3.3 effect of flow rate and sample volume
The influence of the sample flow rate on the adsorption percentage(>90%)of As (III)DMA,MMA,AS(v) were also investigated in the ranges of 1-9 ml min-1,while keeping the other conditions constant ( the maximal flow rate is depended on the maximal tolerable pressure of the pump and the mini-column). The results in fig 6 (a) demonstrated that the adsorption efficiency were almost kept constant till5 ml min-1,then began to fall with further increase of flow rate,indicating adsorption-desorption equilibrium has not been achieved at higher rate for inadequate adsorption and easy desorption. Therefore,all the experiments at a flow rate of 5 ml min-1 , a relative high rate compared to the literatures,were adopted in this study. This good tolerance to high rate is ascribed to the stable mechanical properties of the novel PPMs material,which could increase the analysis efficiency.
It is also necessary to examine the maximum applicable sample volume especially when the practical samples containing a very low level of targets were analyzed. The influence of sample volume on the recoveries of a 0.5 ngml-1 mixed standard solution was investigated in the range of 10-35 ml .The results depicted in fig 6 (b) shown that the recoveries were found to be stable only till 25 ml,then decreased with further increasing of volumn.It might be due to the limited adsorption capacity of the home made mini-column and the loss of adsorbed arsenic species rinsed by the larger volume of sample . Thereby,25 ml was selected as the optimal sample volume in this study.
3.4 effect of wash time after adsorption
The residual As targets inside the system especially at the pipeline and interface after the adsorption process will probably interfere the subsequent measurement.Moreover,the complicated matrix in real environmental water sample could cause the instability of the plasma and may be deposited on the sample cone wash the analysis system after enrichment procedure.The results shown that 1 min duration was sufficient to completely remove the residue with negligible effect on the next analytical run at a flow rate of 5 ml min-1