Measurement of part per billion N2O in air

ในงานแก๊สชีวภาพนั้น มีความต้องการวัด ไนตรัสออกไซค์ที่ระดับต่ำมากๆ (ppb range)  และ เทคนิคที่คุ้นกันส่วนใหญ่ก็มี ECD, HID และวิธีการดั้งเดิม PED ก็ยังเป็นที่่นิยมอยู่  แต่ประเด็นเรื่องความปลอดภัย ของกำมันตภาพรังสีทุกวันนี้ ทำให้วิธี ECD ไม่เป็นที่สนใจนัก ถึงแม้ว่าการแพร่กระจายของกำมันตรังสีจะอยู่ในระดับต่ำก็ตาม  การวัด ppb of N2O จึงนิยมใช้แบบ HID และวิธีพื้นฐาน PED  แต่ก็ยังมีข้อจำกัดเรื่อง selectivity และ sensitivity เรามาดูวิธีการพลาสม่า กับเครื่องmicroGC ในการวัด laughing gas นี้ดูนะคะ

It is well known that the measurement of N2O can be achieved with different kinds of detectors using specific chromatography techniques. The ECD, HID and conventional PED are the mostly used type of detectors for this greenhouse application. The radioactive source coming from the ECD makes this detector less attractive due to the latest worldwide regulations about radioactivity, even for low source of radioactivity. The HID and conventional PED are relatively good detectors for such analysis, but create more chromatography challenges caused by lack of selectivity and sensitivity. With the LDetek PlasmaDetek-E, the enhanced N2O in ambient air with reduced interference mostly coming from the high concentration carbon dioxide and moisture.

The improved operation’s mode and the optical design of the PlasmaDetek-E combined with the right chromatography configuration in the MultiDetek-2 compact GC remove the interference usually coming from the carbon dioxide and moisture.

The MultiDetek-2 system is configured with one injection diaphragm valve V1, which is used for injecting the sampling volume into the chromatography diaphragm valve V2 and a second packed Shincarbon column to catch the desired N2O peak and flush to vent the undesired interference gases. The N2O peak is then going to the PlasmaDetek-E where proper analysis is performed.

This combination makes it ideal to measure extremely low concentration N2O in air without having the N2O peak integrated in the high concentration CO2 tailing. This technique has been tested up to 5000ppm CO2 for measuring as low as 1ppb N2O as demonstrated in the chromatograms.

The MultiDetek-2 can be configured with different sampling mode to get the sample gas ready for analysis. The drawing of this application note shows the use of a micro pump that is mounted just before the sampling loop. The pump is fully controlled from the software interface and can be switched On/Off adequately for a specific period of time to allow proper sample gas quantity going to the sampling loop mounted on the diaphragm valve V1.

This way, the loop is filled just before running an analysis. This technique allows to minimize the sample gas volume required for analysis since in many cases, the sample gas comes from bags, balloons, canisters or others. It is then critical to minimize the sample gas volume for every analysis.

An optional sample line purge system can also be added to the sampling mode of the MultiDetek-2. This system uses the carrier gas to purge the sample line prior to pump the sample gas from its source. This way, the sample flow path and sampling loop are totally purged with N2O free gas since the carrier gas used as purge gas comes from the heated LDetek, model LDP1000 getter that removes N2O down to 0.01ppb level.

The MultiDetek-2 can also have an injector system with septum for syringe injection mode. The system is directly mounted on the injection diaphragm valve V1.

CONCLUSION:

With its user-friendly interface and the simple configuration of this compact GC, the MultiDetek-2 is a perfect fit for this greenhouse application. The enhanced sensitivity and selectivity of the PlasmaDetek-E allow extreme low limits of detection. This new measurement technique combined with the integrated purged low volume sampling gas system brings this technology over the existing conventional measurement methods and systems.