A Study On Method Of Arsenic Speciation

Arsenic is one of the ordinary elements in the nature. Its content is an important testing item in environmental monitoring because of the strong toxicity and carcinogenic characteristic. Recently, as the development of investigating on arsenic, people have realized that the chemical and biological toxicity of arsenic is dependent on its chemical forms in which it exists. So the total content of arsenic could not reflect the real influence of it on environment or human beings. Arsenic speciation should be introduced. It is qualitative and quantitative analysis of different arsenic species. For the numbers of arsenic species, the toxicity of inorganic arsenic compounds is much greater than that of organic arsenic compounds. The much organic group arsenic links, the less toxic it is. Thus, our study selects As(III),As(V),MMA,DMA as analytical subject to establish speciation method because of their toxicity.Now, arsenic speciation is achieved by combined method involving HPLC and ICP MS. But the ICP MS is too expensive to be accepted by most laboratories. In the other hand, HG-AFS, which is much cheaper, has comparable sensitivity to ICP MS. But the HG efficiency of organic arsenic compounds is so poor that it could badly influence the testing of environmental samples. Consequently, the study could make use of UV photo-oxidation instead of micro-wave oven to decompose organic arsenic compounds, AFS as a sensitive detector, IC-Anion-PW anion exchange column as separator for the first time to establish the method of trace arsenic speciation. We hope that the method is accurate, stable, has high sensitivity and wide range of linearity and can be applied to test natural water samples. It will do contributions to enrich the data of environmental samples.Firstly, the testing condition of AFS is optimized. Carrier solution is 10%(V/V) hydrochloric acid, reducing agent of HG is 2% (m/V) KBH₄+0.5% (m/V) KOH solution, samples are kept in 5%(V/V) acidity with concentrated hydrochloric acid and 12%(V/V) dosage of reducing and screen agent with 10%(m/V) Tu-Vc mixture.Secondly, the separation condition of the four arsenic species is studied. The pH is 6.0, the flow rate is 1mL/min, and the phosphate is used as mobile phase, gradient programe is 0-1 min 1mmol/L KH₂PO₄; 1-1.Imin step to 15mmol/L KH₂PO₄; 1.1-7min, remains in 15mmol/L KH₂PO₄. The four arsenic species are baseline separated.Thirdly, the decomposition condition of organic arsenic compounds is studied. It is achieved by oxidation reation with oxidant and UV-irradiation. The oxidant is 2%(m/V) K₂S₂O₈ solution and the irradiation time is 1min. All the organic arsenic compounds are coverted to As(V). No influence on separation is observed.Fourthly, the method is evaluated. It shows that the four arsenic species has good linearity in the range of 0-20mg/L, independently, the DL is lower than 0.1μg/L, the recovery is between 95% - 110%, and the RSD is lower than 3%. It indicates that the method has the charactertistic of wide range of linearity, high sensitivity, good accuracy and stability.Finally, the method is applied to test natural water samples. The result is proved to be realiable and stable by measuring reovery, RSD and compareing the total arsenic content before and after separation. Therefore, the method can be widely used in testing natural samples.