以逆向流層析分析水中酸性除草劑

Abstract

定量水樣品中的酸性除草劑，通常先以固相萃取法做濃縮前處理，再以其他的分析儀器分析之，在萃取的過程中通常會有相當大量的腐植酸一起萃取出來，此時若以HPLC分析，腐植酸會對除草劑的訊號造成嚴重的干擾，造成定性和定量上的困難，即使將層析管柱結合一RAM(restricted access media)管柱，以便降低腐植酸對訊號的影響，但是依然無法得到一滿意之層析圖譜。本研究先以固相萃取做濃縮前處理，再使用分析型的逆向流層析儀來分析五種酸性除草劑，溶劑系統由正己烷-乙酸乙酯-水(15:7:20,v/v/v)所組成，以下層當動相，利用逆向流層析儀結合紫外光/可見光偵測器做線上偵測，成功分離了五種除草劑，並降低了基質中腐植酸對基線所造成的干擾。由於除草劑Bromoxynil的分析時間太長，因此只做四種除草劑的定量；不論除草劑在高濃度(12 mg/L)或低濃度(6 mg/L)的腐植酸水溶液中，以及在真實水樣中，其回收率大部分都在70 %以上，最佳的接近100 %。偵測極限在2μg/L ~ 14μg/L之間。

Quantitative analysis of acidic herbicides from water can be done by solid-phase extraction first, then analyzed using high performance liquid chromatography (HPLC) or other instruments. During the extraction, considerable amount of humic substances would inevitably be extracted with the herbicides from water matrixes. The humic substances can cause a severe baseline deviation using UV/Vis detectors, thus make identification and quantification difficult. In addition, the HPLC column would be contaminated by the humic substances due to permanent adsorption to the stationary phase and shorten the column life time. Combining a “restricted access media” column with the analytical column helps reduce the baseline deviation, but significant baseline signals are usually still observed. We use a countercurrent chromatography for analysis of five acidic herbicides in place of HPLC after solid-state extraction. Since no packing material is needed for CCC columns, damage to the column due to adsorption will not happen. The lower phase of a n-hexane-ethyl acetate-water (15:7:20, v/v/v) solvent system is used as the mobile phase. Since the humic substances are more hydrophilic than the herbicides, they are eluted first and separated from the herbicides. Although five herbicides are well resolved, quantification of compound 2,4-D is somewhat affected by the tailing signal of humic substances. In addition, compound bromoxynil is excluded for quantification due to its elongated retention time. The detection limits range from 2-14 mg/L for four herbicides.