The invention is directed to fluid-containing pouches and to methods for forming fluid-containing pouches. In one embodiment, the invention is to a fluid-containing pouch, comprising first and second opposing sheets, and a fluid (e.g., a calibrant fluid, a reactant fluid or a wash fluid) disposed between the first and second opposing sheets. The first sheet and the second sheet have a substantially liquid and gas impermeable perimeter seal. The sheets may be sealed, for example, by one or more of heat crimping, pressure crimping, heat and pressure crimping, ultrasonic welding, metal-to-metal welding or laser welding. Fluid-containing pouches sealed according to the disclosed methods and apparatuses show substantially improvement in terms of reduced gas exchange, notably CO
2 pressurization levels.