Development of Head Space Method Based on Rapid Vapor-Liquid Equilibrium with Ink-Jet and Its Application to On-Line Analysis for Volatile Organic Compounds in Water

Head space (HS) extraction is known as a sample-preparation method of gas chromatography (GC) for volatile organic compounds (VOCs). It is classified into a static HS method and dynamic HS method, such as the purge-and-trap method. Though the purge-and-trap method accelerates sample evaporation, it has drawbacks, such as a poor reproducibility and a need for large equipment. The static HS method is based on the vapor-liquid equilibrium. Though the static HS method is very easy to perform, it takes a long time to attain an equilibrium state. In this work, a compact and rapid vapor-liquid equilibrium system was developed. To attain the acceleration of vapor-liquid equilibrium, an ink-jet was used to generate ultra-small sample liquid droplets. The ink-jet is a well-known technology for printers. It can eject many small liquid droplets in a short time with good reproducibility, and it is expected to increase the area of the vapor-liquid interface of unit volume of the liquid. An ink-jet microchip was fitted on the cap of an extraction vial with epoxy glue. The proposed system with the ink-jet was compared with the conventional static HS method by applying it to the extraction of VOCs in water. The vapor-liquid equilibrium for both methods was evaluated by ECD-GC with the peak area. The results showed that the proposed method accelerated the vapor-liquid equilibrium. The length of the vial was effective in reproducibility. When the length was changed from 80 mm to 98 mm, RSDs were improved from 3.2 similar to 22% to 3.0 similar to 14%. Considering the flight time of the droplet from ejection to touchdown to the bottom of vial, and the diffusion time of the target ingredients, the vaporizing efficiency would be thought to depend on the droplet flight time. The proposed system showed good linearity for 0.01 similar to 1 ppm VOCs in a short equilibrium time. The regression coefficients for an extraction time of 0 min were 0.992 similar to 0.999. It was applied to tap water and wastewater, then demonstrated to be useful for the detection of VOCs.