Synthesis, properties and applications of silica-immobilized 8-quinolinol

Analytica Chimicu Acta, 287 (1994) 111-117 Elsevier Science B.V., Amsterdam

111

Synthesis, properties and applications of silica-immobilized 8-quinolinol Part 2. On-line column preconcentration of copper, nickel and cadmium from sea water and determination by inductively-coupled plasma atomic emission spectrometry Chi-Ren Lan and Mo-Hsiung Yang Institute of Nuclear Science, National Tsing Hua University, 30043 Hsinchu (Taiwan) (Received 9th February 1993; revised manuscript received 27th September 1993)

Abstract The coupling of an on-line column preconcentration manifold with continuous flow ultrasonic nebulixer equipped with a desolvation system for simultaneous determination of metal ions by inductively coupled plasma atomic emission spectrometry (ICP-AES) is described. The silica-immobilized S-quinolinol prepared by an improved synthetic route has been evaluated as a preconcentration material for metals in a flow system. Dynamic exchange capacities of this material were ascertained by determining the breakthrough volumes of radiotracer %I solution under different flow parameters. Optimization of the on-line column preconcentration with ICP-AES measurement was investigated. The feasibility of the proposed method was tested for the determination of Cu, Ni and Cd in open sea water samples. Results obtained for the determination of these elements in NASS-2 sea water sample agreed very well with certified values. The limits of detection based on a sample volume of 25 ml for Cu, Ni and Cd were 0.07, 0.054 and 0.016 ng ml-‘, respectively. Keywords Atomic Emission spectrometry; Waters

Flow system; On-line column preconcentration;

The determination of trace metal ions in sea water is important for environmental monitoring and for understanding their role in the biological and geochemical cycles of the oceans [l]. Recently, inductively-coupled plasma atomic emission spectrometry (ICP-AES) has been recommended for measurements of a large suite of elements in sea water [2]. Although the method is very sensitive, it sometimes poses problems for direct determination at the concentrations of most trace elements in sea water. The concentrations

Correspondence to: M.-H. Yang, Institute of Nuclear Science, National Tsing Hua University, 30043 Hsinchu (Taiwan).

Ultrasonic

nebulixer;

are generally low compared with their detection limits and the elements must be determined also in presence of large amounts of interfering matrix constituents [2,3]. Preconcentration of the trace metals prior to analysis may be mandatory and serves the dual purpose of removal of the interfering major elements and increasing the concentration of the trace elements. Thus, many processes such as solvent extraction [4,5], coprecipitation [61, sorption on chelating agents either on cellulose collector [7], C,, bonded silica gel [S] or chemically bound to a polymer chain [9-111, have been extensively studied with sea water. Each of the methods has its own advantages and the choice between them depends on the elements

0003-2670/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved .SSDI 0003-2670(93)E0590-4

112

required to be analyzed and the method used for the detection of these elements. During the enrichment and related steps, contaminants containing the desired trace elements may be introduced into the sample from the external source, and therefore minimization of contamination is required in order to achieve accurate analytical results, especially in inorganic trace analysis. On-line column preconcentration combined with flame atomic absorption spectrometry [12,131, inductively-coupled plasma atomic emission spectrometry [14,15], and inductively-coupled plasma mass spectrometry [15-181 are found to be useful in solving these complex analytical problems. These methods provide high enrichment factors and lower detection limits by selective extraction from the complex matrix. It not only helps in reducing the time, but also the closed system for sample handling makes it contamination-free. In this study, metal ions released from an on-line preconcentration column containing silica-immobilized 8-quinolinol as described in Part 1 [19] are analyzed by simultaneous ICP-AES after ultrasonic nebulization. Optimization of instrumental operating conditions in ICP-AES analysis was studied, and the feasibility of on-line column preconcentration of trace amounts of Cu, Ni and Cd from sea water to decrease matrix problems was investigated. Subsequently, the proposed method which optimally combined the online column preconcentration with the ICP-AES measurement was employed for the determination of trace metals of interest in open sea water samples.

EXPERIMENTAL

Reagents All water used was treated by reverse osmosis, mixed-bed ion exchange unit deionization, followed by double distillation in a quartz still equipped with an immersion heater. All reagents were analytical grade except the ammonia solution which was Merck ultra pure NH,. High purity nitric acid and hydrochloric acid were prepared by subboiling distillation in a quartz still

C.-R Lan and M.-H. Yang/Anal. Chim. Acta 287 (1994) Ill-117

from reagent grade feed stocks. 0.5 M Ammonium maleate buffer solution was prepared and purified by passing through a column (50 x 4mm i.d.) packed with 0.5 g of silica-immobilized 8quinolinol at a flow rate of 2 ml min-i. The preparation of purified sea water was as follows. Coastal sea water collected was filtered through a 0.45-Frn Millipore filter. After adjustment to pH 6, sea water was passed through the silica-immobilized 8-quinolinol column at a flow rate of 2 ml min-’ to remove metal impurities. The Merck Titrisol stock standard solution (1000 mg 1-i) of the elements of interest were diluted to the desired concentrations with purified sea water. Open sea water samples from a vertical profile were collected from a station in the Pacific ocean (22” 35’N, 122” lO’E), by using Go-Flo sampling system (General Oceanics, USA). Sea water samples collected at required depths were obtained from the Institute of Oceanography, National Taiwan University. The sea water had been acidified to pH < 2, and stored in precleaned polypropylene bottles. All sample preparations were carried out in a clean room equipped with laminar flow benches, providing a class 100 working environment. All laboratory ware was thoroughly cleaned by soaking. in nitric acid (1: 1) at least for 24 h. Immediately prior to use, all acid washed ware was rinsed with deionized, double-distilled water. Apparafus The Therm0 Jarrell Ash inductively-coupled plasma atomic emission spectrometer, Model ICAP 9000, was used for this work. Introduction of the trace metals concentrate for simultaneous analysis by ICP-AES was accomplished by means of an ultrasonic nebulizer (CETAC Model U5000) equipped with a desolvation system. The instrumental operating conditions were shown in Table 1. The on-line preconcentration setup (Fig. 1) consisted of a Watson Marlow 502s minipulse peristaltic pump used for conveying dilute acid solution, two Dionex Model DQP-1 pumps used for transporting samples, buffer solution and acid eluant, two three way rotatory valves (Vi, V,), and two silica-immobilized 8-quinolinol columns (50 x 4 mm id.). The measurement of transient

C.-R Lan and M-H. Yang/Anal. Chitn. Acta 287 (1994) 111-117

113

signals was performed by using the time scan mode, and the integration of peak area was done numerically. All pH measurements were carried out with a Radiometer Copenhagen Model pHM82 pH meter equipped with a glass pH electrode. Column preparation An empty column (Dionex) 50 x 4 mm i.d. was packed with silica-immobilized 8-quinolinol by the dry packing technique. The procedure consisted of a batch-wise addition of small amounts of chelating material, by hand, into a vertically mounted coiumn. During filling, the outside of the column was tapped gently and the suction was conducted with a vacuum pump connected at the bottom end of the column, in order to facilitate settling of the particle to a dense packing and to prevent size segregation near the tube walls. The last step in column packing was to wash the column with 10 ml 1 M HCI-1 M I-NO, mixture, 5 ml of maleate buffer solution, and then with deionized, double-distilled water. The same columns were used throughout the experiment. Dynamic exchange capacity studies A similar procedure, described by Marshall and Mottola [12], was employed for investigating the dynamic exchange capacities of the silica-immobilized 8-quinolinol column. The procedure for dynamic exchange capacities was performed by using radiotracer 64Cu as the metal ion probe at pH 5. A test solution [137.5 pg ml-r, Ctr(II)] was passed through the column at various flow TABLE 1 Simultaneous ICP-AES conditions Jarrell Ash ICAP 9000 RF generator forward power reflected power Gas flow argon coolant gas argon plasma gas argon carrier gas Sample uptake rate View height (from coil)

1.1 kW
Fig. 1. Schematic diagram of manifold for the on-line column preconcentration for ICP-AES equipped with an ultrasonic nebulizer. P,, peristaltic pump; Pr, DQP-1 pump; VI and V,, three-way valves; W, four-way connector.

rates (0.5, 1, 2 and 3 ml min-‘1. Fractions were collected in lo-ml vials at 2-ml intervals. The dynamic exchange capacities were ascertained by determining the breakthrough volumes of test solution, and the breakthrough volumes were calculated on 1% detection of relative activity of copper solution. Sea water analyses An ultrasonic nebulizer equipped with a desolvation system was used for the introduction of trace elements. The two-column on-line preconcentration manifold for these studies is shown in Fig. 1. In sea water analyses, one column was used for eluent to direct the output from the column to the ultrasonic nebulizer, and the other one served as a preconcentration column for introducing the sample solution. The preconcentration procedure was as following. Typically, the acidified sample (100 ml of sea water) was neutralized by using 10 ml buffer of 0.5 M ammonium maleate at pH 6.5. The buffered sample was passed through a silica-immobilized Squinolinol column (ca. 0.5 g dry weight, 200 mesh particle size) where some of the metals present in the sample were extracted. After a suitable washing time, the metal ions were eluted from the silica-immobilized 8-quinolinol column by introducing the 1 HCl-1 HNO, mixture through the acid injection valve. The eluted metals were determined by ICP-AES after ultrasonic nebulization. The resulting transient signals were recorded and integrated. The maximum flow rate allowed by the back pressure produced in the system was 2 ml min-‘.

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RESULTS AND DISCUSSION

Dynamic exchange capacity studies The initial total capacity of silica-immobilized B-quinolinol is an important parameter which directly affects the feasibility of these materials used for the preconcentration of trace metals in natural water. It is usually measured under batch conditions in which an excess of a known metal ion is allowed to equilibrate with the solid chelating ion exchanger at a fixed pH and the total capacity is then determined by the amount of metal ion extracted [20]. However, in column operation, the dynamic exchange capacity is a more useful quantity. In this work, the dynamic exchange capacity of this material is ascertained by determining the breakthrough volumes of radiotracer 61Cu solution by collecting the fractions and passing them directly to the Na(T1) scintillation detector. Copper(H) is chosen as metal ion probe because it has a large formation constant with 8-quinolinol and has also been widely used in previous work [12,21]. A series of breakthrough curves for the column at various flow rates is shown in Fig. 2. Breakthrough occurs earlier at high flow rates and as the flow rate decreases the dynamic exchange capacity increases. At lower flow rates, more of the complexation sites are accessible for reaction since the longer residence

Fig. 2. Breakthrough curves as a function of volume. Flow rates are A: 0.5 ml min-‘; B: 1 ml min-‘; C: 2 ml min-‘; D: 3 ml min-‘. Test solution is 137.5 pg ml-’ C&I), pH 5. A /A,,: effluent activity/influent activity.

”

^_ “.D

I.3

I

Flow

rote

_

( ml

2

.?.s

f 1

min-‘1

Fig. 3. Dynamic exchange capacity for copper(M) as a function of flow rate.

time of the solution in the column should increase the beneficial conditions for the rate of exchange, and then, should also result in a concomitant increasing in the dynamic exchange capacities. Figure 3 shows the dynamic exchange capacity as a function of flow rate. An approximate exponential decay in capacity is observed as the flow rate increases. At a flow rate of 2 ml min-‘, the dynamic exchange capacity is 138 pmol gg ‘. For the silica-immobilized 8-quinolinol prepared by an improved synthetic route, having a relatively high exchange capacity, can be used for analytical separation of trace levels of metal ions in natural water. Interferences studies Elements such as calcium and magnesium in sea water at high concentrations often provide interferences in ICP-AES measurements. Thus it is desirable to investigate the extraction behaviors of different metal ions that might be presented in sea water samples. The results for the extractabilities of Cu, Ni and Cd elements in sea water as a function of pH have been investigated in Part 1 of this series [19].From these results, it is possible to concentrate trace metals completely when the investigated pH is higher than 4. Figure 4 shows the pH dependence of the signal responses of Ca and Mg retained in the column. The total amounts of Ca and Mg are estimated to be in the range of l-3 pg, when 25 ml of sea water was passed

C.-R. Lan and M.-H. Yang/Anal. Chim. Acta 287 (1994) Ill-117

115

through the column. Generally, the dependences are similar to those obtained in the literature 1121. All the elements of interest and concomitants showed higher recoveries at high pH. However, as these extraction studies indicate that a pH 5-6 for the preconcentration of transition metal ions are effective since these ions will be combined with the immobilized ligands at such a pH, while significant amounts of alkaline earth ions will be excluded from chelation. /

On-line column preconcentration / USN / ICPAES Berman et al. [221 reported that some trace metal ions in sea water could not be determined by ICP-AES with pneumatic nebulization using a 20-fold preconcentration as their levels in such concentrates remain below values at which reliable analysis could be performed. Fassel et al. 1231,however, pointed out that a continuous flow ultrasonic nebulizer equipped with a desolvation system for generating dehydrated aerosol particles prior to their injection into analytical inductively coupled plasma could increase the detection sensitivity. However, difficulty may still be encountered for determination of trace metals in sea water by ICP-AES with pneumatic nebulization or ultrasonic nebulization due to high salt content in the sample. In order to reach the

(LIO’I

Loo2

Fig. 4. Signal responses of Cl’+ and Mg’+ as a function of pH for the silica-immobilized 8quinolinol. (e) CA’+, (m) Mg2+.

0

0

, 50

100 Time

150

I 200

(sl

Fig. 5. Typical elution profiles for standard solution of Cu, Ni and Cd. The amounts of Cu, Ni and Cd in a 25 ml sea water sample are 125 ng, respectively.

required sensitivity as well as to avoid the interference caused by the high salt matrix, recent studies have indicated that an on-line column preconcentration technique combined with ICPAES should be the most promising technique to sea water analyses [14,15]. Because many experimental variables are involved in the operation of the incorporation of an on-line column preconcentration/matrix separation system into a simultaneous ICP-AES equipped with ultrasonic nebulization set up, it is desirable to define the optimization steps that are taken to select compromise operating conditions. First, the compromise plasma operating conditions typically employed for simultaneous multielement determination are used, then a compromise carrier gas flow and sample uptake rate that yielded the overall highest relative net intensity is selected for ultrasonic nebulizer. The instrumental parameters are given in Table 1. Figure 5 shows the elution characteristics of trace elements from silica-immobilized 8-quinolinol column using 1 M HCl-1 M HNO,, the desorption of the collected trace elements at a flow rate of 2 ml min-’ is completed within 120 s. Analysis of sea water Table 2 gives the results for the recoveries of Cd, Ni and Cu from 10 ml of sea water spiked at total concentration of 100 ppb. Even at this low concentration there is nearly a 100% recovery

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C.-R Lan and M.-H. Yang/Anal. Chim. Acta 287 (1994) 11X-117

TABLE 2 Recovery tests carried out by spiking seawater samples with Ni, Cu and Cd (sample volume, 10 ml; ppb = kg 1-l) Ni

CU

Cd

Spiked Found Spiked Found Spiked Found (mg l-‘)(mgl-l)(mg I-‘)(mg I-‘)(mg I-‘)(mg I-‘) 100 100 100 Average:

101 98 99 99%

100 100 100

99 100 102 100%

100 100 100

102 96 100 99%

(about 96-102%) for the three elements. In order to validate the method proposed in this study, a certified reference water (NASS-2, Seawater Reference Material for Trace Metals) from the National Research Council of Canada has been analyzed as a quality control material during the measurement of the elements of interest in the sea water. The analytical results are given in Table 3. The resulting values are in good agreement with the certified values. Since the preconcentration of other transition metal ions can also be effective with the immobilized 8-quinolinol at pH 5-8, the present on-line system can be expected to extend to the determination of these elements in the sea water samples. In this study, the detection limit is defined as that concentration of analyte which gives a response equal to three times the standard deviation of the blank or background value. Blank values were measured by using sea water as the metal-free solution after passing through the silica-immobilized 8-quinolinol column. Analysis of three samples of the blank, gave the limits of detection of Cu, Ni and Cd based on a sample volume of 25 ml were 0.07, 0.054 and 0.016 ng

ml-‘, respectively. Comparing these results with that obtained by the flow injection-ICP systems based on preconcentration with Chelex-100 resin [14,24], the detection power achievable in this work is 2-4 times better for the elements studied. Further comparison of the proposed method with the on-line preconcentration on either silica-immobilized 8-quinolinol[17] or Chelex-100 (181 followed by ICP-mass spectrometric (MS) determination reveals that in both cases the detection limits are low enough to permit the analysis of open sea water. With the ICP-MS method the preconcentration from NASS-2 sea water requires lo-ml aliquots, whereas 100~ml aliquots are needed in this work with ICP-AES as measuring instrument. The excellent resistance to swelling of silica with changes in solvents and acids is an advantage of silica-immobilized 8 quinolinol over polymer-based ion exchanger such as Chelex-100 [121. The use of silica-immobilized 8-quinolinol, however, suffers from the disadvantage of rapid hydrolysis of the silica framework occurred at high pH range [19]. Thus, this tech-

02

o.tsCadmium

0 _z

Nickel

0.8 -

Copper

TABLE 3 Analytical results for the determination of Ni, Cu and Cd in the reference standard seawater sample (NASS-2) Concentration (pg 1-l)

Certified Founda

Ni*+

CU2f

Cd’+

0.257 f 0.027 0.238 f 0.038

0.1O9f0.011 0.119+0.008

0.029 f 0.004 0.025 f 0.006

a Results given are mean f standard deviations (n = 3).

o3 0

2

Depth

3

.

5

(km)

Fig. 6. Concentration-depth profiles of Cu, Ni and Cd in sea water.

C.-R L.an and M.-H. Yang/Anal. Chim. Acta 287 (1994) 111-117

nique is not applicable to all the sample solutions without a preliminary treatment prior to analysis. The method proposed was subsequently employed for the determination of Cu, Ni and Cd in open sea water samples. The concentration-depth profiles of Cu, Ni and Cd in sea water collected from the Pacific Ocean are shown in Fig. 6. The concentrations of Cu, Ni and Cd are less depleted in surface sea water and enriched in the deep ocean. For the three metals, their characteristic vertical profiles presented in this work are similar to the vertical profiles investigated in the California current reported by Bruland et al. [25]. The authors wish to express their thanks to Professor S.C.Pai of the Institute of Oceanography, National Taiwan University for providing the open sea water samples collected from the Pacific ocean. We thank the National Science Council of the Republic of China for the support of this work (Grant No. NSC 81-020%MOO7-106)

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