Preconcentration of Copper with Solid Phase Extraction and its Determination by Flame Atomic Absorption Spectrometry

A new method for the solid phase extraction (SPE) and determination of copper ions at low levels is presented. Extraction percent and the effects of some factors were evaluated. The detection limit was in the range of 2.26 μg•L-1. This procedure has been successfully applied to determination of copper in water samples.


Introduction
Determination of copper is usually carried out by atomic absorption spectrometry 1,2 as well as spectrophotometric methods 3,4 .Preconcentration step prior to determination of copper ions is usually necessary, because of low levels of copper in environmental samples.Liquid-liquid extraction of copper has been used elsewhere 5,6 .Nevertheless, several techniques for the preconcentration or determination of copper have been proposed including spectrophotometry 7 , SPE using C18 cartridge 8 , modified microcrystalline naphthalene 9 , modified silica gel 10 and 4-phenylenediamine propyl silica xerogel 11 as sorbents, FIA with spectrophotometric determination 12 , and differential pulse adsorption stripping voltammetry 13 .Disadvantages of these methods are: using toxic solvents 8,9 , small preconcentration factor 10 , and existing of interfering ions 7,10,12,13 .SPE has received much attention in recent years.This technique reduces consumption of and exposure to solvent, disposal costs, and extraction time 14,15 .Recently, modified activated carbon (AC) have been used as SPE sorbent 16,17,18 .
Many methods based on different SPE sorbents have been applied to the preconcentration of copper.Okutani et al. 19 applied chitosan as adsorbent.There was interference effect on the determination of copper.In other research by Okutani et al. 20 AC was used as a sorbent.There were interfering ions with the determination of copper.Knezevic et al. 21used chelating ion exchanger.In this study the recovery wasn't complete.Granados et al. 22 applied adsorbent of Lewatit.There were interfering ions in this procedure.Cellulose nitrate was used by Soylak et al. 23 In this research sorbent acted as a filter.Tokman et al. 24 applied modified silica gel.Cesur 18 used modified AC.This study was laborious and toxic compounds were applied.
The ligands derived from salicylaldehyde can form very stable chelates with transition metal ions 25,26 .There are a few papers on the use of these substances in preparing solid phases [27][28][29][30][31] .Application of AC as adsorbent and use of a new Schiff base is another way for preconcentration of copper ions.In this paper, application of modified AC with 5 -(( 4heptyloxyphenyl ) azo ) -N -( 4-butyloxyphenyl)-salicylaldimine (HPBS) (Figure 1) as a new sorbent for preconcentration of Cu(II) ions are explained.

Reagents
All reagents and substances were of the highest purity available from Fluka.AC was purchased from Merck.HPBS was synthesized as described elsewhere 32 .
The stock standard solution of Cu(II) was prepared by dissolving 3.9292 g of the Cu(SO 4 ) 2 .5H 2 O in water and diluted to one liter.A HPBS solution (0.6% w/v) was prepared by dissolving 0.3 g of HPBS in 50 mL chloroform.

Instrumentation
The determination of Cu(II) was performed on a Perkin Elmer Aanalyst 300 atomic absorption spectrometer.Spectrophotometric measurements were carried out with a Camspec M350 spectrophotometer.A Metrohm model 691 digital pH meter was used for the pH measurements.

Preparation of the modified AC
The AC was first pounded in a porcelain mortar and sieved with 40-mesh sieve, then was stirred with concentrated HCl for 1h, and was allowed to stand for 2h.After filtration, the AC was washed with water, and then dried at 120°C for 1h.Then, 20 g of purified AC was put into a 1000 mL stoppered bottle and 50 mL of 0.6% w/v HPBS solution was added to bottle and stirred for 1h by mechanical shaker.After filtration, the modified AC was dried at 60°C for 1h.Then 0.1 g of the sorbent was packed into a polypropylene column (10 mm i.d., and 50 mm height).

Procedure
The preconcentration of Cu(II) ions on the modified AC was as follows: A 50-250 mL of the sample solution containing 100 µg Cu(II), buffered at pH=6, was passed through the column at a flow rate of 0.5 mL•min -1 by suction.After the sorption, the column was washed with 10 mL of water, and the extracted copper was then eluted from the adsorbent using 10 mL of 4 M of HNO 3 containing 20% w/v of acetone.The copper concentration was determined by FAAS.
The water samples were first passed through a filter, then enough buffer was added to achieve the pH=6.The copper ions were extracted from the thus treated aliquot and quantified as described above.

Results and Discussion
HPBS is a Schiff base.The results of IR spectrum have showed that the OH group of HPBS has deprotonated and coordinated to the copper ions, and C=N group of ligand coordinated to the copper ions via N atom.Also, elemental analysis results showed that the ligand coordinated to Cu(II) in 2:1 ratio 32 .It is suggested square planar form for this kind of compounds 33 .Also, HPBS could be adsorbed onto AC, due to producing a strong π-π interaction 17 .

Capacity of AC for HPBS
Into a series of stoppered bottles, 5 to 33 mL of a 0.03% w/v solution of HPBS was transferred and each one mixed with 0.1 g of AC.After 1h, the mixtures were filtered and the absorbances of the filtrates were measured spectrophotometrically.From the results (Figure 2), for further experiments AC loaded with 1.5% w/v of HPBS.

Influence of pH
The effect of the pH of sample containing 2 µg•mL -1 was studied in the pH range of 1-12.The results showed that Maximum retention of Cu(II) by sorbent were at pH 6.In an acidic solution the protonation of HPBS occurs and there is a weak tendency for reaction between Cu(II) and HPBS, but at higher pHs (pH>6) Cu(OH) 2 is formed.Thus, buffer with pH 6 was used for the sorption step (Figure 3).

Effect of different eluents
After the adsorption step, the copper ions were eluted with different eluents.From the data given in Table 1 and Figures of 4 and 5, it is obvious that the best eluent is 10 mL of 4 M HNO 3 containing 20% v/v acetone.

Analytical performance
When solutions of 50 µg copper in 50, 100, 200, and 250 mL solutions were passed through the sorbent under optimum conditions, the Cu(II) was quantitatively retained in all cases.Therefore, the breakthrough volume for the method must be greater than 250 mL, providing a concentration factor of 25.This preconcentration factor was superior to the reported papers 21,24 .The detection limit [35][36][37] of (DL) of the method is 2.26 µg•L -1 .The presented method has been achieved better DL relative to elsewhere 28,22 .The CV of the method was 1.4%.The repeatability of the method surpassed those in the previous reports 10, [19][20][21][22][23]34 .

Effect of foreign ions
The effect of other ions was studied by adding a given amount of the desired ion to 50 mL aliquot of solution containing 50 µg Cu(II).The results (Table 2) show that the Cu(II) ions in binary mixtures are retained completely by the adsorbent.Table 2. Effect of foreign ions a .

Analysis of water samples
Tap water of Ardabil city, river water of Borjloo and one synthetic sample were analyzed.For demonstrating the accuracy of the developed method, it was compared with the standard method 2 for determination of Cu(II).Results (Table 3) show that, the added copper ions can be quantitatively recovered from the samples.
Table 3. Recovery of copper added to 250 mL of different water samples.
a Cu(II) found by standard method.b Values in parentheses are CVs based on three replicate analysis.

Conclusions
Results of this work show powerful ability of SPE using modified AC with HPBS.The method was simple, and low cost.Also, the presented method was free of interference.The method successfully applied to the preconcentration and determination of copper in binary mixtures.
Cu(II) found a µg•mL

Figure 2 .
Figure 2. Capacity of AC for the adsorption of HPBS.

Figure 3 .
Figure 3.Effect of pH on the recovery.

Figure 4 . 5 .
Figure 4. Effect of %volume of acetone in the eluent Figure 5.Effect of volume of eluent

Table 1 .
%Recovery of Cu(II) from the sorbent using 10 mL of different eluents.