrane technology, to obtain a gas sample from water in the field. A continuous flow of water is equilibrated with a finite volume of gas until the gas pressure matches the total dissolved gas pressure of the water stream. Samples collected in this manner can be analyzed to determine original water concentrations for potentially any dissolved gas. The sampler requires neither compressed carrier gas nor a vacuum pump to extract the dissolved gas sample; its power consumption is minimal and it fits within a 30 L plastic case. During the development stages, both major atmospheric gases (N2, O2, and Ar) and trace gases (CO2, SF6, and SF5CF3) were measured to confirm the equilibrium condition and to quantify the response time. Equilibration studies were conducted in the laboratory and at the site of a borehole CO2 injection experiment on the Lamont campus of Columbia University. The time required to achieve solubility equilibrium depends on the dissolved gas content and the water flow rate; we determined an e-folding response time of 9–12 min, under air-saturated conditions and with a flow rate of 2 L/min. Typically, equilibrium is achieved within 30–45 min. We compare the system function and analytical results to conventional sampling methods during the recovery phase of a push-pull experiment and find a generally good agreement within 10% of conventional analyses for each of the gases.