zinc is an essential element in the 
 nutrition of animals , human beings , and plants [ 1 , 2 ] . zinc plays an important role in replications , gene expressions , and the metabolism of 
 nucleic acids and different proteins there are many methods for 
 zinc determination such as spectrophotometry 
 [ 35 ] , atomic absorption spectrometry [ 6 , 7 ] , flame atomic absorption spectroscopy , graphite furnace atomic absorption 
 spectrometry [ 9 , 10 ] , 
 inductively coupled plasma atomic emission spectroscopy   , 
 and inductively coupled plasma mass 
 spectrometry . but some are expensive , 
 some are time - consuming , some are troublesome , and others are toxic . preconcentration 
 technique [ 1316 ] can improve the detection limit as well as the selectivity of 
 the method . flow injection analysis ( fia ) [ 1719 ] can improve analytical 
 procedures such as high analytical throughput , precision , accuracy , low 
 reagents , and sample consumption . therefore , a new method was proposed for online preconcentration and 
 spectrophotometric determination of low - concentration zn ( ii ) in water . the 
 proposed method is based on the fact that zn reacts with pan to produce zn ( ii)-pan complex 
 formation in the ph 9.3 buffer solution . to avoid the extraction procedure , it 
 has been found that the pan reacts 
 with zinc to form a water - soluble complex at the presence of tween-80 . a model zj - la automatic metallic element 
 analyzer developed and produced successfully by professor xinshen zhang of our own laboratory was used . the analyzer has the functions of flow injection analysis ( fia ) , ion 
 chromatography , and automatic sample injection . the function of automatic 
 reference fia was applied to determine zn ( ii ) . two analytical pumps ( shanghai 
 huxi analytical instrument plant , shanghai , 
  china ) of fia 
 were used to deliver solutions . one was used to deliver elution solution and 
 reagent solution while the other delivered sample solution . polytetrafluoroethylene ( ptfe ) tubing of 0.5 mm in internal diameter was used as the channels 
 for all solutions to circulate . ultraviolet and visible spectra were 
 obtained using a spectrumlab s54 ( lengguang technology co. , ltd , shanghai , 
  china ) . data 
 acquisition    and processing were performed 
 with hw-2000 chromatography software ( qianpu software co. , ltd , shanghai , china ) running under windows xp . all reagents used including 1-(2-pyridineazo)-2-naphthol 
 ( pan ) ( tianjin ruijinte chemical reagent plant , tianjin , china ) , tween-80 , sodium tetraborate , 
 o - phenanthroline , ddtc , and sodium hexametaphosphate ( chengdu fang zhou chemical reagent plant , chengdu , china ) ,   oxalic acid dihydrate , lithium 
 hydroxide monohydrate , ammonium thiocyanate , and citric acid   ( beijing donghua chemical reagent plant , beijing , china )   were of analytical grade and all 
 solutions were prepared with deionized water . zinc 
 stock solution : zinc stock standard   solutions at a concentration of 
 1000 mg l were obtained from china environmental monitoring terminus , beijing , china . working solutions were prepared by suitable 
 dilution of the stock solution . mixed 
 chromogenic reagent solution ( r ) : 10 ml of 4  10 mol l pan 
 ethanol solution , 20 ml of 8% tween-80 , 20 ml of ph 9.3 of sodium tetraborate 
 suffer solution , 1.5 ml of 10% sodium hexametaphosphate , 1 ml 
 of 0.1% ddtc , and 2 ml of o - phenanthroline 
 were mixed and the mixture solution was 
 diluted to 100 ml . elution 
 solution : a total of 1.26 g of oxalic acid dihydrate , 1.576 g of citric acid monohydrate , and 1.15 g of lithium hydroxide 
 monohydrate were dissolved in 1000 ml 
 of water . the preconcentration column developed by our laboratory was filled with 
 macroporous spherical resin with 4-(2-pyridylazo ) resorcinol ( par ) functional 
 group . the par functional group can form chelate complex with determined metal 
 ion in the preconcentration column and the chelate complex can be eluted by the 
 eluent solution of oxalic acid - citric acid - lithium hydroxide . the filling 
 granularity of resin was 80100 m . the 
 preconcentration column length was 40 mm and its internal diameter was 5 mm . by filling the preconcentration column , mix resin and deionized water into 
 pasty matter and drop slowly into column with burette . after sedimentating for 
 a moment , resin was dropped slowly until the preconcentration column became 
 full . then , cover strainer , cover on the column , and wash a few minutes with 
 deionized water for further work . the sample 
 solution was injected into preconcentration column to preconcentrate with a 
 six - way injection valve . when the instrument was put into analyzing position , the concentrated zn 
 ( ii ) was eluted by eluent solution , pushed to the three - way cock , and mixed 
 with chromogenic reagent solution in the 
 reaction coil . the zn ( ii)-pan complex was 
 then formed and carried into the flow cell . the 
 absorbance intensity of zn ( ii)-pan complex was 
 determined at 560 nm by detector and transformed to a signal of the peak which 
 was recorded by hw-2000 software on a pc . in the flow 
 injection analysis , the absorbance of complex is affected by ph of the reaction 
 medium . the effect of ph on 
 the zn ( ii)-pan complex 
 was tested in the ph range 411 . the test results showed that the maximum peak 
 height could be obtained when ph range was between 8 and 10 . the effect of pan concentration on the peak height were tested from 5  10 
  mol l to 1.2  10 the results showed 
 that the peak height rose when concentration of pan changed from 5  10  mol  l to 4  10  mol  l , and the peak height declined 
 when concentration of pan changed from 4  10 mol l to 1.2  10 mol l. therefore , 4  10  mol l pan was selected for further 
 work . according to 
 the reaction kinetics , the reaction rate will inevitably increase when 
 increasing concentrations of the reagent . however , the reaction rate will not 
 increase when concentrations of the reagent reaches a certain value . therefore , 
 when pan consumption reaches a certain value , the concentration of pan which 
 reacts with zinc should be certain because zinc concentration is fixed in the 
 sample . thereby , the absorbency will not increase when increasing pan 
 concentration . on the contrary , the background color ( baseline noises ) will 
 increase because the free state pan in the system will increase . thus , tween-80 was selected as reagent which could increase 
 solubility of the system . the test results in figure 3 showed that the peak 
 height increased when concentration of tween-80 changed from 1.0% to 1.6% and 
 that the peak height did not increase even when concentration of tween-80 
 changed from 1.6% to 2.2% . therefore , 1.6% tween-80 was selected as the optimal 
 concentration for the further work . in the fia , the 
 flow rate could affect the sensitivity of the proposed method . in the study , 
 the flow rate was adjusted by the interior diameter of pump pipe . therefore , 
 the effect of the flow rate of reagent solution on the peak height was studied 
 from 0.10 ml min to 0.72 ml min . the test results 
 showed that the best flow rate could be obtained when the flow rate of reagent 
 ( r ) was 
 set at 0.23 ml min .   in the 
 flow injection analysis , the reaction time is adjusted by the length of 
 reaction coil . the 
 study results in figure 4 showed that 3 m of the 
 reaction coil length was the 
 optimum and further increase of reaction coil length did not increase peak 
 height . length of preconcentration column and 
 filling granularity could affect the effect of preconcentration . in the study , 
 filling granularity of 80  100 m was selected and length of concentration 
 column from 20 mm to 100 mm was tested . the results showed that the shorter the 
 length of concentration column , the higher the sensitivity . however , if the 
 length of concentration column was too short , the concentration of 
 preconcentration zn ( ii ) in samples was not adequate , which lowered the 
 sensitivity . the highest sensitivity was obtained when the length of 
 concentration column was 40 mm . the results in figure 5 showed 
 that the preconcentration time was longer , the zinc concentration of the 
 preconcentration column would be higher which causes the analysis sensitivity 
 to be higher but the sampling frequency to be 
 lower . taken a comprehensive consideration of analysis sensitivity and 
 sampling frequency , preconcentration time of 5 minutes was decided for further 
 work . too high - preconcentration rate would 
 increase analysis sensitivity but augment pressure of preconcentration column 
 which would cause leakage or pump pipes rupture . too low - preconcentration rate 
 would reduce analysis sensitivity and sampling frequency but augment the 
 preconcentration time . taken a comprehensive consideration of analysis 
 sensitivity and sampling frequency , preconcentration rate of 1.5 min l 
  was chosen for further work . if eluent concentration was too low , zn ( ii ) in 
 preconcentration column would elute incompletely . if eluent concentration was 
 too high , it would waste reagent . in the study , nitric acid and oxalic 
 acid - citric acid - lithium hydroxide were tested as eluents . the experimental 
 results showed the sensitivity of the eluent of oxalic acid - citric acid - lithium hydroxide was higher than that of nitric acid . thus , oxalic 
 acid - citric acid - lithium hydroxide was selected as eluent solution and its 
 concentrations were 0.01 mol l , the sensitivity was greatly affected by flow rate of 
 eluent . if flow rate was too high , the sensitivity augmented and baseline 
 noises increased . if flow rate was too slow , zn ( ii ) in concentration column 
 would elute incompletely , which caused sensitivity decline . the result showed that flow rate of 
 0.5 ml min of eluent was optimal for further work . the interference of some 
 possibly coexisting foreign metallic ions was examined to make sure the proposed 
 method can be applied to determine zn ( ii ) in water . the tolerance limit was 
 defined as the concentration of added solutions causing less than  5% relative 
 error during the determination of 10 g l of zn ( ii ) . no 
 interference was observed from a great deal of k , na , 
 nh , mg , ca , cr , cr , 
 al , co , hg , as , and so 
 forth . however , the metallic ions of ni , cd , mn , 
 pb , cu , and fe interfered . the 
 interference was eliminated ( 100 g l of each ion of ni , 
 cd , mn , pb , cu , and 
 fe ) by adding 1.5 ml of 
 10% sodium hexametaphosphate , 1 ml of 0.1% ddtc , and 2 ml of o - phenanthroline as appropriate 
 masking reagents into mixed 
 chromogenic reagent solution . thus , the results showed that the proposed 
 method had good selectivity . to assess the reproducibility and accuracy of the proposed method , under 
 the selected conditions above the linearity was tested by a series of working standard 
 solutions of zn ( ii ) ranging from 0.5 g l to 400 g l. 
 test showed that the peak height versus zn ( ii ) concentration was linear within 
 the range of 2.0 g l to 360 g l. the curve was h = 0.427c + 3.638   ( h : peak height ; c : 
 concentration of zn ( ii ) ( g l ) ) and the correlation coefficient 
 obtained was 0.9995 and the detection limit ( 3 multiples of baseline noises ) 
 was 0.42 g l. the test of precision was conducted by eight 
 injections of 5.0 g l and 50 g l of zn ( ii ) 
 standard solution , respectively . the results 
 showed that the relative standard deviation ( rsd ) 
 was calculated as 3.55% and 2.14% , respectively . the proposed method was applied to determine zn ( ii ) in water samples 
 using the zj - la automatic metallic element analyzer . the analysis results of 
 samples were shown in table 1 . to prove the 
 accuracy of the method , a test of recovery would be conducted by accurately 
 adding known concentration of zn ( ii ) into sample whose original zn ( ii ) 
 had been determined . the 
 test results in table 1 showed that the recovery was between 97.1% and 104.8% 
 and analytical results of the proposed method were satisfactory . therefore , the 
 proposed method was suitable for determination of trace amounts of zn ( ii ) in 
 water . the proposed method was significant with regard to the development of a 
 simple , reliable , and sensitive flow injection method for online 
 preconcentration and determination of trace amounts of zinc ( ii ) . high sensitivity , 
 selectivity , broad determination range ( 2.0 g l to 360 g l ) 
 of zn ( ii ) , low - detection limit ( 0.42 g l ) , and fully automated analysis were just some of the 
 advantages of the proposed method . especially , compared with other existing 
 methods of zinc preconcentration , eluent concentration is even low to mmol  l. 
 the detection limit of the proposed method can be further improved by 
 increasing preconcentration time . the model zj - la automatic metallic 
 elements analyzer employed in this study was cheap , small , and easy to move . 
 the proposed method has been successfully applied to determine zinc ( ii ) in 
 water . the method is suitable for determination of zn ( ii ) in both water samples and other samples for its good 
 selectivity and the above mentioned advantages .