An automatic system for accurate preparation of gas mixtures

• Automatic preparation of gas mixtures with molar fractions specified by the analyst.
• Good homogeneity resulting from forced circulation of the mixture.
• High correlation between programmed mixture composition and resulting chromatographic signal.
• Recovery study with good percent results (98.5 ± 2.3, 100.5 ± 4.6, and 98.9 ± 6.4) for methane, ethane and propane.

This paper presents a novel automatic system for preparation of gas mixtures with suitable accuracy for quantitative applications. The system is managed by a purpose-built software, which controls the opening of gas admission valves in order to achieve the desired partial pressures. A piston-driven diaphragm pump is used to force the homogenization of the mixture by closed-loop circulation. The proposed system was tested by preparing binary mixtures of nitrogen with methane, ethane and propane. As a result, good correlation (R2 > 0.99) was obtained between the programmed molar fractions of the mixture components (as set by the user in the system management software) and the corresponding analytical signals measured by a gas chromatograph. A recovery study involving gas mixtures with certified composition was also carried out. On the overall, good recovery was obtained, with percent values of 98.5 ± 2.3, 100.5 ± 4.6, and 98.9 ± 6.4 for methane, ethane and propane, respectively. Such findings indicate that the standard additions were properly introduced and suitably mixed with the gas samples, thus corroborating the correct operation of the proposed system.