Development of a new gas-permeation system and its application to the spectrophotometric determination of ammonium ion by FIA

A new on-line gas-permeation system was developed to enhance the efficiency and stability of gas permeation for a long time, and was applied to the determination of ammonium ion in river-water samples by the flow- injection method. The proposed gas-permeation system consists of a newly designed gas-permeation unit, a sample injection valve, a reaction coil and two six-way rotary valves in a constant-temperature oven. The gas- permeation unit was assembled using an inner micro porous polytetrafluoroethylene (PTFE) membrane tubing and an outer glass tube, which could be easily connected with one another using connectors with ferrules and o-rings. A sample solution was injected into a carrier containing 0.02 M sodium hydroxide, in which ammonium ion was converted to gaseous ammonia. The gaseous ammonia generated in the carrier passed through the micro porous PTFE membrane, was absorbed in a reagent solution and changed the pH of the reagent solution, which resulted in a color change to cresol red. The absorbance change of cresol red at 550 nm was measured using a visible detector with a flow cell (8μl, 10mm path). After the measurement, two six-way rotary valves were switched over from the flowing state to the drain state of the gas-permeation unit, and the residual solutions in the gas-permeation unit were removed and the PTFE membrane tubing was dried. By keeping the temperature of the oven at 40℃, the sensitivity and the reproducibility for the determination of ammonium ion was improved. Furthermore, the efficiency of gas permeation through the inner tubing could be kept high and constant for a long period without any procedure for reactivating the porous PTFE membrane tubing. Calibration graphs for ammonium ion were liner over ranges of 0 to 10 ppm and 0 to 1.0 ppm of N-NH(4)(+), and the sampling rate was 30 samples per hour. The detection limit for N-NH(4)(+) was about 0.01 ppm when the effective length of the membrane tubing was 5 cm. By using the proposed FIA system, ammonium ion in river-water samples was determined. The analytical results obtained by the proposed method showed a good correlation with those obtained using the indophenol derivatization/FIA method. The relative standard deviations of ten injections were 0.51% and 0.83% with concentrations of 2.73 ppm and 4.11 ppm, respectively. The recoveries of ammonium ion were found to be 97～98%.ガス透過分離におけるガス透過の長期間の安定性と高効率化,更にガス透過効率の長期間の再現性を向上させた,新しいオンラインガス透過システムを構築した.このシステムでは新しく設計・製作したガス透過ユニットを組み込み,ユニット中を溶液が下方から上方に流れるようにし,気泡の発生を防ぐ工夫がほどこされ,更に測定後にガス透過ユニット内に滞留する溶液を排除するために2個の六方切り替えバルブを内蔵させている.ガスの安定的透過のために温度制御できる小型恒温槽を装着した.この装置中にガス透過ユニット,アンモニウムイオン定量用の反応コイルなどを組み入れ,精密測定の向上を目指した.ガス透過ユニットは,多孔質ポリテトラフルオロエチレン(PTFE)チューブとガラス管からなり,接続には樹脂製フェラル及びO-リングを用い,組み立てを容易にし,デッドボリュームも小さくした.ガス透過ユニット,反応コイル及び試料注入器などを恒温槽の中で一定温度に保つことにより,安定した再現性の良い測定が可能となった.ガス透過ユニット内に滞留する溶液を簡単かつ迅速に強制排除する機能も備えており,測定後はガス透過ユニット中の溶液を除いておくことにより,多孔質チューブの透過性能が長期間維持され,再活性化の操作をすることなく,高いガス透過効率を維持することができた.本システムを用いると,検量線は0～10,0～1.0ppmの範囲で良好な直線性を示し,1時間当たり30試料の分析が可能となった.実際に,河川水中のアンモニウムイオンの定量を行ったところ,インドフェノール誘導体/FIA/吸光光度法による定量値と良く一致し,試料に対する相対標準偏差は0.51,0.83%で,回収率は97～98%と良好であった