| The field of modern technology has made enormous stride in recent years. This rapid development of modern industry inevitably brings serious environmental pollution problems, especially of heavy metal pollution problems. The heavy metals in environmental water samples can be directly or indirectly exposure to people, which may threaten to human health. It is urgent to establish comprehensive on-line monitoring for heavy metal ions in environmental water samples. However, the traditional instruments use for detecting of heavy metal elements analysis, usually are large size, need large power consumption, auxiliary gas such as carrier gas, which adverse to realizing online analysis of heavy metals. Liquid discharge microplasma used as an excitation source which has small volume, low power consumption, and the advantages of simple, portable, it is very suitable for online and on-site detection of heavy metals and thus cause widely public concern.But one common problem of liquid discharge microplasma methods is that the sensitivity is not enough. For example, our team had developed a new kind of alternating current liquid discharge microplasma device (ac-EALD), which achieved the detection of elements (Na, Cd, etc.) under atmospheric pressure with low sample flow rate, the detection limit of Na and Cd were0.04mg L-1,0.09mg L-1. The sensitivity is not satisfied, limiting the application of this method. In order to solve this problem, we propose new approach to improve the analytical performance of liquid discharge microplasma emission base on organic sensitization.We develop a new method to improve the performance of alternating-current electrolyte atmo-spheric liquid discharge (ac-EALD) optical emission spectrometry for the determination of elements. Significant enhancement of emission intensity was achieved by adding organic substance into the nitric acid electrolyte solutions. Liquid discharge device is mainly composed of tungsten electrode, electrolyte pool, quartz capillary and graphite core. Electrolyte solution is pumped into the electrolyte pool through a capillary containing organic substances and elements. The discharge is ignited between the top of the quartz capillary and the tip of the tungsten electrode. Under the optimized conditions,3%(v/v) formic acid in nitric acid (pH1.0) produced13times enhancement for Ag and7%(v/v) formic acid resulted in17times enhancement for Cd. The emission of Pb was even enhanced78times in the presence of3%formic acid. In addition, the signal stability was also improved compared with that in the absence of organic substances. Repeatability was0.8%for0.1mg L-1Ag,0.7%for0.2mg L-1Cd and2.6%for1mg L-1Pb standard solutions (n=5). The limits of detection of Ag, Cd and Pb were1,17and45m gL L-1, respectively. The accuracy of the method was demonstrated by determination of elements in simulated natural water samples (GBW(E)080402and GBW(E)080399). The results show that the accuracy of this method is very good.Based on the previous study, we investigate the enhancement performance of organic sensitization agent in direct-current liquid discharge under the atmospheric pressure. Results show that small molecular organic acids under dc discharge can also enhance the emission intensity of elements, which can improve the sensitivity. Under the optimized condition parameters, the signal of silver enhancement is about5.8times, the signal of lead enhancement is about5.4times and the signal of cadmium enhancement is about1.6times. The detection limit of silver, cadmium and lead were0.3μg/1,1.7μg L-1and19μg L-1respectively. It is found that the stability of the plasma can also be improved with the addition of organic matter under the dc discharge. After preliminary discussion of the sensitization mechanism of organic sensitization agent, we put forward several possible mechanisms, but the details of the sensitization mechanism remains to be further exploited. The liquid discharge microplasma emission spectrometry in the presence of organic substances can achieve the analysis of trace heavy metals in environmental water samples under atmospheric pressure, and the device is simple, portable, promising to be used for developing new type of portable analyzer of heavy metals. |
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