# Online Preconcentration and Determination of  Trace Amounts of Zinc in Nature Waters

Authors: Liang Wei; Xinshen Zhang; Yichun Dai; Jin Huang; Yong-hong Xie; Kai Xiao
Journal: Journal of Automated Methods and Management in Chemistry
 (2008)

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## Abstract

A simple, sensitive, reliable and flexible flow injection spectrophotometric method is proposed for on-line preconcentration and determination of trace amounts of zinc in water. At the presence of Tween-80 in pH 9.3 buffer solutions, the shade of color of Zn (II)-PAN complex is in a linear relation to the zinc amount at the point of the maximum absorption peak of 560 nm. The optimal experimental conditions, including reaction conditions and preconcentration conditions, had been obtained. The linear range of the proposed method was between 2.0 and
360μg L−1 and the detection limit was 0.42 μg L−1. The relative standard deviation was 3.55% and 2.14% for
5.0 μg L−1
and 50 μg L−1
of zinc standard solution (n=8). The method had been successfully applied to zinc determination in water samples and the analytical results were satisfactory.


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## Body

## 1. Introduction

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. However, zinc can be toxic when
exposures exceed physiological needs. Zinc is present in food, soil, and water.
Hence, determination of zinc (II) is indispensable. There are many methods for
zinc determination such as spectrophotometry
[3–5], atomic absorption spectrometry [6, 7], flame atomic absorption spectroscopy [8], graphite furnace atomic absorption
spectrometry [9, 10],
inductively coupled plasma atomic emission spectroscopy  [11],
and inductively coupled plasma mass
spectrometry [12]. The above methods mentioned are of high sensitivity
and accuracy. But some are expensive,
some are time-consuming, some are troublesome, and others are toxic. Modern
environmental monitor calls for automatic, safe, and simple analytical methods.Preconcentration
technique [13–16] can improve the detection limit as well as the selectivity of
the method. Flow injection analysis (FIA) [17–19] can improve analytical
procedures such as high analytical throughput, precision, accuracy, low
reagents, and sample consumption. Therefore, a new method was proposed for on-line 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.


## 2. Experiment

### 2.1. Apparatus

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).
The absorbance intensity was recorded at 560 nm on an IBM-PC. Data
acquisition   and processing were performed
with HW-2000 chromatography software (Qianpu software Co., Ltd, Shanghai, China) running under Windows XP.


### 2.2. Reagents

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 ZhouChemical Reagent Plant, Chengdu, China),  oxalic acid dihydrate, lithium
hydroxide monohydrate, ammonium thiocyanate, and citric acid  (Beijing DonghuaChemical Reagent Plant, Beijing, China)  were of analytical grade and all
solutions were prepared with deionized water.


### 2.3. Preparation of Solution

Zinc
stock solution: zinc stock standard  solutions at a concentration of
1000 mg L−1 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 of4×10−3 mol L−1 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.


### 2.4. Preparation of Preconcentration Column

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 80–100μm. 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.


### 2.5. Analytical Procedure

The FIA manifold used was outlined in Figure1 with PTFE pipes. 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.Figure 1
Schematic diagram of the flow
injection concentration online.


## 2.1. Apparatus

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).
The absorbance intensity was recorded at 560 nm on an IBM-PC. Data
acquisition   and processing were performed
with HW-2000 chromatography software (Qianpu software Co., Ltd, Shanghai, China) running under Windows XP.


## 2.2. Reagents

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 ZhouChemical Reagent Plant, Chengdu, China),  oxalic acid dihydrate, lithium
hydroxide monohydrate, ammonium thiocyanate, and citric acid  (Beijing DonghuaChemical Reagent Plant, Beijing, China)  were of analytical grade and all
solutions were prepared with deionized water.


## 2.3. Preparation of Solution

Zinc
stock solution: zinc stock standard  solutions at a concentration of
1000 mg L−1 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 of4×10−3 mol L−1 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.


## 2.4. Preparation of Preconcentration Column

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 80–100μm. 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.


## 2.5. Analytical Procedure

The FIA manifold used was outlined in Figure1 with PTFE pipes. 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.Figure 1
Schematic diagram of the flow
injection concentration online.


## 3. Results and Discussion

### 3.1. Effect of Reaction Conditions

#### 3.1.1. Effect of pH

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 4–11. The test results showed that the maximum peak
height could be obtained when pH range was between 8 and 10. Thus, pH 9.3 of borate
solution was selected as the buffer solution.


#### 3.1.2. Effect of PAN Concentration

The reagent concentration was another important factor in flow injection
analysis. The effect of PAN concentration on the peak height were tested from5×10−5 mol L−1 to 1.2×10−3 mol L−1 in Figure 2. The results showed
that the peak height rose when concentration of PAN changed from 5×10−5 mol L−1 to 4×10−4 mol L−1, and the peak height declined
when concentration of PAN changed from 4×10−4 mol L−1 to 1.2×10−3 mol L−1. Therefore, 4×10−4 mol L−1 PAN was selected for further
work.Figure 2
Effect of PAN concentration on
the peak height.According to
the reaction kinetics, the reaction rate will inevitably increase when
increasing concentrations of the reagent. Thus, the absorbency will increase
when increasing concentrations of PAN. 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.


#### 3.1.3. Effect of Tween-80 Concentration

The Zn (II)-PAN complex   was
hydrophobic. Thus, Tween-80 was selected as reagent which could increase
solubility of the system. The test results in Figure3 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.Figure 3
Effect of Tween-80
concentration on the peak height.


#### 3.1.4. Effect of Flow Rate

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−1 to 0.72 mL min−1. 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−1.


#### 3.1.5. Effect of the Reaction Coil Length

In the
flow injection analysis, the reaction time is adjusted by the length of
reaction coil. If reaction coil was too short, the
reagents would react incompletely. If reaction coil was too long, it would be
time-consuming. Therefore, reaction coil length from 1 to 5 m was tested. The
study results in Figure4 showed that 3 m of the
reaction coil length was the
optimum and further increase of reaction coil length did not increase peak
height. Thus, 3 m of the reaction coil length was chosen as optimal parameter.Figure 4
Effect of reaction coil length
on the peak height.


### 3.2. Effect of Preconcentration Conditions

#### 3.2.1. Effect of Filling Granularity and Length of Preconcentration Column

Length of preconcentration column and
filling granularity could affect the effect of preconcentration. In the study,
filling granularity of80∼100 μm 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. Thus, 40 mm of concentration column length was
selected for further study.


#### 3.2.2. Effect of Preconcentration Time

Preconcentration
time can affect analysis sensitivity and sampling frequency. Preconcentration
time from 2 minutes to 13 minutes was tested. The results in Figure5 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.Figure 5
Effect of pre-concentration time on the peak height.


#### 3.2.3. Effect of Preconcentration Rate

Either too high or too low preconcentration rate was not proper. 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−1
was chosen for further work.


#### 3.2.4. Effect of Eluent Concentration

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−1, 0.0075 mol L−1 and
0.05 mol L−1, respectively.


#### 3.2.5. Effect of Eluent Flow Rate

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−1 of eluent was optimal for further work.


### 3.3. Effects of Foreign Ions

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 μg L−1 of Zn (II). No
interference was observed from a great deal of K+, Na+,
NH4+, Mg2+, Ca2+, Cr3+, Cr6+,
Al3+, Co2+, Hg2+, As3+, and so
forth. However, the metallic ions of Ni2+, Cd2+, Mn2+,
Pb2+, Cu2+, and Fe2+ interfered. The
interference was eliminated (100 μg L−1 of each ion of Ni2+,
Cd2+, Mn2+, Pb2+, Cu2+, and
Fe2+) 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.


### 3.4.  Linearity, Precision, and Detection Limit of the
Proposed Method

To assess the reproducibility and accuracy of the proposed method, under
the selected conditions above, tests of linearity, precision, and detection
limit were conducted. The linearity was tested by a series of working standard
solutions of Zn (II) ranging from 0.5μg L−1 to 400 μg L−1.
Test showed that the peak height versus Zn (II) concentration was linear within
the range of 2.0 g L−1 to 360 μg L−1. The curve was H = 0.427C + 3.638  (H: peak height; C:
concentration of Zn (II) (μg L−1)) and the correlation coefficient
obtained was 0.9995 and the detection limit (3 multiples of baseline noises)
was 0.42 μg L−1. The test of precision was conducted by eight
injections of 5.0 μg L−1 and 50 μg L−1 of Zn (II)
standard solution, respectively. The results
showed that the relative standard deviation (RSD)
was calculated as 3.55% and 2.14%, respectively.


### 3.5. Application

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 Table1.Table 1
Determination of zinc in water samples by the proposed method.
SampleMeasuredSpikedFoundRecovery(μg L−1)(μg L−1)(μg L−1)(%)River water 1#38.5030.0067.6397.1River water 2#59.2650.00108.5698.6Lake water4.485.009.72104.8Spring water7.2610.0017.1298.6To 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 Table1 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.


## 3.1. Effect of Reaction Conditions

### 3.1.1. Effect of pH

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 4–11. The test results showed that the maximum peak
height could be obtained when pH range was between 8 and 10. Thus, pH 9.3 of borate
solution was selected as the buffer solution.


### 3.1.2. Effect of PAN Concentration

The reagent concentration was another important factor in flow injection
analysis. The effect of PAN concentration on the peak height were tested from5×10−5 mol L−1 to 1.2×10−3 mol L−1 in Figure 2. The results showed
that the peak height rose when concentration of PAN changed from 5×10−5 mol L−1 to 4×10−4 mol L−1, and the peak height declined
when concentration of PAN changed from 4×10−4 mol L−1 to 1.2×10−3 mol L−1. Therefore, 4×10−4 mol L−1 PAN was selected for further
work.Figure 2
Effect of PAN concentration on
the peak height.According to
the reaction kinetics, the reaction rate will inevitably increase when
increasing concentrations of the reagent. Thus, the absorbency will increase
when increasing concentrations of PAN. 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.


### 3.1.3. Effect of Tween-80 Concentration

The Zn (II)-PAN complex   was
hydrophobic. Thus, Tween-80 was selected as reagent which could increase
solubility of the system. The test results in Figure3 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.Figure 3
Effect of Tween-80
concentration on the peak height.


### 3.1.4. Effect of Flow Rate

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−1 to 0.72 mL min−1. 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−1.


### 3.1.5. Effect of the Reaction Coil Length

In the
flow injection analysis, the reaction time is adjusted by the length of
reaction coil. If reaction coil was too short, the
reagents would react incompletely. If reaction coil was too long, it would be
time-consuming. Therefore, reaction coil length from 1 to 5 m was tested. The
study results in Figure4 showed that 3 m of the
reaction coil length was the
optimum and further increase of reaction coil length did not increase peak
height. Thus, 3 m of the reaction coil length was chosen as optimal parameter.Figure 4
Effect of reaction coil length
on the peak height.


## 3.1.1. Effect of pH

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 4–11. The test results showed that the maximum peak
height could be obtained when pH range was between 8 and 10. Thus, pH 9.3 of borate
solution was selected as the buffer solution.


## 3.1.2. Effect of PAN Concentration

The reagent concentration was another important factor in flow injection
analysis. The effect of PAN concentration on the peak height were tested from5×10−5 mol L−1 to 1.2×10−3 mol L−1 in Figure 2. The results showed
that the peak height rose when concentration of PAN changed from 5×10−5 mol L−1 to 4×10−4 mol L−1, and the peak height declined
when concentration of PAN changed from 4×10−4 mol L−1 to 1.2×10−3 mol L−1. Therefore, 4×10−4 mol L−1 PAN was selected for further
work.Figure 2
Effect of PAN concentration on
the peak height.According to
the reaction kinetics, the reaction rate will inevitably increase when
increasing concentrations of the reagent. Thus, the absorbency will increase
when increasing concentrations of PAN. 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.


## 3.1.3. Effect of Tween-80 Concentration

The Zn (II)-PAN complex   was
hydrophobic. Thus, Tween-80 was selected as reagent which could increase
solubility of the system. The test results in Figure3 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.Figure 3
Effect of Tween-80
concentration on the peak height.


## 3.1.4. Effect of Flow Rate

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−1 to 0.72 mL min−1. 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−1.


## 3.1.5. Effect of the Reaction Coil Length

In the
flow injection analysis, the reaction time is adjusted by the length of
reaction coil. If reaction coil was too short, the
reagents would react incompletely. If reaction coil was too long, it would be
time-consuming. Therefore, reaction coil length from 1 to 5 m was tested. The
study results in Figure4 showed that 3 m of the
reaction coil length was the
optimum and further increase of reaction coil length did not increase peak
height. Thus, 3 m of the reaction coil length was chosen as optimal parameter.Figure 4
Effect of reaction coil length
on the peak height.


## 3.2. Effect of Preconcentration Conditions

### 3.2.1. Effect of Filling Granularity and Length of Preconcentration Column

Length of preconcentration column and
filling granularity could affect the effect of preconcentration. In the study,
filling granularity of80∼100 μm 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. Thus, 40 mm of concentration column length was
selected for further study.


### 3.2.2. Effect of Preconcentration Time

Preconcentration
time can affect analysis sensitivity and sampling frequency. Preconcentration
time from 2 minutes to 13 minutes was tested. The results in Figure5 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.Figure 5
Effect of pre-concentration time on the peak height.


### 3.2.3. Effect of Preconcentration Rate

Either too high or too low preconcentration rate was not proper. 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−1
was chosen for further work.


### 3.2.4. Effect of Eluent Concentration

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−1, 0.0075 mol L−1 and
0.05 mol L−1, respectively.


### 3.2.5. Effect of Eluent Flow Rate

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−1 of eluent was optimal for further work.


## 3.2.1. Effect of Filling Granularity and Length of Preconcentration Column

Length of preconcentration column and
filling granularity could affect the effect of preconcentration. In the study,
filling granularity of80∼100 μm 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. Thus, 40 mm of concentration column length was
selected for further study.


## 3.2.2. Effect of Preconcentration Time

Preconcentration
time can affect analysis sensitivity and sampling frequency. Preconcentration
time from 2 minutes to 13 minutes was tested. The results in Figure5 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.Figure 5
Effect of pre-concentration time on the peak height.


## 3.2.3. Effect of Preconcentration Rate

Either too high or too low preconcentration rate was not proper. 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−1
was chosen for further work.


## 3.2.4. Effect of Eluent Concentration

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−1, 0.0075 mol L−1 and
0.05 mol L−1, respectively.


## 3.2.5. Effect of Eluent Flow Rate

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−1 of eluent was optimal for further work.


## 3.3. Effects of Foreign Ions

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 μg L−1 of Zn (II). No
interference was observed from a great deal of K+, Na+,
NH4+, Mg2+, Ca2+, Cr3+, Cr6+,
Al3+, Co2+, Hg2+, As3+, and so
forth. However, the metallic ions of Ni2+, Cd2+, Mn2+,
Pb2+, Cu2+, and Fe2+ interfered. The
interference was eliminated (100 μg L−1 of each ion of Ni2+,
Cd2+, Mn2+, Pb2+, Cu2+, and
Fe2+) 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.


## 3.4.  Linearity, Precision, and Detection Limit of the
Proposed Method

To assess the reproducibility and accuracy of the proposed method, under
the selected conditions above, tests of linearity, precision, and detection
limit were conducted. The linearity was tested by a series of working standard
solutions of Zn (II) ranging from 0.5μg L−1 to 400 μg L−1.
Test showed that the peak height versus Zn (II) concentration was linear within
the range of 2.0 g L−1 to 360 μg L−1. The curve was H = 0.427C + 3.638  (H: peak height; C:
concentration of Zn (II) (μg L−1)) and the correlation coefficient
obtained was 0.9995 and the detection limit (3 multiples of baseline noises)
was 0.42 μg L−1. The test of precision was conducted by eight
injections of 5.0 μg L−1 and 50 μg L−1 of Zn (II)
standard solution, respectively. The results
showed that the relative standard deviation (RSD)
was calculated as 3.55% and 2.14%, respectively.


## 3.5. Application

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 Table1.Table 1
Determination of zinc in water samples by the proposed method.
SampleMeasuredSpikedFoundRecovery(μg L−1)(μg L−1)(μg L−1)(%)River water 1#38.5030.0067.6397.1River water 2#59.2650.00108.5698.6Lake water4.485.009.72104.8Spring water7.2610.0017.1298.6To 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 Table1 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.


## 4. Conclusion

The proposed method was significant with regard to the development of a
simple, reliable, and sensitive flow injection method for on-line
preconcentration and determination of trace amounts of zinc (II). High sensitivity,
selectivity, broad determination range (2.0μg L−1 to 360 μg L−1)
of Zn (II), low-detection limit (0.42 μg L−1), 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−1.
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.



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*Source: 742319-2008-03-19.xml*